In Situ Transmission Electron Microsopy of Oxide Shell-Induced Pore

http://pubs.acs.org/journal/aelccp

In Situ Transmission Electron Microsopy of Oxide Shell-Induced Pore Formation in (De)lithiated Silicon Nanowires Emily R. Adkins,† Taizhi Jiang,† Langli Luo,§ Chong-Min Wang,§ and Brian A. Korgel*,†

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†

McKetta Department of Chemical Engineering and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States § Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *

ABSTRACT: Silicon (Si) nanowires with a silicon oxide (SiOx) shell undergoing lithiation and delithiation were examined by in situ transmission electron microscopy (TEM). Large pores formed in the nanowires during the delithiation cycle. We found that the oxide shell constrains the expansion of the Si nanowires during lithitation and then induces pore formation in the nanowires. We propose that the SiOx shell prevents the vacancies that result from the loss of lithium from escaping the Si core, leading to pore nucleation and growth. It is also possible that the difference in mechanical properties of the expanding and contracting Si nanowire and SiOx shell contribute to the observed pore formation. This in situ study reaffirms the need to directly observe structural changes that occur during cycling in battery materials, especially when modified by coatings.

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expansion during lithiation, which has been correlated to improved cycling stability in coin cell experiments, although with reduced capacity.14,19 Here we show that SiOx shells on Si nanowires can also lead to the rapid evolution of pores in the material during delithiation. We discovered this unusual evolution of pores in Si nanowires coated with SiOx while observing nanowires undergoing lithiation and delithiation by in situ TEM. In situ TEM allows real-time, direct investigation of morphology changes that occur during cycling, such as inhomogeneous lithiation of the material, self-limiting lithiation, and as in this study, pore formation, which may not be noticed by ex situ characterization.16,18,20,27−34 In the materials studied here, a thin SiOx surface layer that is about 10 nm thick, constrains the expansion of the Si nanowire and prevents complete lithiation. This SiOx shell also induces pore formation in the nanowire during delithiation. The resulting pore volume in the nanowires is significant, increasing the total volume of the Si nanowires by more than 40% after just one lithiation and delithiation cycle. We propose that the oxide shell restricts the migration of vacancies to the nanowire surface, leading to the

any emerging technologies require lithium ion batteries (LIBs) with improved energy density, rate capability, cost, and safety.1−3 Silicon (Si) is an attractive next-generation anode material because it is inexpensive and abundant and has a high theoretical gravimetric storage capacity of 3579 mAh/g, nearly 10 times that of present graphite electrodes.4−6 Si, however, expands significantly during lithiation, increasing in volume by about 300% to reach its fully lithiated state.7,8 Although nanostructured Si can withstand these lithium-induced volume changes without pulverization,9 the repeated expansion and contraction can lead to loss of electrical connection between the nanowire and conductive additives, destabilize the solidelectrolyte-interphase (SEI) layer, and lower Coulumbic efficiency over time.10−12 One strategy to improve the performance of nanostructured Si is to add a surface coating layer. A wide variety of coating materials have been explored, including silicon oxide (SiOx), carbon, Al2O3, and TiO2.10−26 The surface coating can increase electrical conductivity, improve mechanical stability, provide stronger adhesion to added binder, and protect the active material from reaction with the electrolyte. Silicon oxide is an especially interesting coating because it can be grown conformally on the Si surface by straightforward oxidation processes. Ex situ transmission electron microscopy (TEM) studies have found that such shells can limit the volume © 2018 American Chemical Society

Received: October 6, 2018 Accepted: October 19, 2018 Published: October 19, 2018 2829

DOI: 10.1021/acsenergylett.8b01904 ACS Energy Lett. 2018, 3, 2829−2834

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Cite This: ACS Energy Lett. 2018, 3, 2829−2834

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ACS Energy Letters

nanowire in Figure 1 has not fully lithiated, there is still crystalline Si within the core of the nanowire surrounded by the amorphized, lithiated region of the nanowire. This thin Si core region remains crystalline after the first delithiation cycle, but eventually becomes amorphous with further cycling. During the first delithiation cycle, the nanowire with the SiOx shell evolves a large number of pores, as shown in Figure 1d. Similar densely packed pores have been observed in a Si nanoparticle with an oxide shell after delithiation.18 After delithiation, the nanowire remains expanded by 40% compared to its initial state because of the additional pore volume in the nanowire. The pores disappear when the nanowire is again lithiated and then reappear when delithiated. The total volume occupied by the pores further increases after the second cycle, expanding by an additional 25% compared to its size after the first delithiation cycle. This is significantly larger than the typical amount of plastic deformation that can occur as a result of the lithiation−delithiation cycling of Si.35 Figure 2 shows

nucleation and accumulation of pores. Differences in the mechanical properties of the lithiating and delithiating shell and core materials could also be playing a role. If the shell burst during the lithiation cycle, the nanowires did not form pores upon delithiation. Si nanowires were made by supercritical fluid−liquid−solid (SFLS) growth in toluene with Au nanocrystal seeds and trisilane as a reactant and then heated in air at 800 °C for 3 h to generate a surface oxide layer that was about 10 nm thick. Most of the nanowires used in the study had ⟨211⟩ growth direction with (111) twins extending down their length (see the Supporting Information for scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and elemental mapping by energy-dispersive X-ray spectroscopy (EDS) of a Si nanowire with a SiOx shell). In situ TEM studies were carried out using a Nanofactory TEM-STM holder inside a Titan 80-300 scanning/transmission electron microscope (S/TEM). Figure 1 shows a time-series of TEM images of a Si nanowire with a SiOx surface oxide shell undergoing lithiation and

Figure 2. Time series of in situ TEM images showing the evolution of pores in a Si nanowire coated with SiOx undergoing its first delithiation cycle (see the Supporting Information for the accompanying video file, PoreVideo_2.mp4).

Figure 1. TEM images of a Si nanowire (42 nm diameter) with a SiOx shell (11 nm thick) during the first (a) lithiation and (b) delithiation cycle. The nanowire increased in volume by 130%. A 24 nm diameter crystalline core of Si is still present after lithiation (see the Supporting Information for the accompanying video file, LithiationVideo_1.mp4.) (c) Illustration of the in situ TEM nanobattery setup. (d) A higher-magnification TEM image of pores that form in the amorphous Si region of the nanowire after delithiation.

another time series of TEM images of a Si nanowire with a SiOx shell exhibiting pore formation during the delithiation step (see the Supporting Information for additional TEM images). In some instances, the SiOx shell ruptured during lithiation and the nanowire expanded to its fully lithiated state. Figure 3 shows an example of a time series of TEM images where the SiOx shell bursts and the nanowire expands by 270% and becomes fully amorphous by the end of this first lithiation cycle. In this case, pores do not form in the nanowire upon delithiation. Without an intact oxide shell, there is no pore formation. For example, the TEM images in Figure 4 show a nanowire undergoing lithiation and delithiation with an oxide shell that has burst at a specific point along the nanowire. Pores did not form near the point of rupture of the oxide shell; the rest of the wire still coated by intact oxide filled with pores during delithiation (see the Supporting Information for more

delithiation. Lithiation initially proceeds down the length of the Si surface of the nanowire and then extends radially into the core. This mechanism of surface-into-core lithiation is wellestablished for Si nanowires.31,32 Lithiation expands the nanowire in both the radial and axial directions and stops when the nanowire has increased in volume by about 130%. This amount of expansion is significantly less than the 300% volume expansion16 that is typical for complete lithiation of a Si nanowire. In this case, the oxide shell has constrained the volume expansion and as a result prevented complete lithiation. This is consistent with published coin cell data showing that Si anodes with SiOx surface coatings tend to have reduced specific capacity during cycling.14,19 Because the 2830

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Figure 3. TEM images of a Si nanowire with a SiOx shell that bursts during the first lithiation cycle. In this case, the SiOx shell does little to restrict the extent of lithiation and the nanowire expands by 270%. The crystalline core also becomes fully amorphous (see the Supporting Information for the accompanying video file, BurstWireVideo_3.mp4).

rate of Li diffusion, but it is rare for Si nanowires because the Li diffusion rate is intrinsically quite slow.16,28,30,33,34 It is true that pore formation has also been observed in delithiating Si nanowires by in situ TEM, but in those cases, the rate of Li diffusion had been significantly enhanced by doping so that the relative rates of Si self-diffusion and vacancy migration were slow enough for pores to nucleate and grow.34 In Si, the energy barrier to vacancy migration is 0.45 eV, which is close to the energy barrier of 0.47 eV for Li diffusion in amorphous lithiated Si.30 The rate of vacancy migration is similar to the rate of self-diffusion of Si atoms. In a typical Si nanowire, vacancies migrate to the nanowire surface before pores can nucleate and grow. The rate of vacancy migration is much slower in a SiOx shell because the diffusion barrier for Li in SiOx is significantly higher (0.72 eV).36 The vacancy diffusion barriers in lithiated SiOx are even higher: 2.41 eV for Li2Si2O5, 1.19 eV for Li2SiO3, and 0.91 eV for Li4SiO4.36 The in situ TEM images show that the SiOx shell also undergoes lithiation and delithiation. Like the Si core, the shell layer increases in thickness during lithiation, expanding in volume by nearly 150%. This layer also becomes rougher with each subsequent cycle (see Figure S4b in the Supporting Information). Various

Figure 4. TEM images of a Si nanowire with a SiOx shell that ruptures during the initial lithiation cycle at the indicated “burst point.” When the nanowire delithiates, pores do not form near the point of rupture, but do form in the region that is still coated by an intact oxide layer (see the Supporting Information for the associated video, BurstWirePoresVideo_5.mp4).

TEM images of nanowires with a burst shell and additional video, BurstWireVideo_4.mp4). Pore formation is relatively common in germanium (Ge) nanowires undergoing delithiation because of the relatively fast

Figure 5. Mechanism of pore formation in SiOx-coated Si nanowires. (1) The lithiation front appears along the nanowire surface and then (2) diffuses radially into the nanowire core. The lithiation of the nanowire is eventually halted as the shell prevents further volume expansion, leaving a crystalline core in the nanowire. Delithiation creates vacancies, which are prevented from diffusing out of the nanowire by the SiOx shell, eventually nucleating and growing into pores. 2831

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ACS Energy Letters lithiation products of silicon oxide have been discussed in the literature, including Li2Si2O5, Li4SiO4, Li2O, Si, and LixSi, and in situ TEM studies have reported crystalline Li4SiO4 and Li2O in a lithiated silicon oxide.15 In our case, we also observed crystalline Li2O in some cases by electron diffraction (see Figure S4a), although this species could simply be due to the diffusion of Li2O electrolyte along the nanowire surface from the counter electrode, as well as lithium interactions with the SiOx shell. Nonetheless, all lithiated SiOx species have significantly higher energy barriers to vacancy diffusion than Si. Enhanced pore formation in Ge nanowires during delithation has also been observed.30 Our proposed mechanism for pore formation in oxide-coated Si nanowires is shown in Figure 5, in which the oxide shell stalls vacancy migration. Additionally, the Si/SiOx interface is known to provide a pathway for even faster Li diffusion out of the nanowire,30,37,38 which could further increase the number of vacancies and enhance the rate of pore formation. The vacancies then accumulate from the nanowire surface into the nanowire core, nucleating and growing pores. The shell could also contribute to pore formation by creating additional strain at the Si−SiOx interface due to differences in mechanical properties of the core and shell layer as the nanowire shrinks. A detailed mechanistic understanding of the pore formation process in shell-coated Si nanowires requires further examination. In summary, these in situ TEM experiments of Si nanowires undergoing lithiation and delithiation show that a SiOx coating will significantly limit the extent of lithiation, but only when the coating is mechanically robust and remains intact during cycling. This kind of oxide shell can also induce the formation of pores in the nanowires during delithiation. Pores do not form in Si nanowires without the shell, or in nanowires in which the SiOx shell ruptures. The formation of these pores is an important consideration in the design of Si nanowire batteries that utilize shell materials to restrict the volume expansion of the materials. Even though the volume expansion of the nanowires is constrained by the oxide, the nanowire ends up with a neat increase in volume after cycling because of the significant void volume created by the pores. As a general conclusion, these results show that the deposition of a shell on battery materials can lead to unexpected morphology changes, and in situ TEM provides a convenient way to observe directly changes in the materials under battery cycling conditions.

crucible (Mettler-Toledo). The temperature was increased at 15 °C/min with nitrogen flow to 100 °C to ensure that all residual solvent was evaporated. After 20 min, the temperature was increased to 800 °C (or 900 °C for thicker oxide shells). After 3 h with air flow at 10 mL/min, the sample was cooled back to room temperature. In Situ TEM Measurements. Nanowire samples were dropcast from a toluene dispersion by micropipette onto a platinum probe wire and inserted into the holder. The counter electrode was a tungsten probe mounted with lithium metal as shown in Figure 1c. A small amount of Li2O was allowed to grow on the surface during transfer from the glovebox to the TEM vacuum which was used as a solid-state electrolyte.41 In the TEM, the nanowire probe was moved using a piezo-positioner and brought into contact with the lithium. The nanowires were lithiated and delithiated by applying a voltage between −5 V and 2 V vs Li/Li+. This larger voltage range than in a traditional lithium ion battery is due to the slow ion diffusion through the solid electrolyte.41

EXPERIMENTAL METHODS Si Nanowires Coated with Oxide. Si nanowires were synthesized by supercritical fluid−liquid−solid growth using trisilane, Au nanocrystal seeds, and toluene as the solvent, following published procedures.39 Gold (Au) nanocrystals capped with 1-dodecanethiol were prepared using the method of Brust et al.40 with an average diameter of 2 nm. Because trisilane (>99%, Voltaix, LLC) is pyrophoric, the synthesis is performed in a nitrogen-filled glovebox. The reaction is carried out at 13.8 MPa and 490 °C in anhydrous deoxygenated toluene in a 10 mL titanium tubular reactor connected to a high-pressure liquid chromatography (HPLC) pump. A reaction solution of 50 mg/mL Au nanocrystals in 2 M trisilane (>99%, Voltaix, LLC) in deoxygenated anhydrous toluene was fed to the reactor at 3 mL/min for 2 min and then cooled. After purification, a thermal oxide was grown on the Si nanowires. To control the heating rate and temperature, the oxidation was carried out in a Mettler-Toledo Thermogravimetric Analyzer/ DSC1 (TGA) with 4 mg of Si nanowires in a 70 μL alumina

ACKNOWLEDGMENTS Funding for this work was provided by the Robert A. Welch Foundation (Grant No. F-1464) and the National Science Foundation (Grant No. CHE-1308813). T.J. also acknowledges funding from the Center for Dynamics and Control of Materials (CDCM) supported by the National Science Foundation under NSF Award Number DMR-1720595. C.M.W. acknowledges the support of the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 6951379 under the advanced Batteries Materials Research (BMR) Program. The in situ TEM experiments were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC0576RLO1830.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01904. TEM and STEM images; descriptions of Supporting Information videos (PDF) Videos as described in the text (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 1-512-471-5633. Fax: 1512-471-7060. ORCID

Langli Luo: 0000-0002-6311-051X Chong-Min Wang: 0000-0003-3327-0958 Brian A. Korgel: 0000-0001-6242-7526 Notes

The authors declare no competing financial interest.

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(21) Jeong, M.-G.; Du, H. L.; Islam, M.; Lee, J. K.; Sun, Y. K.; Jung, H.-G. Self-Rearrangement of Silicon Nanoparticles Embedded in Micro-Carbon Sphere Framework, for High-Energy and Long-Life Lithium-Ion Batteries. Nano Lett. 2017, 17, 5600−5606. (22) Cui, L.-F.; Yang, Y.; Hsu, C.-M.; Cui, Y. Carbon-Silicon CoreShell Nanowires as High Capacity Electrode for Lithium Ion Batteries. Nano Lett. 2009, 9, 3370−3374. (23) Shen, C.; Fang, X.; Ge, M.; Zhang, A.; Liu, Y.; Ma, Y.; Mecklenburg, M.; Nie, X.; Zhou, C. Hierarchical Carbon-Coated BallMilled Silicon: Synthesis and Applications in Free-Standing Electrodes and High-Voltage Full Lithium-Ion Batteries. ACS Nano 2018, 12, 6280−6291. (24) Basu, S.; Suresh, S.; Ghatak, K.; Bartolucci, S. F.; Gupta, T.; Hundekar, P.; Kumar, R.; Lu, T.-M.; Datta, D.; Shi, Y.; Koratkar, N. Utilizing van der Waals Slippery Interfaces to Enhance the Electrochemical Stability of Silicon Film Anodes in Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2018, 10, 13442−13451. (25) Shen, D.; Huang, C.; Gan, L.; Liu, J.; Gong, Z.; Long, M. Rational Design of Si@SiO2/C Composites Using Sustainable Cellulose as a Carbon Resource for Anodes in Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2018, 10, 7946−7954. (26) Guo, S.; Li, H.; Li, Y.; Han, T.; Chen, K.; Xu, G.; Zhu, T.; Hu, X. SiO2-Enhanced Structural Stability and Strong Adhesion with a New Binder of Konjac Glucomannan Enables Stable Cycling of Silicon Anodes for Lithium-Ion Batteries. Adv. Energy Mater. 2018, 8, 1800434. (27) Liu, X. H.; Zheng, H.; Zhong, L.; Huang, S.; Karki, K.; Zhang, L. Q.; Liu, Y.; Kushima, A.; Liang, W. T.; Wang, J. W.; et al. Anisotropic Swelling and Fracture of Silicon Nanowires during Lithiation. Nano Lett. 2011, 11, 3312−3318. (28) Liu, X. H.; Huang, S.; Picraux, S. T.; Li, J.; Zhu, T.; Huang, J. Y. Reversible Nanopore Formation in Ge Nanowires during Lithiation− Delithiation Cycling: An In Situ Transmission Electron Microscopy Study. Nano Lett. 2011, 11, 3991−3997. (29) Liu, X. H.; Fan, F.; Yang, H.; Zhang, S.; Huang, J. Y.; Zhu, T. Self-Limiting Lithiation in Silicon Nanowires. ACS Nano 2013, 7, 1495−1503. (30) Lu, X.; He, Y.; Mao, S. X.; Wang, C.; Korgel, B. A. Size Dependent Pore Formation in Germanium Nanowires Undergoing Reversible Delithiation Observed by In Situ TEM. J. Phys. Chem. C 2016, 120, 28825−28831. (31) Liu, X. H.; Wang, J. W.; Huang, S.; Fan, F.; Huang, X.; Liu, Y.; Krylyuk, S.; Yoo, J.; Dayeh, S. A.; Davydov, A. V.; et al. In Situ Atomic-Scale Imaging of Electrochemical Lithiation in Silicon. Nat. Nanotechnol. 2012, 7, 749−756. (32) Gu, M.; Wang, Z.; Connell, J. G.; Perea, D. E.; Lauhon, L. J.; Gao, F.; Wang, C. Electronic Origin for the Phase Transition from Amorphous LixSi to Crystalline Li15Si4. ACS Nano 2013, 7, 6303− 6309. (33) Lu, X.; Adkins, E. R.; He, Y.; Zhong, L.; Luo, L.; Mao, S. X.; Wang, C.-M.; Korgel, B. A. Germanium as a Sodium Ion Battery Material: In Situ TEM Reveals Fast Sodiation Kinetics with High Capacity. Chem. Mater. 2016, 28, 1236−1242. (34) Lu, X.; Bogart, T. D.; Gu, M.; Wang, C.; Korgel, B. A. In Situ TEM Observations of Sn-Containing Silicon Nanowires Undergoing Reversible Pore Formation Due to Fast Lithiation/Delithiation Kinetics. J. Phys. Chem. C 2015, 119, 21889−21895. (35) Zhao, K.; Pharr, M.; Cai, S.; Vlassak, J. J.; Suo, Z. Large Plastic Deformation in High-Capacity Lithium-Ion Batteries Caused by Charge and Discharge. J. Am. Ceram. Soc. 2011, 94, s226−s235. (36) Ostadhossein, A.; Kim, S.-Y.; Cubuk, E. D.; Qi, Y.; van Duin, A. C. T. Atomic Insight into the Lithium Storage and Diffusion Mechanism of SiO2/Al2O3 Electrodes of Lithium Ion Batteries: ReaxFF Reactive Force Field Modeling. J. Phys. Chem. A 2016, 120, 2114−2127. (37) Wang, J. W.; Liu, X. H.; Zhao, K.; Palmer, A.; Patten, E.; Burton, D.; Mao, S. X.; Suo, Z.; Huang, J. Y. Sandwich-Lithiation and Longitudinal Crack in Amorphous Silicon Coated on Carbon Nanofibers. ACS Nano 2012, 6, 9158−9167.

REFERENCES

(1) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (2) Thackeray, M. M.; Wolverton, C.; Isaacs, E. D. Eletrical Energy Storage for Transportation–Approaching the Limits of, and Going Beyond, Lithium-Ion Batteries. Energy Environ. Sci. 2012, 5, 7854− 7863. (3) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243−3262. (4) Su, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon, B. W.; Wu, J. Silicon-Based Nanomaterials for Lithium-Ion Batteries: A Review. Adv. Funct. Mater. 2014, 4, 1300882. (5) Zuo, X.; Zhu, J.; Muller-Buschbaum, P.; Cheng, Y.-J. Silicon Based Lithium-Ion Battery Anodes: A Chronicle Perspective Review. Nano Energy 2017, 31, 113−143. (6) Bogart, T. D.; Chockla, A. M.; Korgel, B. A. High Capacity Lithium Ion Battery Anodes of Silicon and Germanium. Curr. Opin. Chem. Eng. 2013, 2, 286−293. (7) Obrovac, M. N.; Christensen, L. Structural Changes in Silicon Anodes during Lithium Insertion/Extraction. Electrochem. Solid-State Lett. 2004, 7, A93−A96. (8) Beaulieu, L. Y.; Eberman, K. W.; Turner, R. L.; Krause, L. J.; Dahn, J. R. Colossal Reversible Volume Changes in Lithium Alloys. Electrochem. Solid-State Lett. 2001, 4, A137−A140. (9) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. High-Performance Lithium Battery Anodes Using Silicon Nanowires. Nat. Nanotechnol. 2008, 3, 31−35. (10) McDowell, M. T.; Lee, S. W.; Nix, W. D.; Cui, Y. 25th Anniversary Article: Understanding the Lithiation of Silicon and Other Alloying Anodes for Lithium-Ion Batteries. Adv. Mater. 2013, 25, 4966−4985. (11) Li, P.; Zhao, G.; Zheng, X.; Xu, X.; Yao, C.; Sun, W.; Dou, S. X. Recent Progress on Silicon-Based Anode Materials for Practical Lithium-Ion Battery Applications. Energy Storage Mater. 2018 DOI: 10.1016/j.ensm.2018.07.014. (12) Korgel, B. A. Nanomaterials Developments for HigherPerformance Lithium Ion Batteries. J. Phys. Chem. Lett. 2014, 5, 749−750. (13) Chockla, A. M.; Harris, J. T.; Akhavan, V. A.; Bogart, T. D.; Holmberg, V. C.; Steinhagen, C.; Mullins, C. B.; Stevenson, K. J.; Korgel, B. A. Silicon Nanowire Fabric as a Lithium Ion Battery Electrode Material. J. Am. Chem. Soc. 2011, 133, 20914−20921. (14) Sim, S.; Oh, P.; Park, S.; Cho, J. Critical Thickness of SiO2 Coating Layer on Core@Shell Bulk@Nanowire Si Anode Materials for Li-Ion Batteries. Adv. Mater. 2013, 25, 4498−4503. (15) Zhang, Y.; Li, Y.; Wang, Z.; Zhao, K. Lithiation of SiO2 in LiIon Batteries: In Situ Transmission Electron Microscopy Experiments and Theoretical Studies. Nano Lett. 2014, 14, 7161−7170. (16) Bogart, T. D.; Oka, D.; Lu, X.; Gu, M.; Wang, C.; Korgel, B. A. Lithium Ion Battery Peformance of Silicon Nanowires with Carbon Skin. ACS Nano 2014, 8, 915−922. (17) Kim, S. J.; Kargar, A.; Wang, D.; Graham, G. W.; Pan, X. Lithiation of Rutile TiO2-Coated Si NWs Observed by in Situ TEM. Chem. Mater. 2015, 27, 6929−6933. (18) He, Y.; Piper, D. M.; Gu, M.; Travis, J. J.; George, S. M.; Lee, S.-H.; Genc, A.; Pullan, L.; Liu, J.; Mao, S. X.; et al. In Situ Transmission Electron Microscopy Probing of Native Oxide and Artificial Layers on Silicon Nanoparticles for Lithium Ion Batteries. ACS Nano 2014, 8, 11816−11823. (19) McDowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y. Novel Size and Surface Oxide Effects in Silicon Nanowires as Lithium Battery Anodes. Nano Lett. 2011, 11, 4018− 4025. (20) Liu, X. H.; Zhang, L. Q.; Zhong, L.; Liu, Y.; Zheng, H.; Wang, J. W.; Cho, J.-H.; Dayeh, S. A.; Picraux, S. T.; Sullivan, J.; et al. Ultrafast Electrochemical Lithiation of Individual Si Nanowire Anodes. Nano Lett. 2011, 11, 2251−2258. 2833

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Letter

ACS Energy Letters (38) McDowell, M. T.; Lee, S. W.; Harris, J. T.; Korgel, B. A.; Wang, C.; Nix, W. D.; Cui, Y. In Situ TEM of Two-Phase Lithiation of Amorphous Silicon Nanospheres. Nano Lett. 2013, 13, 758−764. (39) Heitsch, A. T.; Akhavan, V. A.; Korgel, B. A. Rapid SFLS Synthesis of Si Nanowires Using Trisilane with In situ Alkyl-Amine Passivation. Chem. Mater. 2011, 23, 2697−2699. (40) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-Derivatised Gold Nanoparticles in a Two-Phase Liquid−Liquid System. J. Chem. Soc., Chem. Commun. 1994, 801− 802. (41) Liu, X. H.; Zheng, H.; Zhong, L.; Huang, S.; Karki, K.; Zhang, L. Q.; Liu, Y.; Kushima, A.; Liang, W. T.; Wang, J. W.; et al. Anisotropic Swelling and Fracture of Silicon Nanowires during Lithiation. Nano Lett. 2011, 11, 3312−3318.

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