Sulfur and phosphorus co-doped hard carbon derived from oak seeds enabled reversible sodium spheres filling and plating for ultra-stable sodium storage

Hard carbon has been considered as a promising anode material for sodium ion batteries (SIBs), while suffers from poor rate performance and some safety issues caused by irreversible or undesirable sodium plating. Heteroatom doping is an effective strategy to further improve the electrochemical performance of hard carbon. Herein, biomass oak seeds are selected as carbon source to prepare S/P co-doped hard carbon through a simple one-step pyrolysis process. The hard carbon pyrolyzed from oak seeds at 1000 °C (C1000) exhibits excellent rate performance (initial discharge capacity is 136.1 mA h g⁻¹) and remarkable cycling stability (1000 cycles with 88% capacity retention at 1000 mA g⁻¹). The unique S/P co-doping is very important to the high electrochemical performances, since sulfur and phosphorus co-doping enlarges interlayer distance, modifies the electronic structure, as well as provides more effective active sites, thereby enabling hard carbon with a fast, complete charge transfer kinetics of sodium storage.