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fairly constant with increasing number of cycles. Therefore, contact impedance among the active particles and the current collector is not affected by cycling. Evidently, the nano-Si@C anodes are not dras- tically affected by the volume expansion of a typical Si-based anode. The (EIS) measurements performed after 1st, 3rd, 5th, 7th, and 9th cycles show two distinct arcs. The high frequency semicircle corre- sponds to SEI film and contact impedance while the mid frequency semicircle corresponds to charge transfer impedance on electrode- electrolyte interface34. The Warburg element represents impedance due to diffusion of ions into the active material of the electrode35. The low-frequency (,200 MHz) Warburg impedance tail can be attrib- uted to bulk diffusional effects in nano-Si This includes the diffusion of salt in the electrolyte and lithium in the nano-Si@C electrodes36. We observe that the biggest change occurs in impedance between the 1st and the 5th cycle. The change in impedance hereafter (from 5th cycle to 9th cycle) is relatively less pronounced, confirming that the anode tends to stabilize as it is repeatedly cycled. The ability to mitigate the volume expansion related effects is due to the ability to produce a highly porous interconnected 3D network of nano-Si. This is achieved via the addition of a relatively large amount or NaCl, which serves to absorb the large amount of heat generated in this highly exothermic Mg reduction, as in Eq. 1. remove NaCl and then etched in 5 M HCl for 12 h to remove Mg2Si and unreacted Mg. Powders were then etched in 10% HF to remove unreacted SiO2, washed several times with DI and EtOH, and then dried. Preparation of Electrodes. To increase conductivity, nano-Si powder was loaded into a quartz boat and placed in a 1" quartz tube furnace. Under ambient pressure, the system was heated to 950uC in 25 min under a flow of Ar and H2. At 950uC, C2H2 was introduced and kept on for 20 min to produce a 4 nm C-coating. C-coated nano-Si powder was mixed with acetylene black and PAA (Sigma-Aldrich) in a 75152 weight ratio, spread onto copper foils, and dried for 4 h. The mass loading density was 0.5– 1.0 mgcm22. Electrochemical Characterization. Electrochemical performance of electrodes was characterized vs. Li using CR2032 coin cells with an electrolyte comprising 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC:DEC 5151, v/v) with a 2% vol. vinylene carbonate (VC) additive for improved cycle life. Cells were assembled in an Argon-filled VAC Omni-lab glovebox. All cells were tested vs. Li from 0.01 to 1.0 V using an Arbin BT2000 at varying current densities. Cyclic voltammetry and electrochemical impedance spectroscopy measurements were conducted on a Biologic VMP3 at a scan rate of 0.02 mVs21. 1. Iwamura, S., Nishihara, H. & Kyotani, T. Effect of Buffer Size around Nanosilicon Anode Particles for Lithium-ion Batteries. J. Phys. Chem. C 116, 6004–6011 (2012). 2. 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. 25, 4966–4985 (2013). 3. Cho, J. Porous Si anode materials for lithium rechargeable batteries. J. Mater. Chem. 20, 4009–4014 (2010). 4. Liu, X. H. et al. Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 6, 1522–1531 (2012). 5. Ryu, I., Choi, J. W., Cui, Y. & Nix, W. D. Size-dependent fracture of Si nanowire battery anodes. Jour. Mech. Phys. Solids 59, 1717–1730 (2011). 6. Lee, S. W., McDowell, M. T., Berla, L. A., Nix, W. D. & Cui, Y. Fracture of crystalline silicon nanopillars during electrochemical lithium insertion. PNAS 109, 4080–4085 (2012). 7. Ye, J. C. et al. Enhanced lithiation and fracture behavior of silicon mesoscale pillars via atomic layer coatings and geometry design. J. Power Sources 248, 447–456 (2014). 8. Wu, H. et al. Stable cycling of double-walled silicon nanotube battery anodes through solid-electrolyte interphase control. Nat. Nanotechnol. 7, 310–315 (2012). 9. Ge, M., Rong, J., Fang, X. & Zhou, C. Porous Doped Silicon Nanowires for Lithium Ion Battery Anode with Long Cycle Life. Nano Lett. 12, 2318–2323 (2012). 10. Hassan, F. M., Chabot, V., Elsayed, A., Xiao, X. & Chen, Z. W. Engineered Si electrode nanoarchitecture: A scalable treatment for the production of next- generation Li-ion batteries. Nano Lett. 14, 277–283 (2014). 11. Yoo, J.-K., Kim, J., Jung, Y. S. & Kang, K. Scalable Fabrication of Silicon Nanotubes and their Application to Energy Storage. Adv. Mater. 24, 5452–5456 (2012). 12. Liu, N. et al. A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Anodes. Nano Lett. 12, 3315–3321 (2012). 13. Yao, Y. et al. Interconnected Silicon Hollow Nanospheres for Lithium-Ion Battery Anodes with Long Cycle Life. Nano Lett. 11, 2949–2954 (2011). 14. Kondo, S., Tokuhashi, K., Nagai, H., Iwasaka, M. & Kaise, M. Spontaneous ignition limits of silane and phosphine. Combust. Flame 101, 170–174 (1995). 15. Vlad, A. et al. Roll up nanowire battery from silicon chips. PNAS 109, 15168–15173 (2012). 16. Huang, Z. et al. Extended Arrays of Vertically Aligned Sub-10 nm Diameter [100] Si Nanowires by Metal-Assisted Chemical Etching. Nano Lett. 8, 3046–3051 (2008). 17. Vlad, A. et al. Roll up nanowire battery from silicon chips. PNAS 109, 15168–15173 (2012). 18. Sun, X., Huang, H., Chu, K.-L. & Zhuang, Y. Anodized Macroporous Silicon Anode for Integration of Li-Ion Batteries on Chips. J. Electron. Mater. 41, 2369 (2012). 19. Chang, W.-S. et al. Quartz (SiO2): a new energy storage anode material for Li-ion batteries. Energy Environ. Sci. 5, 6895–6899 (2012). 20. Mitchell, B. S. An Introduction to Materials Engineering and Science: For Chemical and Materials Engineers. (John Wiler & Sons, Inc., 2004). 21. Favors, Z. J., Wang, W., Bay, H. H., George, A., Ozkan, M. & Ozkan, C. Stable Cycling of SiO2 Nanotubes as High-Performance Anodes for Lithium-Ion Batteries. Sci. Rep. 4 (2014). 22. Dingsoyr, E. & Christy, A. A. Effect of reaction variables on the formation of silica particles by hydrolysis of tetraethyl orthosilicate using sodium hydroxide as a basic catalyst. Surf. Colloid Sci. 116, 67–73 (2001). 23. Richman, E. K., Kang, C. B., Brezesinski, T. & Tolbert, S. H. Ordered Mesoporous Silicon through Magnesium Reduction of Polymer Templated Silica Thin Films. Nano Lett. 8, 3075–3079 (2008). 24. Liu, N., Huo, K., McDowell, M. T., Zhao, J. & Cui, Y. Rice husks as a sustainable source of nanostructured silicon for high performance Li-ion battery anodes. Sci. Rep. 3 (2013). Mg (g )zSiO2 ?Si(s)zMgO (s) ð1Þ ð2Þ Mg (g )zSi(s)?Mg 2 Si(s) Mg reduction evolves a large amount of heat that can cause local melting of Si and, consequently, aggregation of nano-Si particles (Mg (g): DH 5 2586.7 kJ/molSiO2)26. However, by surrounding the milled quartz particles with a large amount of NaCl (DHfusion 5 28.8 kJ/mol) the heat is used in the fusion of NaCl rather than in the fusion of Si. Additionally, NaCl is a highly abundant, low cost, and environmentally benign salt that can be subsequently recycled for further reductions. We also observe that the addition of NaCl also serves to reduce the presence of Mg2Si, an unwanted product that can result from excess Mg alloying with Si, as in Eq. 2. Etching of this silicide with HCl produces silane, which is a highly toxic and pyro- phoric gas. The presence of Mg Si also reduces the overall yield of the reduction process. In conclusion, we have demonstrated a highly scalable, cheap, and environmentally benign synthesis route for producing nano-Si with outstanding electrochemical performance over 1000 cycles. The out- standing performance of the nano-Si@C electrodes can be attributed to a number of factors including the highly porous interconnected 3D network of nano-Si, the conformal 4 nm C-coating, and the use of PAA as an effective binder for C and Si electrodes. Nano-Si@C electrode fabrication follows conventional slurry-based methods uti- lized in industry and offers a promising avenue for production of low cost and high-performance Si-based anodes for portable electronics and electric vehicle applications. Methods Synthesis of Nano-Si. Collected sand was first calcined at 900uC to burn off organic impurities. The sand was wet etched in 1 M HCl for 1 hour, 49% HF for 24 h, and then alkaline etched in 1 M NaOH. DI water washing was used after each step to remove previous etchant solution. Purified sand was hand-milled in an alumina mortar for several minutes, ultrasonicated for 1 h, and then left to settle for 3 h. Suspended particles in solution were collected and allowed to dry at 110uC under vacuum for 4 h, while larger settled particles were later re-milled. Dried quartz powder was milled in an alumina mortar with NaCl (Fisher, molecular biology grade) in a 1510 SiO2:NaCl weight ratio. The SiO2:NaCl powder was added to DI water, vigorously stirred and ultrasonicated for 4 h, and then dried overnight at 110uC under vacuum. Dried SiO2:NaCl powder was then milled with 250 mesh Mg powder (Sigma-Aldrich) in a 150.9 SiO2:Mg ratio. The resultant powder was loaded into Swagelok-type reactors and sealed in an Ar-filled glovebox. Reactors were immediately loaded into a 1" quartz tube furnace (MTI GSL1600X). The furnace was ramped to 700uC at 5uCmin21 and held for 6 h with a 0.472 sccm Ar flow under vacuum. Resultant powders were washed with DI water and EtOH several times to 2 www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 5623 | DOI: 10.1038/srep05623 6PDF Image | Scalable Synthesis of Nano-Silicon from Beach Sand
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