Structural design of biomass-derived hard carbon anode materials for superior sodium storage via increasing crystalline cellulose and closing the open pores

Abstract

Biomass-derived hard carbon is a promising anode material for sodium-ion batteries. However, ensuring the consistency of hard carbon poses a challenge due to the intricate properties of biomass precursors, impacting their large-scale and stable supply. In this study, we used a two-step acid treatment and a two-stage heat treatment process to modulate the properties of the biomass precursor and design the structure of hard carbon materials. During the acid treatment, the removal of both the amorphous region of cellulose and hemicellulose fosters the formation of additional graphite microcrystals while concurrently decreasing the specific surface area of the resulting hard carbon. Moreover, adjustments in the pre-carbonization conditions lead to the enlargement of the carbon interlayer spacing, reduction of open pores, and the introduction of more oxygen-containing groups into the carbon structure. As a result, the optimized sample demonstrates an impressive specific capacity of 342.4 mA h g⁻¹ with an initial coulombic efficiency (ICE) of 87.19% at 30 mA g⁻¹, sustaining a reversible capacity of 128.3 mA h g⁻¹ at 10 A g⁻¹. Additionally, the sodium storage mechanism of the resulting hard carbon material was confirmed to be “adsorption-intercalation-filling” by kinetics and structural evolution analysis. This work sheds light on the effective obtaining of biomass-derived hard carbon for enhanced sodium ion storage.