Revealing the Double-Edged Behaviors of Heteroatom Sulfur in Carbonaceous Materials for Balancing K-Storage Capacity and Stability

Heteroatoms in the carbon matrix are generally considered as active sites to enhance potassium storage capacity, while their adverse effects on ion batteries remain unclear. Herein, a series of sulfur doped carbon (SCDPx) with adjustable S content and crystallinity are accurately synthesized in the closed autoclave by controlling the ratios of precursors. Electrochemical measurements exhibit that heteroatom sulfur displays double-edged electrochemical activities with a high initial potassium storage capacity but poor cycling stability for carbon anode. Combined with solid-state nuclear magnetic resonance (NMR), catalytic tests, and various ex-situ characterizations, it is demonstrated that abundant S in the carbon would not only form C-S-C bonds, acting as active sites to reversibly adsorb/desorb potassium ions for high capacity, but also significantly catalyze the reduction and decomposition of the electrolyte including KPF₆ and ethylene carbonate/diethyl carbonate (EC/DEC) to form thicker solid electrolyte interface (SEI) and degrade electrolyte, resulting in rapid capacity decay. As a result, the optimized sample (SCDP2) with the appropriate sulfur doping content exhibits the best electrochemical performance with high capacity (688.4 mA h g⁻¹ at 100 mA g⁻¹), long-term cycling stability (198.4 mA h g⁻¹ at 2000 mA g⁻¹ after 10 000 cycles), and excellent rate capability (238.8 mA h g⁻¹ at 5000 mA g⁻¹).