Silicon/carbon composite anodes from rice husk for lithium-ion batteries: optimizing the Si/C ratio to enhance battery performances and durability

Rice husk (RH), the outer covering of a rice kernel, is an abundant agricultural by-product that can be used as source of anode materials for lithium-ion batteries (LIBs). RHs are in fact a source of both carbon (from the organic constituents like cellulose, hemi-cellulose, and lignin), and silicon (as RHs contain ~ 20% wt of hydrated silica). [1] As anode material for LIBs, Si has a theoretical capacity ten folds that of standard graphite electrodes (~ 3500 mAh/g vs. 372 mAh/g), but it is subjected to huge volumetric expansion upon lithiation (> 300% for bulk Si) which leads to high mechanical instability and thus a very short battery life cycle. Compositing Si with C is an effective route towards mechanical stability while also increasing the anode conductivity. [2] Our main scope is to obtain Si/C composite anodes for LIBs with an optimized Si/C ratio in order to enhance the battery performances and life cycle. The raw RH undergoes three different reactions to obtain the composite material: an acidic pre-treatment, a carbonization process, and a low temperature magnesiothermic reduction (MgR). Different acidic pre-treatments (using HNO3 or H2SO4) have been performed at reflux with varying reaction times (1h, 2h and 4h) and acid concentration (16 N, 8 N and 4 N) to assess their effects on the final Si/C ratio and on the composite structure. Preliminary results show that HNO3, also having a strong oxidizing activity against C, gives the highest Si/C ratios. Carbonization at 800°C in argon atmosphere allowed for the reduction of residual organic components to carbon. MgR is an effective and simple way to reduce SiO2 without destroying the oxide (nano)structure. We mixed the Mg powder with a eutectic mixture of AlCl3 and NaCl, which has been proved to allow for a low temperature MgR (as low as 200°C) [3], as AlCl3 plays an active role with Mg in silica reduction [4], thus leading to a double advantage: saving energy and reducing the risk of undesirable side reactions that are favoured at high temperature (such as the formation of SiC). The reactants are ball milled with the carbonized RH with a Si:Mg:AlCl3 molar ratio of about 1:2.2:6.6 and then the MgR is performed at 250°C under Ar atmosphere. The obtained materials have been characterized (e.g., by scanning electron microscope energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Brunauer–Emmett–Teller surface analysis) and electrochemically tested in a half-cell configuration with a standard electrolyte formulation.