nanopillar anodes for -ion batteries using nanoimprint lithography with flexible molds Eric Mills, John Cannarella, Qi Zhang, Shoham Bhadra, Craig B. Arnold, and Stephen Y. Chou

Citation: Journal of Vacuum Science & Technology B 32, 06FG10 (2014); doi: 10.1116/1.4901878 View online: http://dx.doi.org/10.1116/1.4901878 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/32/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.112.142.131 On: Tue, 30 Dec 2014 22:19:15 Silicon nanopillar anodes for lithium-ion batteries using nanoimprint lithography with flexible molds Eric Mills NanoStructure Laboratory, Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 John Cannarella Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544 Qi Zhang NanoStructure Laboratory, Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 Shoham Bhadra and Craig B. Arnold Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544 Stephen Y. Choua) NanoStructure Laboratory, Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 (Received 5 September 2014; accepted 4 November 2014; published 19 November 2014) The lithium ion battery, a preferred energy storage technology, is limited by its volumetric and gravimetric energy densities, as well as its capacity retention with prolonged cycling. In this work, the authors exploited the extremely high lithium storage capacity of Si as an anode material and tackled the issue of lithium-induced volume expansion by patterning the Si into a nanopillar array using nanoimprint lithography and reactive-ion etching. Arrays of 200 nm-pitch Si pillars of 50–70 nm diameter and 200–500 nm height were fabricated on stainless steel substrates, assembled into coin cells, and tested against lithium counter electrodes. Initial charge capacities in excess of 3000 mAh/g, and a low rate-dependence, were obtained with these Si pillar anodes. This represents an improvement over previously reported nanoimprint-patterned Si anodes. Though this initial capacity is roughly equivalent to previously reported values for bulk Si anodes, our nanopillar ano- des exhibit far superior capacity retention with subsequent charge–discharge cycles. VC 2014 American Vacuum Society.[http://dx.doi.org/10.1116/1.4901878]

I. INTRODUCTION integrity is compromised during battery cycling. For the pop- ular Si nanowire-based anode, first demonstrated by Cui The lithium ion battery has become the energy storage me- 9 dium of choice for almost all applications requiring recharge- et al. in 2008, this critical radius has been shown to be on the order of 300 nm for initially crystalline Si.8 Randomly able batteries, due to its favorable performance characteristics relative to other rechargeable battery chemistries.1,2 However, packed nanowire systems are also popular because they do for applications with size and weight constraints, such as elec- not require binders or conductive additives (which reduce energy density), and they have intrinsic free space to accom- tric vehicles, it is still necessary to achieve significant increases in energy density. Consequently, the development of modate volumetric expansion of the Si electrode material battery electrode materials with higher lithium storage capaci- during lithiation. The accommodation of volumetric expan- sion is critical for reducing mechanical stress in the individ- ties has remained an area of intense research. One promising anode material is silicon, which shows theoretical Li-storage ual structures as well as stress within the cell as a whole, which can lead to overall cell degradation.10 A number of specific charge capacities of 3600 mAh/g (Ref. 3)atroom deposition-based methods have been used to create viable temperature, nearly 10 that of current commercial anodes, 11 which are typically graphite (372 mAh/g).4 Si anodes made Si Nanowire (NW) anodes, including thin film deposition, creation of random wire networks,12 and coating existing from 45 lm-sized powders have shown initial capacities of 13 5 fibers with Si via chemical vapor deposition (CVD). nearly 4000 mAh/g, though anodes made from such “bulk” 6 Etching-based fabrication techniques, involving chemical Si typically lose 80%–90% of their capacities with 5 cycles. 14 15 This rapid degradation is due to the 310% volume expansion7 etching or deep reactive ion etching, have also been implemented, often in combination with self-assembled col- that occurs during lithiation, resulting in rapid capacity losses 14 due to mechanical pulverization of the electrode. loidal monolayers to create patterns. Random nanowire net- To prevent Si pulverization, it has been widely accepted works, such as those grown by CVD, typically show NWs growing in many directions12,16 and are thus extremely ineffi- that Si electrodes must be nanostructured, as Si systems ex- hibit a critical “cracking radius,”8 above which structural cient at filling volume. To efficiently fill volume, NWs with well-defined diameter, pitch, and orientation are required. Nanoimprint lithography (NIL), in combination with a)Electronic mail: [email protected] appropriate deposition and etching techniques, is an ideal

06FG10-1 J. Vac. Sci. Technol. B 32(6), Nov/Dec 2014 2166-2746/2014/32(6)/06FG10/5/$30.00 VC 2014 American Vacuum Society 06FG10-1

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way to generate precisely ordered arrays of wires with mini- Inc.) PFPE was chosen due to its low surface energy, high mal defects.17 In addition, soft-mold NIL’s compatibility elastic modulus, and impressive solvent resistance.22 Ather- with roll-to-roll processes,18 and ability to produce patterns mal nanoimprint was conducted at <80 C and 200 psi using on substrates too rough for standard “hard-mold” NIL, make a Nanonex, NX-2500 nanoimprinter. it the perfect candidate for creating such patterns on rela- In step (3), the trilayer was etched using reactive ion etch- 19 tively “rough” metal substrates. To our knowledge, only ing (RIE) (PlasmaTherm, 720 SLR), with O2 (10 sccm, two previous attempts to apply NIL to the realm of nanopat- 3 mTorr, 50 W) for the residual imprint resist, CF4/H2 19,20 terned Si Li-ion battery anodes have been published. In (33/7 sccm, 50 mTorr, 300 W) for the SiO2 layer, and O2 this paper, we aimed to improve the specific charge capacity (10 sccm, 3 mTorr, 100 W) for the XHRiC polymer layer. of nanostructured Si electrodes fabricated via NIL by apply- E-beam evaporation of 25 nm Cr was then performed, fol- ing High-Fidelity Flexible Mold (HiF2M) NIL technology to lowed by liftoff of the trilayer in RCA1 (5:1:1 DI H2O:29% the production of Si nanopillars. Nanopillar structures are NH4OH:30% H2O2) solution. Si etching was accomplished preferable to those previously fabricated by NIL (nanobars with a pseudo-Bosch process (PlasmaTherm, 720 SLR), and nanoporous Si) due to their smaller size and greater sur- using an SF6/O2/H2/Ar recipe adapted from Mohajerzadeh face area, and are expected to exhibit excellent performance et al.,23 (17/2.5/7.5/0.5 sccm, 10 mTorr, 200 W, 9 s) with a at high cycling rates, as well as good tolerance of the CHF3 passivation step (20 sccm, 100 mTorr, 90 W, 15 s). A mechanical stresses imposed by lithiation.21 minimum 25% overetch was used, as residual Si would arti- ficially inflate capacity results. To obtain accurate values for Si loading, some samples were soaked in chromium-etchant II. EXPERIMENT (Cyantek, CR7), which dissolves enough underlying Cr in A. Fabrication of Si nanopillar anodes the substrate to knock over wires without damaging them. The fabrication has three key steps: (1) Si film deposition, These wires were examined via scanning electron micro- (2) nanoimprint lithography and metal liftoff, and (3) Si pillar scope (SEM) (Zeiss, Leo 1550), and the images were ana- etching (Fig. 1). In step (1), 1.5 in. diameter #8 polished stain- lyzed using ImageJ software. The examinations show that less steel disks (Stainless Supply) were cleaned ultrasonically fabricated pillars have heights between 200 and 500 nm, and in toluene, rinsed with isopropanol, dried with N2,andfinally widths between 30 and 75 nm. Pillars show a slight undercut cleaned in O2 plasma to remove any residual organics and from etching, which was accounted for during pillar volume enhance adhesion. E-beam evaporation of Si was then per- calculations. formed at <1 105 Torr, at a rate of 6nm/min. In step (2), a trilayer of nanoimprint resist, consisting of B. Assembly of anodes 150 nm thermally cross-linking polymer (XHRiC 16, The cell preparation and assembly process has four key Brewer Sciences), 10 nm e-beam evaporated SiO2 (99.99%, steps: (1) transfer to an Ar-filled glovebox, (2) Li-disk prepa- Kamis Inc.), and 130 nm thermal imprint resist (NXR-1025, ration, (3) separator preparation, and (4) final assembly. In Nanonex), was deposited by spin-coating, and e-beam evapo- step (1), after etching, the anodes were moved into an ration. A HiF2M was used, because the stainless steel sub- Ar-filled glovebox (MBraun)—with <0.1 ppm H2Oand strates were unsuitable for standard “hard mold” imprinting O2—and loaded into stainless steel CR2032 coin cell casings (due to local surface roughness and debris at the substrate (MTI). In step (2), a 7/16-in. diameter Li disk was punched edges). The HiF2M mold is comprised of a cross-linked per- from 0.5 mm-thick Li foil (Alfa Aesar) and used as a counter fluoropolyether (PFPE) patterned layer, attached to chemical- electrode in a two-electrode measurement configuration. The resistant PET backing (Melinex 054, Dupont Teijin Films) thick Li foil represented an essentially unlimited supply of with UV-curing optical adhesive (NOA73, Norland Products lithium, so that any capacity limitations are due to degrada- tion of the Si electrode structure. In step (3), microporous dry-stretched polypropylene separators of 1/2-in. diameter (Celgard 2500) were soaked in a solution of 1M LiPF6 in 1:1 ethylene carbonate: dimethyl carbonate electrolyte (Novolyte Technologies). In step (4), the cells were assembled from the above components, sealed using a coin cell crimper (MTI), and tested in ambient atmosphere at 20–23 C using a poten- tiostat (Arbin Instruments, BT2000).

C. Testing of anodes All cells were cycled using a constant current constant voltage (CCCV) methodology, with two variants: constant current over all cycles for a given cell, and changing the charging current every 3 cycles in a given cell. The latter FIG. 1. (Color online) Process flow for Si nanopillar fabrication. The key steps are (1) Si deposition (a), (2) nanoimprint [(a)–(c)] and metal lift-off were done to probe the rate dependence of capacity for our [(d) and (e)], and (3) pillar etching of Si (f). nanopillar anodes. The specific capacities of the electrodes

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were determined by normalizing the measured capacity steel substrates. After pseudo-Bosch etching, pillars similar to (mAh) by the estimated mass of Si (g) on the electrode. This those seen in Fig. 3 were created across the entire sample area. was found by estimating the average pillar volume from SEM images, multiplying by feature density (#/cm2) and an- B. Capacity measurements 2 3 ode size (cm ), and converting to mass (2.33 g/cm ). The pil- 1. Voltage versus capacity lars exhibited deviation in shape from ideal cylinders, so the reported radius corresponds to that of a cylinder having A plot of cell voltage versus capacity for a 314 nm tall, equivalent volume and height to the imaged pillars. The 55 nm diameter Si nanopillar anode cycled at 0.3 C is shown mass loadings are assumed to be uniform among all electro- in Fig. 4. The voltage curves are representative of the volt- des of a given pillar size, and any nonuniformities would age curves of all Si nanopillar electrodes tested in this work. translate to uncertainty in specific capacity. The voltage curves corresponding to both lithiation and The capacity-normalized charging rate (C-rate) was deter- delithiation of the Si nanopillar electrode are shown, labeled mined by dividing the charging current (mA) by the full the- with their corresponding cycle number. The first cycle oretical capacity of the Si on each wafer, calculated based on curves are not shown because the first lithiation cycle exhib- the estimated mass loading (g), and the theoretical specific its anomalously high specific capacity, followed by a high charge capacity (3579 mAh/g). Our “1 C” charging rate is initial degradation rate, often seen with Si nanostructures. thus equivalent to 3579 mA/g Si, and our “2 C” rate is equiv- The maximum lithiation/delithiation capacities monotoni- alent to 7158 mA/g Si. All cells were cycled using a CCCV cally decrease, denoting continued nanopillar anode methodology as follows. The Si is lithiated with a constant destruction. Degradation has slowed by the tenth cycle, current until the cell voltage decreases to 10 mV. After where this graph begins. In nanostructured Si electrodes, the reaching 10 mV, the cell is held at 10 mV for 2 h to ensure often-seen fast initial irreversible capacity loss is partly complete lithiation of the Si. The 10 mV cut off is chosen to attributed to side reactions including electrochemical prevent the undesirable onset of lithium plating on the Si sur- decomposition of the electrolyte, as well as the reduction of face, which occurs below 0 V. The Si is subsequently delithi- SiOx, which will naturally be present in amorphous Si film ated at a constant current until the cell voltage rises to 1.3 V, samples subjected to the RCA1 liftoff technique employed here. It has been previously shown that SiOx phases can marking the completion of a single cycle. For alternating- þ rate experiments, charging current was switched every 3 react with Li to form Li2OandLi4SiO4, which appear to cycles, between three different values.

III. RESULTS AND DISCUSSION A. Nanopattern transfer onto rough substrates The as-provided stainless steel disks were unsuitable for hard-mold NIL, showing 120 nm height variations across the substrate, in addition to debris from the laser-cutting process at the disk edges. However, thermal imprint with the HiF2M molds has achieved large-area high-fidelity pattern transfer, demonstrated by the SEM micrographs shown in Fig. 2. Figure 2 shows an SEM micrograph of a pillar pattern trans- ferred onto one of our highly nonuniform, Si-coated stainless

FIG. 3. Cross-sectional SEM image of 450 nm-high, 106-nm wide Si nano- FIG. 2. SEM image of uniform 200 nm-pitch Cr dot pattern on highly inho- pillars on Si substrate (a), and 30 SEM image of 400 nm-high, 62 nm wide mogeneous stainless steel substrate. Dots in the darkest region could not be Si nanopillars on stainless steel substrate (b), created by pseudo-Bosch fluo- brought into sharp focus simultaneously with the rest of the image. rine-based process in conventional RIE.

JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

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FIG. 4. (Color online) Charge/discharge curves for 314 nm high and 55 nm diameter nanopillars. Shown are the curves for the tenth, 20th, 30th, and 40th lithiation and delithiation cycles. Note the lithiation and delithiation curves shift to the left with increasing cycle number, denoting Si degradation. FIG. 5. (Color online) Gravimetric capacity data for 314 nm tall and 55 nm diameter nanopillars over a period of 50 cycles, compared with capacity val- stabilize Si-based anode structures, though at reduced ues for other NIL-fabricated samples from the literature (Refs. 19 and 20). capacities.24,25 The presence of irreversible side reactions can be quanti- our more aggressive utilization of our Si electrodes. In both fied through measurements of coulombic efficiency, which is Refs. 19 and 20, the extent of lithiation of their electrodes the ratio of lithiation capacity to delithiation capacity during a was limited in order to help improve capacity retention. cycle. The value for our nanopillar Si electrodes is typically Note that other Si nanopillar electrodes with varying diame- around 80% on the first cycle. Subsequently, the coulombic ter and pillar heights were fabricated and tested, but no cor- efficiency increases rapidly, reaching stable values around relations between specific capacity and these parameters 96% after about 10 cycles and remains stable during subse- were observed within our range of fabrication dimensions. quent cycles. The initially low values of coulombic efficiency Figure 5 shows the cell performance at three different support the idea of SiO reduction, as once the SiO has been x x cycling rates (0.3 C, 0.7 C, and 1.4 C), where the rate is exhausted, the coulombic efficiency would stabilize. The sta- switched every 3 cycles. Cycling the cells at different rates ble coulombic efficiency of 96% is indicative of ongoing side demonstrates the rate capability of our Si electrodes, with reactions in the cell during cycling. These side reactions likely large capacity drops at faster rates being indicative of higher proceed through a crack and growth mechanism in which vol- cell impedance. Our nanopillar electrodes show a drop in umetric expansion of the Si electrodes during charging cracks capacity of about 5% when the cycling rate is increased by a any formed passivation layers, allowing further electrolyte factor of 4.6 from 0.3 to 1.4 C. This is significantly improved decomposition. Increasing the coulombic efficiency of these over previous Si electrodes fabricated by NIL, which have electrodes remains a challenge and has previously been shown drops in capacity between 25% and 40% over similar addressed through the fabrication of composite structures.26 ranges of charging rates. This increase in rate capability is attributed to the lower characteristic lengths for both solid 2. Specific capacity phase lithium in our Si structures, and liquid phase The specific capacity for 314 nm tall and 55 nm diameter ion diffusion between the Si nanopillars, compared to previ- nanopillars during cycling is shown in Fig. 5. The data plot- ously reported NIL-fabricated electrode geometries. From a ted in Fig. 5 represent an average of three cells, with error simple 1D diffusion standpoint, complete diffusion into a bars marking þ/ one standard deviation. The variance in 50 nm-diameter pillar should take less than a minute, given a þ 11 2 27 capacity data is attributed to slight variations to local geome- Li diffusion coefficient on the order of 10 cm /s in Si. try of the nanopillars that occur during processing. The aver- age initial delithiation capacity of the cells in Fig. 5 was C. Capacity retention 3100 mAh/g when delithiated between 10 mV and 1.3 V at a The capacity of the Si electrodes in Fig. 5 can be seen to 0.3 C rate. This capacity is lower than the theoretical decay during cycling, with “nanopillar”-labeled samples capacity of Si, but is typical of capacity values reported in approaching 60% of their initial capacity by 50 cycles. This the literature for other nanostructured Si electrodes. The loss of 40% capacity is attributed to degradation of the Si measured specific capacities of our electrodes are higher electrode structure, presumably through loss of electrical than those previously reported for Si electrodes structured contact to portions of the Si. using NIL. The higher capacities compared with the struc- The attribution of the capacity fade to contact loss follows tures reported in Refs. 19 and 20 are due at least in part to from the fact that other degradation mechanisms such as loss

J. Vac. Sci. Technol. B, Vol. 32, No. 6, Nov/Dec 2014

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.112.142.131 On: Tue, 30 Dec 2014 22:19:15 06FG10-5 Mills et al.: Silicon nanopillar anodes for lithium-ion batteries 06FG10-5

IV. SUMMARY AND CONCLUSIONS We have applied HiF2M NIL to the production of Si nanopillars on nonuniform substrates, creating high specific- capacity Si anodes with initial capacities in excess of 3000 mAh and good rate capability at charging rates up to 1.4 C, representing a significant improvement over previ- ously reported nanostructured Si electrodes fabricated via NIL. Our work comparing nanopillars to thin films reaffirms the idea that nanostructuring of Si anodes is crucial for any commercial Si-based battery to be run at high rates. This work represents an excellent progress toward using NIL to mass-produce next-generation battery electrode technology.

ACKNOWLEDGMENTS The authors would like to acknowledge the assistance of Hao Chen for profilometry measurements, Raleigh Davis for AFM measurements, and the financial support of the FIG. 6. (Color online) Long-term cycling performance of nanostructured sam- Department of Defense through their NDSEG fellowship ples vs comparably thick film samples run at 1 and 2 C. Each dataset is nor- program. malized with respect to its initial capacity. Pillar samples show noticeably better capacity retention than film samples when run at the same cycling rate. 1M. Armand and J.-M. Tarascon, Nature 451, 652 (2008). 2 of lithium inventory and impedance rise can be ruled out. J. B. Goodenough and Y. Kim, Chem. Mater. 22, 587 (2010). 3J. Li and J. R. Dahn, J. Electrochem. Soc. 154, A156 (2007). Loss of lithium inventory is ruled out because the lithium foil 4M. R. Zamfir, H. T. Nguyen, E. Moyen, Y. H. Lee, and D. Pribat, J. Mater. counter electrode in the cells acts as an essentially infinite Chem. A 1, 9566 (2013). source of lithium. Impedance rise is ruled out by Fig. 5,which 5F. Wang, L. Wu, B. Key, X.-Q. Yang, C. P. Grey, Y. Zhu, and J. Graetz, shows no significant changes in rate capability during cycling. Adv. Energy Mater. 3, 1324 (2013). 6H. Li, Electrochem. Solid-State Lett. 2, 547 (1999). Significant rises in impedance would manifest themselves as 7L. Y. Beaulieu, K. W. Eberman, R. L. Turner, L. J. Krause, and J. R. a more severe reduction in capacity when stepping to higher Dahn, Electrochem. Solid-State Lett. 4, A137 (2001). cycling rates. The electrical contact loss is a consequence of 8S. W. Lee, M. T. McDowell, L. A. Berla, W. D. Nix, and Y. Cui, Proc. the high volume expansion of the electrodes and can proceed Natl. Acad. Sci. U.S.A. 109, 4080 (2012). 9C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins, through either fracturing of the Si features or delamination of and Y. Cui, Nat. Nanotechnol. 3, 31 (2008). the features from the stainless steel current collector. Due to 10J. Cannarella and C. B. Arnold, J. Power Sources 245, 745 (2014). the small characteristic dimensions of the electrodes, it is 11S. Ohara, J. Suzuki, K. Sekine, and T. Takamura, J. Power Sources 136, assumed that fracture is unlikely and that delamination from 303 (2004). 12H. T. Nguyen et al., Adv. Energy Mater. 1, 1154 (2011). the current collector is the more likely cause. 13S. A. Klankowski, R. A. Rojeski, B. A. Cruden, J. Liu, J. Wu, and J. Li, As a control experiment to confirm that nanostructuring J. Mater. Chem. A 1, 1055 (2013). of the Si films had a positive effect on capacity retention, a 14A. Vlad, A. L. M. Reddy, A. Ajayan, N. Singh, J.-F. Gohy, S. Melinte, and P. M. Ajayan, Proc. Natl. Acad. Sci. U.S.A. 109, 15168 (2012). batch of nanopillar samples was fabricated alongside a batch 15 J. Li et al., J. Mater. Chem. A 1, 14344 (2013). of thin film samples of the same thickness, and charged at 16S. Sharma, T. I. Kamins, and R. S. Williams, Appl. Phys. A 80, 1225 comparable rates. A comparison of the structured (2005). 17 “nanopillar” versus unstructured “thin-film” Si electrodes is K. J. Morton, G. Nieberg, S. Bai, and S. Y. Chou, Nanotechnology 19, 345301 (2008). shown in Fig. 6 for cells cycled at 1 C and 2 C rates. The 18J. John, Y. Tang, J. P. Rothstein, J. J. Watkins, and K. R. Carter, capacity data presented in Fig. 6 are normalized by dividing Nanotechnology 24, 505307 (2013). the capacity of each cell by the cell’s initial capacity. In this 19J. Wan et al., J. Mater. Chem. A 2, 6051 (2014). 20 manner, the only comparison between the cells is of capacity K. Nishio, S. Tagawa, T. Fukushima, and H. Masuda, Electrochem. Solid- State Lett. 15, A41 (2012). fade. Structured samples show a clear improvement in 21I. Ryu, J. W. Choi, Y. Cui, and W. D. Nix, J. Mech. Phys. Solids 59, 1717 capacity retention compared to films, decaying to 60% of (2011). initial capacity after 50 cycles and 40% of initial capacity 22S. S. Williams, S. Retterer, R. Lopez, R. Ruiz, E. T. Samulski, and J. M. after 150 cycles; while film samples rapidly decay to 20% of DeSimone, Nano Lett. 10, 1421 (2010). 23S. Azimi, M. Mehran, A. Amini, A. Vali, S. Mohajerzadeh, and M. their initial capacities within 20 cycles and <5% after 40 Fathipour, J. Vac. Sci. Technol., B 28, 1125 (2010). cycles. This long-term stability of pillars versus thin films 24J. Wang, H. Zhao, J. He, C. Wang, and J. Wang, J. Power Sources 196, confirms previous work suggesting that the diffusion- 4811 (2011). 25A. Veluchamy et al., J. Power Sources 188, 574 (2009). gradient-limiting effects of extremely small structures would 26 21 H. Wu and Y. Cui, Nano Today 7, 414 (2012). limit stress incurred during lithiation, yielding better long- 27R. Ruffo, S. S. Hong, C. K. Chan, R. A. Huggins, and Y. Cui, J. Phys. term performance, particularly at high cycling rates. Chem. C 113, 11390 (2009).

JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

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