Recycling rice husks for high-capacity lithium battery anodes

Abstract

The rice husk is the outer covering of a rice kernel and protects the inner ingredients from external attack by insects and bacteria. To perform this function while ventilating air and moisture, rice plants have developed unique nanoporous silica layers in their husks through years of natural evolution. Despite the massive amount of annual production near 10 8 tons worldwide, so far rice husks have been recycled only for low-value agricultural items. In an effort to recycle rice husks for high-value applications, we convert the silica to silicon and use it for high-capacity lithium battery anodes. Taking advantage of the interconnected nanoporous structure naturally existing in rice husks, the converted silicon exhibits excellent electrochemical performance as a lithium battery anode, suggesting that rice husks can be a massive resource for use in high-capacity lithium battery negative electrodes.R ice is one of the most widespread food crops for human sustenance (Fig. 1A). It is currently cultivated in about 75 countries, and more than one-third of the global population eats rice as a staple food. Its worldwide annual production amounts to ∼422 million metric tons (1). 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, a very large amount, considering the massive scale of global rice production. The utilization of RHs has been an extensive research topic for decades (2). However, 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 (2). There is a large opportunity for further research targeting more valuable applications.Although RHs contain a variety of components such as lignin, cellulose, and silica, the present study pays attention mainly to recycling of the silica component. It has been known (3, 4) that silica accounts for ∼15-20 wt% of the entire RHs (Fig. 1B, Lower Inset) and originates from monosilicic acid that is first introduced into rice plants through their roots and is then moved to the rigid outer epidermal walls of the plants where it is converted into silica. The silica in RHs plays an important role in protecting rice from external attack by insects and bacteria (3, 5, 6), 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.In an effort to recycle RHs toward high-value applications, in the present investigation, the RH silica possessing unique nanostructures has been applied in high-capacity lithium ion battery (LIB) anodes by reducing the silica to silicon (Si). Si has recently attracted considerable attention as an LIB anode due to its unparalleled theoretical capacity around 4,000 mAh/g ...

Nanopurification of silicon from 84% to 99.999% purity with a simple and scalable process

“…Si is also one of the most important materials for thermoelectrics, where both nanosize and heavy doping are necessary to effectively scatter phonons and tune the electronic properties, respectively (13)(14)(15)(16). With rapid development in the past decade, Si has become one of the most promising candidates for lithium ion battery anode (17)(18)(19)(20)(21)(22)(23)(24)(25)(26), where Si nanoparticles with purity above 99% (wt %) and sizes below 150 nm have shown very high capacity without mechanical fracture during electrochemical cycling (27)(28)(29).…”

Si-containing precursors for Si-based anode materials of Li-ion batteries: A review

“…62 As a result, silica derivation from natural resources, especially from rice husks, is much more sustainable and lower cost than alternative methods, and also is attracting considerable research attention. [63][64][65][66] Recently, many researchers employed natural sources as the silica precursors, and then converted such kinds of nature-derived silica to silicon for energy storage applications. Cui's group prepared pure silica directly from rice husks and converted these silica particles to SiNPs with a conversion yield as high as 5% by mass (Figure 3a and 3b).…”

“…Similarly to Cui's report, Jung et al also employed rice husks as the precursor via the same process to prepare silicon-based anodes. 66 Silicon-based anodes with an ideal porous structure and much improved electrochemical performances were successfully obtained.…”

Influence of roasting-quenching pretreatment on the rice husk silica prepared by calcination method

“…However, the obtained silica will have different properties for different process or methods used. Thus, many RH silica-based products can be produced, such as silica gel [3], catalysts [4][5], biomedical applications [6], semi-conductor materials [7], dental composite filler [8], lithium battery anode [9], nanotube material [10], as well as solar grade silicon feedstock [11].…”

“…Therefore, many researchers succeeded in obtaining high purity silica from RH, for instance Mishra et al [13], Della et al [14], Swatsitang et al [15], Wang et al [16] and Jung et. al [9]. Other milder acids have been used as chemicals in pretreatment leaching were acetic acid [17], oxalic acid [18] and citric acid [19].…”