Revealing the storage mechanism of plateau-dominated S-doped hard carbon as high-performance anode for sodium-ion batteries

Hard carbon (HC) materials have poor rate performance and low initial Coulombic efficiency, which limits their practical use in sodium-ion batteries. An effective structural design with a suitable porous structure and an optimized graphitic degree is much required to address these issues. Herein, a novel S-doped commercial HC, consisting of hierarchically porous channels offering additional active sites and storage capacity to facilitate sodium ion transport from a simple thermal treatment, is reported. S-doped HC delivers a plateau-dominated reversible capacity of 429 mAh g⁻¹ at 0.3 mA g⁻¹ and 252 mAh g⁻¹ at 30 A g⁻¹ with 90 % capacity retention after 1000 cycles. as the SIB anode. Utilizing a commercial NFM cathode and a pre-sodiated S-doped HC anode, the full cell produced a high energy density of 237 Wh kg⁻¹ at an average operating potential of 3.25 V. The 3-stage sodium-ion storage mechanism of the S-doped material is revealed and confirmed through ex-situ XRD, Raman, XPS, EPR, and SEM. A tool to complement and differentiate the storage mechanisms through ex-situ Raman is reported to enable the design and development of plateau-dominated HCs in SIBs.