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OPEN SUBJECT AREAS: BATTERIES SYNTHESIS AND PROCESSING Received 31 March 2014 Accepted 20 June 2014 Published 8 July 2014 Correspondence and requests for materials should be addressed to C.S.O. (cozkan@engr. ucr.edu) or M.O. (mihri@ee.ucr.edu) Scalable Synthesis of Nano-Silicon from Beach Sand for Long Cycle Life Li-ion Batteries Zachary Favors1, Wei Wang1,2, Hamed Hosseini Bay1, Zafer Mutlu1, Kazi Ahmed2, Chueh Liu2, Mihrimah Ozkan2 & Cengiz S. Ozkan1 1Materials Science and Engineering Program, Department of Mechanical Engineering, University of California Riverside, CA 92521 (USA), 2Department of Electrical Engineering, Department of Chemistry, University of California, University of California Riverside, CA 92521 (USA). Herein, porous nano-silicon has been synthesized via a highly scalable heat scavenger-assisted magnesiothermic reduction of beach sand. This environmentally benign, highly abundant, and low cost SiO2 source allows for production of nano-silicon at the industry level with excellent electrochemical performance as an anode material for Li-ion batteries. The addition of NaCl, as an effective heat scavenger for the highly exothermic magnesium reduction process, promotes the formation of an interconnected 3D network of nano-silicon with a thickness of 8-10 nm. Carbon coated nano-silicon electrodes achieve remarkable electrochemical performance with a capacity of 1024 mAhg21 at 2 Ag21 after 1000 cycles. Silicon is considered the next generation anode material for Li-ion batteries and has already seen applications in several commercial anodes. This is due to its high theoretical capacity of 3572 mAhg21 corresponding to 1,2 ambient temperature formation of a Li Si phase . However, silicon has major drawbacks stemming from 15 4 the large volume expansion upwards of 300% experienced during lithiation3. Depending on the structure, lithia- tion-induced mechanical stresses cause silicon structures to fracture when the characteristic dimension is as small as 150 nm, which promotes pulverization and loss of active material4–6. Despite scaling the dimensions of silicon architectures below this critical dimension, the large volume expansion deteriorates the integrity of the solid electrolyte interphase (SEI)7. Expansion upon lithiation and subsequent contraction during delithiation leads to the constant fracturing and reformation of new SEI, resulting in irreversible capacity loss8. Several structures such as double-walled silicon nanotubes, porous silicon nanowires, and postfabrication heat-treated silicon nanopar- ticle (SiNP) anodes have alleviated this issue via protecting the crucial SEI layer after its initial formation8–10. While a myriad of silicon nanostructures have exhibited excellent electrochemical performance as anode materials, many of them lack scalability due to the high cost of precursors and equipment setups or the inability to produce material at the gram or kilogram level11,12. Silicon nanostructures derived from the pyrolization of silane, such as silicon nanospheres, nanotubes, and nanowires, have all demonstrated excellent electrochemical performance9,11,13. However, chemical vapour deposition (CVD) using highly toxic, expensive, and pyrophoric silane requires costly setups and cannot produce anode material on the industry level14. Metal assisted chemical etching (MACE) of crystalline silicon wafers has been extensively investigated as a means of producing highly tunable silicon nanowires via templated and non-templated approaches15,16. However, electronic grade wafers are relatively costly to produce and the amount of nanowires produced via MACE is on the milligram level17. Crystalline wafers have also been used to produce porous silicon via electrochemical anodization in an HF solution18. Quartz (SiO2) has been demonstrated as a high capacity anode material without further reduction to silicon, with a reversible capacity of ,800 mAhg21 over 200 cycles19. However, SiO2 is a wide bandgap insulator with a conductivity ,1011 times lower than that of silicon20. Additionally, SiO2 anodes carry 53.3% by weight oxygen which reduces the gravimetric capacity of the anodes. The highly insulating nature of SiO2 is also detrimental to the rate capability of these anodes21. Tetraethyl Orthosilicate (TEOS) has garnered significant attention recently due its ability to produce nano-SiO2 via hydrolysis22. The SiO2 has been subsequently reduced to silicon in such structures as nanotubes and mesoporous particles11,23. However, examining Fig. 1a reveals the extensive produc- tion process needed to produce TEOS. Conversely, Liu et al. have demonstrated a method of synthesizing nano-Si SCIENTIFIC REPORTS | 4 : 5623 | DOI: 10.1038/srep05623 1PDF Image | Scalable Synthesis of Nano-Silicon from Beach Sand
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