Elemental regulation and ultra-micropore defects engineering for enhancing low-potential Na⁺ storage in biomass-derived hard carbon

The commercialization of sodium-ion batteries is significantly hindered by the limited plateau capacity of hard carbon, caused by insufficient closed pores within the carbon matrix. Herein, we employ palm kernel shells as a representative precursor to synthesize closed-pore-rich hard carbon through elemental regulation and ultra-micropore defects engineering. Pre-carbonization followed by acid washing enhances the structural stability of the carbon matrix and prevents excessive graphitization induced by internal impurities. Subsequent alkali activation fabricates abundant ultra-micropore defects within the carbon-based precursor, acting as developmental sites to support the formation of closed pores during carbonization. As the temperature increases, the defects are gradually transformed into initially enclosed pores. These initially enclosed pores restrict the bridging and excessive growth of carbon layers, facilitating the formation of short and thin carbon layers that serve as structural boundaries, thus forming a significant number of closed pores. Ultimately, the optimized hard carbon exhibits excellent rate performance with 219.7 mAh g^-1 and cycling stability of 95.4 % after 1000 cycles at 10 C. The assembled full-cell delivers high energy densities of 242.1 Wh kg^-1 and 182.6 Wh kg^-1 at 25 °C and -40 °C, respectively. Our study provides a new insight into the preparation of hard carbon with high plateau capacity through the utilization of ultra-micropore defects.