O-Targeted Carbon Hybrid Orbital Conversion to Produce sp²-Rich Closed Pores for Sodium-Storage Hard Carbon

Biomass-based hard carbon has the advantages of a balanced cost and electrochemical performance, making it the most promising anode material for sodium-ion batteries. However, due to the structural limitations of biomass (such as macropores and impurities), it still faces the problems of low specific capacity and initial Coulombic efficiency (ICE). Herein, an integrated strategy of biomass liquefaction and oxidation treatment is proposed to fabricate hard carbon with low ash content and sp²-rich closed pores. Specifically, liquefaction treatment can break through the inherent constraints of biomass, while oxidation treatment with O-targeted effect can directionally convert C─C/C─O bonds into C═O/O═C─O bonds, which would promote the formation of closed pores and the rearrangement into sp²-carbon within the graphene layer. Moreover, it is well demonstrated that the hard carbon interface rich in sp² hybridization can induce the generation of an inorganic-rich solid electrolyte interface, contributing to fast ion migration and excellent interfacial stability. As a result, the optimized hard carbon with maximum closed pore volume and sp²/sp³ ratio can exhibit a high capacity of 347.3 mAh g^-1 at 20 mA g^-1 with the ICE of 90.5%, and a capacity of 110.4 mAh g^-1 at 5.0 A g^-1 after 10 000 cycles.