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

Abstract 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 (SCDP x ) 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 KPF 6 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 ).