Axial Si-Ge Heterostructure Nanowires as Lithium-ion Battery Anodes

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Axial Si-Ge Heterostructure Nanowires as Lithium-ion Battery Anodes Killian Stokes, Grace Flynn, Hugh Geaney, Gerard Bree, and Kevin M. Ryan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01988 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Axial Si-Ge Heterostructure Nanowires as Lithium-ion Battery Anodes Killian Stokes,†,# Grace Flynn,†,# Hugh Geaney,† Gerard Bree,† and Kevin M. Ryan†,* † Bernal Institute and Department of Chemical Sciences, University of Limerick, Ireland # Both Authors contributed equally to this work

ABSTRACT: Here, we report the application of axially heterostructured nanowires consisting of alternating segments of silicon and germanium, with a tin seed, as lithium-ion battery anodes. During repeated lithiation and delithiation, the heterostructures completely rearrange into a porous network of homogenously alloyed Si1-xGex ligaments. The transformation was characterized through ex-situ: TEM, STEM and Raman spectroscopy. Electrochemical analysis was conducted on the heterostructures nanowires with discharge capacities in excess of 1180 mAh/g achieved for 400 cycles (C/5) and capacities of up to 613 mAh/g exhibited at a rate of 10C. KEYWORDS: Si-Ge, heterostructure, nanowires, restructuring, lithium-ion battery

Nanostructured configurations of Si and Ge are among the most promising candidates to replace commercially widespread graphitic anodes in Li-ion batteries, as they possess theoretical capacities several multiples higher (3579 mAh/g for Si and 1384 mAh/g for Ge) than graphite (372 mAh/g) and the ability to mitigate the volume changes associated with lithiation/delithiation that have rendered them unusable as bulk anodes.1-10 Nanostructured

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anodes have additional benefits including improved conductivity and shortened Li diffusion distances.11 Furthermore, the requirement for non-active components, such as binders and conductive additives, can be removed by growing nanostructured active materials directly from the current collector, leading to significant weight savings.12 Due to it having one of the highest known theoretical capacities and lower material costs (in comparison to Ge), an ideal anode would be entirely composed of a Si active material. However, pure Si electrodes are limited by steady capacity fade during long-term cycling and poor rate capability performance at faster charge and discharge rates.13-15 Several strategies have been used to improve the performance of Si anodes, including: coating Si NWs with carbon, Cu, TiO2 or Al2O3, depositing a layer of graphene on the current collect prior the NW growth and AAO-templated growth.16-22 While these techniques have been successful in improving the capacity retention over extended cycling, Si still performs poorly at faster rates and these approaches typically involve the addition of electrochemically inactive materials, resulting in an overall reduction in gravimetric capacity for the electrode. Ge has a high material cost and a theoretical capacity that is less than half of Si but it has a much faster rate of Li ion diffusion (400 x) and superior conductivity (10,000 x).23-26 These fundamental properties allow it to display exceptional rate capability performance.27,28 While the majority of research into the use of nanostructured Li-alloying materials has either focused on pure Si or Ge, there has recently been a number of reports illustrating the benefits of combining Si and Ge within the same battery electrode. Some of these investigations have included the use of alloyed nanowires (NWs),29-30 core/shell NWs,31 branched heterostructures,32 crystalline nanotubes,33,34 nanostructured thin films,35 templated nanostructures25 and co-sputtered architectures.36-38 These composites allow the respective benefits of Si (high capacities, lower relative costs) and Ge (high conductivity and Li diffusivity) to be harnessed. Since the first reported synthesis by Lew et al, Si/Ge axial hNWs


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have been investigated for a range of semiconductor applications such as photonics or electronics.39-41 Surprisingly, this morphology has not been investigated as directly grown anode materials for Li-ion batteries even though they offer the potential benefits of both Si and Ge in a single NW structure, with a higher Si to Ge segment length ratio allowing greater specific capacities and a higher Ge to Si segment ratio allowing for higher-power (greater rate capability) anodes.While Au is the typically used catalyst for axial hNW synthesis, it is expensive and is not suitable for Li-ion battery applications as it remains present as additional, inactive mass on the current collector.42 Furthermore, Au seeded Si/Ge hNWs have diffuse interfaces, with an intermixed Si/Ge region typically the same size as the catalyst seed diameter.43-46 The use of Sn for the growth of hNWs means that the catalyst can alloy with Li (Sn has a specific capacity of 994 mAh/g), thus contributing to the overall specific capacity of the electrode.9, 47 An additional benefit of using Li-alloying NWs as anode materials is the mechanically robust morphology that forms during repeated cycling.27 The active material has been observed to undergo a complete restructuring from single NWs to a porous network that is made up of intertwined nanometre sized ligaments of the starting material. Postmortem studies into the effects of lithiation/delithiation on Li alloying materials have observed this effect for Si, Ge, Sn and Sb containing electrodes.48-52 This restructuring has been attributed to a combination of Li assisted electrochemical welding between neighbouring nanostructures and pore formation that results from Li extraction.53-58 It was observed that anodes consisting of Ge NW backbones with secondary Si branches formed a porous, intermixed Ge/Si alloy structure after extended lithiation/delithiation cycling.32 This rearrangement of Si and Ge within the anodes is an interesting phenomenon that warrants further investigation for other mixed Si/Ge anode architectures. Axial Si-Ge hNWs grown with Sn seeds possess well-defined, compositionally sharp interfaces between the


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segments.59,60 As a result, they are an ideal model system to track Si/Ge compositional evolution associated with the conversion from crystalline NWs to the restructured amorphous morphology that occurs during early battery cycling. Here, we demonstrate the electrochemical performance of the Si-Ge hNWs that are grown using a low-energy wet chemical approach, as Li-ion battery anodes. The synthesis follows a previously reported protocol where the hNWs were grown directly from the stainless steel current collector using liquid precursors of Si and Ge in a high boiling point solvent growth system. The as-grown NWs on the electrode required no further processing prior to electrochemical testing.61 The active material displayed stable cycling capacities of over 1180 mAh/g after 400 cycles and a promising rate capability performance, retaining capacities of 613 mAh/g at 10C.The morphological changes for axial Si-Ge hNWs, associated with Li-cycling were also investigated. HRTEM revealed the restructuring of the active material morphology from Sn seeded hNWs with distinct Si and Ge segments to a highly porous interconnected network of the electrochemically active materials. Raman spectroscopy was used to confirm that this restructured material was a homogenous SiGe alloy. The hNWs investigated were synthesized through an injection of a liquid silicon precursor, phenyl silane, onto an evaporated layer of Sn on stainless steel. This resulted in the formation of a Si NW array with a secondary injection of triphenylgermane, after a growth interval, yielding axial heterostructures (Scheme 1). 59,60 The anode mass loading was in the range of 0.085 – 0.100 mg/cm2. An as-synthesized hNW is shown in Figure 1a where there is a clear interface between the Si and Ge segments. Typically, the diameters of the Si segment ranged from 40 to 70 nm and the Ge segments were found to be between 70 and 100 nm with Sn seeds of 100 – 110 nm present at the tip (Supporting information: Figure S1). Reflections


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corresponding to cubic Si, cubic Ge and tetragonal Sn were observed in XRD analysis of the hNWs (Supporting information: Figure S2).

Scheme 1. Overview of the synthetic protocol used to prepare Sn seeded Si-Ge hNWs directly from current collectors and the changes in morphology, due to repeated cycling with Li, from segmented crystalline structures to an amorphous alloyed network, after 50 cycles.

A number of identical NW anodes were formed and respectively subjected to 5, 6, 7, 8 and 9 half-cell cycles followed by ex-situ analysis to assess the effects of lithiation and delithiation on the Si and Ge segments and the interface separating them. The cells consisted of Si-Ge hNWs grown from the current collector as the working electrode and Li foil as the counter and reference electrode. In the anode subjected to 5 cycles prior to ex-situ analysis, the NWs appear slightly textured due the Li cycling related volume changes. By this point, the active material has become amorphous and is no longer a highly ordered crystalline structure. The hNW in Figure 1b roughly retains the initial structure with both the Ge and Si segments, and the interface separating them, still easily distinguishable. The anode subjected to 6 cycles (Figure 1c) has more pronounced pores from Li extraction during delithiation. It has been reported that Li-alloying materials show a “pore memory” and that the sites of Li


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extraction are repeated on each cycle which in turn, results in the agglomeration and formation of larger pores.53 The representative anodes after 7 (Figure 1d) and 8 cycles (Figure 1e) clearly show evidence of a much more porous and textured morphology. The interface separating the two segments has also become progressively less defined, yet remains visually distinguishable. In comparison, the hNW anode subjected to 9 cycles (Figure 1f) shows minimal contrast difference between Si and Ge. While the NW has retained its shape, it no longer resembles the solid, dense crystalline structure (seen in Figure 1a) but now consists of nanometre sized ligaments. Associated EDX elemental line scans are available in supporting information, Figure S3.

Figure 1. TEM images of an (a) uncycled Si-Ge hNW and of hNWs after they have been cycled (b) 5 (c), 6 (d), 7 (e), 8 (f) and 9 times.

DFSTEM and EDX elemental mapping were used to investigate the structural changes of the NWs from heterostructure morphology to the alloyed Si-Ge interconnected


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network that forms after repeated lithiation/delithiation (Scheme 1). Again, the following discussion refers to a number of identical anode cells that have been subjected to a fixed number of cycles before extraction for ex-situ analysis. After one cycle (Figure 2a), the NW morphology remains distinguishable with a clear contrast difference between the Si and Ge segments. In the Ge segment, there is clear indication of pore formation caused by Li extraction even after the first cycle. The EDX elemental map shows no evidence of mixing with no signals for Ge appearing in the location of the Si segment (and vice versa). Although the overall outline of a NW appears in Figure 2a, the structure itself has become extensively textured and some restructuring has occurred. After the first cycle, the hNWs do not return to their initial diameters due to volume changes associated with lithiation. This can be attributed to the formation of void space inside the active material in order to reduce the surface energy of the structure.62-64 After 10 cycles, the irreversible volume changes from repeated cycling have put surrounding hNWs in contact with each other. This enables Li assisted electrochemical welding which results in fusion between once independent hNWs.54 This is evident in SEM images taken at this point where individual hNW structures can still be defined but there is clear agglomeration between neighbouring hNWs (Supporting information: Figure S4). Due to repeated cycling, the number of points of fusion increase, and when coupled with pore formation and agglomeration, an interconnected porous structure forms. By 50 cycles, the NW morphology is completely lost and has rearranged into a continuous porous structure. The EDX elemental map (Figure 2c) displays a homogeneous alloy that consists of Si1-xGex ligaments. At this point, the hNWs have transitioned into an electrochemically stable form of the active material whereby the morphology can accommodate the volume changes and related stress caused from alloying/dealloying with Li. It was observed that there is little change to the morphology of the active material from this point onwards, with a SEM analysis after 100 cycles showing a structure similar to the one in


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presented in Figure 2c (supporting, S5). Signals for Sn were picked up by the EDX (supporting, Figure S6) showing that the electrochemically active seed material is also incorporated into the porous matrix. Raman spectroscopy was conducted on the uncycled NWs and anodes after they had been cycled 1, 10 and 50 times, respectively (Figure 3d). Peak sizes were normalized to the intensity of the Si peak of each recorded spectrum as it was the most prominent. The spectrum corresponding to the uncycled sample produced clear peaks corresponding to signals for Si-Si and Ge-Ge bonding and Si-Ge bonding.65,66 The Si – Ge signal was a less intense peak relative to those seen for pure Si – Si and Ge – Ge and corresponds to the bonding at the interfaces of the hNWs. Progressively, as the number of cycles increases, the size of the Si-Ge signal also increases and signals related to the Ge – Ge and Si – Si bonding decrease, respectively; confirming the gradual formation of a Si1-xGex alloy. Raman analysis for pure Si and Ge NWs is presented as Figure S7 in supporting information where respective peaks corresponding to Si – Si and Ge – Ge were observed, overlapping with those seen in Figure 2d.


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Figure 2. STEM images and, corresponding EDX elemental maps for Si and Ge, showing the effects of repeated lithiation and delithiation on the hNW morphology. (a) After 1 cycle (b), after 10 cycles (c) and after 50 cycles. (d) Raman spectra obtained for: the uncycled hNWs and the hNWs after 1, 10 and 50 cycles.

The electrochemical performance of the Sn seeded Si-Ge hNWs was investigated by galvanostatic cycling of the material in a two electrode Swagelok-type cell cycled in the potential range of 0.01 to 1.0 V versus Li/Li+ in a 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v) + 3% vinylene carbonate electrolyte. VC was added to the standard electrolyte as it has been found to encourage the formation of polymers in the SEI layer which results in a more flexible SEI layer to help accommodate the restructuring of the active material.49,

67,68

The entire Li-active mass of the anode was used to calculate the

charge/discharge rates and capacity values for the plots presented which was determined using weighing and quantitative EDX analysis (Supporting, S8). The as-grown anode materials on SS current collectors were used directly as the working electrodes and cycled against Li-foil counter electrodes at a rate of C/5 (Figure 3a). The Si-Ge hNWs exhibited enhanced electrochemical performance when compared to individual (Sn seeded) Si NWs and Ge NWs, displaying improved charge and discharge capacities versus Ge and superior long-term cyclability and versus Si.27, 49 The initial charge and discharge capacities for the sample were 2200 mAh/g and 1644 mAh/g respectively with a 74.7 % first cycle coulombic efficiency. The high initial charge capacity is likely due to SEI formation and other irreversible processes occurring during the first cycle. The first discharge capacity was under the maximum theoretical capacity of 1959 mAh/g, calculated for the anode. Reasons for this reduced capacity may be related to the existence of a native oxide layer on the material as samples were exposed to air prior to being assembled in the battery. This effect has been previously reported for the oxides of Li-alloying materials and results in an irreversible capacity loss.35, 69,70 The early charge and discharge capacities were all above the maximum


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theoretical value for Ge NWs (1384 mAh/g), showing the capacity boosting effects of having Si in the anode. These electrodes also benefit from the exceptional stability offered by Ge, where capacities greater than 1100 mAh/g were maintained after 400 cycles, which compares favourably with previously reported Si/Ge composite anodes (supporting information, Table S1). The voltage profiles of the 1st, 10th, 50th, 100th, 250th and 400th cycles are shown in Figure 3b (with the respective differential capacity plots presented as Figure S9). Over the initial 50 cycles, a capacity loss of 0.2 % per cycle was observed, with a considerably lower 0.05 % loss per cycle then observed for the remaining 350 cycles. It is around this point that the coulombic efficiencies (CE) were all consistently above 99 % (from cycle 45 onwards). Interestingly, this coincides with the transformation of the hNW active material to the highly porous, interconnected alloyed structure, suggesting this restructured configuration is the most stable for electrochemical cycling. Over the initial number of cycles there is extensive SEI formation (and reformation), due to continuous material restructuring resulting in reduced CE values.71-75 This is a well-known issue for Li-alloying materials and can be mitigated by preconditioning and prelithiation protocols that allow for stable long-term fullcell cycling without irreversible Li inventory depletion.76-78 In Figure 3c the cyclic voltammetry (CV) curves for the first five cycles of the Si-Ge hNWs are plotted. On the cathodic scan overlapping peaks observed were at: 0.6, 0.38, 0.35 and 0.2 V. The lithiation peaks that appeared in the range 0.38 to 0.1 V relate to the lithiation of Si and Ge and can be accounted for when compared to the respective CVs for Si and Ge NWs (supporting, S10). The peak which appeared at between 0.65 and 0.55 V corresponds to the lithiation of Sn.27 For the anodic scan, peaks appeared at: 0.33, 0.55, 0.61, 0.73 and 0.80 V. The prominent peaks between 0.3 and 0.65 V can be related to the delithiation for Si and Ge and remaining signals are due to the Sn dealloying.


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Figure 3. (a) Charge and discharge capacities (mAh/g) and the related coulombic efficiency as a percentage plotted against cycle number. (b) Voltage profiles for charge and discharge cycles: 1, 10, 50, 100, 250 and 400. (c) Cyclic voltammetry for the first 5 lithiation/delithiation cycles.


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Figure 4. (a) Charge and discharge capacities (mAh/g) demonstrated at different current rates. The rates tested were: C/10, C/5, C/2, C, 2C, 5C, 10C before returning to C/10. (b) The percentage discharge capacity retention exhibited at different rates. The average discharge capacity at each rate was used to calculate this. (c) Differential capacity plot of the first C/10 charge/discharge cycle.

The rate capability performance of the Si-Ge hNWs was evaluated by charging and discharging the anodes for 5 cycles at rates of C/10, C/5, C/2, C, 2C. 5C, 10C and again at C/10 (Figure 5a). The average discharge capacities that were exhibited at each respective rate were: 1670, 1639, 1575, 1478, 1310, 961, 613 and 1591 mAh/g. Over the first three different rates, little capacity loss is seen and the anode has retained approximately 94 % of its initial capacity at C/2 (Figure 4b). When compared to the capacities demonstrated for Sn seeded Si NWs and Sn seeded Ge NWs at 10C (supporting, S12), the hNWs are on par with those


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observed for Ge NWs. In contrast, the Si electrode retained capacities of just 186 mAh/g at this rate, clear showing high power capability offered by Ge. The exceptional rate performance from the Si-Ge NWs is due the greater electrical conductivity and higher rate of Li diffusivity offered by utilising Ge in a Li-ion battery electrode and, is an effect that has been previously seen for electrodes that combine Si and Ge.35 After cycling at 10C, the rate was returned to C/10 and the hNWs exhibited good capacity recovery, regaining 95% of their initial capacity (Figure 4b). A DCP corresponding to the 1st cycle of the C/10 rate capability can be seen in Figure 4b. Lithiation peaks for the Si-Ge hNWs were observed at 0.1 and 0.25 V, which are intermediate peaks to those corresponding to Si (0.08 V and 0.23 V) and Ge (0.1, 0.2 and 0.36 V), confirming that both Si and Ge contributed during cycling (Supporting information: Figure S12). Sn lithiation peaks were observed at 0.42 and 0.63 V and the corresponding delithiation peaks appeared at 0.78, 0.71 and 0.59 V. The delithiation peaks for the Si-Ge hNWs can be viewed at 0.5 V, which overlaps with the broad delithiation peak for Ge, and 0.43 V for Si (one of two delithiation peaks that was observed for Si) and again there is no direct overlap of the Si-Ge DCP with those for Si or Ge NWs. The voltage profiles and differential capacity plots of the first charge and discharge cycle for each rate tested are available in supporting information (S13 and S14). Electrical impedance spectroscopy (EIS) was used to investigate the changes in the resistance of the hNW anodes, as the material converts from crystalline to amorphous, showing that the resistance for cell decreases once the active material has become amorphous (Figure S15). Axially heterostructured Si-Ge NWs, grown directly from a stainless steel current collector were investigated as Li-ion battery anodes. The heterostructuring allows the individual properties of both components to be exploited as a hybrid anode with the high theoretical capacity of Si and better rate capability of Ge combined in a single structure. The structural and morphological transformation in these Si-Ge hNW arrays occurs during


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repeated cycling, with the progression from crystalline segments with abrupt interfaces through to a homogenous porous network occurring over 50 cycles. We have tracked this morphology evolution through ex-situ analysis of a number of identical electrodes subjected to a different number of cycles focussing on the representative changes that occur both at the individual wire level and in the array. In individual wires, the pore formation and distortion on cycling is evident which when combined with neighbouring wires in close proximity (in an array) results in Li assisted electrochemical welding to form an amorphous porous network with a homogenous distribution of Si and Ge.In long term cycling, the hNWs exhibited high initial capacities of 1644 mAh/g, and long-term stability, maintaining capacities of 1180 mAh/g after 400 cycles. The NW also showed good rate performance with capacities in excess of 600 mAh/g retained at 10C, suggesting these structures have potential for high power rate applications. The results show that linear heterostructuring of wires is an effective pathway to create electrode materials that can collectively harness the optimal properties of each material type for higher performant lithium ion batteries


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ASSOCIATED CONTENT Supporting Information Low-magnification SEM and TEM images highlighting Si and Ge diameters for the Sn seeded Si-Ge hNWs, XRD analysis of Si and Ge NWs and Si-Ge hNWs, BFSTEM and EDX elemental line scans of the hNWs between 5 and 9 cycles. Ex-situ SEM images of the hNW morphologies after 1, 5, 10, 50 and 100 cycles, DFSTEM images and elemental maps of the hNWs morphology after 50 cycles, Raman spectroscopy of Ge NWs and Si NWs and quantitative EDX analysis of the uncycled hNWs. Differential capacity plots of the 1st, 5th, 10th, 50th, 100th, 200th, and 400th for the Si-Ge hNWs, cyclic voltammetry, rate capability data and differential capacity plots for Si and Ge NWs, voltage profiles and associated differential capacity plots for the Si-Ge hNWs at each rate tested. EIS of the uncycled material and of the anodes after lithiation and first delithiation. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions # K.S. and G.F. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by Science Foundation Ireland (SFI) under the Principal Investigator Program under Contract No. 11-PI-1148. K.S. thanks the Irish Research Council for funding through the Government of Ireland Postgraduate Scheme and G.F. acknowledges Intel Ireland and the Irish Research Council for funding through the Enterprise Partnership Scheme. H.G. acknowledges Enterprise Ireland under Contract No. CF20144014. We thank Iobhar Stokes-Rodriguez for help in preparing the schematics. REFERENCES 1. Bogart, T. D.; Chockla, A. M.; Korgel, B. A., Curr. Opin. Chem. Eng. 2013, 2, 286-293. 2. Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J., Chem. Soc. Rev. 2010, 39, 3115-3141. 3. Su, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon, B. W.; Wu, J., Adv. Energy Mater. 2014, 4, 1300882. 4. Zhang, W.-J., J.Power Sources 2011, 196, 13-24. 5. Chan, C. K.; Zhang, X. F.; Cui, Y., Nano Lett. 2008, 8, 307-309. 6. Chan, C. K.; Ruffo, R.; Hong, S. S.; Huggins, R. A.; Cui, Y., J. Power Sources 2009, 189, 34-39. 7. Osiak, M.; Geaney, H.; Armstrong, E.; O'Dwyer, C., J. Mat. Chem. A 2014, 2, 9433-9460. 8. Obrovac, M. N.; Christensen, L., Electrochem. Solid State Lett. 2004, 7, A93-A96. 9. Winter, M.; Besenhard, J. O., Electrochim. Acta 1999, 45, 31-50. 10. Chockla, A. M.; Klavetter, K. C.; Mullins, C. B.; Korgel, B. A., Chem. Mater. 2012, 24, 37383745. 11. Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y., Highperformance lithium battery anodes using silicon nanowires. Nat. Nanotechnol. 2008, 3, 31-35. 12. Wu, H.; Cui, Y., Nano Today 2012, 7, 414-429. 13. Li, J.; Dudney, N. J.; Xiao, X.; Cheng, Y. T.; Liang, C.; Verbrugge, M. W., Adv. Energy Mater. 2015, 5, 1401627. 14. Esmanski, A.; Ozin, G. A., Adv. Funct. Mater. 2009, 19, 1999-2010. 15. Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y., ACS Nano 2012, 6, 1522-1531. 16. Cui, L.-F.; Yang, Y.; Hsu, C.-M.; Cui, Y., Nano Lett. 2009, 9, 3370-3374. 17. Bogart, T. D.; Oka, D.; Lu, X.; Gu, M.; Wang, C.; Korgel, B. A., ACS Nano 2014, 8, 915-922. 18. Memarzadeh Lotfabad, E.; Kalisvaart, P.; Cui, K.; Kohandehghan, A.; Kupsta, M.; Olsen, B.; Mitlin, D., Phys. Chem. Chem. Phys. 2013, 15, 13646-13657. 19. Chen, H.; Xiao, Y.; Wang, L.; Yang, Y., J. Power Sources 2011, 196, 6657-6662. 20. Xia, F.; Kwon, S.; Lee, W. W.; Liu, Z.; Kim, S.; Song, T.; Choi, K. J.; Paik, U.; Park, W. I., Nano Lett. 2015, 15, 6658-6664. 21. Cho, J.-H.; Picraux, S. T., Nano Lett. 2013, 13, 5740-5747.


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