Potassium escaping balances the degree of graphitization and pore channel structure in hard carbon to boost plateau sodium storage capacity†

Abstract

Biomass holds significant potential for large-scale synthesis of hard carbon (HC), and HC is seen as the most promising anode material for sodium-ion batteries (SIBs). However, designing a HC anode with a rich pore structure, moderate graphitization and synthesis through a simple process using a cost-effective precursor to advance SIBs has long been a formidable challenge. This is primarily because high temperatures necessary for pore regulation invariably lead to excessive graphitization. Herein, innovative guidelines for designing such HC structures are reported by leveraging the inherent potassium in biomass to optimize the pore structure and alleviate graphitization through a novel carbothermal shock (CTS) method. During CTS, potassium-related compounds are effectively released and counteract the tendency of the carbon layers to graphitize by competing for thermal adsorption, thus forming pore channels while mitigating graphitization. The resulting HC anode exhibits an outstanding sodium storage capacity of 357.1 mA h g⁻¹ and a high initial coulombic efficiency of 90.7% at 50 mA g⁻¹. This work provides a new insight into balancing the pore structure and the degree of graphitization of HC to keep sufficient space for Na⁺ diffusion.