Rice husk as anode material for Li-ion batteries and beyond

Introduction: Rice husk (RH) the outer covering of a rice kernel, is an abundant agricultural byproduct that can be source of anode materials for lithium-ion batteries (LIBs). From RHs both carbon (from the organic constituents), and silicon (RHs contain ~ 10/15% wt of silica) can be derived. As anode material for LIBs, Si has a theoretical capacity ten folds that of standard graphite electrodes, but it is subjected to huge volumetric expansion upon lithiation (> 300% for bulk) which leads to high mechanical instability and thus to rapid battery failure. Compositing nanosized Si domains with C is an effective route towards mechanical stability while also increasing the anode conductivity. Direct carbonization of RH at high temperature gives hard carbon, so RH-C is also suitable for sodium intercalation. In this work, different C/SiO2 and C/SiO2/Si composites derived from RH are tested in half-cell configuration vs. Li as well as Na, both with conventional (LP30) and non-conventional glyoxal based electrolyte (i.e., LiTFSI in TEG:PC, NaTFSI in TEG:PC). Material and Methods: As a first trial, RH has been carbonized in Ar atmosphere either up to 800°C in a tubular oven (sample RH800) or up to 1000°C (RH1000). These two samples were then used as active material for preparing electrodes without any further treatment. Moreover, after the same carbonization at 800°C, a third sample has undergone a magnesiothermic reduction at 700°C in a tubular oven under Ar. The thus obtained silica-reduced sample (RHMgR) has been washed with HCl and filtered with distilled water (H20dist) to remove by-products (mainly MgO). All the samples have been characterized by XRD, BET surface analysis, XPS, SEM-EDX. Electrodes were prepared by depositing on Cu foil a slurry of active material (90%), carbon black (5%) and CMC (5%) in H20dist. Results: All the samples were characterized by cyclic voltammetry (CV) and galvanostatically cycled in Li-half cells with standard LP30 electrolyte and the alternative glyoxal-based electrolyte. Directly carbonized samples have also been similarly tested in Na-half cells. Discussion Preliminary results show that the directly carbonized samples behave like hard-carbons: no clear redox peaks during CVs, good capacity retention at 1C cycling (calculated on graphite-rates) although an increase of capacity over cycling may suggest SiO2 activation. Charge-discharge cycling of the RH-derived anodes has been also proved to work with Na-disc as counter-electrode. The effect of glyoxal-based electrolyte on all samples and the electrochemical behaviour of the Si-containing sample are currently under investigation.