|Fig. 1: a, Comparison between existing methods and the new method (highlighted with red color) for producing nano-Si. Si(OR)4 denotes silicon alkoxide. b, A panicle of ripe rice at a rice farm in China (photographed by Huo, K.). c, Flow chart of the process for recovering SiNPs from RHs. R.T. denotes room temperature. d - g, Optical images of the intermediate substances. The inset of (f) shows an optical microscopy image (0.9 mm × 0.6 mm) of one piece of heat treated RH.  (Courtesy of Nature.)|
Silicon has large theoretical specific charge storage capacity (4,200 mA-h g-1), which is more than ten times the theoretical capacity of conventional graphite anodes. However, due to large volume changes upon Li insertion and extraction, silicon anode structures experience problems with fracture and pulverization. Recent pioneering works have shown that reducing the size of silicon to the nanoscale can result in effective accommodation of the strain and can even prevent fracture.
Although Si is the second most abundant element in the Earth's crust, the processes to form Si nanomaterials are usually complex, costly and energy-intensive. It is recent reported that by recycling rice husks, an abundant agricultural byproduct as the raw material source, it is possible to synthesize low-cost and functional Si nanomaterials for Li-ion battery anodes. [1,2]
Rice is the second-most widespread crop species worldwide, just below corn. Its worldwide annual production amounts to ~700 million tons.  The cultivation of rice plants generates a waste product, so-called rice husks (RHs), and upon the complete harvest of rice, the content of the RH reaches ~20 wt% of the entire rice kernel, amounting to 140 million tons of RHs per year across the globe. This huge amount of waste by-product is an environmental nuisance; hence, developing uses for this waste resources is in accord with the global paradigm shift towards sustainable development. Practical applications of RHs have been limited to a narrow range of low- value agricultural items, such as fertilizer additives, stockbreeding rugs, and bed soil, because of their tough and abrasive properties.  It is still needed to develop more valuable applications.
RHs contain a variety of components such as lignin, cellulose, and silica. Silica, accounting for ~15 - 20 wt% of the entire RHs, plays an important role in protecting rice from external attack by insects and bacteria, but simultaneously facilitates ventilation between inside and outside RHs to preserve moisture and nutrients inside the kernels. To perform these critical dual functions, the silica in RHs has developed unique porous nanostructures through years of natural evolution.  By reducing the silica to silicon while retaining the porous morphology, RHs can become a raw material source for Li-ion batteries.
In this method, preserving the nanostructure of the SiO2 species in the RHs during the whole recovery process, especially at steps involving elevated temperature, is key to producing high quality nano-Si. As shown in the flow chart and optical images in Fig. 1c - g, the raw RHs are first converted to nano-SiO2 by thermally decomposing the organic matter. In this step, simply burning RHs in air will produce bulk SiO2, and it is necessary to first perform HCl leaching and then only heat at moderate temperatures 700°C to avoid the fusion of the SiO2. In the next step, the nano-SiO2 is further reduced to obtain nano-Si. In industry, bulk Si is usually produced by the carbothermic reduction of silica, which requires temperatures greater than 2000°C, which are well above the melting point of silicon (1410°C). In order to preserve the nanostructure during silicon formation, magnesium is used in this step, as a reducing agent. The reaction temperature could decrease to 650°C. In order to increase scalability and yield, Mg and SiO2 powder are premixed together in conducting this reaction.
The existing methods for producing nano-Si anodes are high temperature or high energy pyrolysis of silane/polysilane/halosilane precursors, or laser ablation of bulk Si (Fig. 1a), which constrains the utilization of nano-Si in Li-ion batteries due to the high cost. Compared with these methods, this method of nano-Si synthesis from HRs has several advantages: (i) the recovered silicon inherits the intrinsic and unique nanostructure of the silica in RHs, which allows for excellent battery performance by mitigating pulverization; (ii) RHs are an abundant and sustainable materials source with a supply that far exceeds the demand for Li-ion battery anode materials; (iii) the overall method is simple, energy-efficient, and easy to scale-up; and (iv) the overall process does not use expensive Si precursors or reagents. Mg metal, one of the most commonly used structural metals, is produced by electrolytic process or Pidgeon process with a relatively low cost compared to other nano-Si precursors. And Mg can be regenerated from the MgCl2 byproduct by electrolysis as shown in Fig. 1c. Thus, the whole process only consumes HCl and converts it to Cl2 after the electrolysis. The overall process is green. 
© Shuang Wang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 N. Liu et al., "Rice Husks as a Sustainable Source of Nanostructured Silicon For High Performance Li-ion Battery Anodes", Sci. Rep. 3, 1919 (2013).
 D. S. Jung et al., "Recycling Rice Husks for High Capacity Lithium Battery Anodes", Proc. Nat. Acad. Sci. 110, 12229 (2013).
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