Green Silicon Carbide (SiC) and SiC/Graphite Composite Anodes for Lithium-Ion Batteries

Arising from concerns of continuously deteriorating environmental issues, worldwide efforts are dedicated to developing more sophisticated energy storage techniques to replace traditional nonrenewable sources. Given the significant greenhouse gas emissions that occur coincident with current battery production and operation, the introduction of agricultural waste rice hull ash (RHA), a byproduct of biomass combustion, as a source for battery components could facilitate the actual realization of green and sustainable energy storage. Herein, we focus on using RHA to produce SiC with hard carbon (HC) nanocomposites and their mixtures with graphite as Li+ battery anodes. Using a simple method developed in our laboratories to control SiO2:HC ratios, the intrinsic nanoscale mixing of SiO2 and HC in RHA provides a natural starting material for carbothermal reduction producing high purity SiC without the need to add an external carbon source. The optimized SiC/HC nanocomposites contain ≈13 wt. % HC, showing BET surface areas >200 m2/g. We previously reported the SiC/HC nanocomposites derived from RHA exhibit charge/discharge capacities 3× of current graphite anodes at > 950 mAh g-1, accompanied by distinct capacity increases. In particular, SiC can be lithiated to Li1.1-1.4SiC without forming LixSi or LixC. Investigations of the lithiation/delithiation mechanisms using multiple techniques coupled with modeling suggest a cubic (3C-SiC) to hexagonal phase transition (6H-SiC) after long-term cycling, which may explain the origins of extraordinary capacity increments. Moreover, lithiation/delithiation occur without significant volume changes according to XRD and SEM, suggesting that SiC can compete with metallic Si as an anode material as free volumes equivalent to 3× the charged Si are required. Most recently, inspired by the developed concept of adding graphite to improve the performance of silicon-based anodes, we find adding graphite to SiC/HC nanocomposites results in a 3× improvement in capacity increases on cycling, reaching >950 mAh g-1 after 150 cycles. The proposed interactions within the composites will also be addressed in this presentation. It should be noted most silicon/graphite composite anodes use low Si contents of < 30 wt. % to limit the total electrode volume expansion and ensure contacts and conductivities between components, which restrains the practical energy density improvements from using Si in the anodes. In comparison, mixing only 30 wt. % graphite with SiC/HC in the current work results in specific capacities competitive with Si. Furthermore, Si metal can be synthesized via the carbothermal reduction at ≈1900 °C, while battery-grade graphite used today is produced in an environmentally harmful and expensive multistep method that involves heating to ≈3000 °C. The SiC/HC need only be heated to 1450 °C, with hard carbon forming coincidentally providing positive effects on anode performance. In this aspect, directly using SiC/HC or adding a low amount of graphite to further boosts its performance in an apparently more environmentally friendly route to alternative anodes.