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On-Chip High Power Porous Silicon Lithium Ion
Andrew S. Westover1,2, Daniel Freudiger1, Zarif S. Gani1, Keith Share1,2, Landon Oakes1,2,
Rachel E. Carter1, and Cary L. Pint1,2*
Department of Mechanical Engineering, Vanderbilt University, Nashville TN 37235
Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, TN 37235
Nanoscale Accepted Manuscript
Batteries with Stable Capacity over 10000 Cycles
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We demonstrate the operation of a graphene-passivated on-chip porous silicon material as a high
rate lithium battery anode with over 50X power density, and 100X energy density improvement
compared to identically prepared on-chip supercapacitors. We demonstrate this Faradaic storage
nanoporous silicon, preventing the degradation and poor cycling performance attributed to deep
storage in the bulk silicon. This device exhibits cycling performance that exceeds 10000 cycles
with capacity above 0.1 mAh/cm2 without notable capacity fade. This demonstrates a practical
route toward high power, high energy, and long lifetime all-silicon on-chip storage systems
relevant toward integration into electronics, photovoltaics, and other silicon-based platforms.
Nanoscale Accepted Manuscript
behavior to occur at fast charging rates (1-10 mA/cm2) where lithium locally intercalates into the
From portable electronics to solar cells, power delivery systems that can be integrated
into unused components in devices are critical to meet the increasing power requirements of
future applications. In electronics, as the number of transistors per unit area increases, so do the
corresponding power requirements for operation. For solar cells, as residential solar usage
becomes more prominent, integrated energy storage allows continuous power generation amidst
intermittent periods of sunshine.1-3 In both of these cases, as well as in other silicon-based onchip applications, a key differentiating factor for integration is the notion that energy storage
must be developed on a two-dimensional, planar substrate that must concurrently support
operation of the component on the front side of the device. Unlike conventional energy storage
systems that are packaged into three-dimensional architectures with performance assessed
relative to mass or volume,4 on-chip devices must be assessed for their ability to store charge that
scales with the total available chip area, or footprint. This has led to the development of on-chip
devices called micro-supercapacitors or micro-batteries,5-8 which are most practical when the
native chip material is used as real estate for energy storage with minimal additional
manufacturing steps. Silicon is an ideal choice due to its relevance to batteries, supercapacitors,
and its broad application in electronics, MEMS, solar energy conversion, and sensing.
Specifically for lithium storage applications, silicon exhibits the highest known storage
capacities (up to 4200 mAh/g), but exhibits rapid capacity fade upon cycling associated with the
large volume expansion in silicon during lithiation.9-11 Whereas elegant routes have been
developed to combat such degradation in nanoparticle electrodes, such as yolk-shell structures12
and pomegranate-like architectures,13 seamless integration of these techniques to on-chip
platforms is not straightforward. Previous studies which have demonstrated on-chip storage
using porous silicon build upon a native architecture that enables efficient volumetric expansion
Nanoscale Accepted Manuscript
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of the silicon during cycling, but exhibit limitations in cycle lifetime due to the porous
silicon/silicon interface,14-16 even though some studies indicate some intercalation control
through modulating the rates or capacities.16, 17 This has motived the development of superior
macroparticle silicon particle networks.18
Therefore, despite the promise of large energy
lifetime of these materials. On the other hand, electrochemical supercapacitors exhibit the power
capability sought for on-board electronics, and are often rated for up to a million consecutive
cycles, but are limited to energy densities at best 10X lower than conventional batteries due to
the physical double layer charge storage mechanism.19-21 However, porous silicon materials used
for supercapacitors have demonstrated unique integration capability with applications,22-24 such
as the ability to form supercapacitors to store energy directly in the unused material in solar
In this work, we demonstrate hybrid performance bridging these two device platforms
based on carbon-passivated porous silicon material etched into bulk silicon wafers and cycled as
a lithium-ion battery electrode at high currents. Using identically structured supercapacitors as a
control comparison, we demonstrate over 50X improved power density, and over 100X
improved energy density in this high rate porous silicon battery which operates over 10000
cycles with no significant capacity fade and stores more total energy on chip than both an ideal
deeply intercalated battery anode, or an electrochemical supercapacitor operating for 250 and a
million cycles, respectively. This unique operation mode provides ideal characteristics of power,
energy, and lifetime sought for on-chip integration.
In order to prepare electrodes, B-doped silicon wafers (0.01 – 0.02 Ωcm) were
electrochemically etched using a 3:7 v/v HF (50% H2O by volume) and ethanol solution at 45
Nanoscale Accepted Manuscript
densities, the bottleneck for on-chip silicon batteries is the low power density and short cycle
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mA/cm2 for 180 seconds in either a home-built etching cell or an AMMT porous silicon etching
system. This yields porous Si with 75% porosity and a ~ 4 µm depth (figure 1A). To passivate
the porous silicon from the effects of electrochemical degradation or inhibitory solid electrolyte
interphase layer formation, we used chemical vapor deposition using a ramp procedure described
that is known to generate few-layer coatings of graphene-like carbon on the
surface of the porous silicon material. Briefly, this involves consecutive temperature ramps from
650-750oC, and 750-850oC for 10 minutes each, with a 1:20:100 ratio of C2H2:H2:Ar gas mixture
carried out under atmospheric conditions. A representative TEM image of the carbon passivated
silicon material used in these experiments is shown in Figure 1B. In order to assess the storage
performance of these materials, we constructed lithium-ion batteries using the passivated porous
silicon as an electrode combined with Li foil, a Celgard battery separator, and electrolyte
consisting of 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate, sealed in a coin cell
assembly. One general observation is that the charging rate dictates the penetration depth of the
intercalation process, and a transition exists between what we indicate as “high-rate” behavior,
where the current is high enough to only allow intercalation into the nanoporous active material,
and “deep-charge” behavior where lower charging currents allow the slower bulk intercalation
kinetics into bulk silicon material underneath the porous silicon layer. In order to assess the nonFaradaic (surface) storage per unit footprint in a material with an identical porous structure
(thickness, porosity, etc.), we utilized an electrochemical supercapacitor as an ideal control
experiment. Here, a supercapacitor only stores energy by the assembly of a double-layer of ions
at the surface of the porous silicon, allowing a robust comparison to Faradaic, bulk redox storage
in batteries using an identical material. Supercapacitors were made in a manner reported
elsewhere22, 23 by sandwiching two symmetric porous silicon electrodes with a Celgard separator
Nanoscale Accepted Manuscript
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and 1-ethyl-3-methylimidazolium tetraflouroborate (EMIBF4) ionic liquid electrolyte. A visual
comparison of these three different storage modes in the same material are shown in Figure 1C
representing (top) double-layer storage of ions in a supercapacitor, (middle) fast lithium
intercalation into the nanoscale porous active material, and (bottom) deep intercalation at slow
In order to assess the performance of these on-chip devices, we performed both
Galvanostatic charge-discharge measurements and cyclic voltammetry analysis.
With this
specific porous silicon material employed in a Li-ion battery, we observe a distinct transition
near charging currents of 1.2 mA/cm2 separating the “high rate” and “deep-intercalation”
processes. Above 1.2 mA/cm2, we observe reversible storage capacities up to ~ 0.1 mAh/cm2,
with energy stored at voltages vs. Li/Li+ above the energy of intercalation into bulk silicon (e.g.
above 0.6 V vs Li/Li+) extending up to ~ 3 V (Figure 2A). In comparison to a device charged at
1 mA/cm2 (inset, Figure 2A), signatures of deep Li bulk intercalation (below 0.6 V) and Li
intercalation into the passivated porous silicon layer (0.6 V – 3 V) are observed, distinguishing
these two intercalation processes. To better understand this, we performed CV measurements of
high-rate devices at rates of 5, 10, and 20 mV/s (Figure 2B). Evident from these curves is the
lack of a bulk intercalation signature below 0.5 V, and a redox couple for insertion/deinsertion at
voltages centered around ~ 1.6 V and ~ 1.2 V vs. Li/Li+, respectively. CV curves taken at the
same rates, but without the carbon passivation layer indicate similar intercalation and
deintercalation kinetics, emphasizing that this behavior is not due to the graphene passivation
(supplementary information).
Furthermore, we performed control experiments where
freestanding layers of identical porous silicon were isolated on Ti substrates and assessed at high
rates, with similar cyclic voltammetry profiles observed in comparison to on-chip integrated
Nanoscale Accepted Manuscript
rates into both the porous layer and the bulk silicon wafer.
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devices emphasizing the porous material as being responsible for the lithium storage.
The high
rate intercalation behavior is distinguished from that observed in CV scans taken at slow rates of
0.1 and 0.5 mV/s, respectively (Figure 2C), where sharp intercalation peaks are observed at 0.36
V and 0.56 V and a deintercalation peak is observed at 0.14 V that emphasizes the redox couples
measurements indicate less distinctly defined Faradaic storage energies, we can distinguish this
storage from capacitive double-layer storage by direct comparison to supercapacitors formed
from identical carbon passivated porous silicon. CV measurements for supercapacitors (Figure
2D) indicate smooth, featureless curves representative of non-Faradaic storage processes, in
comparison to CV curves for high-rate batteries, which implies storage through Faradaic
chemical redox processes. This indicates the high-rate storage energetics and process for carbonpassivated porous silicon electrodes is distinguished both from bulk Li storage observed in
conventional silicon battery electrodes and the surface double-layer storage behavior observed in
electrochemical supercapacitors.
One of the key challenges in utilizing silicon materials in batteries is the poor cycling
lifetime of the active materials. Our work demonstrates that we can overcome this limitation for
an on-chip carbon passivated porous silicon battery by operation in the high power mode, thus
inhibiting deep bulk intercalation that leads to irreversible capacity fade. Figure 3 shows the
capacity measured as a function of the number of cycles for an on-chip porous silicon Li ion
battery charged and discharged at 3 mA/cm2, following 3000 preliminary cycles for stabilization
at 3 mA/cm2. We speculate that the preliminary 3000 cycles represents a continuous activation of
the porous silicon electrical conductivity by formation of a Li-Si alloy, which has been observed
to improve conductivity of amorphous silicon by over 3 orders of magnitude.29 As porous
Nanoscale Accepted Manuscript
associated with bulk Li storage in silicon.26-28 Furthermore, whereas high-rate Galvanostatic
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silicon exhibits high resistance after electrochemical etching, we anticipate that lowering the
resistance of the active material in combination with the high surface area for the electrodeelectrolyte interface that leads to low cell resistance, allows this device to overcome the
resistance polarization that ultimately limits conventional batteries from high power cycling
slightly fluctuating but constant capacity that exceeds 13,000 cycles. This result is in direct
contrast to the slow charged porous Si anodes which rapidly degrade over 10-30 cycles with the
capacity fade showing an exponential decay upon cycling (supplementary information).
Whereas the best silicon nanoparticle electrode materials have demonstrated cycling lifetime up
to 1000 cycles,12, 13 these materials must be combined with binders and cast into coatings applied
to a conductive charge collector. In our case, the electrode active material is mechanically
integrated directly into a silicon wafer with no binder materials, resulting in deep intercalation
processes that inhibit cyclability as observed in other studies with porous silicon materials.10, 11, 14
However, by using the Galvanostatic charging conditions to control the intercalation depth to
only penetrate the nanoscale active layer, we observe this battery to exhibit exemplary cycling
stability that significantly exceeds the capability of other Si materials. This mechanism and the
device lifetime is similar to that exhibited in a conventional pseudocapacitor, except with
significantly improved capacities and using conventional carbonate electrolytes. To support this
concept, we performed SEM imaging of porous Si materials cycled both at high power
conditions, and under deep intercalation conditions. For the deep intercalation (~ 20 cycles),
significant cracking that extends up to ~ 40 microns into the surface is observed. However, for
the high power cycling experiments, the porous structure after cycling is clearly in-tact, with no
evident damage to the silicon material residing underneath the porous layer. This confirms, in
Nanoscale Accepted Manuscript
behavior. After capacity stabilization, the device shows excellent cycling capability, with a
agreement with cycling data performed on freestanding porous silicon films, that the high rate
cycling is correlated to storage in the porous material (supplementary information). To further
emphasize this point, normalizing the capacity of the deep intercalation battery to only the
material in the porous layer yields ~ 124,600 mAh/g, which is clearly well above the maximum
capacity of Si for Li storage. Similar assessment of the high rate storage normalized to the
porous layer indicates a capacity of ~ 570 mAh/g, which is a reasonable capacity for silicon
materials at high charge-discharge cycling rates, and still respectable in comparison to
conventional anodes such as bulk carbons (maximum capacity of 372 mAh/g).
Additionally, it is important to assess the energy and power capability of these devices
based on their available chip-based footprint. For devices produced using identical carbonpassivated porous silicon materials, we compared the energy and power performance of
supercapacitors, high power batteries, and deep intercalation batteries (Figure 4A). These curves
were each assessed by direct integration of Galvanostatic discharge curves at different cycling
rates, and analysis of the average power that is represented by the total energy divided by the
discharge duration. A distinguishing factor between the high-power Li-ion battery and the
electrochemical supercapacitor is that the high power battery can exhibit up to 100X the power
density per footprint of the supercapacitor, and exhibit energy densities up to almost 250X
greater than the supercapacitor per unit footprint. We anticipate this is due to the greater
electrode-electrolyte surface area interface that minimizes electrode resistance polarization losses
that inhibit bulk materials from high power storage.
These relative values significantly
distinguish the bulk storage in the nanoscale porous layer from the surface double-layer storage
in a supercapacitor since both materials have the exact same structure. Comparing the power and
energy storage capability measured in these high rate devices to the best state-of-the-art 3D
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graphene supercapacitors per unit active mass (40-85 Wh/kg, 25 kW/kg),30 the specific power
and energy capability, scaled to the porous active material, still exhibits comparable power
density of ~ 20 kW/kg, and much greater energy density of ~ 220 Wh/kg. It should further be
noted that higher power carbon supercapacitors are usually realized from ultrathin highly porous
thickness making the comparison of such materials challenging.4 This means thick active layers
of storage material, such as graphene, are likely to both inhibit good areal on-chip performance,
in addition to being poorly suited for integration due to a heterogeneous van der Waals interface
with a collector material – in contrast to the directly integrated high power porous silicon battery
electrode. Further, comparison of the high power battery to a slow-charged, deep intercalation
battery indicates a 100-150X lower energy density, but a power density that remains between 10100X improved per unit footprint.
The unique nature of a secondary battery emphasizes that both the energy stored per
cycle and the total stored energy over the lifetime of the device are important metrics for device
performance. In order to compare the total energy stored, we provide an optimistic comparison
between the electrochemical supercapacitor and deep intercalation battery performance,
compared to the measured performance of the high power battery. The total energy stored by the
supercapacitor, over the lifetime of the device was estimated by multiplying the maximum
energy stored by the supercapacitor times the anticipated supercapacitor lifetime of 1,000,000
cycles. For the deep intercalation battery, we assess the maximum capacity and optimistically
assume this capacity to be achievable for 250 cycles, despite our experimental measurements of
~ 60% capacity fade for the first 10 cycles, emphasizing the highly conservative nature of this
estimate. Finally, we calculated the total energy stored in the high power Li ion battery based on
Nanoscale Accepted Manuscript
electrodes, and the energy and power performance is known to decrease or change with electrode
experimental analysis of the 10,000 active cycles that were experimentally measured and
assessed. Whereas this device is still active after 10,000 cycles and our cycling experiments
extend beyond 13,000 cycles, we observe this high rate battery platform to exhibit significantly
more total energy stored in comparison to both a conservatively estimated ideal deep
intercalation battery and an ideal supercapacitor that operates based on an identical electrode
structure. Furthermore, the higher power capability of the fast-rate porous silicon battery that is
enabled by Li metal stored in the nanoscale silicon material enables it to exhibit better power
capability in storing its energy over the course of its lifetime.
Finally, whereas our results emphasize that the on-chip porous layer is responsible for the
high power, long lifetime cycling capability that we demonstrate here, we emphasize that
modifications of the porous silicon porosity, pore structure, thickness, and passive layer
composition are all factors that could lead to improved and controllable performance relevant to
a spectrum of long-term integrated applications.
As an example, for energy storage directly
integrated into photovoltaics, a concept we recently demonstrated as being feasible for porous
silicon supercapacitors,25 most respectable photovoltaic devices exhibit current densities greater
than 5 mA/cm2 and are rated for between 20-30 years of stable operation. Such charging
currents kinetically inhibit deep bulk intercalation storage in silicon (Figure 2), requiring
asymmetry in power generation and power storage footprint areas, which makes integration
challenging in addition to the large mismatch between storage and generation lifetimes. For a
high-power silicon battery device that can operate at the native photogenerated current density of
a silicon solar cell exposed to 1 sun of illumination, and charged and discharged once per day,
the ability to achieve 10,000 cycles without noticeable capacity loss indicates the capability to be
charged and discharged once per day through a period of almost 30 years – which is the
Nanoscale Accepted Manuscript
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maximum rated lifetime of a solar cell.1 Beyond this unique integration scheme for solar cells,
integration of on-chip high power, long-lifetime silicon-based energy storage into other
applications such as silicon-based integrated circuits and electronics, MEMS devices, and
sensors, provides a unique pathway to provide on-board energy storage in excess silicon material
We thank Adam Cohn for useful discussions, Jeremy Mares for insight on electropolishing, and
Sharon Weiss for early use of facilities for porous silicon etching. This work was supported by
National Science Foundation (NSF) CMMI grant # 1334269, and A.S.W. was supported by NSF
grant EPS 1004083.
Nanoscale Accepted Manuscript
real estate in these devices.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure Captions
Figure 1. (A) Side-view SEM image of carbon-passivated porous silicon materials utilized in
this work, (B) High resolution TEM image of the carbon passivated silicon material that
indicates the quality of the carbon and crystalline nature of the silicon nanomaterials, and (C)
materials explored and discussed in this work including supercapacitors, high-power batteries,
and deep intercalation batteries.
Figure 2. (A) Galvanostatic charge-discharge curves taken for carbon-passivated porous silicon
high power batteries at rates between 1.2 – 12 mA/cm2. A corresponding deep intercalation
battery charge-discharge curve is inset in order to distinguish bulk Si intercalation versus high
power storage in the nanoscale carbon-passivated porous silicon material.
(B) Cyclic
voltammetry (CV) curves of carbon-passivated porous silicon batteries at fast scan rates of 5, 10,
and 20 mV/s demonstrating the high power storage behavior of the porous silicon. (C) CV
curves for carbon-passivated porous silicon batteries at slow scan rates of 0.1 and 0.5 mV/s
indicating standard Si bulk intercalation behavior. (D) CV curves for carbon passivated porous
silicon supercapacitors with EMIBF4 electrolytes at 50 mV/s to demonstrate charge storage
through a surface double layer formation.
Figure 3. Footprint capacity as a function of cycle number over more than 13000 consecutive
cycles with charge and discharge capacities plotted separately to indicate the ~ 100% Coulombic
efficiency achieved. A straight line is included to guide the eye.
Figure 4. (A) Energy-power analysis based on the integration of discharge curves from devices
prepared with identical carbon-passivated porous silicon materials, including supercapacitors,
Nanoscale Accepted Manuscript
Scheme visually depicting the three modes of energy storage in carbon-passivated porous silicon
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high-power lithium-ion batteries, and deep-intercalation lithium-ion batteries. (B) Total energy
stored over the measured or extrapolated lifetime of the three devices studied in this work. For
supercapacitors, the cycle lifetime is extrapolated to 1 million cycles, and for deep-intercalation
batteries the cycle lifetime is extrapolated to 250 cycles. As both of these are optimistic and
energy over 10000 cycles conducted in this study.
Nanoscale Accepted Manuscript
ideal extrapolations, the total energy for the high power device is based on the measured stored
Page 19 of 21
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