Phosphoric acid deconstruction of lignocellulose structure enables an ultra-wide plateau capacity in hard carbon anode

Abstract

Biomass-derived hard carbon, thanks to its natural advantages as an anode material for sodium-ion batteries (SIBs), has been receiving growing attention. However, the structure and performance of hard carbon can vary remarkably depending on the precursor materials and carbonization processes used. Thus, how to regulate the structure of biomass materials to achieve superior properties remains an open question. In this study, we propose a design for a porous, biomass-derived hard carbon anode material featuring a broad low-voltage plateau, high capacity, and excellent stability. Walnut shells serve as the biomass precursor, while phosphoric acid acts as both the regulator and dopant. Phosphoric acid promotes the disruption of microfibrillar crystalline regions and etches the tight structures of lignin and hemicellulose, developing more and larger enclosed pore spaces. The material carbonized at 1500 °C demonstrates a high capacity of 422 mAh g⁻¹ and a wide plateau capacity of 322 mAh g⁻¹ at 20 mA g⁻¹, with an initial coulombic efficiency (ICE) reaching 93.99 %. Importantly, the optimized hard carbon displays a higher plateau capacity, achieving an increase of 192 mAh g⁻¹ over directly carbonized hard carbon. This represents nearly double the capacity of the directly carbonized material. This simple and reliable method offers an easy-to-implement route for mass production of high-capacity, biomass-derived hard carbon for use in SIBs.