Unraveling the structure evolution and sodium storage in hard carbon enabled by an in situ self-template strategy

The insufficient low-potential sodium storage of hard carbon anodes remains a major obstacle for developing high-energy-density sodium-ion batteries. While conventional templating methods can improve the plateau capacity, they usually involve corrosive agents and complex post-treatment steps, resulting in high costs and limited efficiency. Herein, an innovative fluorine-induced self-templating strategy was designed to overcome these challenges. By molecularly incorporating fluorine into phenolic resin precursors, volatile HF is in situ generated during pyrolysis, serving as a dynamic pore-former to create a hierarchical architecture without any post-removal. The optimized hard carbon delivers a remarkable reversible capacity of 445.8 mAh g⁻¹ and an initial Coulombic efficiency of 86.2%. Most strikingly, it achieves an ultrahigh plateau capacity of 333.4 mAh g⁻¹, ranking among the highest reported values and approaching the theoretical capacity of graphite in lithium-ion batteries. A sequential adsorption-intercalation-pore filling-sodium cluster formation mechanism was elucidated, combined with DFT simulations that identify the optimal pore size for sodium storage. This work provides a novel and scalable paradigm for constructing high-performance hard carbon anodes.