Step-temperature strategy enables closed-pores in hard carbon achieving high plateau capacity for sodium ion storage

Abstract

Hard carbon (HC) has become the most promising anode material for sodium ion batteries (SIBs) due to its high reversible capacity, adjustable conductivity and low platform potential. However, the presence of numerous defects often results in relatively low capacity in the low-voltage plateau region (<0.1 V) and poor initial Coulombic efficiency (ICE). Closed pores in HC are widely regarded as a critical factor for enhancing plateau capacity. In this study, we utilized step-temperature strategy to stabilize the oxygen-centered radicals in biomass, which in turn mediated the topological reconstruction process of carbon microcrystals. This strategy increases the closed porosity of HC by accurately adjusting the curvature of the carbon layer, promotes the orderly reorganization of the carbon layer, and inhibits the problem of low closed porosity caused by excessive structural crosslinking. On this basis, the optimized HC has a high specific capacity of 339 mAh g⁻¹ at 100 mA g⁻¹, and has an ultra-high capacity retention rate of 97.5 % after 100 cycles. The platform capacity is increased from 142 mAh g⁻¹ to 232 mAh g⁻¹, and ICE is increased to 80.1 %. Furthermore, the electrochemical mechanism of sodium storage under pore structure regulation was investigated using in situ Raman spectroscopy, elucidating the electrochemical reactions during different sodiation-desodiation processes.