Zero-waste: Carbon and SiO2 composite materials from the solid residue of the hydrothermal liquefaction of anaerobic digestion digestate for Li-ion batteries

Hydrothermal liquefaction (HTL) of wet biomass waste (e.g., from anaerobic digestion (AD) digestate) is an efficient technology to produce bio-crude, a hydrocarbon based renewable fuel. However, the solid waste by-product has limited use and could be an environmental nuisance if not managed appropriately. Herein, we demonstrate a sustainable way of transforming the solid residue generated through HTL of AD of sugarcane bagasse digestate into potential electrodes for lithium-ion battery (LIB). Briefly, the solid residue from the HTL process was converted into hard carbon (HC@WA) or converted through KOH activation to a carbon with a higher specific surface area carbon material (HC@AA). The structural and morphological characterizations confirm the encapsulation of SiO2 nanoparticles in the carbon matrix to form composite materials. When employed as anode materials in LIB, HC@WA and HC@AA achieve specific capacities of 452 mAh/g and 319 mAh/g respectively, and with long term cycling stability. The practical feasibility of the electrode materials was assessed by fabricating full cells with commercial LiFePO4 (LFP) cathode material. The full cells delivered good specific capacities and cycling stabilities, confirming the potential of the composite materials as electrodes for LiB. Thus, the solid residue after HTL of AD digestate could be one of the sustainable sources to develop materials for energy storage systems, thereby paving the way towards zero-waste and clean energy for a circular economy.