Researchers at the University of California, Riverside’s Bourns College of Engineering have synthesized a porous nano-silicon material from beach sand (SiO2) via a highly scalable heat scavenger-assisted magnesiothermic—i.e., using a combination of heat and magnesium—reduction. The addition of NaCl as a 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.
Coated with carbon, the nano-silicon electrodes achieve high electrochemical performance with a capacity of 1024 mAhg−1 at 2 Ag−1 after 1,000 cycles. A paper on their work is published in the Nature open access journal Scientific Reports.
A number of approaches have been devised to synthesizing silicon anode materials in an attempt to exploit their high theoretical capacity while eliminating the major limiting factor of volume expansion of upwards of 300% experienced during lithiation, which promotes pulverization and loss of active material.
Such approaches include double-walled silicon nanotubes; porous silicon nanowires; and post-fabrication heat-treated silicon nanoparticle (SiNP) anodes, all of which protect the material and the crucial SEI layer after its initial formation.
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 level. Silicon nanostructures derived from the pyrolization of silane, such as silicon nanospheres, nanotubes, and nanowires, have all demonstrated excellent electrochemical performance. However, chemical vapour deposition (CVD) using highly toxic, expensive, and pyrophoric silane requires costly setups and cannot produce anode material on the industry level. 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 approaches. However, electronic grade wafers are relatively costly to produce and the amount of nanowires produced via MACE is on the milligram level. Crystalline wafers have also been used to produce porous silicon via electrochemical anodization in an HF solution.
Quartz (SiO2) has been demonstrated as a high capacity anode material without further reduction to silicon, with a reversible capacity of ~800 mAhg-1 over 200 cycles. However, SiO2 is a wide bandgap insulator with a conductivity ~1011 times lower than that of silicon. 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 anodes.
… Thermic reduction of SiO2 can be accomplished via a few well-known mechanisms including carbothermal, magnesiothermic, alu- minothermic, and calciothermic reduction. … Recently, magnesiothermic reduc- tion has gained attention due its much lower operating temperatures (~650 ˚C). … Herein, we propose a facile and low cost alternative to production of nano-Si with excellent electrochemical performance using a highly abundant, non-toxic, and low cost Si precursor: sand.—Favors et al.
|Schematic Schematic of the heat scavenger-assisted Mg reduction process to create the nano-silicon material from sand. Credit: UC Riverside. Click to enlarge.|
Lead author Zachary Favors, a graduate student working with Cengiz and Mihri Ozkan, both engineering professors at UC Riverside, and colleagues began by milling sand grains to reduce the original grain size of ~0.10 mm to the micrometer and nanometer scale. They removed organic species via calcining in air at 900 °C, and then sequentially washed the sand with HCl, HF (which removed unwanted silicate species), and NaOH.
Following this purification, the resulting quartz powder and NaCl were ground together, ultrasonicated and stirred. To this mix was slowly heated at 5 °C min-1 to 700 °C and held for 6 hours to ensure complete reduction of all SiO2.
The result was washed to remove NaCl and then etched with 1 M HCl for 6 hours to remove Mg, Mg2Si, and MgO. The MgCl2 produced via HCl etching of MgO can be recycled back to Mg via electrolysis—the predominant industrial synthesis route for Mg production. The powder is washed several times to remove the etchant and dried overnight under vacuum.
The resulting nano-Si powder is composed of a highly porous network of interconnected crystalline silicon nanoparticles (SiNPs). The team attributed the high porosity to the selective etching of imbedded MgO and Mg2 Si particles after reduction. the use of NaCl as a heat scavenger during the reduction process, enabled the synthesis of a highly uniform porous structure throughout the width of the particle by avoiding localized melting of Si.
To address silicon’s relatively low electrical conductivity, they conformally coated the nano-Si powders with a ~4 nm amorphous carbon coating to enhance conductivity across all surfaces. The nano-Si@C derived from sand was then electrochemically characterized using a half-cell configuration with Li-metal as the counter-electrode.
|Cycling data of nano-Si@C anodes with selected C-rates (C = 4 Ag-1). Favors et al. Click to enlarge.|
…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 utilized 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.—Favors et al.
In addition to Favors and the Ozkans, authors were: Wei Wang, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed and Chueh Liu. All five are graduate students working in the Ozkan’s labs.
The Ozkan team is now trying to produce larger quantities of the nano-silicon beach sand and is planning to move from coin-size batteries to pouch-size batteries that are used in cell phones.
The research is supported by Temiz Energy Technologies. The UCR Office of Technology Commercialization has filed patents for inventions reported in the research paper.
Zachary Favors, Wei Wang, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Chueh Liu, Mihrimah Ozkan & Cengiz S. Ozkan (2014) “Scalable Synthesis of Nano-Silicon from Beach Sand for Long Cycle Life Li-ion Batteries,” Scientific Reports 4, Article number: 5623 doi: 10.1038/srep05623