Interface Stability of Sulfide/PVDF‐HFP Solid Composite Electrolyte with High Voltage NMC Cathode

Abstract Solid composite electrolytes (SCEs) have attracted serious attention for solid‐state Li metal batteries. In particular, SCEs that incorporate inorganic sulfide into polymer electrolytes provide a feasible approach to address the air sensitivity and (electro)chemical instability of sulfides. Nevertheless, there is still little research on pairing sulfide‐SCEs with high‐voltage cathodes. In this work, reports on efforts to synthesize and compare SCEs that embedding sulfides (Li₇PS₆ and Li₃PS₄) into PVDF/HFP polymer using a strong polar solvent (DMF). Two sulfides show distinct behaviors when dispersed in the DMF solvent. The Li₇PS₆‐SCE exhibits an ionic conductivity of 2.5 × 10⁻⁴ S cm⁻¹ at room temperature, higher than the Li₃PS₄‐SCE (1.75 × 10⁻⁴ S cm⁻¹). Moreover, Li₇PS₆‐SCE displays better electrochemical cycling performance in solid‐state Li metal batteries with LiNi_1/3Mn_1/3Co_1/3O₂ (NMC 111) cathode.. When increasing upper cut‐off voltages from 4.0 to 4.4 V, Li| Li₇PS₆‐SCE |NMC111 cells deliver higher discharge capacities but exhibit worse cycling stability. Interface analysis using X‐ray photoelectron spectroscopy (XPS) reveals the formation of LiF under a high voltage of 4.4 V, while t not present with 4.0 V. This work explores the synthesis of SCEs with different sulfides in a strong polar solvent and highlights the interface reactions between sulfide/PVDF‐HFP SCEs with oxide cathodes.