Solid interface electrochemistry between LiCoO₂ and solid-state electrolytes

Strategies to optimize the cathode/solid electrolyte interphase (CSEI) have been developed to improve the high voltage durability of LiCoO₂ (LCO), yet the underlying interfacial mechanism remains unclear. Here, we construct a stable oxyhalide-derived CSEI for all-solid-state batteries (ASSBs) to identify the key interfacial features required for high-voltage operation. At 4.6 volts, commercial LCO coupled with Li₆PS₅Cl (LPSC) suffers from severe interfacial instability, sluggish Li⁺ transport, and continuous side reactions. By introducing LiNbOCl₄ (LNOC), an in situ formed CSEI rich in Li-Cl/Nb-O/Nb-O-Cl species is established, lowering the interfacial energy barrier to 0.363 electron volts and enhancing interfacial toughness and ionic conductivity. This stabilized interface suppresses lattice oxygen activity, accelerates Li⁺ transport, and improves the reversibility of O3/H1-3 phase transitions. Consequently, LCO|LNOC|LPSC|Li-In ASSBs deliver 95.8% capacity retention after 500 cycles at ∼1.0 C-rate with an LCO loading of 15.31 milligrams per square centimeter. The pouch cell achieves 90% initial Coulombic efficiency and stable cycling over 50 cycles.