ARTICLE
Received 24 May 2013 | Accepted 11 Oct 2013 | Published 13 Nov 2013
DOI: 10.1038/ncomms3756
Biologically enhanced cathode design for improved
capacity and cycle life for lithium-oxygen batteries
Dahyun Oh1,2,*, Jifa Qi1,2,*, Yi-Chun Lu1,3,w, Yong Zhang4, Yang Shao-Horn1,3,5 & Angela M. Belcher1,2,6
Lithium-oxygen batteries have a great potential to enhance the gravimetric energy density of
fully packaged batteries by two to three times that of lithium ion cells. Recent studies have
focused on finding stable electrolytes to address poor cycling capability and improve practical
limitations of current lithium-oxygen batteries. In this study, the catalyst electrode, where
discharge products are deposited and decomposed, was investigated as it has a critical role in
the operation of rechargeable lithium-oxygen batteries. Here we report the electrode design
principle to improve specific capacity and cycling performance of lithium-oxygen batteries by
utilizing high-efficiency nanocatalysts assembled by M13 virus with earth-abundant elements
such as manganese oxides. By incorporating only 3–5 wt% of palladium nanoparticles in the
electrode, this hybrid nanocatalyst achieves 13,350 mAh g 1c (7,340 mAh g 1c þ catalyst) of
specific capacity at 0.4 A g 1c and a stable cycle life up to 50 cycles (4,000 mAh g 1c,
400 mAh g 1c þ catalyst) at 1 A g 1c.
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 2 The David H. Koch
Institute for Integrative Cancer Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 3 Electrochemical Energy
Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 4 Center for Materials Science and Engineering, Massachusetts
Institute of Technology, Cambridge, Massachusetts 02139, USA. 5 Department of Mechanical Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, USA. 6 Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,
USA. * These authors contributed equally to this work. w Present address: Department of Mechanical and Automation Engineering, The Chinese University of
Hong Kong, N.T. Hong Kong SAR, China. Correspondence and requests for materials should be addressed to Y.S.H. (email: shaohorn@mit.edu) or to A.M.B.
(email: belcher@mit.edu).
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3756
R
echargeable Li-O2 batteries operate through the reversible
reaction between lithium ions and oxygen molecules1–3.
When Li-O2 batteries discharge in non-aqueous
electrolytes, Li þ reacts with reduced oxygen (oxygen reduction
reaction; ORR) yielding the reaction product, mainly Li2O2 that is
deposited on the catalyst electrode at a thermodynamic voltage of
2.96 V (versus Li/Li þ )4–6. On charging, the electrical energy is
stored by decomposing Li2O2 back into Li þ and O2 (oxygen
evolution reaction; OER). To promote these ORR and OER for
Li-O2 batteries, several materials have been investigated as
catalyst electrodes, such as noble metals (for example, Au, Pt,
Ru and Pd)7–9, transition metal oxides (for example, a-MnO2
(ref. 10), Co3O4 (ref. 11), MnCo2O4 (ref. 12), RuO2 (ref. 13)) and
carbon-based materials (for example, graphene14,15, carbon
fibre16 and doped carbon nanotube17). However, these
materials suffer either from high material costs, low
conductivities or side reactions with electrolytes, which limits
their use as high-performance Li-O2 battery electrodes. Although
these difficulties restrain the capability of Li-O2 batteries, the
rational design and selection of electrode materials may fully
deliver these battery chemistries to a high gravimetric energy
storage system.
To enhance structural functionality, organisms in nature form
skeletal tissues from soft to hard through the interactions between
nucleating protein matrices and inorganic ions18–20. Inspired by
these biominerals, researchers have fabricated novel nanoarchitectures using peptides or proteins as scaffolds applied in
various functional structures21. For example, numerous types of
platforms such as synthetic peptide nanofibers22, nanorings23 and
ferritin cages24 provide simple and uniform ways to synthesize
interesting materials at the nanoscale. In electrochemical energy
storage devices, nanostructured materials enhance Li ion batteries
by shortening the diffusion length of Li ions25–27 and benefit
capacitors by providing electrodes with large surface areas28,29.
For Li-O2 batteries, as the oxygen is in principle derived from the
air, both oxygen flow into electrodes and the high efficiency to
promote discharge/charge reactions are additional important
factors30. Constructing a new catalyst that combines porous
nanostructures and high catalytic activity would allow us to
harness the advantages of Li-O2 batteries for practical
applications, such as long-range electric vehicles.
Here we built the catalyst structures to achieve high Li-O2
battery performances by forming a nanocomposite of biotemplated manganese oxide nanowires (bio MO nanowires) with
incorporation of a small weight percent (3 wt%) of Pd
nanoparticles (Fig. 1). We believe this is the first bio-directed
synthetic method demonstrated for Li-O2 battery applications.
Results
M13 virus-mediated synthesis of manganese oxide nanowires.
Bio MO nanowires with spherulitic surface morphology were
synthesized through a facile and moderate chemical reaction by
utilizing the biomolecule, M13 virus. M13 viruses have been used
as versatile templates for high aspect ratio nanowire synthesis
(for example, semiconductors31, metal oxides32–34) under
environmentally benign conditions. Among the five different
types of capsid proteins on this virus template, p8 is a major coat
protein with 2,700 copies surrounding the single-stranded DNA.
In addition to the material specificity of these surface proteins
(p8 or five copies of minor proteins existing at one end of virus
particle, p3), electrostatic interactions between the opposite
charge of precursor ions and functional groups of p8 coat
protein broaden the template functionality. In this work, the virus
clone with the p8 peptide sequences ADVYESALPDPAEAAFE
(named FC#2)35 was used because it has two additional negative
2
Biotemplating
MnOx
M13 virus
Carbon
particle
p8
ADVYESALPDPAEAAFESL..
Porous biotemplated
NWs electrodes
Li+
Li2O2
e
PAA wrapping
on biotemplated
MnOx NWs
O2
Incorporation of
metal NPs
Porous and catalytic
biotemplated
NWs electrodes
Figure 1 | Schematic of a nanocomposite structure Synthesis step of the
metal nanoparticle/M13 virus-templated manganese oxide nanowires
(bio MO nanowires) and the operational reaction inside Li-O2 battery cells.
functional groups (-COOH from glutamic acid on 13th and 17th)
than M13KE (New England Biolabs) under basic conditions. Using
this clone, bio MO nanowires were synthesized by first binding
Mn2 þ ions to the FC#2 virus p8 proteins followed by reacting with
KMnO4 at room temperature. From this simple aqueous
biotemplating reaction, homogeneous bio MO nanowires with
high aspect ratio (B80 nm in diameter and B1 mm in length) were
obtained. These nanowires showed spherulitic surface morphology
(transmission electron microscopy (TEM) image in Fig. 2a,b
and Supplementary Fig. S1a). Although the flower-like spheres of
MO were previously synthesized36,37, the nucleation of MO
along the virus particle developed this spherulitic surface of
materials into higher-dimensional nanowires. Thus, the
hierarchical nanostructures of these bio MO nanowires are
expected to be advantageous in Li-O2 battery performances.
Although previous studies utilized smooth surface MO nanowires
for Li-O2 battery electrodes10,38, the rough surface of these bio MO
nanowires can provide large catalytic area as well as enough storage
space for discharge products. To the best of our knowledge, this is
the first report of M13 virus-directed synthesis of manganese oxide
nanowires, which is applicable not only to Li-O2 battery electrodes
but also to various applications such as electrochemical capacitors39,
Li ion battery electrodes40 and water purifications41.
With the aqueous, room temperature approach using M13
virus templates, multivalent (Mn3 þ /Mn4 þ ), birnessite-like MO
nanowires were developed. The crystallographic and chemical
properties of bio MO nanowires were investigated with X-ray
diffraction (XRD), high-resolution TEM (HRTEM), the chemical
titration method and X-ray photoelectron spectroscopy (XPS).
First, the XRD pattern of virus-templated MO nanowires showed
three broad peaks centred at 2y of 37°, 53.5° and 65.5°
(Supplementary Fig. S1b), partially matching to d-MnO2
(birnessite, space group C2/m, powder diffraction file number
01-073-2509) XRD pattern. In addition, the surface morphology
of bio MO nanowires was similar to that of typical birnessites42,
as can be seen in the TEM image (Fig. 2b). The crystal sizes of bio
MO nanowires were 2–5 nm and the coexisting amorphous phase
was observed by HRTEM (Fig. 2c). The average oxidation state of
bio MO nanowires was determined to be 3.51 by chemical
titration methods43, suggesting a multivalence state (Mn3 þ /
Mn4 þ ¼ 0.96) of nanowires. This mixed-valence property of MO
was also observed from XPS spectra in Supplementary Fig. S2.
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3756
1.6 Å
2.4 Å
Mn
Mn
Pd
Au
BET surface area: 98.5 m2 g–1
BET surface area: 271.7 m2 g–1
Figure 2 | Electron microscope images of catalyst materials. TEM images of M13 virus-templated MO nanowires (a) at low magnitude (scale bar,
500 nm), (b) zoomed at the end of an individual bio MO nanowire (scale bar, 50 nm) and (c) HRTEM image showing amorphous phase mixed with
crystalline phase (scale bar, 5 nm, inset: the fast Fourier transform (FFT) pattern with two major rings matching to the atomic planes with the spacing of 2.4
and 1.6 Å, respectively). The synthetic procedure of bio MO nanowires was easily reproducible. We synthesized, characterized and made electrodes
from over 30 times independent experiments. (d) STEM image of Pd/bio MO nanowires (left) and the corresponding energy dispersive X-ray (EDX)
elemental mapping images of manganese (Mn, red, upper right) and palladium (Pd, blue, lower right; scale bars, 100 nm). (e) STEM image of Au/bio MO
nanowires (left) and the corresponding EDX elemental mapping images of manganese (Mn, red, upper right) and gold (Au, yellow, lower right; scale bars,
50 nm). The SEM image and BET surface area of (f) MO nanoparticle showing bulky phase of aggregated particles and (g) homogeneously dispersed bio
MO nanowires with larger pores (scale bars, 500 nm). The two BET surface areas are 98.5 (f) and 271.7 m2 g 1 (g).
We confirmed that the average oxidation state of bio MO
nanowires lies between Mn3 þ and Mn4 þ by observing two
features: the peak position of Mn 2p3/2 and the multiplet splitting
of Mn 3s. First, the Mn 2p3/2 peak position of bio MO nanowires
(642.1 eV in Supplementary Fig. S2a) was close to that of MnO2
(Mn4 þ , 642.2B642.4 eV) as reported in the literature44,45.
However, the width of Mn 3s multiplet splitting (5.2 eV in
Supplementary Fig. S2b) of bio MO nanowires showed a larger
separation than that of MnO2 (4.5B4.7 eV) but close to that of
Mn2O3 (5.2B5.4 eV)44,45. As it is known that the separation
between Mn 3s splitting peaks becomes larger as the oxidation
state of Mn is lower46, the larger separations of Mn 3s splitting
compared with Mn4 þ imply the presence of a lower valence state
in bio MO nanowires.
Nanocompositing ORR catalyst on bio MO nanowires. To
enhance the ORR activity and surface conductivity of porous bio
MO nanowires for Li-O2 battery electrodes, 1–3 wt% of ORR
catalytic metal nanoparticles were homogeneously incorporated
onto the surface of the nanowires. It is reported that noble metal
nanoparticles (for example, Pd, Pt, Ru and Au) have higher
intrinsic ORR activities compared with carbon with 0.1 M lithium
perchlorate (LiClO4) in dimethoxyethane (DME) electrolyte7. In
particular, very recent studies reported a great stability (Au)47 and
high ORR catalytic activity (Pd)7 in Li-O2 battery applications.
However, those electrodes contained 40–100 wt% of noble metal
nanoparticles, making them less practical because of costefficiency. Thus, we suggest the composite structure of bio MO
nanowires combined with noble metal nanoparticles to decrease
the loading of precious metals in cathodes while maximizing the
catalytic activity of Li-O2 batteries.
The nanocomposites were designed to nucleate the metal
nanoparticle on the surface of the bio MO nanowire templates to
maximize the contact with discharge products in Fig. 1 (red lines,
right middle). In addition, the homogeneous distribution of ORR
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3756
metal catalyst along the bio MO nanowires was important in
maintaining the porous structure of the electrodes. To facilitate
these requirements, poly acrylic acid (PAA) was incorporated
around the bio MO nanowires to nucleate the metal nanoparticle.
The PAA-wrapped nanowires increased the colloidal stability of
the nanowires during the synthesis so that a more homogeneous
solution of bio MO nanowires was observed (Supplementary
Fig. S3). Control experiment without PAA wrapping resulted in
free Pd nanoparticle formation and aggregated MO nanowires as
shown in the TEM image (Supplementary Fig. S4). This
composite synthesis method was applied to two different ORR
metal catalysts, Au or Pd (Au/Pd). To nucleate these metal
nanoparticles on PAA-wrapped bio MO nanowires, ethylene
glycol was used as a mild and selective reducing agent to react
with noble metal precursors (HAuCl4 and Na2PdCl4) slowly,
avoiding reactions with virus-templated MO nanowires. This
metal nanoparticle incorporation method by stabilizing and
trapping precursor ions with PAA is expected to be applicable not
only to biotemplated nanowires but also to various nanosized
substrates, such as nanofibers.
The homogeneous distribution of Au/Pd nanoparticles (with a
diameter of 2–6 nm Supplementary Fig. S5), was observed by
elemental mapping using scanning TEM (STEM) in Fig. 2d,e. The
amount of PAA used to form the composite nanostructure was
determined with thermogravimetric analysis (TGA) to be 2.5 wt%
of the electrode by comparing the weight loss difference between
bio MO nanowires and Pd incorporated bio MO nanowires at
500°C (Supplementary Fig. S6)48. The amount of Au/Pd
nanoparticle incorporated in the bio MO nanowires was
measured by inductively coupled plasma atomic emission
spectroscopy (ICP-AES) analysis and varied B1–3 wt% of the
total electrode, depending on the metal precursor amount that
was added during synthesis.
Improved Li-O2 battery performances with bio MO nanowires.
The virus-templated MO Li-O2 battery cathodes showed a
9,196 mAh g 1c (955 mAh g 1c þ catalyst þ Li2O2) first discharge
capacity with 92.6% of coulombic efficiency at a current density
of 0.4 A g 1c. The electrochemical tests were performed with
0.1 M LiClO4/DME electrolyte under 1 atm of oxygen (further
details on measurement methods are described in the experimental section). Although ether-based electrolytes may undergo
some side reactions during Li-O2 battery charging49, it is one of
the most stable electrolytes against the oxygen electrode for Li-O2
batteries known to date50 without the reactivity with Li metal like
DMSO has.47 Further control experiments with different
electrolytes are included in the Methods section and
Supplementary Fig. S7. For galvanostatic tests, the operating
voltage was limited to 2.2–4.15 V (versus Li/Li þ ) to minimize
possible parasitic reactions. First, the effect of MO electrode
structures on Li-O2 battery performance was studied by varying
the architecture from nanoparticles to bio MO nanowires. Under
the same synthetic conditions, MO nanoparticles (60 nm
diameter) were synthesized without viruses and their
morphology was compared with bio MO nanowires with
scanning electron microscope (SEM) images (Fig. 2f,g). The
surface area of both nanoparticles and nanowires was measured
by the Brunauer–Emmett–Teller (BET) method giving 98.5 and
271.7 m2 g 1, respectively. Bio MO nanowires showed a 40%
improvement of Li2O2 storage capability (9,196 mAh g 1c at
0.4 A g 1c) compared with MO nanoparticles (6,545 mAh g 1c),
as can be seen from the first discharge capacity in Fig. 3a. In
addition, the discharge voltage of the bio MO nanowires
(B2.68 V) is higher than that of MO nanoparticles (B2.6 V)
by B80 mV and the charge voltage of the bio MO nanowires
4
(B3.65 V) is lower than that of MO nanoparticles (B3.77 V) by
B120 mV. In other words, the bio MO nanowires showed
decreased discharge and charge overpotentials compared with the
MO nanoparticles. As the oxidation state of both MO
nanoparticles and nanowires was determined to be similar by
XPS analysis (Supplementary Fig. S2a,b), the difference in Li-O2
battery discharge capacity was mainly attributed to their different
surface area51 and morphologies. As MO nanoparticles
agglomerate during the synthesis, the low surface area electrode
materials (Fig. 2f) limit the surface-mediated reaction so that it
results in a poor specific discharge capacity. In our biotemplated
system, the high aspect ratio as well as larger surface areas
(Fig. 2g) formed highly active Li-O2 battery cathodes where the
electrode spaces can be fully utilized while minimizing the effect
of discharge product blocking the electrodes.
The major discharge product obtained after the first discharge
of bio MO nanowire electrodes was Li2O2 confirmed by XRD in
Fig. 3b. The XRD data of discharged electrode match well to the
reference XRD pattern of Li2O2 (orange star, powder diffraction
file number 01-074-0115). There was no other major discharge
product observed except Li2O2. In previous reports, MOs have
been shown to produce a LiOH phase during the discharge38,49
because of the structural water or hydroxyl residues. The Li-O2
battery performance of dehydrated (thermal treatments at 160 °C
for 2 h) bio MO nanowires was investigated to eliminate any
possible contributions from water molecules or hydroxyl groups.
The thermally treated nanowires showed a similar discharge
capacity of 9,206 mAh g 1c at 0.4 A g 1c (Supplementary Fig. S7,
blue) compared with nanowires without heat treatments,
although structural water decreased from 3.2 to 1.5 wt% of
electrodes after heat treatment (Supplementary Fig. S6). Thus, the
LiOH formation is negligible in this Li-O2 battery performance.
Battery performances of Pd/Au-loaded bio MO nanowires. The
hybrid nanostructure composed of Pd nanoparticles and bio MO
nanowires exhibited improvement of the Li-O2 battery discharge
capacity. The first galvanostatic cycle of Au/bio MO nanowires
and Pd/bio MO nanowires were compared with approximately
the same amount of Au (1.7 wt%) and Pd (1.1 wt%) incorporations in each nanocomposited electrode. After the addition of
only B1 wt% of Au or Pd nanoparticles, the first discharge
capacities of Au/bio MO nanowires and Pd/bio MO nanowires
(cutoff voltage at 2.2 V versus Li/Li þ ) were improved by 8% (Au)
and 15.5% (Pd) compared with bare bio MO nanowires (at
0.4 A g 1c, Fig. 3c, Au: blue straight, Pd: red dashed). In addition,
the Pd/bio MO nanowire electrodes showed B60 mV higher
discharge voltage than Au/bio MO nanowires as can be observed
from Fig. 3c. The higher discharge voltage of Pd/bio MO nanowires than Au/bio MO nanowires in the first galvanostatic cycle
of Li-O2 electrodes may be attributed to the attractive interaction
between Pd and oxygen52. Moreover, although Pd has a lower
contrast than Au on TEM observation because of its lower atomic
mass, large (greater than 5 nm) Pd nanoparticles on bio MO
nanowires were rarely observed than Au nanoparticles-decorated
bio MO nanowires (Supplementary Fig. S5); hence, the smaller
nanoparticle distribution of Pd/bio MO nanowires in a given
synthetic conditions may improve the battery performances.
Thus, the surface electrocatalytic functionality of bio MO
nanowires was easily tuned by the composite synthesis method
developed here.
The capability to store lithium peroxide was further
improved by increasing Pd incorporations from 1.1 to 3 wt% of
Pd/bio MO nanowires electrodes. With 3 wt% of Pd incorporation, a 45% improvement of the first specific capacity of bio
MO nanowires (Fig. 3c, red straight, 13,347 mAh g 1c,
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Specific capacity (mAh g–1
)
c
0
2,000
4,000
6,000
8,000
10,000
4.5
* Li2O2
(100)
(101)
Intensity (a.u.)
3.5
3.0
2.5
2.0
MnOx NPs
*
Discharged
Pristine
Biotemplated MnOx NWs electrodes
Biotemplated MnOx NWs
1.5
0
Charged
**
(110)
Cell voltage (V)
4.0
1,000 2,000 3,000 4,000 5,000
30
40
50
60
70
2 (degree)
Specific capacity (mAh g–1
c+catalyst)
Specific capacity (mAh g–1
)
c
3,000
6,000
9,000
12,000 15,000
Cell voltage (V)
4.0
3.5
3.0
2.5
PAA/Au NPs (1.7 wt%)
PAA/Pd NPs (1.1 wt%)
PAA/Pd NPs (3 wt%)
2.0
8,000
Specific capacity (mAh g–1
c+catalyst)
Biotemplated MnOx NWs nanocomposites
1.5
14,000
12,000
6,000
10,000
8,000
4,000
6,000
4,000
2,000
2,000
0
0
0
2,000
4,000
6,000
Specific capacity (mAh gc–1)
0
4.5
MnOx
NPs
8,000
Specific capacity (mAh g–1
c+catalyst)
Biotemplated Biotemplated Biotemplated
MnOx
MnOx
MnOx NWs
NWs
NWs/PAA
with
PAA/Pd NPs
Specific capacity (mAh g–1
)
c
1,000
2,000
3,000
4,000
4.5
Cell voltage (V)
4.0
3.5
3.0
MnOx NPs
2.5
Biotemplated MnOx NWs
Biotemplated MnOx NWs with PAA/Pd NPs
Specific capacity (mAh g–1
c+catalyst)
500
Electrodes with 8 wt % of carbon
5,000
Electrodes with 8 wt % of carbon
400
4,000
300
3,000
200
2,000
100
2.0
0
100
200
300
400
1,000
MnOx NPs
Biotemplated MnOx NWs
Biotemplated MnOx NWs with PAA/Pd NPs
0
0
10
20
30
40
50
60
Specific capacity (mAh gc–1)
0
0
70
Cycle number
Specific capacity (mAh g–1
c+catalyst)
Figure 3 | The electrochemical performance of Li-O2 batteries. (a) The first galvanostatic cycle of MO nanoparticles (blue) and bio MO nanowires
(red) at a current density of 0.4 A g 1c (B0.044 mA cm 2) with 0.1 M LiClO4/DME electrolytes under 1 atm oxygen. (b) XRD patterns of bio MO
nanowire electrodes at the pristine state (blue), the end of the first full discharge until 2.2 V (dark yellow) and the end of the first full charge until 4.15 V
(red) under 0.4 A g 1c of current density. (c) The first galvanostatic cycle of bio MO nanowires with the similar weight percent of Pd (red dash)
and Au (blue straight) nanoparticle composited at the current density of 0.4 A g 1c. Further improvement was observed with 3 wt% addition of Pd
nanoparticles (red straight) onto bio MO nanowires. (d) The specific capacities of MO nanoparticles (orange), bio MO nanowires (blue), bio MO
nanowires with PAA wrapping (dark yellow), Pd/bio MO nanowires (red) electrodes including s.d. error bars (154B854 mAh g 1c). Each sample type was
made and tested at least triplicate. (e) The first cycling voltage profile of Li-O2 batteries with the fixed discharge capacity of 4,000 mAh g 1c
(400 mAh g 1c þ catalyst) for MO nanoparticles (blue), bio MO nanowires (dark yellow) and Pd/bio MO nanowires (red) at the current density of 1 A g 1c.
Each electrode was composed of 8 wt% of carbon and was cycled in 2–4.15 V voltage windows. (f) The cycling performance of Li-O2 battery
electrodes composed of MO nanoparticles (blue), bio MO nanowires (dark yellow) and Pd/bio MO nanowires (red) at the current density of 1 A g 1c
(0.1 A g 1c þ catalyst) with the two different fixed discharge capacities of 2,000 and 4,000 mAh g 1c (200, 400 mAh g 1c þ catalyst). The circular shape
indicates discharge capacities (for 2,000 and 4,000 mAh g 1c), the triangle indicates charge capacities (until 4.15 V) for 2,000 mAh g 1c and the inverted
triangle indicates charge capacities for 4,000 mAh g 1c. Each electrode was composed of 8 wt% carbon. All of electrochemical tests reported in this work
were repeated for at least six different devices.
1,008 mAh g 1c þ catalyst þ Li2O2) was achieved at 0.4 A g 1c. This
is the highest specific capacity under a high applied current
(0.4 A g 1c and 0.22 A g 1c þ catalyst) among all manganese
oxide-based electrodes reported to date38,53. The formation of
Li2O2 after discharge and its decomposition during the charge
(oxygen evolution was detected by gas chromatography in
Supplementary Table S1) with Pd/bio MO nanowire electrodes
was also confirmed by Raman spectroscopy in Supplementary
Fig. S8. No significant amount of side product (for example,
Li2CO3) can be detected by XRD and Raman, suggesting that
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3756
DME is largely stable during discharge54. We believe the
enhanced discharge capacity of Pd/bio MO nanowires
originated from the better utilization of porous electrode spaces
by improving the electrochemical active surface area and surface
conductivity of bio MO nanowires. The Pd nanoparticles located
on the surface of the bio MO nanowires facilitated ORR reactions
and improved the electronic conduction throughout the Li2O2
layers allowing further discharge product depositions.
The differences in the charging profile of the 3 wt% Pd-MO
nanowires compared with the bare bio MO nanowires (Fig. 3a)
can be associated with the increased thickness of Li2O2 layer of
the 3 wt% Pd/bio MO nanowires (B4 nm Li2O2) compared with
the bare bio MO nanowires (B2.6 nm Li2O2) because of larger
discharge capacity. However, the charging potential of Pd/bio
MO nanowires after decomposing 0.05 mAh cm 2true of Li2O2
was still B100 mV lower than that of catalyst-free, carbon
electrode55 at the same depth of discharge, B1 mAh cm 2true (the
true electrode surface area-specific capacities; the detail
calculation method is included in the Methods section). This
lower OER potential compared with catalyst-free electrodes
suggests the catalytic function of Pd/bio MO nanowires even
after the deep discharge in Li-O2 battery. The following (second)
cycle profile of MO nanoparticles, bio MO nanowires, Au
(1.7 wt%)/bio MO nanowires, Pd (1.1 wt%)/bio MO nanowires
and Pd (3 wt%)/bio MO nanowires are included in
Supplementary Fig. S9 and Supplementary Table S2. The
second charging profile showed enhanced performance over
previous manganese oxides-based electrode at B6 times higher
current density38, and further studies to improve the cycle life
after the deep discharge remains as future work.
To clarify the role of Pd nanoparticles, PAA-wrapped bio MO
nanowires were tested as Li-O2 battery electrodes under the same
test conditions. The first discharge capacity of the PAA/bio MO
nanowires electrode was 9,722 mAh g 1c (Supplementary Fig. S7,
green), which is only a 5% improvement of bio MO nanowire
electrodes. This small improvement of discharge capacity could
be attributed to the decreased aggregation between bio MO
nanowires after PAA stabilization. The average value of discharge
capacity can be seen in Fig. 3d. Therefore, the increased first
discharge capacity of Pd/bio MO nanowire composites originated
from Pd nanoparticle incorporation onto bio MO nanowires, not
from the PAA wrapping. We expect that this methodology can be
easily applied to other promising ORR or OER candidate
materials to develop efficient catalyst structures for Li-O2
batteries in future studies.
Lowered carbon mass in cycling electrodes. The catalyst
nanostructure designed in this work, Pd/bio MO nanowires,
allowed for an electrode with greatly decreased carbon content
(8 wt%) in the catalyst electrodes to improve cycle life of Li-O2
battery. Although carbon functions as a conducting agent and an
ORR catalyst, it can also reduce the cycle life because side products
can be formed at the carbon/electrolyte or carbon/Li2O2 interface
in electrodes with ether-based electrolytes56. By reducing carbon
amounts in our electrodes, cycle life of Li-O2 batteries was
enhanced at high-current densities (1 A g 1c, 0.1 A g 1c þ catalyst)
even under relatively low charging cutoff voltage (4.15 V). The
cycling tests were conducted by limiting the discharge capacity to
2,000 mAh g 1c (200 mAh g 1c þ catalyst) or 4,000 mAh g 1c
(400 mAh g 1c þ catalyst, 299 mAh g 1c þ catalyst þ Li2O2) following
the previous works method3,10. Similar to the first dischargespecific capacity, the cycle life of Li-O2 batteries was gradually
enhanced as the catalyst structure changed from MO
nanoparticles to nanowires and finally to composite Pd/bio MO
nanowires (Fig. 3e,f, Supplementary Figs S10–12). In particular,
6
the OER overpotential was lowered as the surface area of catalyst
electrodes as well as the surface conductivity of bio MO nanowires
was improved by Pd incorporation (Fig. 3e) after being discharged
to 4,000 mAh g 1c (400 mAh g 1c þ catalyst). Thus, the first cycling
round trip efficiencies of MO nanoparticles, bio MO nanowires
and Pd/bio MO nanowires was improved from 75%, 77.7 to
79.2%, respectively, as the cathode was rationally designed. The
aggregated microstructure of MO nanoparticles limited the
Li-O2 battery cycle life to 10 cycles with 4,000 mAh g 1c
(299 mAh g 1c þ catalyst þ Li2O2) at 1 A g 1c (0.1 A g 1c þ catalyst).
The bio MO nanowires showed cycle retentions up to 20 cycles
because of the high porosity and large catalytic surface area of the
electrodes. After incorporating Pd nanoparticles, the discharge
capacity of Li-O2 batteries maintained stable to 50 cycles (Fig. 3f,
Supplementary Fig. S10). When the depth of discharge capacity
was fixed to 2,000 mAh g 1c (at 1 A g 1c, 0.1 A g 1c þ catalyst),
these electrodes showed a longer cycle life with a similar cycling
performance trend. Each MO nanoparticles, bio MO nanowires
and Pd/bio MO maintained the 2,000 mAh g 1c of discharge
capacity up to 22, 47 and 58 cycles, respectively (Fig. 3f,
Supplementary Fig. S11). The reversible cycling number with
Pd/bio MO nanowire electrodes is smaller than the recently
reported3,47, which might be because of the instability of
electrolyte or the reactivity between carbon and oxygen
reduction intermediates56,57. This can be observed from the
charging cycle profile in Supplementary Figs S10–12, resulting in
the faster decay of charging capacity than the discharging capacity,
as cycling number increased in Fig. 3f. Ongoing and future work
will be focused on developing stable electrolytes and carbon-free
electrodes to enhance the cycling of these electrodes.
To investigate the effect of carbon on cycling of Li-O2 batteries,
electrodes were prepared with 44 wt% of carbon. The cycle life of
bio MO nanowires and Pd/bio MO nanowires decayed faster than
those of low-carbon electrodes under the same fixed discharge
capacity (728 mAh g 1c, 400 mAh g 1c þ catalyst) and current
density (1 A g 1c, 0.1 A g 1c þ catalyst). The observed cycle
number was 17, 21 and 30 for MO nanoparticles, bio MO
nanowires and Pd/bio MO nanowires, respectively
(Supplementary Fig. S12). Although the electronic conductivity
was higher for 44 versus 8 wt% carbon electrodes, side products
can be formed in the high-carbon electrodes during cycling56
counteracting the kinetic advantage in Li-O2 battery performance.
Therefore, we believe that Pd/bio MO nanowires are a viable
material platform for long-lasting, high-capacity Li-O2 batteries.
Discussion
Using a biological template, a new catalyst electrode was
developed
that
increased
capacity
(13,350 mAh g 1c;
1
1
7,340 mAh g c þ catalyst at 0.4 A g c) and cycle life (50 cycles
with 4,000 mAh g 1c; 400 mAh g 1c þ catalyst at 1 A g 1c;
0.4 A g 1c þ catalyst) of Li-O2 batteries at the highest gravimetric
current density compared with other transition metal oxide-based
electrodes in literatures10,38. These templates allowed the
formation of high aspect ratio manganese oxide nanowires
wrapped in 3 wt% Pd nanoparticles facilitating compositional
control that outperforms materials made by mechanical mixing58.
These high aspect ratio bio MO nanowires formed a porous
network that maximizes the interaction between the catalyst and
the discharge products (Li2O2) thus increasing reversibility and
generating a stable cycle life of Li-O2 batteries. Further, the Pd/bio
MO nanowires allowed the implementation of low-carbon
electrode batteries improving battery cycle life. We believe that
this template is readily adaptable for other material combinations
and can provide a platform to test various catalytic electrode
materials for Li-O2 batteries and other catalytic systems.
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3756
In addition, the application of biomaterials will broaden the
material selection scope into diverse areas in designing Li-O2
battery electrodes thus contributing to the development of energy
storage systems.
Methods
Material synthesis. Manganese sulphate monohydrate (MnSO4 H2O; SigmaAldrich, 2.4 ml of 50 mM) and sodium sulfate (Na2SO4; Sigma-Aldrich, 2.4 ml of
50 mM) solutions were added together into 50 ml of deionized (DI) water. FC#2
viruses (1.63 1013) were added into this solution and stirred at room temperature
overnight. After the overnight incubation, potassium permanganate (KMnO4; Alfa
Aesar, 16 ml of 10 mM) solution was added and stirred at room temperature
overnight. The final products were washed with DI water by centrifugation and
collected after lyophilization. To incorporate the noble metal nanoparticles (Pd and
Au), the bio MO solution was dialyzed against DI water overnight and diluted with
DI water to make a final solution volume of 200 ml. PAA (Mw: 2,000 g mol 1, 0.6 g
in 20 ml of DI water) was added and stirred overnight. The PAA-wrapped bio MO
nanowires were collected by centrifugation and dispersed again with 240 ml of DI
water. Au/Pd precursor solutions were added into the previous solution (for Au:
gold (III) chloride trihydrate, HAuCl43H2O, 2.17 ml of 25.4 mM for 1.7 wt% of Au
in the final Au/bio MO nanowire composite electrodes. For Pd: sodium tetrachloropalladate (II), Na2PdCl4, Sigma-Aldrich, 0.612 ml of 34 mM for 3 wt%
(0.408 ml of 34 mM for 1.1 wt%) of Pd in the final Pd/bio MO nanowire composite
electrodes) and stirred overnight at room temperature. To reduce the metal ion,
3 ml of ethylene glycol was added to each Au/bio MO nanowire and Pd/bio MO
nanowire solution for 24 h inside a brown bottle because of light sensitivity. The
solution was washed three times by centrifugation with DI water and lyophilized.
Material characterizations. For SEM images, FEG-SEM 6700F (JEOL) was used
under 5 kV with 5.9 mm of working distance (or 0.5 kV with 3 mm of WD) after
coating the sample with Pd. HRTEM images were obtained with 2100, 2010 FEG
(JEOL) under an accelerating voltage of 200 kV. The sample was loaded on formvar/carbon or quantifoil grids (Electron Microscopy Sciences). XRD (PANanalytical Multipurpose Diffractometer, Cu Ka radiation) patterns were collected with
0.18° per minute scan speeds under the following settings: 2° of anti-scatter slit,
6 mm of irradiated length of automatic mode and 0.04 rad of soller slit. The
discharged/charged samples were collected after disassembling batteries inside an
Ar-filled glove box and were covered by Kapton tape to prevent any air contamination. ICP-AES (Horiba Jobin Yvon ACTIVA-S) was used to determine the
atomic ratio between manganese, gold, palladium ions of Au/bio MO nanowires
and Pd/bio MO nanowires. The sample was dissolved in nitric acid (ACS reagent
grade, 70%) at 50 °C overnight and diluted with DI water (Mediatech, Inc. meets
WFI quality standards) to make the final nitric-acid concentration of 2% v/v for
ICP-AES measurements. XPS (Physical Electronics Versaprobe II) was measured
with the monochromatic Al X-ray source (Ka, 1486.6 eV) under the pass energy of
23.5 eV and the step size of 0.1 eV. The acquired XPS spectra were calibrated with
adventitious carbon peak (1s) as a reference positioned at 284.8 eV. To quantify the
PAA mass of the metal nanoparticles/bio MO nanowires composites, TGA (Q50
TA instrument) was used with 10 °C min 1 increasing temperature rate under
nitrogen.
Chemical titration for Mn oxidation state determination. The average oxidation
state of MO was determined by two separate chemical titration steps reported in
the literature43. The colour change indicator (sodium diphenylamine-4-sulphonate,
C12H10NNaO3S, Alfa Aesar) was further added during the titration. MO
nanoparticles were used for titrations because the manganese oxidation state is
expected to be similar to bio MO nanowires from XPS analysis and the protein
residue of bio MO nanowires would cause significant errors during the titration.
Electrochemical tests. The lyophilized catalyst materials were further dried in a
vacuum oven at 90 °C overnight before electrode preparations. The Li-O2 electrodes were composed of Ketjan black (EC600JD, AkzoNobel), catalyst materials,
LiTHion dispersion (Ion Power, USA) with the mass ratio of 44: 36: 20 for highcarbon-containing (44 wt%) electrodes and the mass ratio of 8: 72: 20 for lowcarbon-containing (8 wt%) electrodes. These three components were dissolved into
2-propanol and mixed overnight using a bath sonicator to get homogeneous slurry
mixtures. The mixture was spread on the separator (Celgard, PP2075) by tape
casting methods and dried in a vacuum oven at 80 °C for 2 h. The dried slurry was
punched into circle electrodes with diameters of 1.27 cm, and the mass of each
electrode was measured (0.1–0.3 mg cm 2) following further drying in vacuum
oven at 80 °C overnight before the electrochemical test. Inside the Ar-filled glove
box, the catalyst electrode was soaked with the electrolyte in advance (0.1 M LiClO4
in DME, Novolyte) and assembled into the Li-O2 battery cell designed by our lab.
Metallic Li foils (negative electrode), two layers of separators, catalyst electrodes
and a positive electrode current collector (316 stainless steel mesh) were sequentially put on the cell and additional electrolyte (100 ml) was added. After the
assembly, the Li-O2 battery cell was purged with oxygen (Airgas, Ultra Pure Carrier
grade) and closed at 1.08 atm. The purged cell was allowed to rest at least 2 h before
the electrochemical test. The first cycle of Li-O2 battery cell was tested by galvanostatic measurements at 0.4 A g 1c with a Solartron Analytical 1470E potentiostat. For the cycling tests, the pressure of the Li-O2 battery cell was adjusted again
to 1.08 atm after 16 cycles and 1 A g 1c of current was applied.
The Li-O2 battery performance with different electrolytes. The first galvanostatic cycle of bio MO nanowires was tested with 1 M lithium triflate (LiCF3SO3) in
tetra(ethylene)glycol dimethyl ether (TEGDME) to investigate the Li-O2 performance dependence on electrolytes. Here bio MO nanowire electrodes with
LiCF3SO3/TEGDME exhibited higher charging overpotential as well as lower first
discharge capacity (Supplementary Fig. S7, red, 8,766 mAh g 1c at 0.4 A g 1c).
Further, the coulombic efficiency of nanowire electrodes dropped from 92.6%
(0.1 M LiClO4 in DME) to 61.5% (1 M LiCF3SO3 in TEGDME) at the same
charging cutoff voltage, 4.15 V. As increasing the test voltage windows higher than
4.15 V may evoke the electrolyte decomposition, DME was suitable for bio MO
nanowire electrodes to achieve higher coulombic efficiency in lower voltage ranges.
The true electrode surface area-specific capacity. Here we make estimation
in calculating the thickness of the discharge product, Li2O2. It is assumed that
Li2O2 is formed as films throughout the overall catalyst electrode with the
volumetric capacity, 2698.6 mAh cm 3. First, the true electrode surface area was
calculated. The BET surface area of bio MO nanowires is 271.7 m2 g 1 and the
reported BET surface area of Ketjan black (EC600JD, AkzoNobel) is 1,400 m2 g 1.
Thus, the true surface area can be obtained by considering the weight percentage
(catalyst: Ketjan black ¼ 36: 44) of each component in electrode, resulting in
271.7 m2 g 1 0.36 þ 1,400 m2 g 1 0.44 ¼ 713.8 m2true g 1c þ catalyst. As
13,347 mAh g 1c corresponds to 7,340.8 mAh g 1c þ catalyst, the true electrode
surface area-specific capacity is E1.0 mAh cm 2true.
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Acknowledgements
This study was supported by the Institute for Collaborative Biotechnologies through
grant W911NF-09-0001 from the U.S. Army Research Office. The content of the
information does not necessarily reflect the position or the policy of the Government,
and no official endorsement should be inferred. In addition, this study was supported in
part by the MRSEC Program of the National Science Foundation under award number
DMR-0819762. The authors appreciate the assistance of TEM analysis by Dr Dong Soo
Yun and the gas chromatography measurement by Dr Nimrod Heldman. D.O. is grateful
to Kwanjeong Educational Foundation for scholarship. The authors dedicate this paper
to the memory of Officer Sean Collier, for his caring service to the MIT community and
for his sacrifice.
Author contributions
D.O., J.Q., Y.S.H. and A.M.B. conceived and designed the experiments. D.O. and J.Q.
performed experiments and analysed the data. Y.C.L and Y.S.H. analysed the electrochemistry data. Y.Z. conducted high-resolution transmission electron microscopy. Y.S.H.
and A.M.B. supervised the project. All authors discussed the results and commented on
the manuscript.
Additional information
Supplementary Information accompanies this paper at http://www.nature.com/
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Competing financial interests: The authors declare no competing financial interests.
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How to cite this article: Oh, D. et al. Biologically enhanced cathode design for improved
capacity and cycle life for lithium-oxygen batteries. Nat. Commun. 4:2756 doi: 10.1038/
ncomms3756 (2013).
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