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October 2015
Inkjet Printed Thin Film Electrodes for Lithium-Ion
Batteries
Stephen D. Lawes
The University of Western Ontario
Supervisor
Dr. Andy (Xueliang) Sun
The University of Western Ontario
Graduate Program in Mechanical and Materials Engineering
A thesis submitted in partial fulfillment of the requirements for the degree in Master of Engineering Science
© Stephen D. Lawes 2015
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INKJET PRINTED THIN FILM ELECTRODES FOR LITHIUM-ION BATTERIES
(Thesis format: Integrated Article)
by
Stephen Lawes
Graduate Program in Mechanical and Materials Engineering
A thesis submitted in partial fulfillment
of the requirements for the degree of
Master of Engineering Science
The School of Graduate and Postdoctoral Studies
The University of Western Ontario
London, Ontario, Canada
© Stephen Lawes 2015
Abstract
With the miniaturization of wireless electronics, the demand for ever-smaller energy
storage devices has increased. Thin film batteries can meet this need by providing higher
energy densities at smaller scales than conventional lithium-ion batteries. However, the
fabrication of thin films batteries by vapor deposition methods typically involves
expensive equipment and high temperatures, which limits their commercial application.
This thesis reports the development of an inexpensive inkjet printing method of
fabricating thin film electrodes for thin film lithium-ion batteries. Inks containing various
electrode materials were first developed and optimized in terms of physical properties to
ensure ideal jetting conditions. Then, thin film anodes comprised of silicon and titanium
dioxide were fabricated with a household inkjet printer and their physical and
electrochemical properties were characterized. Critical parameters involved in inkjet
printing (e.g. the polymer binder used and the electrode thickness) were thoroughly
studied, based on which high-capacity and stable anodes were finally achieved. Overall,
this work demonstrates the efficacy and future potential of using inkjet printing for
fabricating thin film battery electrodes.
Keywords
Inkjet Printing, Lithium-ion Batteries, Thin Film Batteries, Silicon, Titanium Dioxide,
PEDOT:PSS
ii
Co-Authorship Statement
Title: Printing nanostructured carbon for energy storage and conversion applications
Authors: Stephen Lawes, Adam Riese, Qian Sun, Niancai Cheng, and Xueliang Sun
This review paper was organized by Stephen under the guidance of Dr. Xueliang Sun and
Dr. Qian Sun. Stephen Lawes wrote the entirety of the manuscript except for the section
on fuel cells, which was written by Adam Riese and Niancai Cheng. The parts of this
paper used in this thesis were written by Stephen Lawes; no part of this thesis deals with
fuel cells. The final version of this manuscript has been published in Carbon 2015, 92,
150-176.
Title: High performance inkjet-printed silicon anodes for lithium-ion batteries
Authors: Stephen Lawes, Qian Sun, Andrew Lushington, Biwei Xiao, Yulong Liu, and
Xueliang Sun
The experimental work was carried out by Stephen Lawes under the supervision of Dr.
Xueliang Sun and Dr. Qian Sun. The manuscript was organized and written by Stephen
Lawes under the guidance of Dr. Xueliang Sun and Dr. Qian Sun. Co-authors contributed
by helping with related characterization, providing valuable discussions, and polishing
the draft. The final version of this manuscript is to be submitted for peer-reviewed
publication.
Title: High performance inkjet-printed titanium dioxide anodes for lithium-ion batteries
Authors: Stephen Lawes, Qian Sun, Hanting Guo, Mohammad Norouzi Banis, and
Xueliang Sun
iii
The experimental work was carried out by Stephen Lawes and Hanting Guo under the
supervision of Dr. Xueliang Sun and Dr. Qian Sun. The theoretical work was carried out
by Stephen Lawes and the manuscript was organized and written by Stephen Lawes
under the guidance of Dr. Xueliang Sun and Dr. Qian Sun. Co-authors contributed by
helping with related characterization, providing valuable discussions, and polishing the
draft. The final version of this manuscript is to be submitted for peer-reviewed
publication.
iv
Acknowledgments
Over the past two years that I’ve spent in Dr. Sun’s Nanomaterials and Clean Energy
Group, I’ve had the chance to meet and work with a number of great people. First, I’d
like to thank Dr. Andy Sun for providing me with this opportunity and for his full support
throughout my master’s degree. He always provided guidance and encouragement during
my thesis project and is an invaluable resource for anything related to nanomaterials and
energy storage and conversion.
I am also grateful to our lab manager and research engineer, Kathy Li. She has always
been interested in the well-being of all our group members and her kindness is unending.
Her hard work to keep the lab running smoothly has played a large role in the success of
this group and she has always been willing to help whenever I needed it.
I would also like to express my greatest thanks to Dr. Qian Sun. He has supervised and
guided me from my very first day in the lab and has trained me on nearly everything I
know about batteries. Despite his tremendous workload, he has always been available to
spare some time for my questions. He is an excellent mentor. Qian is the most
knowledgeable person that I know in this field and our many discussions have taught me
more than any textbook or course ever could.
I would also like to thank my family for their support throughout my life, both my
parents and my brother.
Lastly, thank you to all my friends and fellow lab members who have contributed to
making UWO and London a great place to work and live.
v
Table of Contents
Abstract ............................................................................................................................... ii
Co-Authorship Statement................................................................................................... iii
Acknowledgments............................................................................................................... v
List of Tables ..................................................................................................................... ix
List of Figures ..................................................................................................................... x
Chapter 1 ............................................................................................................................. 1
1 Introduction and Literature Review ............................................................................... 1
1.1 Introduction to Lithium-Ion Batteries ..................................................................... 1
1.1.1
Fundamental Principles of Lithium-Ion Batteries....................................... 1
1.1.2
Thin Film Lithium-Ion Batteries................................................................. 3
1.1.3
Anode Materials for Lithium-Ion Batteries ................................................ 5
1.2 Introduction to Printing Technologies .................................................................. 11
1.2.1
Principles of Inkjet Printing ...................................................................... 12
1.2.2
Other Printing Technologies ..................................................................... 16
1.3 Battery Electrode Fabrication Methods ................................................................ 20
1.3.1
Conventional Fabrication Methods ........................................................... 20
1.3.2
Inkjet Printing of Battery Electrodes ........................................................ 22
1.4 Thesis Objectives .................................................................................................. 25
1.5 Thesis Organization .............................................................................................. 26
References .................................................................................................................... 27
Chapter 2 ........................................................................................................................... 36
2 Experimental and Characterization Methods ............................................................... 36
2.1 Inkjet printing of nanomaterials ............................................................................ 36
vi
2.1.1
Ink preparation .......................................................................................... 36
2.1.2
Inkjet printing............................................................................................ 38
2.2 Characterization methods...................................................................................... 39
2.2.1
Physical and compositional characterization ............................................ 39
2.2.2
Electrochemical characterization .............................................................. 42
References .................................................................................................................... 45
Chapter 3 ........................................................................................................................... 46
3 High performance inkjet-printed silicon anodes for lithium-ion batteries ................... 46
3.1 Introduction ........................................................................................................... 46
3.2 Methods................................................................................................................. 48
3.2.1
Silicon ink preparation .............................................................................. 48
3.2.2
Electrode and coin cell preparation........................................................... 48
3.2.3
Characterization ........................................................................................ 49
3.3 Results ................................................................................................................... 49
3.3.1
Electrode fabrication and physical characterization ................................. 49
3.3.2
Electrochemical performance ................................................................... 51
3.3.3
Physical characterization .......................................................................... 55
3.4 Discussion ............................................................................................................. 57
3.5 Conclusions ........................................................................................................... 59
Acknowledgements ...................................................................................................... 60
References .................................................................................................................... 60
Supporting Information ................................................................................................ 65
Chapter 4 ........................................................................................................................... 70
4 High performance inkjet-printed titanium dioxide anodes for lithium-ion batteries ... 70
4.1 Introduction ........................................................................................................... 70
vii
4.2 Methods................................................................................................................. 72
4.2.1
TiO2 ink preparation ................................................................................. 72
4.2.2
Electrode and coin cell preparation........................................................... 73
4.2.3
Characterization ........................................................................................ 73
4.3 Results and Discussion ......................................................................................... 74
4.3.1
Printing process and morphological characterization of the inkjetprinted thin films ....................................................................................... 74
4.3.2
Structural and compositional characterization of the inkjet-printed thin
films .......................................................................................................... 76
4.3.3
Electrochemical characterization .............................................................. 78
4.4 Conclusions ........................................................................................................... 83
Acknowledgements ...................................................................................................... 83
References .................................................................................................................... 84
Chapter 5 ........................................................................................................................... 88
5 Conclusions and Future Work...................................................................................... 88
5.1 Conclusions ........................................................................................................... 88
5.2 Recommendations for Future Work...................................................................... 90
References .................................................................................................................... 91
Curriculum Vitae .............................................................................................................. 93
viii
List of Tables
Table 1.1. Alloys of silicon and lithium with their respective unit cell volumes. Data
gathered from [23]. ............................................................................................................. 8
Table 1.2. Comparison of the four printing techniques discussed in this chapter. ........... 20
ix
List of Figures
Figure 1.1. Schematic illustration showing the basic operating principles of LIBs [2]...... 2
Figure 1.2. Cross-sectional schematic illustration of a TFB. .............................................. 3
Figure 1.3. Weight and volume percentages of the different components in a typical LIB.
Data gathered from [4, 7]. ................................................................................................... 4
Figure 1.4. Volumetric and gravimetric energy densities of different types of batteries [4].
............................................................................................................................................. 5
Figure 1.5. The specific capacities and operating voltages of various anode materials for
LIBs..................................................................................................................................... 6
Figure 1.6. Plot showing how the full cell theoretical specific capacity is related to the
capacity of the anode, for a cell with a silicon anode and LiCoO2 cathode [30]. ............... 9
Figure 1.7. Crystal structures of TiO2 polymorphs commonly used for LIB anodes [33].
........................................................................................................................................... 11
Figure 1.8. Schematic illustration of the inkjet printing process [47]. ............................. 13
Figure 1.9. Influence of ink properties on (a) droplet formation [57] and (b) droplet
spreading [58]. The shaded area in (b) is the region of high quality inkjet printing. ....... 15
Figure 1.10. Schematic illustration of screen printing [46]. ............................................. 18
Figure 1.11. Schematic illustration of the transfer printing process [84]. ........................ 19
Figure 1.12. Standard electrode fabrication process for LIBs [94]................................... 21
Figure 1.13. Schematic illustration of the inkjet printing process for battery electrodes. 23
Figure 1.14. SEM images of the (a) surface and (b) cross-section of an inkjet-printed
LTO anode (the inset is a TEM image of LTO particles in the film) [38]. ...................... 24
x
Figure 1.15. (a) Graphene/TiO2 ink for printing flexible thin film LIB electrodes.
(b)Schematic diagram showing the major components of the cell. (c) Demonstrating the
flexibility of a printable graphene electrode [97]. ............................................................ 25
Figure 2.1. A schematic illustration (left) and photo (right) of the U-tube viscometer used
to measure ink viscosity. ................................................................................................... 37
Figure 2.2. Injecting a silicon ink solution into an HP 61 ink cartridge (left) and the HP
Deskjet 2540 inkjet printer used to print LIB anodes (right). ........................................... 38
Figure 2.3. A photo of the Hitachi S-4800 field emission SEM equipped with and EDX
spectrometer. ..................................................................................................................... 40
Figure 2.4. A photo of the Bruker D8 Advance XRD instrument. ................................... 40
Figure 2.5. A photo of the Nicolet 6700 FTIR spectrometer. ........................................... 41
Figure 2.6. A photo of the HORIBA Scientific LabRAM HR Raman spectrometer. ...... 41
Figure 2.7. A photo of the TA Instruments SDT Q600 TGA. .......................................... 42
Figure 2.8. A photo of the Arbin BT-2000 battery test station. ........................................ 44
Figure 2.9. A photo of the VMP3 multichannel potentiostat 3/Z for CV and EIS
measurements. ................................................................................................................... 45
Figure 3.1. Procedure used to print SiNP anodes on copper foil. (a,b) First, the ink was
prepared by mixing SiNPs, carbon black, and the polymer binder in water. (c) After 3
hours of sonication, the solution was well-mixed and (d) injected into an inkjet printer
cartridge and printed. (e) Photograph of the Western University logo printed with the
SiNP ink. (f) Optical photographs and SEM images of the inkjet-printed SiNP anode
films on copper foil. Scale bars red, white, and black represent 3 cm, 5 cm, and 500 nm,
respectively. ...................................................................................................................... 51
Figure 3.2. (a) Cycling performance of inkjet-printed silicon anodes prepared with
different polymer binders at 0.1C. (b) Voltage profiles of selected cycles for the
xi
PEDOT:PSS (DMC:FEC) cell from (a). (c, d) Rate capability measurements of Si anodes
with PEDOT:PSS binder in DMC:FEC electrolyte. (e) Limited depth-of-discharge tests
performed to a capacity cut-off of 1000 mAh g-1. The Coulombic efficiency shown is for
PEDOT:PSS (DMC:FEC). (f) Voltage profiles of selected cycles for the PEDOT:PSS
(DMC:FEC) cell shown in (e)........................................................................................... 54
Figure 3.3. Ex-situ (a) SEM images and (b) FTIR spectra of SiNP anodes with
PEDOT:PSS binder taken at three stages of a discharge/charge cycle: before cycling
(pristine), after first discharge (lithiated), and after first full discharge and charge
(delithiated). The green highlighted regions in the FTIR spectra indicate the PEDOT:PSS
thiophene C=C and C-S stretching vibrations, the blue highlighted regions indicate SEI
formation and residual electrolyte salt, and the purple highlighted regions indicate the
sulfonic acid groups. (c) The structure of PEDOT:PSS. Scale bars are 500 nm. ............. 57
Figure 3.4. Schematic illustration of the proposed mechanism explaining the
electrochemical performance of anodes prepared with different binders. The use of nonconductive binders (CMC, Na-alginate, and PVP) leads to electrical isolation of SiNPs.
In the case of CMC and Na-alginate, electrical isolation occurs from the start, leading to
poor initial capacity, while in the case of PVP the conductive carbon black network is
destroyed during large volume changes. With PEDOT:PSS, the SiNPs remain electrically
connected throughout charging/discharging and are therefore able to maintain a stable
cycling capacity. ............................................................................................................... 59
Figure S3.1. (a) Photographs of printed films with different numbers of printing passes.
Too few passes (10x) results in non-uniform films that cannot be used for electrodes.
More passes (25x and 40x) result in uniform films. However, too many passes decreases
capacity. (b) Discharge capacity of Si anodes with PEDOT:PSS binder and DMC:FEC
electrolyte, with different numbers of printing passes. ..................................................... 65
Figure S3.2. EDX mapping of inkjet printed Si films with four different binders, each
prepared with 25 printing passes. The top images show the SEM image of where the
mapping was performed and the bottom images show the EDX mapping for Si. All four
xii
films show uniform Si dispersion throughout, with some fluctuations due to variations in
the surface morphology. Scale bars are 2 μm. .................................................................. 65
Figure S3.3. Cycling performance of Si anodes with PEDOT:PSS binder and DMC:FEC
electrolyte at 1 C. The average CE over 2000 cycles is 99.8%. ....................................... 66
Figure S3.4. EIS spectra of inkjet printed SiNP anodes prepared with PEDOT:PSS
binder. Cells were fabricated with EC:DEC:EMC electrolyte. The electrode starts with
relatively high impedance that decreases during cycling. This may be due to increased
electrolyte wetting of the printed anode layers and the higher conductivity of Li-doped Si,
as discussed in [30]. .......................................................................................................... 66
Figure S3.5. Raman spectra taken at three stages of a discharge/charge cycle: before
cycling (pristine), after one discharge cycle (lithiated), and after one full discharge/charge
cycle (delithiated). The peak at 1435 cm-1 represents thiophene ring stretching
vibrations, from the thiophene ring present in the PEDOT molecules. Similar to the C=C
and C-S thiophene stretching shown in the FTIR spectra, the thiophene ring stretching
peak disappears after lithiation, due to stretching being inhibited by the already stretched
polymer chains. However, after delithiation there is no return of this peak. This may be
due to the SEI binding to the PEDOT molecules and preventing further stretching of the
thiophene rings. ................................................................................................................. 67
Figure S3.6. TGA of a film of inkjet printed SiNP anode material prepared with
PEDOT:PSS binder, used to determine the mass percentage of Si in the printed films.
Since Si is stable in the temperature range used here, all mass loss is due to the removal
of the polymer binder and carbon black. We therefore used the minimum point (46%) on
the curve as the % mass of Si in the anode for calculating charge/discharge rates and
capacities. To confirm that this assumption was valid, we also measured a sample of Si
powder. No mass loss was observed; in fact, oxidation of the SiNPs resulted in a mass
gain. This increase is negligible (2%) at the minimum point of the printed Si anode curve
and will anyways result in lower reported capacity values............................................... 68
xiii
Figure S3.7. Cyclic voltammetry curves of inkjet printed SiNP anodes prepared with four
different binders. All cells were fabricated with EC:DEC:EMC electrolyte. ................... 69
Figure 4.1. Ink solutions prepared with three different polymer binders: (a) PVP, (b)
PVDF, and (c) PEDOT:PSS. (d) Optical image of the TiO2 nanoparticle solution with
PEDOT:PSS binder inkjet-printed on copper foil with 25 layers. SEM images of the
printed film (e) before and (f) after cycling. ..................................................................... 75
Figure 4.2. Cross-sectional SEM images of TiO2 films printed with different number of
layers. ................................................................................................................................ 76
Figure 4.3. XRD patterns of inkjet-printed TiO2 films before and after cycling. A
reference of TiO2 powder is shown for comparison. Peaks marked with an asterisk (∗) are
from the underlying copper foil substrate. ........................................................................ 77
Figure 4.4. TGA curves of an inkjet-printed TiO2 anode and each of its components
individually. ...................................................................................................................... 78
Figure 4.5. (a) Galvanostatic cycling at 0.1C and (b) EIS measurements of three inkjetprinted TiO2 anodes with different thicknesses, controlled by the number of printed
layers. 15 layers = 1.9 μm, 25 layers = 3.0 μm, and 35 layers = 4.4 μm. ......................... 80
Figure 4.6. (a) Rate capability of a TiO2-25 anode and (b) the corresponding 2nd-cycle
charge and discharge voltage profiles at each different current rate. (c) Long-term cycling
performance of TiO2-25 anodes at varying charge/discharge rates. ................................. 81
Figure 4.7. (a) CV scans of inkjet-printed TiO2 anodes at varying scan rates. (b) Plot of
peak current vs. square root of the scan rate for both anodic (top) and cathodic (bottom)
peaks. ................................................................................................................................ 82
xiv
1
Chapter 1
1
Introduction and Literature Review
*Parts of this chapter have been published in Carbon, vol. 92, pp. 150-176, 2015.
1.1 Introduction to Lithium-Ion Batteries
Lithium-ion batteries (LIBs) were first commercialized by Sony in 1991. Today, LIBs are
the most widely-used portable energy storage devices, with the global market estimated
to grow from $15 billion in 2012 to $32 billion in 2016 to $76 billion in 2020 [1]. LIBs
have been adopted for a variety of portable electronics, such as cell phones, cameras, and
laptops, and are becoming increasingly used in electric vehicles. This is due to their
unique merits of high energy density, low self-discharge rate, no memory effect, and
relatively low price.
1.1.1 Fundamental Principles of Lithium-Ion Batteries
LIBs are comprised of electrochemical cells connected in series and/or parallel to achieve
a desired voltage and capacity. A typical cell is made of two electrodes separated by a
separator and electrolyte, through which lithium ions can travel. Commercial LIBs are
typically composed of a graphite negative electrode (anode) and a lithium metal oxide
positive electrode (cathode) separated by a polymer separator immersed in a nonaqueous
liquid electrolyte (Figure 1.1). During charging, lithium ions are extracted from the
cathode, migrate through the electrolyte, and intercalate between graphite layers in the
anode, forming LiC6. During discharge, the reverse electrochemical reaction occurs –
lithium ions deintercalate from the graphite and reinsert into the cathode. These processes
produce and consume electrons via oxidation and reduction reactions, respectively. The
electrons are then free to move around the external circuit, storing their energy as
chemical potential energy (charging) or supplying energy to an external load
(discharging).
2
Figure 1.1. Schematic illustration showing the basic operating principles of LIBs [2].
The half-reaction at the anode is given by:
 +  + +  − ↔  
(1.1)
And the half-reaction at the cathode is given by:
2 ↔ 1− 2 +  + +  −
(1.2)
The overall cell reaction is given by:
2 ↔   + 1− 2
(1.3)
where the forward direction is charging and the reverse direction is discharging. MO2 is a
transition metal oxide, typically CoO2, but occasionally NiO2, MnO2, or FePO4.
The miniaturization of electronic devices over the last thirty years has led to an increased
demand for smaller energy sources. However, a smaller battery inherently contains less
electrochemically active material, reducing the amount of energy that can be stored. And
as devices such as laptops, wireless sensors, and medical implants become smaller and
more complex, they require more energy to operate. This compounding problem of size
reduction with increased energy demand has fueled intensive research in the field of
portable energy storage. The United States Department of Energy (DOE) has set a target
energy density for lithium-ion batteries (LIBs) of 750 Wh/L by 2020 [3]. There are two
main approaches that researchers are using to meet this goal: (1) designing novel
fabrication techniques to load more material into a cell and (2) developing new materials
3
with higher energy densities. This thesis will aim to combine both of these methods, by
developing a technique for fabricating thin film batteries comprised of novel anode
materials for next-generation LIBs.
1.1.2 Thin Film Lithium-Ion Batteries
As portable electronics decrease in size, a need has arisen for more compact batteries.
Oftentimes, the size of a device is limited by the size of its battery; it cannot be made
smaller without major tradeoffs in battery life. Over the past two decades, many studies
have focused on reducing the size of LIBs while maintaining their capacity [4, 5]. Thin
film batteries (TFBs) have been developed to meet this challenge, with the first lithium
TFB reported in 1983 [6]. A schematic illustration of the design of a typical TFB is
shown in Figure 1.2.
Figure 1.2. Cross-sectional schematic illustration of a TFB.
The various components of TFBs are typically on the order of microns to hundreds of
microns thick, giving a full cell thickness on the order of millimeters or less. Because of
their reduced size, TFBs are fundamentally comprised of less material. This includes both
the electrochemically active materials and the inactive components.
4
Figure 1.3. Weight and volume percentages of the different components in a typical
LIB. Data gathered from [4, 7].
Typical weight and volume percentages of the various battery components are shown for
a standard LIB in Figure 1.3. As shown, the active materials account for only 57.4% of
the total weight and 30% of the total volume, while the cell packaging, electrolyte, and
other inactive materials account for 42.6% of the weight and 70% of the volume. In
TFBs, a solid-state electrolyte is typically used, which can contribute to reducing both the
weight and volume of the electrolyte and eliminating the need for a separator and
excessive packaging to properly contain a liquid electrolyte. Thus, the total loading
percentage of active materials is increased, resulting in higher volumetric and gravimetric
energy densities, as shown in Figure 1.4.
Although the energy density can be improved by using TFBs, a number of challenges still
exist. A shorter distance between electrodes makes the battery more likely to short
circuit, a major safety concern [8]; the use of a solid-state electrolyte can prevent this
problem, but results in lower lithium-ion diffusion rates. And mechanical stresses during
cycling of the battery may cause the layers to separate from one another, resulting in poor
cycle life [9, 10]. The fabrication of TFBs can also pose a problem due to adhesion issues
between the various layers as well as the temperature limits of the substrate [11].
Conventional methods for fabricating full TFBs or thin film electrodes for TFBs mainly
include sputtering [12, 13], chemical vapor deposition [14, 15], pulsed laser deposition
[16, 17], spin coating [18], and sol-gel methods [19, 20]. However, these can require
5
expensive equipment, high temperatures, and/or post-annealing treatments that can
damage the films and the substrate. Therefore, a facile and cost-efficient method of thin
film electrode fabrication for LIBs is in high demand. Standard fabrication methods for
LIBs and thin film LIBs are discussed in more detail in section 1.3.
Figure 1.4. Volumetric and gravimetric energy densities of different types of
batteries [4].
1.1.3 Anode Materials for Lithium-Ion Batteries
Graphite has been the most common anode material for LIBs over the past two decades
due to its decent specific capacity, long cycle life, chemical stability, low cost, and flat
working potential. However, as LIBs are used for new applications requiring high energy
outputs, particularly electric vehicles and household energy storage, a number of
problems with graphite anodes have arisen. First, the theoretical capacity of graphite (372
mAh g-1) is not sufficient to achieve the high energy densities required for future
demands. Second, graphite’s relatively low lithium-ion diffusion rate limits its cycling
performance at high current densities and therefore its power density [21]. And third,
when a LIB is fully charged, the graphite anode becomes lithiated and has a similar
reactivity to lithium metal [22]. Upon catastrophic failure of the cell, a highly exothermic
6
reaction can take place, leading to ignition of the flammable electrolyte and a possible
explosion, which is a major safety concern. Therefore, new anode materials with higher
specific capacities, higher rate capabilities, and improved safety are needed.
A number of alternative anode materials have been intensively developed and studied
over the past decade, and are shown in Figure 1.5. The anode’s specific capacity and
working potential are both important to developing cells with high energy density. We
want to maximize the capacities of the anode and cathode as well as the potential
difference between them to maximize the energy density. However, these are not the only
considerations when choosing an electrode material. Cycle life, rate capability, cost, and
safety must also be taken into account. For example, lithium metal has a very high
theoretical specific capacity of 3860 mAh g-1. However, dendrite formation, chemical
reactivity, and safety concerns have so far prevented its widespread adoption in
rechargeable batteries.
Figure 1.5. The specific capacities and operating voltages of various anode materials
for LIBs.
The criteria for LIB anode materials are summarized as follows:
(1) High specific capacity. This results in a higher energy density of the cell.
7
(2) Low working potential. A greater difference between the anode and cathode
working potentials leads to higher energy density of the cell.
(3) Long cycling life (high Coulombic efficiency). The capacity must not fade too
rapidly during cycling to ensure the cell can be charged/discharged for hundreds
or thousands of cycles.
(4) High rate capability. The cell must be able to be charged and discharged in a
reasonable amount of time
(5) Chemical stability. The anode material should not continuously react with the
electrolyte or undergo other side reactions within the electrochemical window of
the cell.
(6) Low cost. For commercial purposes, the cost of the material and processing must
be justified by the cell’s performance.
(7) Safety. The anode material should have low toxicity and not react violently when
exposed to the atmosphere.
The definitions of “high specific capacity” or “long cycling life”, for example, depend on
the particular application and there are typically trade-offs between these criteria. As
examples, cycling at higher rates tends to decrease the cycling life, and safer anode
materials usually operate at higher potentials. It is important to understand the needs of
the specific application when choosing or developing an anode material.
In this thesis, two promising candidates for next-generation LIB anodes are investigated
by inkjet printing fabrication technology: silicon and titanium dioxide. The advantages
and challenges of using these alternative anode materials in LIBs are introduced here.
1.1.3.1.1
Silicon Anodes
Silicon shows great potential to be used as an anode material for LIBs in the near future.
It possesses a very high theoretical capacity of 4200 mAh g-1, due to its ability to alloy
with up to 4.4 lithium atoms per silicon atom. Silicon also has a long discharge plateau at
a low operating voltage, providing a stable voltage while discharging. In addition, it is
8
nontoxic and is one of the most abundant elements in the earth’s crust, making it an
economical choice for LIB anodes.
Unlike graphite and titanium dioxide, silicon forms alloys with lithium at different stages
of the charging and discharging processes. The half-reaction at the anode is given by:
 +  + +  − ↔  
(1.4)
where x is the molar ratio of lithium to silicon and varies from 0 to 4.4. The forward
direction is charging and the reverse direction is discharging. Silicon forms four distinct
alloys with lithium during electrochemical cycling, which are given in Table 1.1 along
with their respective unit cell volumes.
Table 1.1. Alloys of silicon and lithium with their respective unit cell volumes. Data
gathered from [23].
Crystal Structure
Unit Cell Volume (Å3)
Cubic
160.2
Li12Si7 (Li1.71Si)
Orthorhomic
243.6
Li14Si6 (Li2.33Si)
Rhombohedral
308.9
Li13Si4 (Li3.25Si)
Orthorhomic
538.4
Li22Si5 (Li4.4Si)
Cubic
659.2
Compound
Si
As can be seen, the alloying process leads to a volume expansion up to 400% of the
initial volume of silicon upon full lithiation. This creates larges internal stresses in the
silicon particles, leading to pulverization of the electrode and loss of electrical contact
due to repeated volume expansion and contraction during cycling. This ultimately results
in very short lifetimes for silicon anodes and has so far prevented their widespread
adoption. A lot of research has focused on developing novel nanostructured silicon
electrodes, such as nanowires [24], nanotubes [25], hollow nanospheres [26], and coreshell structures [27, 28] to overcome the poor cycling stability of silicon.
9
Thin film silicon electrodes may alleviate this problem by limiting the total volume
expansion of the electrode and increasing the critical fracture stress, according to the
Griffith-Irwin equation [29]:
 =

(1.5)
√ℎ
where σfracture is the critical fracture stress, KIc is the fracture toughness of the material,
and h is the thickness of the film. Based on this, thinner films require greater stress to
fracture and therefore have better cycling stability than thicker films.
In the design of LIBs, limiting the specific capacity to a lower depth of discharge (DOD)
in order to increase the cycle life is acceptable. In fact, above a certain point, increasing
the capacity of the anode does not increase the capacity of the full cell significantly.
Figure 1.6 shows the theoretical capacity of a full cell with a silicon anode and LiCoO2
cathode as a function of the capacity of the silicon anode. From this graph, it is clear that
only minor gains are made by increasing the capacity of the silicon negative electrode
above about 1000 mAh g-1. Therefore, limiting the anode’s capacity to increase the
lifetime will not significantly reduce the cell’s overall capacity. Until higher capacity
cathodes are developed, this should be considered an acceptable tradeoff. For these
reasons, silicon is considered a very promising candidate for the next generation of LIBs.
Figure 1.6. Plot showing how the full cell theoretical specific capacity is related to
the capacity of the anode, for a cell with a silicon anode and LiCoO2 cathode [30].
10
1.1.3.1.2
Titanium Dioxide Anodes
Titanium dioxide (TiO2) is another promising anode material for LIBs in applications
where safety is a primary concern or where high current densities are required. This is
because TiO2 anodes have a high working potential (1.5-1.8 V vs. Li/Li+), meaning that
side reactions with the electrolyte and lithium deposition on the Cu current collector are
avoided, unlike with graphite anodes. Furthermore, during lithium insertion TiO2
undergoes a volume change of only 4% [31], making it very stable at high cycling rates
and results in long cycle lives.
TiO2 is an intercalation-type compound, in which lithium ions can insert into and be
stored in its crystal structure. The half-reaction at the anode is given by:
2 +  + +  − ↔  2
(1.6)
where x is the insertion coefficient. The forward direction is charging and the reverse
direction is discharging. In theory, TiO2 can host up to 1 mole of lithium per mole of
TiO2, corresponding to a theoretical capacity of 335 mAh g-1. However, experimentally
lower insertion coefficients are measured, due to anisotropic and slow lithium-ion
diffusion rates in lithiated TiO2 [32].
There are four different crystal structures of TiO2 that have been used as LIB anode
materials (Figure 1.7). For bulk anatase and rutile TiO2, which are the most studied for
LIBs, the maximum insertion coefficients are 0.5 and 0.1, respectively [34]. These values
correspond to capacities of 167.5 mAh g-1 and 33.5 mAh g-1, respectively, which are
much lower than that of graphite. However, the capacity dramatically increases when
TiO2 is made into nanostructures, due to a larger electrode/electrolyte interfacial area and
shorter electron and lithium ion diffusion lengths in nano-sized TiO2 [33]. Nanoparticles
of anatase and rutile TiO2 are reported to have maximum capacities of 285 and 251 mAh
g-1, respectively.
11
Figure 1.7. Crystal structures of TiO2 polymorphs commonly used for LIB anodes
[33].
At first glance, it may appear that the low capacity of TiO2 makes it an unattractive anode
material for LIBs. However, its low volume expansion, high stability, and the ability to
easily tailor its structure make it a great candidate for long-life and high-rate LIBs, which
cannot be achieved with other anode materials.
1.2 Introduction to Printing Technologies
The fundamental principles of four major printing technologies are introduced here.
Inkjet printing is by far the most common of these techniques for depositing
nanomaterials onto substrates of varying size, surface energy, and flexibility. Screen
printing and transfer printing are popular for specific applications (e.g. in the textile
industry and for flexible electronics), while 3D printing is an emerging technology with
the potential to replace many conventional prototyping and manufacturing processes.
12
1.2.1 Principles of Inkjet Printing
Inkjet printing is an additive technique for patterning two-dimensional structures onto a
substrate. It precisely deposits ink droplets at desired locations without pre-patterning the
substrate, making it simple to use while minimizing wasted material. It has been widely
adopted in industry as a rapid fabrication technique that can be used on a variety of
substrates, with applications ranging from advertisements to printed circuit boards.
Inkjet printing has also been successfully applied to fabricating energy storage and
conversion devices, such as battery electrodes [35-40], supercapacitors [41-45], and solar
cells [46-50]. It can be used to fabricate thin films or patterns of uniform thickness, which
can be controlled by the number of layers printed on top of one another. Inkjet printing
technology has many advantages over other fabrication techniques, including costeffectiveness, ease of use, minimal wasted material, scalability, and the ability to deposit
designed patterns.
Principally, inkjet printing can be divided into continuous inkjet (CIJ) and drop-ondemand (DOD) methods. CIJ printing involves pumping liquid ink through a nozzle
where a continuous stream of droplets is formed by a vibrating piezoelectric crystal.
Some droplets are charged by passing them through an electric field, which can be varied
to control the degree of charging. The droplets then pass through another electric field,
with the more highly charged droplets deflecting more than those with a lesser charge. In
this way an image can be produced, with unused ink being collected in a gutter and
recycled. On the other hand, DOD printers eject material only when required. This
involves forcing ink out of a series of nozzles mounted on a print head. Because DOD
printers do not recycle ink, which may result in degradation upon exposure to
atmosphere, they are the standard choice for printing functional materials. In addition,
DOD printing generally wastes less material; it is therefore a more suitable technique for
printing expensive materials.
The three main stages of inkjet printing are illustrated in Figure 1.8: droplet ejection,
droplet spreading, and droplet solidification. The print head is first moved to the desired
position, where droplets are ejected through the nozzle and travel to the substrate. Upon
13
impact, they spread along the surface and join with other droplets, forming a thin film of
liquid ink. Finally, the solvent evaporates, leaving the solid contents of the ink remaining
on the substrate.
To achieve droplet ejection in DOD printers, there are two main types of inkjet print
heads: thermal and piezoelectric. Thermal print heads contain a resistor inside the ink
chamber which, upon an applied voltage, will superheat the ink above the bubble
nucleation temperature. The bubble expands, forcing ink out of the chamber and through
the nozzle. Once the ink is ejected, the chamber rapidly cools, allowing more ink to refill
the chamber. This entire process occurs within a few microseconds [51]. Piezoelectric
inkjet print heads, on the other hand, contain a piezoelectric element that pulsates upon
electrical excitation, which forms a pressure wave that forces ink out of the chamber. The
vibration of the piezoelectric material can be precisely tuned to control droplet ejection
from the nozzle. Typically, inkjet print heads comprise of hundreds of ink chambers and
nozzles to achieve high throughput. A higher number of nozzles allows for printing of
higher resolution patterns in shorter time frames, an important metric for large-scale
production.
Figure 1.8. Schematic illustration of the inkjet printing process [47].
Thermal print heads are generally cheaper and require less maintenance than
piezoelectric print heads because they contain no moving parts. The cartridge on which
the print heads are mounted can simply be replaced by the user for relatively low cost if
14
the nozzles become clogged or broken. Piezoelectric print heads, however, typically
require more expensive maintenance procedures by a technician if their piezoelectric
crystals become damaged. On the other hand, piezoelectric print heads are preferable for
printing a wide range of functional materials since they do not require any heating of the
ink, which can result in degradation of active material in the ink. In addition, a wider
range of solvents can be used with the piezoelectric systems, including water, oils, and
organic solvents; thermal print heads are generally limited to aqueous inks due to the
nucleation temperature required for droplet ejection Also, the viscosity, surface tension,
and density of the ink must be more precisely controlled when using thermal print heads.
Typically the ink viscosity should be approximately 10 cP [52, 53]. Therefore,
piezoelectric print heads are more commonly used for inkjet printing of nanomaterials
due to their versatility in terms of the ink’s composition and physical properties.
For both types of print head, droplet ejection is dependent on the viscosity, surface
tension, and density of the ink, the shape and size of the nozzle, and the ejection velocity
of the droplet. These parameters are described by the Reynolds (Re), Weber (We), and
Ohnesorge (Oh) numbers. As shown in Figure 1.9a, there is a region in which Re, We,
and Oh are optimized for ideal jetting. In the figure, Z is defined as the reciprocal of the
Ohnesorge number, 1/Oh. Generally, high Z fluids (high viscosity, low surface tension)
will be unable to form droplets that can eject from the nozzle, whereas low Z inks (low
viscosity, high surface tension) will result in the formation of satellite droplets. Satellite
droplets lead to blurred line edges and misplaced drops, ultimately leading to lower
resolution. Therefore, when developing an ink formulation it is important to control these
physical properties with the addition of surfactants, thickeners, stabilizers, and other
additives.
The second stage of inkjet printing is droplet spreading, which is dependent on the
interactions between the ink and the substrate. When the droplet contacts the surface of
the substrate, inertial and capillary forces will influence the spreading behaviour, while
gravitational forces can be neglected [54]. Again, these forces are related by Re, We, and
Oh, as shown in Figure 1.9b. These parameters determine the surface energy and contact
angle of the liquid droplets on the substrate and can be controlled by varying the
15
viscosity, surface tension, and density of the ink, as well as the morphology, composition,
and temperature of the substrate. Generally, the simplest method to ensure good
spreading behaviour, and therefore high resolution of the printing process, is a surface
treatment on the substrate prior to printing [55].
Figure 1.9. Influence of ink properties on (a) droplet formation [57] and (b) droplet
spreading [58]. The shaded area in (b) is the region of high quality inkjet printing.
Inkjet solutions for printing functional materials are usually comprised of a nanomaterial
dispersed in a solvent, often with a surfactant. As an approximation, the size of the solids
in the ink should be less than one-fiftieth the size of the print head nozzles. Typically,
inkjet printer nozzles have diameters on the order of tens of microns, so the
16
nanomaterials should have dimensions less than a few hundred nanometers to prevent
clogging of the nozzles [56]. Agglomeration of the solids can also lead to clogging;
therefore, the choice of solvent is very important to achieve a uniform dispersion. When
the printer is not in use the solvent around the nozzles will evaporate, increasing the local
viscosity and disrupting ideal droplet formation. The time for this gelation to occur is
referred to as the latency time of the ink and is one of the major challenges of developing
inks for inkjet printing [49]. Inkjet printer inks must have relatively low viscosity,
compared to other techniques such as screen printing and 3D printing.
During and after spreading, the solvent evaporates and leaves behind a solid film.
Solidification is dependent on the solvent used and the temperature of the substrate.
During solvent evaporation there is usually a significant decrease in volume, especially
when the solid loading concentration is low, as is generally the case when printing
nanomaterials. This can be problematic if the ink is not well dispersed, as agglomeration
of the solid content may occur, resulting in the formation of disconnected islands. The
coffee-ring effect is another commonly encountered problem, in which the concentration
of solids becomes higher at the droplet perimeter compared to in the centre upon drying
[59]. This can lead to fluctuations in the conductivity within a printed pattern and
complications in device operation. A number of techniques have been shown to reduce
this coffee-ring effect [60-64]. More detailed explanations of the major stages involved in
inkjet printing can be found in references [57] and [58].
1.2.2 Other Printing Technologies
Three additional printing technologies are discussed here: screen printing, transfer
printing, and 3D printing. While inkjet printing is the most commonly-used technique for
printing nanomaterials, these other technologies have their own unique advantages and
are becoming increasingly popular for printing functional materials, especially 3D
printing.
17
Screen printing is a technique using a mesh mask to deposit ink onto a substrate in a
given pattern. The ink is placed on top of a thin plastic or metal screen that contains open
areas that the ink is forced through with a squeegee (Figure 1.10). Screen printing is
commonly used to apply patterns to textiles, wood, and glass. However, researchers have
also used it to fabricate electronic devices, such as transistors [65, 66], battery electrodes
[39, 67, 68], solar cells [69-72], and fuel cells [73-75].
Screen printing differs from inkjet printing in that it is not an additive process, so there is
more wasted material and generally less control of film thickness. However, it can be
simpler to make films as the ink can be of a wider range of viscosities and surface
tensions, whereas these parameters must be tightly controlled in the inkjet printing
process. Usually screen-printed films are much thicker than inkjet-printed films. Similar
to inkjet printing, screen printing uses inks composed of solids dispersed in a solvent.
However, for screen printing, the inks have a higher viscosity and are less volatile. The
solvent usually consists of water or an organic compound that is more stable at room
temperature, making the drying process slower than for inkjet printing. Due to the
required high viscosity and low volatility, the use of screen printing has been somewhat
limited in the field of energy storage and conversion. However, low solid concentration
and the coffee-ring effect are not problems when screen printing due to the high pressures
used and the fact that the ink is not deposited as droplets. Additionally, screen printing
can be adapted to roll-to-roll processing [69], meaning it may soon be a suitable
fabrication technique for large-scale production of batteries, supercapacitors, fuel cells,
and solar cells.
18
Figure 1.10. Schematic illustration of screen printing [46].
A third printing technique, transfer printing, involves patterning a material onto one
substrate initially and then moving it to a second substrate. It has recently become a
popular technique for printing a wide variety of materials, including graphene [76],
carbon nanotubes [77], quantum dots [78], DNA [79], and metal nanostructures [80].
While inkjet and screen printing are limited to resolutions of approximately 12 μm and 40
μm [57, 81], respectively, transfer printing can be used to pattern features below 100 nm
[82, 83].
The transfer printing process is shown schematically in Figure 1.11. First the material to
be transferred is synthesized or patterned on its initial substrate. It is then brought into
contact with the transfer substrate, generally a flexible elastomeric polymer, and peeled
off from the first substrate. Then the transfer substrate with the transfer material is
applied to the final substrate. In these last two steps, the temperature and pressure must
be tightly controlled in order to get defect-free transfers. Lastly, the transfer substrate is
removed and the transfer material remains.
Device fabrication can be performed separately from the assembly stage, which is
beneficial for a number of materials and applications, especially graphene and flexible
electronics. High quality graphene is usually synthesized by chemical vapour deposition
(CVD) at high temperature, which is limited to only a few substrates such as silicon or
19
copper. However, many applications cannot use graphene in this form and it must
therefore be transferred to a second substrate without introducing defects and
compromising its quality. This is especially critical for flexible device fabrication, which
are assembled on low melting point polymers, usually polyethylene terephthalate (PET).
Transfer printing can accomplish this process without exposing the material to any
chemicals or solvents that may damage it.
Figure 1.11. Schematic illustration of the transfer printing process [84].
Lastly, 3D printing is a rapidly growing printing technique and is finding applications in
an increasing number of fields. Recently it has been used to assemble a 3D battery [85],
supercapacitor electrodes [86], tissue engineering scaffolds [87], strain sensors [88], and
reduced graphene oxide nanowires [89].
3D printing of functional materials involves extruding material through a nozzle onto a
substrate. The pattern is passed over multiple times to build up a three-dimensional
structure. Similar to inkjet and screen printing, 3D printing uses an ink comprised of
solids dispersed in a solvent that dries upon contact with the substrate. By optimizing the
ink’s rheological properties, features below 10 μm have been achieved [90]. 3D printing
will be especially effective for energy storage and conversion device fabrication. For
20
example, it is predicted that 3D-printed architectures can double the volumetric energy
density of conventional batteries [85].
Table 1.2. Comparison of the four printing techniques discussed in this chapter.
Inkjet Printing
Screen
Printing
Transfer Printing
3D Printing
Minimum Line
Width
~12 μm [57]
~40 μm [81]
< 100 nm [82, 83]
~10 μm [90]
Film Thickness
10 nm [91] –
500 μm [46]
400 nm [81] –
500 μm [46]
0.34 nm [92] – 20
μm [93]
> 10 μm [90]
Drying Time
Seconds
Minutes to
hours
N/A
Seconds
Printing
Speed [46]
Slow to
Moderate
Moderate
Moderate to Fast
Slow
Dimensions
[46]
2D or quasi-3D
2D
2D
3D
Roll-to-roll
compatible
[46]
Yes
Yes
Yes
No
Low viscosity,
high volatility
High viscosity,
low volatility
None
Very high
viscosity, high
volatility
Solvent
1.3 Battery Electrode Fabrication Methods
1.3.1 Conventional Fabrication Methods
Battery electrode fabrication processes have been developed over the past few decades
with an emphasis on increasing throughput and decreasing costs, without compromising
the performance of the cells. Different companies may have different production
techniques with differing levels of automation, but their general processes will be similar.
And although overall battery pack assembly methods will vary between different cell
21
types (coin, cylindrical, prismatic, pouch), the production of the electrodes is the same.
Figure 1.12 shows a typical LIB electrode fabrication process developed by Siemens.
Figure 1.12. Standard electrode fabrication process for LIBs [94].
The first step is to mix the constituent electrode materials together (active material,
conductive carbon, and binder) in a non-aqueous solvent, usually N-methyl-2-pyrrolidone
(NMP). It is important to achieve a uniform dispersion with no agglomerations or
precipitates. Second, this mixture is coated onto the current collector (copper foil for
anodes and aluminum foil for cathodes). For thick electrodes, coating is usually
performed with a doctor blading or roller coating process to prepare films with uniform
thicknesses of hundreds of microns. For thin film electrodes, a number of different
techniques are used, which are discussed below. Third, after the solvent has been dried
out of the coated films, the electrodes are compressed to reduce porosity and ensure good
contact with the current collector. Fourth, the electrodes are dried in a low-humidity
chamber to remove any residual water. The fifth stage involves cutting the electrodes into
shape with a die punch or other machinery. And finally, the electrodes are stacked, with
many cells going into each battery to give the desired voltage and capacity.
22
The major difference between the fabrication of thick electrodes and thin electrodes is the
coating step. This is because the methods used for thick electrodes typically cannot be
used to form uniform films thinner than 100 μm. For TFBs, coating the electrode
materials on the current collector can be done using sputtering [12, 13], chemical vapor
deposition [14, 15], pulsed laser deposition [16, 17], spin coating [18], sol-gel [19, 20], or
other methods. All of these techniques can be used to form films with thicknesses below
1 μm. Film thickness can also be finely controlled, meaning the exact desired loading of
active material can be achieved. Additionally, by using techniques such as sputtering,
chemical vapor deposition, or pulsed laser deposition, one can eliminate the need for
polymer binders since the active materials can be strongly bonded to the surface. And if
the films are thin enough, conductive carbons are not necessary to carry electrons through
the electrode. This eliminates the initial mixing step and removes any chance of
inhomogeneity in the electrode [15].
However, these thin film deposition techniques also have their downsides. Sputtering,
chemical vapor deposition, and pulsed laser deposition require expensive, complex
equipment and high temperatures, which can damage the electrode materials or the
underlying substrate. Spin coating and sol-gel methods often require substrate pretreatments and are not easily scalable. Recently, an alternative fabrication method for
LFBs, inkjet printing, has been investigated as a solution to these problems.
1.3.2 Inkjet Printing of Battery Electrodes
Inkjet printing is an alternative technique for fabricating thin film electrodes and provides
a few advantages over the other techniques discussed above. First, it is inexpensive, with
household inkjet printers costing under $100 and larger piezoelectric printers designed
for functional materials costing in the range of a few thousand dollars. Second, inkjet
printing is simple. Once an ink formulation has been developed, printing can be
automated. Third, inkjet printing is scalable, with industrial-sized printers or roll-to-roll
inkjet printers available. And lastly, it can be used as a more efficient way to deposit
patterned electrodes. Instead of cutting the electrodes to size and shape in a punching
23
step, inkjet printers can deposit droplets only where necessary, eliminating wasted
material.
The inkjet printing process for battery electrodes can be divided into two main stages
(Figure 1.13). First, ink solutions must be prepared with tightly controlled physical
properties to ensure proper jetting. This involves optimizing the ink’s viscosity, surface
tension, and density to within specifications discussed in section 1.2 above. Inks are
typically comprised of nanoparticles of active material, conductive carbon, and a binder
dispersed in a volatile solvent. The nanoparticles must be small enough and have minimal
agglomeration in the solvent to prevent clogging of the printer’s nozzles. Second, after an
ink has been prepared it is transferred into a cleaned printer cartridge and inkjet printed
onto the current collector. In all studies in which battery electrodes were fabricated by
inkjet printing, multiple printing passes were necessary to achieve the desired thickness
and uniformity. After printing, the fabrication stages are similar to those discussed for
conventional battery electrodes: pressing, drying, punching (if necessary), and assembly.
Figure 1.13. Schematic illustration of the inkjet printing process for battery
electrodes.
The first instance of inkjet printing used to fabricate battery electrodes was in 2003 when
Xu et al. printed thin film MnO2 electrodes for alkaline (Zn/MnO2) batteries [35].
Following this work, researchers expanded this technique to LIBs, fabricating different
anode and cathode materials by inkjet printing. Tin oxide (SnO2) [36] and lithium titanate
(Li4Ti5O12, LTO) [38] nanoparticles have been printed as anodes, while lithium cobalt
oxide (LiCoO2) [37, 40, 95] and lithium manganese oxide (LiMn2O4) [96] have been
printed as cathodes.
24
As a representative example, Zhao et al. fabricated thin film LTO electrodes via an inkjet
printing process [38]. They synthesized an ink composed of LTO nanoparticles, acetylene
black, and carboxymethyl cellulose sodium (CMCS) binder dispersed in a water and
alcohol solution. Printing with a standard desktop Canon inkjet printer, they were able to
achieve uniform thin films with a thickness of about 1.7-1.8 μm by printing 10
consecutive layers. Figure 1.14 shows both surface and cross-sectional views of the
printed thin films on a gold substrate. The LTO particles form a uniform layer with a
somewhat porous structure.
Figure 1.14. SEM images of the (a) surface and (b) cross-section of an inkjet-printed
LTO anode (the inset is a TEM image of LTO particles in the film) [38].
Flexible thin film LIBs have also been fabricated by inkjet printing ink onto a flexible
substrate. Wei et al. printed a flexible solid-state LIB (Figure 1.15) comprised of anatase
titanate (TiO2) nanoparticles mixed with graphene sheets dispersed in water [97]. The
graphene was modified with either n-type or p-type anionic groups to help stabilize the
graphene dispersions in water, allowing them to be printed without agglomeration and
subsequent clogging of the nozzles. The printable electrodes exhibited stable cycling
performance comparable to other flexible LIBs [98]. Overall, these studies have
demonstrated that inkjet printing is a simple and low-cost fabrication method for thin film
LIB electrodes.
25
Figure 1.15. (a) Graphene/TiO2 ink for printing flexible thin film LIB electrodes.
(b)Schematic diagram showing the major components of the cell. (c) Demonstrating
the flexibility of a printable graphene electrode [97].
1.4 Thesis Objectives
Thin film LIBs are excellent on-board power sources for wireless sensors, implantable
medical devices, and flexible electronics. Their small size in the z-dimension allows them
to be used in applications where traditional LIBs are too large, or to decrease the
thickness of the electronic devices in which they are used. Additionally, higher charging
and discharging rates are possible with thin film batteries due to shorter lithium-ion
diffusion lengths. However, the methods used to fabricate thin film electrodes are limited
by the expensive equipment required, intensive energy needs, and damaging postannealing treatments.
Therefore, the main objective of this thesis was to develop a simple, cost-effective, and
versatile inkjet-printing method for fabricating LIB electrodes. In particular, the goal was
to fabricate thin film electrodes comprised of next-generation anode materials by inkjet
printing. Specifically, silicon and titanium dioxide were chosen for reasons stated above.
Second, the electrochemical performance of these thin film electrodes produced by inkjet
printing was investigated. The objectives of this thesis are summarized as follows:
(1) Develop an inkjet-printing fabrication method for LIB thin film electrodes
26
a) Fabricate silicon nanoparticle anodes by inkjet printing
b) Fabricate titanium dioxide nanoparticle anodes by inkjet printing
c) Explore the effect of printing parameters on film formation and morphology
(2) Investigate and improve the electrochemical performance of inkjet-printed electrodes
a) Establish relationships between printing parameters and the electrochemical
performance of inkjet-printed thin film electrodes
b) Explore the effect of different polymer binders on the specific capacity and
cycling performance of inkjet-printed silicon anodes
c) Study the rate capability of inkjet-printed titanium dioxide anodes
1.5 Thesis Organization
This thesis consists of five chapters, an introduction, a summary of the experimental
methods, two articles, and a concluding chapter, organized in integrated article format. It
has been submitted in accordance with the Thesis Regulation Guide of the School of
Graduate and Postdoctoral Studies at the University of Western Ontario. In more detail,
the following chapters are included:
Chapter 1 gives a general introduction to lithium-ion batteries and printing technologies.
It goes into specific details about thin film batteries, the anode materials examined in this
work, and inkjet printing, as well as summarizes the conventional battery electrode
fabrication techniques and the field of inkjet-printed battery electrodes. Additionally, the
objectives of the thesis are stated.
Chapter 2 summarizes the experimental methods used to develop nanomaterial inks and
inkjet print them for LIB anodes. It also discusses the various characterization tools used
to investigate the morphology, chemical composition, and electrochemical performance
of the inkjet-printed films.
Chapter 3 presents the inkjet printing process for fabricating high performance silicon
anodes for LIBs. The effects of printing parameters on the film morphology and
27
electrochemical performance are examined and the electrochemical effects of using
different polymer binders are studied in detail.
Chapter 4 reports the use of inkjet printing to fabricate titanium dioxide anodes for LIBs.
It details how the thickness of the electrode has a significant effect on its electrochemical
performance and also investigates the rate capability of these inkjet-printed anodes.
Chapter 5 summarizes the results presented in chapters 3 and 4 and reflects on the
original objectives of the thesis. The prospects of using inkjet printing for commercial
fabrication of LIBs are discussed and future work is also suggested.
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36
Chapter 2
2
Experimental and Characterization Methods
This chapter gives an overview of the methods used to inkjet print thin film electrodes for
LIBs and the techniques used to characterize them. It is meant as a brief summary of the
equipment used during the thesis work. More details about the specific processes and
characterization tools used in each study can be found in the methods sections of their
respective chapters.
2.1 Inkjet printing of nanomaterials
2.1.1 Ink preparation
Ink solutions were comprised of three components: (1) an electroactive material, (2) a
conductive agent, and (3) a polymer binder, all dispersed in water. In this thesis,
commercial silicon nanoparticles (50 nm diameter) and titanium dioxide nanoparticles
(21 nm diameter) were used as the electrochemically active components. Carbon black
(50 nm primary particles) was used as the conductive filler. And four different polymer
binders were investigated for their effects on stabilizing the cycling performance of the
printed
electrodes:
poly(3,4-ethylenedioxythiophene)-poly(styrene
sulfonate)
(PEDOT:PSS), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), and Naalginate.
The ratio of these three components was varied to optimize the jetting properties of the
ink, as well as the electrochemical performance of the printed films. For silicon, a ratio
of 2:2:1 active material : carbon black : binder was used, while for titanium dioxide a
ratio of 8:1:1 was determined to be ideal. The solid materials were dispersed in DI-water
and the solutions were sonicated in a bath sonicator (Branson) for at least 3 hours prior to
use to break up particle agglomerations and to ensure the ink was uniformly mixed.
37
The viscosity of the ink was determined with a U-tube viscometer (Cannon Instrument
Company), shown in Figure 2.1. By measuring the time it takes for the solution to pass
between points A and B, the kinematic viscosity, in cSt (= 1 mm2/s), can be calculated by
multiplying by the viscometer constant, in cSt/s, given by the manufacturer. The dynamic
viscosity, in mPa∙s, is related to the kinematic viscosity by equation 2.1:
=


(2.1)
where v is the kinematic viscosity, η is the dynamic viscosity, and ρ is the density. The
concentration of the ink was varied until a dynamic viscosity of 10 mPa∙s was achieved,
which is a standard value for inkjet printing [1]. This typically corresponded to a
concentration of around 20 mg/mL.
Figure 2.1. A schematic illustration (left) and photo (right) of the U-tube viscometer
used to measure ink viscosity.
38
2.1.2 Inkjet printing
The prepared ink solutions were injected into thoroughly-cleaned ink cartridges, as
shown in Figure 2.2. The cartridges were cleaned by first printing out the original black
ink until a “low cartridge” warning was given and then injecting DI-water until the liquid
passing out was clear, indicating that no black ink remained. The ink was then printed
onto copper foil, which acted as the current collector, with the Hewlett-Packard (HP)
Deskjet 2540 inkjet printer shown in Figure 2.2. Multiple layers were printed on top of
one another until a sufficiently thick and uniform film was formed. After each layer was
printed, the films were dried using a compressed air gun. The ideal number of layers was
determined to be 25, as discussed in Chapters 3 and 4. After all layers were printed, the
films were dried in a vacuum oven at 60°C overnight.
Figure 2.2. Injecting a silicon ink solution into an HP 61 ink cartridge (left) and the
HP Deskjet 2540 inkjet printer used to print LIB anodes (right).
39
2.2 Characterization methods
2.2.1 Physical and compositional characterization
A multitude of techniques were used to examine the physical and chemical properties of
the inkjet-printed films, including scanning electron microscopy (SEM), electron
dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), Fourier transform
infrared (FTIR) spectroscopy, Raman spectroscopy, and thermogravimetric analysis
(TGA). All characterization techniques could be used both before and after
electrochemical cycling, to observe changes in the electrodes’ morphologies and
chemical structures. To perform post-cycling tests, cells were disassembled in the
glovebox and the electrodes were washed with DMC to remove any residual electrolyte.
The samples were kept under argon atmosphere until characterization was performed.
A Hitachi S-4800 field emission SEM (Figure 2.3) was used to characterize the surface
morphology of the inkjet-printed electrodes. The resolution of the secondary electron
mode is 2 nm at an accelerating voltage of 1 kV. The attached EDX spectrometer was
used to measure the elemental composition of the films, giving the atomic percentage of
elements present.
A Bruker D8 Advance XRD instrument (Figure 2.4) was used to determine the crystal
structure and composition of the printed electrodes. To analyze the thin films, it was
necessary to operate at grazing incidence angles in order to minimize the contribution
from the copper substrate. When characterizing electrodes after cycling, an airtight
chamber with a beryllium window filled with argon was used to hold the sample.
A Nicolet 6700 FTIR spectrometer (Figure 2.5) was used to gather chemical information
about the printed electrodes, especially concerning the different functional groups present
during charging and discharging. It was a very simple, yet powerful, technique to observe
how the polymer binder’s chemical interactions with its environment changed during
cycling.
40
Figure 2.3. A photo of the Hitachi S-4800 field emission SEM equipped with and
EDX spectrometer.
Figure 2.4. A photo of the Bruker D8 Advance XRD instrument.
41
Figure 2.5. A photo of the Nicolet 6700 FTIR spectrometer.
As a complementary tool to FTIR spectroscopy, a HORIBA Scientific LabRAM HR
Raman spectrometer with a 532.4 nm (green) laser (Figure 2.6) was also used to analyze
the chemical structure of the electrodes during cycling.
Figure 2.6. A photo of the HORIBA Scientific LabRAM HR Raman spectrometer.
To gather compositional data, as well as thermal stability information, a TA Instruments
SDT Q600 TGA was used. By burning off all components of the electrodes except for the
42
active material, it was possible to determine the loading percentage of electroactive
material in the printed films. This value typically differed from the loading percentage in
the ink solutions, due to particles becoming trapped in the cartridge’s sponge or nozzles.
The loading percentage in the printed films measured with TGA was used to calculate the
current densities and capacities for electrochemical characterization.
Figure 2.7. A photo of the TA Instruments SDT Q600 TGA.
2.2.2 Electrochemical characterization
In order to test the electrochemical performance of the inkjet-printed electrodes, they
were first assembled into CR-2032 coin cells in an argon-filled glovebox (oxygen and
moisture concentrations below 1 ppm). The printed films were cut into circular disks with
a diameter of 9/16 inches (14.29 mm) and used as the working electrode, while lithium
metal was used as the counter and reference electrode. Unless otherwise stated, the
electrolyte consisted of LiPF6 salt dissolved in a solution of ethyl carbonate (EC), diethyl
carbonate, (DEC), and ethyl methyl carbonate (EMC), 1:1:1 by volume. In some cases
when measuring the performance of inkjet-printed silicon anodes, the electrolyte was
43
composed of LiPF6 salt dissolved in a solution of fluoroethylene carbonate (FEC) and
dimethyl carbonate (DMC), 1:9 by weight. The assembled coin cells were stored
overnight before testing, to ensure sufficient wetting of the polypropylene separator
(Celgard 2400).
These half-cells were then studied with galvanostatic cycling, cyclic voltammetry (CV),
and electrochemical impedance spectroscopy (EIS). Galvanostatic cycling was performed
on an Arbin BT-2000 battery test station, shown in Figure 2.8. This technique involves
charging and discharging the cell by maintaining a constant current and measuring its
capacity (amount of charge stored or released) and voltage. By repeating charge and
discharge steps many times, one can gather information about the cycling performance of
the cell (i.e how the capacity fades over the cell’s lifetime). It is standard to report the
applied current in terms of a C-rate to normalize it to the electrode’s theoretical capacity.
A rate of 1C corresponds to a full discharge or full charge in one hour, whereas a rate of
0.1C corresponds to a full discharge or full charge in ten hours. A higher C-rate results in
a lower capacity. In this thesis, all capacity values are given with the corresponding Crate used to cycle the cell.
The theoretical capacity, q, of an electrode material can be calculated using equation 2.2:
=


(2.2)
where n is the number of electrons involved in the electrochemical reaction, F is
Faraday’s constant, and M is its molecular weight. The active materials used in this
thesis, silicon and titanium dioxide, have theoretical capacities of 4200 mAh g-1 and 335
mAh g-1, respectively.
44
Figure 2.8. A photo of the Arbin BT-2000 battery test station.
CV and EIS measurements were taken with a multichannel potentiostat 3/Z (VMP3),
shown in Figure 2.9. CV measures the current through the cell at an applied voltage. The
voltage is swept at a constant rate with respect to time until a set potential is reached, at
which point the scan is reversed. CV was used to study the reduction and oxidation
processes of electrochemical species, giving information about the location of redox
potentials and about the reversibility of a reaction. By repeating multiple scans, the
formation and stabilization of the solid-electrolyte interface (SEI) was observed.
Additionally, varying the scan rate and measuring the change in current was used to
calculate the diffusion rate of lithium through the electrode, as discussed in Chapter 4. In
general, a faster scan rate gives results in a higher absolute current at a given potential.
45
Figure 2.9. A photo of the VMP3 multichannel potentiostat 3/Z for CV and EIS
measurements.
EIS was used to measure the internal resistance of the half-cells. Of interest were the
charge transfer resistance and Warburg impedance, which give information about the
kinetics at the electrode-electrolyte interface and lithium-ion diffusion into the electrode,
respectively. Additionally, evidence for the mechanism of capacity fade was collected by
measuring the impedance after galvanostatic cycling.
References
[1]
J. Li, F. Ye, S. Vaziri, M. Muhammed, M. C. Lemme, and M. Östling, "Efficient
Inkjet Printing of Graphene," Advanced Materials, vol. 25, pp. 3985-3992, 2013.
46
Chapter 3
3
High performance inkjet-printed silicon anodes for
lithium-ion batteries
*This chapter is to be submitted for peer-reviewed publication.
Thin film batteries have attracted increasing attention recently as the miniaturization of
wireless electronics demands ever-smaller energy storage devices. Here we report the
fabrication of thin film silicon nanoparticle anodes for lithium-ion batteries by inkjet
printing, which allows for precise control over the thickness of the electrode. Silicon’s
high capacity makes it a promising candidate material for next-generation lithium-ion
batteries, but its large volume expansion during cycling ultimately leads to cell failure.
To overcome this problem, we investigated the effect of four different polymer binders on
the cycling performance of inkjet-printed silicon anodes via ex-situ characterization
techniques. Using the conductive polymer PEDOT:PSS as a binder, we demonstrate high
capacity retention of over 1000 cycles at a limited depth-of-discharge of 1000 mAh g-1.
3.1
Introduction
Portable energy storage has received worldwide interest as the recent demand for mobile
power sources has sky-rocketed. With the miniaturization of wireless devices,
rechargeable batteries have had to decrease in size while maintaining the amount of
energy stored. Thin film batteries are poised to meet this challenge, as they exhibit a
number of unparalleled features including high energy and power densities [1, 2], short
ion diffusion lengths [3, 4], and intrinsic flexibility [5, 6]. The successful development of
high capacity thin film batteries will enable advances in the fields of wireless sensors,
RFID tags, implantable medical devices, and robotics.
47
Inkjet printing is a promising technique for fabricating thin film electrodes. It can be used
to deposit films of precisely controlled thickness, which can be tuned by the number of
layers printed on top of one another. It has many advantages over other fabrication
techniques, including ease of use, cost-effectiveness, minimal wasted material,
scalability, and the ability to deposit patterns. For these reasons, inkjet printing has been
successfully used to fabricate supercapacitors [7, 8], transistors [9, 10], and battery
electrodes [11, 12]. Commercial graphite anodes for lithium-ion batteries (LIBs),
however, cannot meet future energy requirements because of their low theoretical
specific capacity (372 mAh/g). Silicon has been widely studied as a candidate to replace
graphite [13-16], as it exhibits a high theoretical specific capacity (4200 mAh/g), low
lithium extraction potential, and low cost [17, 18].
However, one well-known challenge with using Si as an anode material is its 400%
volume expansion upon lithium insertion [19], causing pulverisation of the particles and
loss of electrical contact which results in decreased capacity during cycling. Many studies
have focused on developing novel nanostructured Si electrodes, such as nanowires [20],
nanotubes [21], hollow nanospheres [22], and core-shell structures [23, 24] to overcome
the poor cycling stability of Si nanoparticles (SiNPs). However, these synthesis methods
typically require large amounts of energy and are not easily translated to commercial
mass production. One simpler alternative is to use a polymer binder that can
accommodate the volume expansion of SiNPs and maintain electron conduction across
the electrode. Previous reports have shown that the mechanical, chemical, and electronic
properties of different binders have a significant effect on the cycling performance of
SiNPs [25-30].
Herein we prepared inkjet-printed SiNP anodes with four commercially available
polymer
binders,
poly(3,4-ethylenedioxythiophene)-poly(styrene
sulfonate)
(PEDOT:PSS), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), and Naalginate, and investigated their effects on the electrochemical performance of the
electrodes in LIBs. We demonstrate that SiNP anodes printed with PEDOT:PSS binder
exhibit the most stable cycling at high discharge capacity, due to its excellent jetting
properties [31] and electrical conductivity. Scanning electron microscopy (SEM) shows
48
that PEDOT:PSS conformally coats the SiNPs as a conductive layer, allowing for rapid
electron transport while binding the electrode together. Ex-situ characterization with
SEM, Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy reveals
that PEDOT:PSS acts as a self-healing polymer, stretching during discharging to
effectively accommodate the volume expansion of SiNPs and shrinking during charging
to preserve this continuous electronically conductive network.
3.2 Methods
3.2.1 Silicon ink preparation
Inks were prepared by mixing SiNPs (50 nm, Hongwu Nano), carbon black (50 nm,
Gunbai), and polymer binder at a ratio of 2:2:1 by weight in an appropriate volume of DIwater to achieve a viscosity of 10 mPa∙s, as measured with a U-tube viscometer (Cannon
Instrument Company). Four polymer binders were used: PEDOT:PSS (Sigma-Aldrich),
PVP (Sigma-Aldrich), CMC (Calbiochem), and sodium alginate (Sigma-Aldrich). Inks
were sonicated for at least 3 hours to break up large agglomerates and ensure uniform
dispersion prior to use.
3.2.2 Electrode and coin cell preparation
The ink was transferred into a well-cleaned HP 61 ink cartridge and printed using a
Hewlett-Packard Deskjet 2540 inkjet printer. Twenty-five layers of each ink were printed
on copper foil to ensure sufficient thickness and uniformity. After printing, the films were
dried in a vacuum oven at 60°C overnight. The printed films were then cut and assembled
in CR-2032 coin cells with lithium metal foil as the counter electrode in an argon-filled
glove box. The two electrolytes used were composed of 1 M LiPF6 salt dissolved in
different solvent mixtures: (1) 1:1:1 ratio by volume EC:DEC:EMC and (2) 1:9 ratio by
weight FEC:DMC. The coin cells were stored overnight at room temperature before
testing.
49
3.2.3 Characterization
A field emission SEM (Hitachi S-4800) equipped with an EDX spectrometer was used to
observe the microstructure and elemental composition of the printed films.
Compositional information was also obtained by XRD (Bruker D8 Advance, Cu Kα Xray source) at a grazing incidence angle of 1° and a step size of 0.02°. Galvanostatic
charge-discharge measurements were performed on an Arbin BT-2000 battery test station
between 0.01 and 1.0V vs. Li/Li+. The charge/discharge rates and capacities were
calculated based on the mass of Si in the anode, as determined by thermogravimetric
analysis (TGA, TA Instruments SDT Q600) (see Figure S3.6). Cyclic voltammetry (CV)
(Figure S3.7) and EIS were performed on a multichannel potentiostat 3/Z (VMP3). Exsitu FTIR spectroscopy (Nicolet 6700) and Raman spectroscopy (532.4 nm laser,
HORIBA Scientific LabRAM HR) were performed to analyze anodes with PEDOT:PSS
binder before cycling, after first cycle lithiation, and after first cycle delithiation.
3.3 Results
3.3.1 Electrode fabrication and physical characterization
Inkjet printing is a simple method to produce highly uniform thin film electrodes with
tunable thicknesses. All inks were prepared with commercially available materials
without modification. Ink formulations were comprised of three materials: an
electrochemically active material, SiNPs; a conducting agent, carbon black; and one of
four polymer binders (Figure 3.1a). These components were mixed in water (Figure 3.1b)
and then sonicated for several hours during which large agglomerations of particles were
broken up, resulting in a dark brown homogenous suspension (Figure 3.1c). Inks were
then transferred into a well-cleaned ink cartridge and printed onto a copper foil current
collector with a commercial desktop inkjet printer (Figure 3.1d). The inkjet printing
process can be divided into three main stages: droplet ejection and travel, droplet
spreading, and droplet solidification. The print head is positioned at the desired location
and droplets of ink are forced through the nozzles and are deposited onto the substrate.
Upon impact, the deposited droplets spread along the surface and join with other droplets
50
to form a thin film of liquid ink. Finally, the solvent evaporates and the solid contents of
the ink remain on the substrate. To demonstrate the patternability of the inkjet printing
technique, the Western University logo was printed using this Si ink on copper foil
(Figure 3.1e). In addition, this printing technique uses aqueous solutions, making it an
environmentally benign, versatile, and safe fabrication method.
Multiple printing passes were performed to achieve highly uniform films of desired
thickness. Printing too few passes resulted in non-uniform films with isolated islands of
deposited material, while printing too many passes reduced the electrode’s capacity due
to higher internal cell resistance (Figure S3.1), consistent with previous reports [32]. The
optical images in Figure 3.1f show printed films with 25 layers using the four different
polymer binders, along with SEM images demonstrating their morphological differences.
These four films were each around 1 μm thick, as measured by cross-section SEM.
Energy dispersive X-ray (EDX) spectroscopy mapping demonstrates the uniform
distribution of Si within each film (Figure S3.2). The printed anodes with PEDOT:PSS
binder exhibited a continuous polymer network, with SiNPs embedded in the matrix.
However, the other binders created more discontinuous films containing isolated
particles. From this, it is clear that the anodes prepared with PEDOT:PSS are comprised
of a single continuous matrix. This is one of the reasons for its superior cycling
performance, as will be discussed in the next section.
51
Figure 3.1. Procedure used to print SiNP anodes on copper foil. (a,b) First, the ink
was prepared by mixing SiNPs, carbon black, and the polymer binder in water. (c)
After 3 hours of sonication, the solution was well-mixed and (d) injected into an
inkjet printer cartridge and printed. (e) Photograph of the Western University logo
printed with the SiNP ink. (f) Optical photographs and SEM images of the inkjetprinted SiNP anode films on copper foil. Scale bars red, white, and black represent
3 cm, 5 cm, and 500 nm, respectively.
3.3.2 Electrochemical performance
To characterize the electrochemical properties of the inkjet-printed silicon anodes with
different polymer binders, deep galvanostatic cycling measurements were performed on
printed Si/metallic Li half cells between 0.01 and 1 V (Figure 3.2). At a current rate of
0.1 C (where 1 C = 4200 mA g-1), Si anodes with PEDOT:PSS binder exhibited the most
stable cycling performance (Figure 3.2a). Anodes with PEDOT:PSS and PVP binders
exhibited very high first-cycle discharge capacities, close to the theoretical value for
silicon. Anodes with CMC and sodium alginate binders displayed low initial capacities
that rapidly faded to zero within a few cycles. This may be due to low electron
52
conduction through the electrode, resulting in electrical isolation of SiNPs not adjacent to
the current collector, possibly as a result of poor ink-jetting compatibility of these
polymer binders. After the first cycle, Si anodes with all binders demonstrated an
irreversible capacity loss, which can be attributed to solid electrolyte interphase (SEI)
formation. Following this initial drop, anodes prepared with PEDOT:PSS show stable
performance for 100 cycles. Furthermore, cells prepared with electrolyte containing
dimethyl carbonate and fluoroethylene carbonate (DMC:FEC) demonstrated increased
stability compared to those with electrolyte containing ethylene carbonate, diethyl
carbonate, and ethyl methyl carbonate (EC:DEC:EMC), with 100th cycle capacities of
1714 mAh g-1 and 961 mAh g-1 based on the mass of Si, respectively. Anodes with
PEDOT:PSS binder and DMC:FEC electrolyte were also cycled at 1 C and show
excellent performance for 2000 cycles (Figure S3.3). The stabilizing effect of FEC has
been previously reported and is attributed to the formation of a more stable SEI layer that
prevents cracking of the Si surface [33, 34].
The voltage profiles of printed anodes using PEDOT:PSS binder and DMC:FEC
electrolyte are shown in Figure 3.2b. The first cycle exhibits a long plateau around 0.1 V
during lithiation, characteristic of crystalline silicon. Subsequent cycles have sloping
plateau regions between 0.3 and 0.01 V vs. Li/Li+, indicative of lithium insertion into
amorphous LixSi [35].
PEDOT:PSS forms a continuous and conductive network that connects SiNPs to the
current collector, providing rapid electron transport at elevated current densities. The
specific capacity of an inkjet-printed silicon anode with PEDOT:PSS binder varied from
2500 mAh g-1 at 0.1 C to 900 mAh g-1 at 5 C (Figure 3.2c). Even at a charge/discharge
rate of 2 C, a sloping plateau region is observed between 0.25 and 0.01 V vs. Li/Li+
(Figure 3.2d), similar to those shown in Figure 3.2b. This demonstrates the ability of
lithium ions to rapidly penetrate the PEDOT:PSS coating and alloy with the Si at high
cycling rates. However, no obvious plateau is observed in the charging profile when
cycling at 5 C, due to increasing polarization under high current densities.
53
Limited depth-of-discharge measurements were also carried out to a capacity cut-off of
1000 mAh g-1 at 0.1C (Figure 3.2e). Cells were also limited to a maximum charging
voltage of 2.5 V to prevent oxidation and dissolution of the copper current collector.
Anodes prepared with binders other than PEDOT:PSS faded rapidly. Cells made with
EC:DEC:EMC electrolyte and PEDOT:PSS binder lasted only 150 cycles at 1000 mAh g1
before fading. When using DMC:FEC electrolyte, printed Si anodes with PEDOT:PSS
binder exhibited further superior cycling stability, with over 600 cycles at 1000 mAh g-1.
This can be attributed to less stable SEI formation of the EC-based electrolyte than of the
FEC-based electrolyte. To compare the inkjet printing technique to conventional
fabrication methods, anodes with Na-alginate and poly(vinylidene fluoride) (PVDF)
binders were also prepared using the conventional doctor-blade casting method. These
anodes were unable to maintain a discharge capacity of 1000 mAh g-1 after 22 and 5
cycles for Na-alginate and PVDF binders, respectively. Previous studies have shown that
PVDF is unable to accommodate the large volume changes during the lithiation of
silicon, causing the binder to detach from the current collector and leading to increased
cell resistance and poor cycling performance [36].
The Coulombic efficiency (CE) of the Si anode with PEDOT:PSS binder in DMC:FEC is
31% and 75% for the first two cycles and increases to around 98.6% for the remaining
cycles. The low CE for the first two cycles is attributed to irreversible capacity loss from
SEI formation. High initial resistance of the cell, as measured by electrochemical
impedance spectroscopy (EIS) (Figure S3.4), may also contribute to the low first-cycle
CE. Once a stable SEI is formed, the CE remains constant and the anodes continue to
exhibit a stable capacity of 1000 mAh g-1. However, by the 5th cycle the charge capacity
reaches nearly 1000 mAh g-1 and the CE remains stable. The discharge plateaus remain
relatively constant during cycling and all discharge plateaus occur at lower voltages than
the first cycle, indicating a decreased internal resistance.
54
Figure 3.2. (a) Cycling performance of inkjet-printed silicon anodes prepared with
different polymer binders at 0.1C. (b) Voltage profiles of selected cycles for the
PEDOT:PSS (DMC:FEC) cell from (a). (c, d) Rate capability measurements of Si
anodes with PEDOT:PSS binder in DMC:FEC electrolyte. (e) Limited depth-ofdischarge tests performed to a capacity cut-off of 1000 mAh g-1. The Coulombic
efficiency shown is for PEDOT:PSS (DMC:FEC). (f) Voltage profiles of selected
cycles for the PEDOT:PSS (DMC:FEC) cell shown in (e).
55
3.3.3 Physical characterization
Ex-situ measurements were taken at three stages of the charge/discharge cycle of anodes
prepared with PEDOT:PSS binder: before cycling, after lithiation, and after delithiation.
As seen in the SEM images shown in Figure 3.3a, the anode began in the pristine state as
a continuous polymer network with embedded SiNPs. Upon lithiation the SiNPs
expanded, causing the polymer binder to stretch into a fibrous structure. After
delithiation, the SiNPs contracted and the morphology of the electrode returned to its
pristine condition, indicating that the PEDOT:PSS network remains intact during volume
expansion and is able to maintain intimate contact with the SiNPs. The continuous
conductive network is preserved, ensuring that all SiNPs remain electrically connected to
the current collector, which is critical to the overall electrochemical performance of the
electrode.
Furthermore, this self-healing effect is also consistent with results obtained from FTIR
(Figure 3.3b) and Raman (Figure S3.5) spectroscopy. The IR peaks at 1635 and 667 cm-1
represent C=C stretching and C-S stretching in the thiophene ring, respectively [37-39].
Upon lithiation the intensity of both these peaks decreases, indicating reduced stretching
vibrations in the thiophene ring of the PEDOT chain. This can be explained by
considering the structure of PEDOT:PSS (Figure 3.3c). In the initial state, positivelycharged thiophene groups exist in the PEDOT molecules and negatively-charged sulfonyl
groups exist in the PSS molecules, which stabilize one another by electrostatic attraction.
The lower electron densities in the positive thiophene rings result in more asymmetric
stretching vibrations compared to the neutral thiophene rings. When the polymer chains
stretch during lithiation, the PEDOT and PSS molecules slide past one another, reducing
the interaction between the two polymer units. The decreased number of sulfonyl groups
nearby to stabilize positively-charged thiophene rings leads to a decreased number of
positive thiophene rings. The increased electron density of the neutral thiophene groups
suppresses the asymmetric stretching modes of the C=C and C-S bonds in PEDOT,
resulting in a lower IR absorbance. During delithiation, the polymer chains contract,
increasing the interaction between PEDOT and PSS molecules and therefore increasing
the number of positively-charged thiophene rings, as evident from the return of the IR
56
intensity to that of the pristine state. This explanation is further supported by changes to
the IR peaks at 1382 and 648 cm-1, which represent S=O stretching and S-O stretching,
respectively [39, 40]. Similar to above, these peaks are present in the IR spectra of the
pristine and delithiated samples, but absent in that of the lithiated electrode. This is due to
the suppressed asymmetric stretching of the sulfonic acid groups in PSS when Li+ ions
are available to ionically stabilize the negative sulfonyl groups that were previously
paired with the positive thiophene ring in the pristine and delithiated electrodes. These IR
results provide more evidence for the mechanism of the self-healing effect of the
PEDOT:PSS binder as shown in the SEM images.
The additional peak at 864 cm-1 corresponds to residual LiPF6 [41, 42] and the peaks at
1435 and 1508 cm-1 are attributable to the formation of Li2CO3 or other organic
carbonates in the SEI [42, 43]. As expected, these peaks appear after lithiation and are not
present in the pristine state. The SEI peaks are still present after delithiation, indicating
irreversible SEI formation. This also supports the cycling performance results above, in
which there is significant irreversible capacity loss after the first cycle. The SEI formed at
the PEDOT-electrolyte interface may differ chemically and mechanically from the SEI
formed at the Si-electrolyte interface, and it has been suggested that this difference may
also contribute to improved cycling stability [44].
57
Figure 3.3. Ex-situ (a) SEM images and (b) FTIR spectra of SiNP anodes with
PEDOT:PSS binder taken at three stages of a discharge/charge cycle: before cycling
(pristine), after first discharge (lithiated), and after first full discharge and charge
(delithiated). The green highlighted regions in the FTIR spectra indicate the
PEDOT:PSS thiophene C=C and C-S stretching vibrations, the blue highlighted
regions indicate SEI formation and residual electrolyte salt, and the purple
highlighted regions indicate the sulfonic acid groups. (c) The structure of
PEDOT:PSS. Scale bars are 500 nm.
3.4 Discussion
The superior performance of the inkjet-printed silicon anodes with PEDOT:PSS binder
can be attributed to the favourable properties of PEDOT:PSS. First, the PEDOT:PSS
formulation used was specifically designed for use in inkjet printers, with optimal
58
viscosity, surface tension, and density for ideal jetting conditions. It therefore readily
formed uniform films without modification. Second, PEDOT:PSS is both electrically and
ionically conductive. A conductive network is maintained during cycling, ensuring that
SiNPs do not become electrically isolated while simultaneously allowing lithium ions to
migrate to and alloy with the SiNPs. Third, PEDOT:PSS is able to reversibly deform,
allowing it to stretch and contract with the SiNPs and accommodate the large volume
changes. It is also rigid enough to confine any particles that fracture during cycling.
Additionally, there may be a chemical interaction between Si and PEDOT:PSS that
stabilizes the electrode structure during charging/discharging [45].
An explanation of the varying electrochemical performance of printed Si anodes prepared
with different binders is proposed in Figure 3.4. First, CMC and Na-alginate binders
surround groups of SiNPs and carbon black, electrically isolating them from each other.
There are no electron conduction pathways to these isolated particles, and therefore only
SiNPs in direct contact with the current collector can contribute to energy storage. This
results in the low initial capacity of electrodes prepared with these binders.
In the case of PVP, a continuous conductive network of carbon black initially exists, so
that SiNPs not in direct contact with the current collector are electrically connected.
Printed Si anodes prepared with PVP therefore exhibited high initial capacities. However,
as the SiNPs expand and contract they lose contact with the conductive carbon black. The
conductive network becomes disrupted after only a few lithiation and delithiation cycles
and the battery’s capacity rapidly decreases.
Similar to PVP, PEDOT:PSS initially forms a conductive network of carbon black.
However, when SiNPs become disconnected from the carbon black network, they
maintain electrical contact with the remainder of the electrode through the conductive
PEDOT:PSS matrix. Printed Si anodes prepared with PEDOT:PSS thereby exhibited high
initial capacities and more stable performance than anodes prepared with non-conductive
polymer binders. This mechanism is also consistent with other reported conductive
binders for Si electrodes, such as polypyrrole [46], polyaniline [30], and poly(9,9dioctylfluorene-co-fluorenone-co-methylbenzoic
acid)
(PFFOMB)
[47].
However,
59
compared to other studies, the PEDOT:PSS formulation used here is commercially
available with no modification or polymerization required, which is favourable for
printing technology.
Figure 3.4. Schematic illustration of the proposed mechanism explaining the
electrochemical performance of anodes prepared with different binders. The use of
non-conductive binders (CMC, Na-alginate, and PVP) leads to electrical isolation of
SiNPs. In the case of CMC and Na-alginate, electrical isolation occurs from the
start, leading to poor initial capacity, while in the case of PVP the conductive carbon
black network is destroyed during large volume changes. With PEDOT:PSS, the
SiNPs remain electrically connected throughout charging/discharging and are
therefore able to maintain a stable cycling capacity.
3.5 Conclusions
In conclusion, SiNP anodes were fabricated for the first time by inkjet printing. Inkjet
printing resulted in very uniform, thin film electrodes with precise control over the
thickness. The effect of the binder on cycling performance was investigated and anodes
60
with PEDOT:PSS binder were the most stable, attributed to its electrical conductivity and
reversible deformation upon electrode expansion. The continuous conductive network
formed by PEDOT:PSS allows for rapid electron transfer and, at the same time, stretches
to accommodate the large volume changes of SiNPs during charging and discharging.
These anodes exhibit very high capacities of greater than 1700 mAh g-1 for 100 cycles, as
well as very stable cycling performance when cycled at a limited depth-of-discharge of
1000 mAh g-1, with over 600 successful cycles. Anodes prepared with non-conductive
polymer binders, on the other hand, had capacities that quickly degraded to zero after
only a few cycles. This was attributed to some SiNPs losing electrical contact with the
remaining electrode each cycle, preventing them from contributing to capacity. This
proposed mechanism was supported with ex-situ SEM, FTIR, and Raman spectroscopy
measurements. Overall, we have shown that inkjet printing is a viable fabrication method
for high capacity thin film Si electrodes and that the polymer binder plays and important
role in the electrochemical behavior of printed electrodes. This technique may be
extended to other electrode materials as well as a printable electrolyte, for the fabrication
of a fully inkjet-printed cell.
Acknowledgements
This work was supported by the Natural Sciences and Engineering Research Council of
Canada (NSERC), Canada Research Chair (CRC) Program, Canada Foundation for
Innovation (CFI), Ontario Research Fund (ORF), and University of Western Ontario.
Stephen Lawes also acknowledges the Province of Ontario and the University of Western
Ontario for the Queen Elizabeth II Graduate Scholarship in Science and Technology.
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Supporting Information
Figure S3.1. (a) Photographs of printed films with different numbers of printing
passes. Too few passes (10x) results in non-uniform films that cannot be used for
electrodes. More passes (25x and 40x) result in uniform films. However, too many
passes decreases capacity. (b) Discharge capacity of Si anodes with PEDOT:PSS
binder and DMC:FEC electrolyte, with different numbers of printing passes.
Figure S3.2. EDX mapping of inkjet printed Si films with four different binders,
each prepared with 25 printing passes. The top images show the SEM image of
where the mapping was performed and the bottom images show the EDX mapping
for Si. All four films show uniform Si dispersion throughout, with some fluctuations
due to variations in the surface morphology. Scale bars are 2 μm.
66
Figure S3.3. Cycling performance of Si anodes with PEDOT:PSS binder and
DMC:FEC electrolyte at 1 C. The average CE over 2000 cycles is 99.8%.
Figure S3.4. EIS spectra of inkjet printed SiNP anodes prepared with PEDOT:PSS
binder. Cells were fabricated with EC:DEC:EMC electrolyte. The electrode starts
with relatively high impedance that decreases during cycling. This may be due to
increased electrolyte wetting of the printed anode layers and the higher conductivity
of Li-doped Si, as discussed in [30].
67
Figure S3.5. Raman spectra taken at three stages of a discharge/charge cycle:
before cycling (pristine), after one discharge cycle (lithiated), and after one full
discharge/charge cycle (delithiated). The peak at 1435 cm-1 represents thiophene
ring stretching vibrations, from the thiophene ring present in the PEDOT
molecules. Similar to the C=C and C-S thiophene stretching shown in the FTIR
spectra, the thiophene ring stretching peak disappears after lithiation, due to
stretching being inhibited by the already stretched polymer chains. However, after
delithiation there is no return of this peak. This may be due to the SEI binding to
the PEDOT molecules and preventing further stretching of the thiophene rings.
68
Figure S3.6. TGA of a film of inkjet printed SiNP anode material prepared with
PEDOT:PSS binder, used to determine the mass percentage of Si in the printed
films. Since Si is stable in the temperature range used here, all mass loss is due to
the removal of the polymer binder and carbon black. We therefore used the
minimum point (46%) on the curve as the % mass of Si in the anode for calculating
charge/discharge rates and capacities. To confirm that this assumption was valid,
we also measured a sample of Si powder. No mass loss was observed; in fact,
oxidation of the SiNPs resulted in a mass gain. This increase is negligible (2%) at the
minimum point of the printed Si anode curve and will anyways result in lower
reported capacity values.
69
Figure S3.7. Cyclic voltammetry curves of inkjet printed SiNP anodes prepared
with four different binders. All cells were fabricated with EC:DEC:EMC
electrolyte.
70
Chapter 4
4
High performance inkjet-printed titanium dioxide anodes
for lithium-ion batteries
*This chapter is to be submitted for peer-reviewed publication.
A facile and economical inkjet printing technique was developed to fabricate thin film
titanium dioxide anodes for lithium-ion batteries with a desktop inkjet printer. The homemade ink, composed of stable dispersions of TiO2 nanoparticles, carbon black, and
polymer binder, was printed on copper foil substrates. The thickness of the thin film
electrodes could be precisely controlled by printing multiple layers in succession, with
each layer having a thickness of about 125 nm. The morphology, composition, and
electrochemical performance of the TiO2 anodes were characterized by scanning electron
microscope,
spectroscopy,
X-ray
and
diffraction,
cyclic
galvanostatic
voltammetry,
cycling
tests,
electrochemical
respectively.
impedance
Electrochemical
measurements revealed that the TiO2 electrodes with an optimal thickness of 3 μm,
corresponding to 25 printed layers, exhibited the best electrochemical performance, i.e. a
large initial capacity of around 290 mAh g-1, stable cycling (above 150 mAh g-1 after 100
cycles, i.e. a capacity retention of ~53%), and high rate performance up to 5C (capacity
retention of 58% after 100 cycles). We have thus demonstrated that inkjet printing can be
used as a simple and low-cost method to fabricate thin film TiO2 electrodes for lithiumion batteries.
4.1 Introduction
As portable electronic devices continue to decrease in size, intensive research efforts
have focused on developing thin film batteries (TFBs) in recent years. Due to their
reduced thickness that enables fast reaction kinetics, thin film batteries have great
potential for applications in RFID tags, wireless sensors, and implantable medical
71
devices. Current fabrication techniques for thin film electrodes or all-solid-state TFBs
mainly include sputtering [1, 2], chemical vapor deposition [3, 4], pulsed laser deposition
[5, 6], spin coating [7], and sol-gel methods [8, 9]. However, many of these techniques
typically require expensive equipment, high temperatures, and/or post-annealing
treatments that can damage the films and the substrate. These obstacles greatly restrict the
development of state-of-the-art TFBs.
Inkjet printing is a simple and inexpensive alternative to fabricate thin film electrodes
with typical thicknesses on the scale of micrometers [10-12]. It is an additive process in
which picoliter-sized droplets of ink are deposited onto a substrate in a desired pattern.
By repeating this process multiple times, the thickness of thin films can be precisely
controlled. Additionally, the structure of the thin film electrodes can be patterned by
following a computer-aided design (CAD). Inkjet printing has many advantages
compared to other thin film fabrication techniques, including cost-effectiveness, ease of
use, minimal wasted material, and the ability to deposit material in any pattern. It has
therefore been used to fabricate thin films for various energy storage and conversion
devices, including battery electrodes, supercapacitors, fuel cells, and solar cells [12].
The key to achieving high performance inkjet-printed thin film electrodes is the
optimization of the ink’s composition and the printing process. The solid contents of the
ink must be uniformly dispersed in solution; if the ink is not well-mixed, the particles can
agglomerate and cause the printer’s nozzles to clog. Additionally, if the solid content of
the ink is too low, ideal jetting cannot be achieved due to satellite droplet formation and a
high number of printing passes required to deposit sufficient material. Previously,
printing processes for various LIB electrode materials have been developed. Tin oxide
[13] and lithium titanate [14] anodes and lithium cobalt oxide [15-17] and lithium
manganese oxide [18] cathodes have all been fabricated by inkjet printing. In general, the
physical properties of the inks in these studies were not optimized and the electrodes did
not exhibit high electrochemical performance. It is therefore critical to control the ink’s
composition to realize high performance inkjet-printed LIB electrodes.
72
Titanium dioxide (TiO2) has been considered a promising alternative anode material for
lithium-ion batteries (LIBs) due to its low cost, non-toxicity, safe lithiation potential, high
rate capability, and excellent cycling stability [19, 20]. In the present study, we report the
fabrication of thin film TiO2 anodes for LIBs by inkjet printing with a conventional
desktop inkjet printer. The TiO2 inks were optimized with different choices of polymer
binders to achieve the best stability and jetting properties. When an ideal ink formulation
was determined, thin film anodes were printed directly onto the current collector using a
household inkjet printer. The electrochemical performance of the thin film electrodes was
examined by charge and discharge cycling, rate performance, cyclic voltammetry (CV),
and electrochemical impedance spectroscopy (EIS). We also investigated the effect of
film thickness on their electrochemical performance in LIBs, and examined their
morphology and chemical composition after extended cycling. The high performance of
the inkjet-printed TiO2 anodes demonstrates that inkjet printing is a viable fabrication
method for thin film electrodes for LIBs.
4.2 Methods
4.2.1 TiO2 ink preparation
Inks were prepared by mixing 80 wt. % TiO2 nanoparticles (21 nm, Sigma-Aldrich), 10
wt. % carbon black (50 nm, GunBai), and 10 wt. % polymer binder in DI-water. The
concentration of the solids in water was optimized to achieve a viscosity of 10 mPa·s, the
ideal viscosity for inkjet printing [21, 22] (approximately 20 mg/mL). The polymer
binders used in this study were poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)
(PEDOT:PSS), polyvinylpyrrolidone (PVP), and polyvinylidene fluoride (PVDF), all
purchased from Sigma-Aldrich. The mixture was then sonicated for at least 3 hours to
break up large agglomerates and ensure uniform dispersion prior to printing.
73
4.2.2 Electrode and coin cell preparation
The ink was transferred into a well-cleaned HP 61 ink cartridge and printed using an HP
Deskjet 2540 inkjet printer (Hewlett-Packard). Different numbers of layers (15, 25, or
35) of TiO2 thin films were printed on copper foil to investigate the effect of the electrode
thickness on electrochemical performance. The inkjet-printed films were dried with a
compressed air gun between printing layers. After printing, the films were dried in a
vacuum oven at 60°C overnight. The printed films were then cut into round disks and
assembled into CR-2032 coin cells with lithium metal as the counter electrode and
Celgard 2400 separators in an argon-filled glove box. The electrolyte was composed of 1
M LiPF6 salt dissolved in a solvent of 1:1:1 ratio by volume of ethylene carbonate (EC) :
diethyl carbonate (DEC) : ethyl methyl carbonate (EMC), from BASF. The coin cells
were stored overnight at room temperature before testing.
4.2.3 Characterization
A field emission scanning electron microscopy (SEM, Hitachi S-4800) was used to
observe the microstructure of the printed films. Compositional information was obtained
by X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα X-ray source) at a grazing
incidence angle of 1° and a step size of 0.02° in 2θ. Galvanostatic charge-discharge
measurements were performed on an Arbin BT-2000 Battery Tester between 1.0 and 2.5
V vs. Li/Li+. The charge/discharge currents (rates) and capacities were calculated based
on the actual mass of active TiO2 material in the anode, as determined by
thermogravimetric analysis (TGA, TA Instruments SDT Q600). Cyclic voltammetry
(CV) and electrochemical impedence spectroscopy (EIS) were performed on a
multichannel potentiostat 3/Z (VMP3).
74
4.3 Results and Discussion
4.3.1 Printing process and morphological characterization of the
inkjet-printed thin films
Inkjet printing is an effective technique for fabricating uniform electrodes with tight
control over thickness. Ink solutions were prepared with TiO2 nanoparticles as the
electrochemically active material, carbon black as the conductive agent, and three
different polymer binders, mixed in water. The as-prepared inks with PVDF, PVP, and
PEDOT:PSS binders are shown in Figure 4.1a-c. As seen, inks with PVDF and PVP
binders could not be well-mixed; even after 5h of sonicating the carbon black separated
from solution, ultimately resulting in clogging of the printer’s nozzles. On the other hand,
when using PEDOT:PSS the ink was well-dispersed and readily formed uniform films on
copper foil. PEDOT:PSS has previously been shown to be an effective binder material for
lithium-ion batteries [23, 24] and serves two purposes here: (1) it enables ideal jetting, as
it is a commonly used material for inkjet printing [25-27] and therefore commercial ink
formulations have been specifically designed with optimized jetting properties; and (2)
PEDOT:PSS is electronically conductive and forms a continuous conductive network
throughout the electrode, ensuring rapid electron transport to and from the insulating
TiO2 nanoparticles.
The optical image in Figure 4.1d demonstrates the high degree of uniformity of the
inkjet-printed TiO2 films with PEDOT:PSS binder. The SEM image in Figure 4.1e shows
the surface morphology of an as-printed TiO2 thin film electrode before cycling. It can be
seen that the PEDOT:PSS binder formed a continuous network in which the TiO2
nanoparticles were embedded, with many small pores that resulted in a large electrodeelectrolyte contact area. After charging and discharging for 100 cycles at 0.1C (Figure
4.1f), the structure of the TiO2 thin film electrode was well-maintained and no visible
physical degradation was observed.
75
Figure 4.1. Ink solutions prepared with three different polymer binders: (a) PVP,
(b) PVDF, and (c) PEDOT:PSS. (d) Optical image of the TiO2 nanoparticle solution
with PEDOT:PSS binder inkjet-printed on copper foil with 25 layers. SEM images
of the printed film (e) before and (f) after cycling.
Figure 4.2 shows cross-sectional SEM images of TiO2 thin films printed with 15, 25, and
35 layers (referred to as TiO2-15, TiO2-25, and TiO2-35, respectively), after postpressing. The thicknesses of the films were measured to be 1.89, 3.02, and 4.40 μm,
respectively. This corresponds to an average thickness of a single printed layer of
approximately 125 nm. Therefore it is straightforward to fabricate thin film electrodes
with sub-micron precision simply by repeating the inkjet printing process until the
desired thickness is achieved.
76
Figure 4.2. Cross-sectional SEM images of TiO2 films printed with different number
of layers.
4.3.2 Structural and compositional characterization of the inkjetprinted thin films
Figure 4.3 shows the XRD patterns of inkjet-printed anodes before and after cycling,
along with the TiO2 powder used. TiO2 has four polymorphs that have been reported for
lithium storage: rutile, anatase, brookite, and bronze (TiO2-B) [19]. In this study, a
commercial TiO2 nanopowder was directly used. Diffraction peaks at 25.3°, 37.0°, 37.8°,
38.6°, and 48.0° in its XRD pattern correspond to the (101), (103), (004), (112), and
(200) crystal faces of anatase TiO2 respectively (COD 9009086), and peaks at 27.4°,
36.1°, 39.2°, 41.2°, and 44.0° correspond to the (110), (101), (200), (111), and (210)
crystal faces of rutile TiO2, respectively (COD 9004141). The XRD pattern of the printed
film well-matched the powder, indicating that a mixture of anatase and rutile TiO2
coexists in the printed electrode. In addition, the fact that the XRD pattern of the inkjetprinted film before cycling closely matches that of the TiO2 powder demonstrates that the
TiO2 maintains its crystal structure during the dispersion and printing processes. Because
the film is very thin, peaks from the copper substrate underneath are clearly visible at
42.7° and 43.4°, marked with asterisks. Interestingly, after charging and discharging for
100 cycles, diffraction peaks from the initial rutile TiO2 phase disappeared in the XRD
pattern, while those belonging to the anatase phase were still clearly visible. This
phenomenon is consistent with what has been observed in a previous study and is
believed to be due to the transformation of rutile TiO2 into an amorphous LixTiO2 phase
[28].
77
Figure 4.3. XRD patterns of inkjet-printed TiO2 films before and after cycling. A
reference of TiO2 powder is shown for comparison. Peaks marked with an asterisk
(∗) are from the underlying copper foil substrate.
It should be noted that the mass percentage of TiO2 in the printed film may differ from
that in the prepared ink solution due to non-ideal jetting conditions or from particles
becoming trapped inside the cartridge sponge or nozzles. To address this difference and
determine an accurate value for the mass of TiO2 in the inkjet-printed electrodes, TGA
was performed on a printed thin film electrode that was scraped off of the copper
substrate (Figure 4.4). Considering the fact that TiO2 is stable in a temperature range of
25°C to 900°C, as confirmed by TGA of pristine TiO2 powder (the weight loss below
100°C is due to the removal of adsorbed water), all observed mass loss of the printed thin
film can be attributed to the removal of carbon black and PEDOT:PSS polymer binder.
Both carbon black and PEDOT:PSS were completely removed, with no remaining mass
above 825°C. We therefore used the measured residual mass of the printed TiO2 anode
(76%) as the mass percentage of TiO2 in the anode when calculating the charge/discharge
rates and capacities during electrochemical testing.
78
Figure 4.4. TGA curves of an inkjet-printed TiO2 anode and each of its components
individually.
4.3.3 Electrochemical characterization
The lithiation and delithiation processes of titanium dioxide can be expressed by the
following insertion/extraction reaction:
 + +  − + 2 →  2
(4.1)
The maximum insertion coefficients, x, for bulk anatase and rutile TiO2 are typically 0.5
and 0.1, respectively [29], corresponding to theoretical capacities of 167.5 mAh g-1 and
33.5 mAh g-1 (with respect to the mass of TiO2). Nanoscale TiO2 is reported to be able to
be further lithiated up to x = 0.85 for anatase and x = 0.75 for rutile [29], corresponding to
capacities of 285 mAh g-1 and 251 mAh g-1, respectively. This improvement in the
electrochemical performance of nanosized TiO2 is a result of shorter electron and lithium
ion diffusion lengths, as well as a larger electrode/electrolyte interfacial area [19]. Figure
79
4.5a shows the cycling performance of inkjet-printed TiO2-15, TiO2-25, and TiO2-35
anodes with different thicknesses at a current rate of 0.1C (1C = 167 mA g-1). The first
lithium insertion capacities were 285 to 300 mAh g-1 for thin film electrodes with
different thicknesses, equivalent to lithium insertion coefficients of 0.85 to 0.90. Some
minor additional lithium storage by the conductive carbon black and PEDOT:PSS binder
may have contributed to these elevated discharge capacities. All cells exhibited
irreversible capacity loss during the first few cycles, originating from irreversible lithium
insertion. This may also be due to the irreversible phase change of the rutile phase, as
observed with XRD. First cycle Coulombic efficiencies of all the TiO2 electrodes were
below 75%; however, after the five cycles their Coulombic efficiencies are above 95%.
The electrode printed with 25 layers, corresponding to a thickness of approximately 3
μm, exhibited the highest reversible capacity over 100 cycles. Electrodes with 15 and 35
layers, 1.9 and 4.4 μm thick, respectively, had more rapid capacity fading over 100
cycles. The poorer cycling performance of the thicker anode can be attributed to the cell’s
higher internal resistance, as measured by EIS (Figure 4.5b). Additionally, the discharge
capacity of thicker films is limited by the diffusion and penetration of lithium ions and
tends to decrease as the electrode thickness increases [30]. The decreased performance of
the thinner anode may be due to non-uniformities in the film created during the inkjetprinting process, such as pin-holes and isolated islands of deposited electroactive
material. With more printed layers these defects are filled in, resulting in an optimized
thickness of 3 μm for inkjet-printed TiO2 anodes.
80
Figure 4.5. (a) Galvanostatic cycling at 0.1C and (b) EIS measurements of three
inkjet-printed TiO2 anodes with different thicknesses, controlled by the number of
printed layers. 15 layers = 1.9 μm, 25 layers = 3.0 μm, and 35 layers = 4.4 μm.
Figure 4.6 shows the rate capability of an inkjet-printed TiO2-25 anode with the optimal
thickness. Its reversible specific discharge capacity varied from 180 mAh g-1 at 0.1C to
37 mAh g-1 at 5C (Figure 4.6a). After cycling at elevated rates, the capacity returned to
165 mAh g-1 at 0.1C, corresponding to irreversible capacity loss of approximately 8%.
The length of the discharge plateau region at approximately 1.7 V, seen in the voltage
profiles (Figure 4.6b), clearly decreased as the cycling rate increased. This plateau
corresponds to the formation of a bi-phase region where Li-rich phases coexist with Lipoor anatase and rutile TiO2, and is where the majority of the anode’s capacity is derived
[31]. The plateau was relatively flat at low cycling rates of 0.1 and 0.2C, but at the higher
rates above 0.5C no obvious plateau was observed in the discharge profiles, due to
increasing polarization under high current densities brought around by sluggish Li-ion
diffusion and intercalation kinetics. Instead, most of the reversible lithium storage at high
rates was surface-confined charge storage that occurs at particle interfaces, indicated by
the sloping region below 1.7 V [32]. This ultimately led to the decreased cycling capacity
observed at 1C and 5C (Figure 4.6c). However, the Coulombic efficiency of the 25
printed-layer electrode was above 99.2% at 1C and 99.5% at 5C after the first few cycles,
compared to 98% at 0.1C, demonstrating stable cycling performance at elevated rates.
81
Figure 4.6. (a) Rate capability of a TiO2-25 anode and (b) the corresponding 2ndcycle charge and discharge voltage profiles at each different current rate. (c) Longterm cycling performance of TiO2-25 anodes at varying charge/discharge rates.
At high current rates, the specific capacity was limited by the diffusion of Li ions into
and out of the TiO2 nanoparticles. To measure the diffusion coefficient in these inkjetprinted anodes, CV curves at varying scan rates were taken, as shown in Figure 4.7a. A
linear relationship exists between the peak current and square root of the applied voltage
scan rate (Figure 4.7b), confirming that the reaction kinetics are diffusion-limited [33,
34]. From this plot, the apparent Li-ion diffusion coefficient, D, can be calculated using
equation 4.2 [35]:
 = 0.4958 (
  1/2
)

(4.2)
82
where ip is the peak current, n and nα are the number of electrons involved in the overall
and rate-determining processes (here, both 1), F is the Faraday constant, A is the surface
area of the electrode (estimated using the method outlined in [36]), C is the concentration
of lithium ions in Li0.5TiO2 (0.024 mol cm-3 [37]), α is the transfer coefficient (0.5 for
anatase TiO2 [34]), v is the scan rate, R is the gas constant, and T is the absolute
temperature (here, 298 K). Apparent diffusion coefficients for lithium-ion intercalation
and extraction are calculated as 1.1 × 10-15 and 5.6 × 10-15 cm2 s-1, respectively, which are
in good agreement with other studies [33, 34, 38, 39]. The higher rate of Li-ion extraction
compared to insertion into the TiO2 host can be attributed to the faster Li-ion diffusion
rates observed in TiO2 compared to lithiated TiO2 (Li0.5TiO2), due to the greater number
of unoccupied octahedral sites available for Li ions in anatase TiO2 [33].
Figure 4.7. (a) CV scans of inkjet-printed TiO2 anodes at varying scan rates. (b) Plot
of peak current vs. square root of the scan rate for both anodic (top) and cathodic
(bottom) peaks.
Herein, an inkjet-printing technology was applied to fabricate TiO2 thin film electrodes.
The parameters for inkjet printing have been optimized to achieve the highest
performance of TiO2 thin film electrodes. The cycling performance of these anodes in
LIBs is similar to other reported values for anatase and rutile TiO2 electrodes prepared by
conventional methods [32, 40, 41]. It should be noted that commercial TiO2 nano-powder
has been used in this study to give a universal demonstration of the ability and potential
83
of inkjet printing technology in the application of LIBs. Further improved performance of
the printed TiO2 electrodes can be expected by replacing it with battery-grade or tailored
nanostructured TiO2 powders. Accordingly, inkjet printing can therefore be a viable
fabrication method for thin film electrodes. It may find a niche in designing various
patterned electrodes for applications in which irregularly-shaped electrodes are needed,
such as terraced batteries or foldable devices. In addition, further work on applying this
technique to cathode materials and polymer electrolytes is ongoing in our group, in the
aim of fabricating a fully inkjet-printed cell.
4.4 Conclusions
In summary, we have fabricated thin film titanium dioxide anodes for LIBs by inkjet
printing and optimized the process in terms of selection of the polymer binder used and
control of thickness. The thickness of the electrode can be precisely controlled by varying
the number of printed layers, with each layer adding approximately 125 nm to the film.
By tuning the thickness of the anode it was possible to optimize the electrochemical
performance, with electrodes 3 μm thick exhibiting the highest reversible capacity, above
150 mAh g-1 for 100 cycles. In terms of rate performance, these inkjet-printed TiO2
anodes also demonstrated excellent capacity retention after 100 cycles up to 5C. Overall,
inkjet printing has been used as a feasible, simple, and inexpensive thin film electrode
fabrication technique and it is expected that inkjet printing will be further applied to
various types of electrode materials in the future.
Acknowledgements
This work was supported by the Natural Sciences and Engineering Research Council of
Canada (NSERC), Canada Research Chair (CRC) Program, Canada Foundation for
Innovation (CFI), Ontario Research Fund (ORF), and University of Western Ontario.
Stephen Lawes also acknowledges the Province of Ontario and the University of Western
Ontario for the Queen Elizabeth II Graduate Scholarship in Science and Technology.
84
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Chapter 5
5
Conclusions and Future Work
5.1 Conclusions
Two studies were conducted for this thesis, both developing an inkjet printing fabrication
process for thin film lithium-ion battery electrodes. Next-generation anode materials,
silicon and titanium dioxide, were inkjet printed for TFBs for the first time. The
application of these thin film electrodes in LIBs was demonstrated and further
experiments were performed to enhance their cycling performance and to investigate the
underlying mechanisms of these improvements.
Both studies followed the same basic plan: first develop nanoparticle inks with ideal
jetting characteristics, print them onto copper foil current collector, and then characterize
the physical, chemical, and electrochemical properties of these anodes. This process
involved determining the relationships between printing parameters and electrochemical
performance of the thin film electrodes. The effects of the polymer binder, number of
printed layers (thickness), and ink concentration on both inkjet printing quality and
cycling of LIBs were investigated.
In the first study, silicon nanoparticle anodes were fabricated by inkjet printing. The ink’s
physical properties were optimized to achieve excellent jetting conditions. The
relationship between the concentration of solid particles in the ink, the number of printed
layers, and the jetting performance was determined to ensure good printing properties.
Once it was possible to repeatedly achieve uniform films, four polymer binders were
investigated for their effect on the electrochemical performance of these printed anodes.
The conductive polymer PEDOT:PSS gave the best results; in cycling tests, inkjet-printed
silicon anodes with PEDOT:PSS binder outperformed those with conventionally-used
binders and those fabricated by the conventional doctor-blading technique. To determine
the reason for this, a number of ex-situ characterization techniques were used to study the
morphological and chemical structure of the anodes during electrochemical cycling. A
mechanism was proposed, which attributed the improved performance to the ability of
89
PEDOT:PSS to accommodate the large volume expansion of silicon during lithiation and
maintain an electrically conductive network. This study demonstrated the efficacy of
using inkjet printing for silicon anode fabrication, as well as furthered our understanding
of the role of the polymer binder in LIB performance.
In the second study, titanium dioxide anodes were fabricated using the same inkjet
printing process. Variations in the ink formulation were made to ensure proper jetting and
good electrochemical performance. Specifically, a higher loading content of active
material was possible for the TiO2 inks (80%) compared to the silicon inks (40%). This
was due to the smaller size of the TiO2 nanoparticles (21 nm) compared to silicon (50
nm), which resulted in shorter electron and lithium-ion diffusion lengths. This meant that
less conductive material was required to ensure sufficient electrical conductivity
throughout the electrode. Similarly, the optimal thickness of the TiO2 anodes (3 μm) was
greater than that of the silicon anodes (1 μm). The inkjet-printed TiO2 anodes exhibited
excellent cycling stability, even at high current densities. In particular, the performance at
high rates was investigated and it was determined that diffusion-limited kinetics
accounted for the decrease in capacity as the charge-discharge rate increased.
Overall, this work demonstrates that inkjet printing is a viable fabrication method for thin
film LIB electrodes. Inkjet printing can be used as a simple, inexpensive, and scalable
production technique for TFBs. Additionally, precise control over thickness, the ability to
deposit patterns, and minimal wasted material are all advantages of using inkjet printing
for TFB electrode fabrication. While the work here demonstrated a proof-of-concept for
the efficacy of inkjet printing for next-generation LIB anode materials, further studies
should be conducted to further improve the printing speed and reliability of this
technique, as well as fabricate other battery components (cathode and electrolyte) for full
cell assembly.
90
5.2 Recommendations for Future Work
Fabrication of battery components by printing is still a relatively new concept and more
work must be done to prove its commercial efficacy. Printing techniques are more suited
for fabricating smaller batteries for consumer electronics; however they may be feasible
for manufacturing larger cells for electric vehicles by using a scaled-up printer or roll-toroll technology. On the research side, additional studies should be performed to develop
inkjet printing fabrication methods for LIBs. To directly continue the work of this thesis,
a few projects of varying complexity and timescales are suggested here.
First, as a natural progression of the work presented here, next-generation cathode
materials for LIBs should be inkjet printed. A number of studies have already developed
inkjet printing techniques for fabricating LCO cathodes [1-3], but none have used highcapacity lithium iron phosphate (LFP) or nickel manganese cobalt oxide (NMC). One
potential challenge may be synthesizing these cathode materials into particles small
enough for inkjet printing (less than a few microns) to prevent clogging of the nozzles.
However, with ball milling or other techniques it should be possible. This would be a
relatively short project, as ink development would be straightforward following the work
presented in this thesis.
A more interesting project would be the development of a printable electrolyte. To date,
no LIB electrolyte has been deposited by inkjet printing. It would require the
development of a low-viscosity electrolyte solution that is stable in air, or operating an
inkjet printer inside a glovebox. Gel polymer electrolytes may be a promising choice, as
they can be stable in air and are initially liquids with sufficiently low viscosity for inkjet
printing before solidifying [4]. This means that they could be printed as a liquid and then
solidified by evaporation or other post-treatment (e.g. UV light).
Moreover, the combination of the above two projects plus the work in this thesis would
be high impact work: a fully inkjet-printed LIB. Ideally, a fully integrated process would
be developed, in which the anode, electrolyte, and cathode are sequentially printed on top
of one another. An insulating polymer could also be printed for the packaging of the cell.
91
It should be noted that the application of inkjet printing technology to other advanced
battery systems, including lithium-sulfur, sodium-ion, and metal-air batteries, has not yet
been demonstrated in any reported study so far. However, the printing techniques
discussed here may also be suitable for these batteries. For example, carbon encapsulated
sulfur (C-S) composite materials are the most commonly used electrodes for Li-S
batteries in the state-of-the-art. These microporous carbons used as the sulfur host have a
similar morphology as the carbon blacks commonly used in black inks for inkjet printing.
It can therefore be expected that such materials can be easily adopted for printed
electrodes. Additionally, porous carbon cathodes are indispensable in metal-air cells.
Printing technology can not only ensure the large-scale uniform fabrication of these air
cathodes, but can also be used to fabricate 3D-structured air electrodes to efficiently
accommodate the discharge products.
The true power of battery printing technology will be realized with 3D printing
techniques in the future, in which electrodes can be readily patterned into micro- and
nano-structures. Interdigitated electrodes can be used to minimized the ionic path length
and achieve high charge/discharge rates, as well as increase the volumetric energy
density by using the limited space within the cell [5]. And 3D printers are able to extrude
much higher viscosity fluids than inkjet printers, making it easier to print polymer
electrolytes and fabricate fully-printed batteries. In brief, the potential of printing
technology for batteries is still far from being fully realized.
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93
Curriculum Vitae
Name:
Stephen Lawes
Post-secondary
Education and
Degrees:
University of Waterloo
Waterloo, Ontario, Canada
2008-2013 B.A.Sc.
The University of Western Ontario
London, Ontario, Canada
2014-2015 M.E.Sc.
Honours and
Awards:
OGS Queen Elizabeth II Graduate Scholarship in Science
and Technology
2014
NSERC Canadian Graduate Scholarship
Masters Level
2015
Related Work
Experience
Teaching Assistant
The University of Western Ontario
2014-2015
Publications:
Lawes S, Riese A, Sun Q, Cheng N, and Sun X. Printing Nanostructured Carbon for
Energy Storage and Conversion Applications. Carbon, 2015.
Lawes S, Sun Q, and Sun X. Printing Graphene and Carbon Nanotubes for Energy
Storage and Conversion Applications. Handbook of Carbon Nano Materials: Volume 8,
World Scientific, 2015.
Lawes S, Sun Q, Lushington A, Xiao B, Liu Y, and Sun X. High performance inkjetprinted silicon anodes for lithium-ion batteries. To be submitted.
Lawes S, Sun Q, Guo H, Banis M, and Sun X. High performance inkjet-printed titanium
dioxide anodes for lithium-ion batteries. To be submitted.
Zhao Y, Yan B, Li X, Li D, Lawes S, and Sun X. Significant impact of 2D graphene
nanosheets on large volume change tin-based anodes in lithium-ion batteries: A review.
Journal of Power Sources, 2015.
Sun Q, Wang B, Xiao B, Liu J, Banis M, Yadegari H, Lawes S, Li R, and Sun X. Selfstacked nitrogen-doped carbon nanotubes as long-life air electrode for sodium-air
94
batteries: Elucidating the evolution of discharge product morphology. Nano Energy,
2015.
Pu N, Shi G, Liu Y, Sun X, Chang J, Sun C, Ger M, Chen C, Wang P, Peng Y, Wu C,
Lawes S. Graphene grown on stainless steel as a high-performance and ecofriendly anticorrosion coating for bipolar plates. Journal of Power Sources, 2015.
Lushington A, Liu J, Banis M, Xiao B, Lawes S, Li R, and Sun X. A Novel Approach in
Controlling the Conductivity of Thin Films using Molecular Layer Deposition. Applied
Surface Science, in press.
Shan H, Xiong D, Li X, Sun Y, Yan B, Li D, Lawes S, and Sun X. Tailored Lithium
Storage Performance of Graphene Aerogel Anodes with Controlled Surface Defects for
Lithium-Ion Batteries. Submitted.