Defect-Engineered LiYO₂ Interlayers Enabling Fast Interfacial Li⁺ Transport in All-Solid-State Batteries

All-solid-state batteries (ASSBs) are desirable for next-generation energy storage systems due to their high energy density and enhanced safety. However, chemical and electrochemical incompatibility between Ni-rich layered oxide cathodes and sulfide solid electrolytes, particularly Li₆PS₅Cl (LPSCl), limits practical applications. Direct contact between these components accelerates oxidative decomposition of LPSCl, forming resistive interphase products, sluggish interfacial Li⁺ transport, and rapid capacity decay. Here, we propose defect-engineered LiYO₂ (LYO) coatings as chemically stable and conductive interlayers for Ni-rich cathodes. To improve the intrinsically limited Li⁺ transport of LYO, aliovalent substitution is employed based on Li⁺ defect chemistry. Zn²⁺-substituted LYO (LYZnO, Li_1.05Y_0.95Zn_0.05O₂) and Zr⁴⁺-substituted LYO (LYZrO, Li_0.95Y_0.95Zr_0.05O₂) are designed to introduce Li-excess and Li-vacancy defects, respectively. Importantly, both configurations successfully maintain the chemically robust Y-O polyhedral framework of LYO. When applied as nanoscale coatings on Ni-rich cathode particles, both LYZnO and LYZrO effectively suppress parasitic decomposition of LPSCl and improve Li⁺ transport across the cathode-electrolyte interface. Li-vacancy-type LYZrO exhibits superior interfacial stabilization compared with Li-excess-type LYZnO, owing to more favorable vacancy-mediated Li⁺ migration. These findings demonstrate that defect engineering of stable oxide coatings is an effective strategy to construct reliable and kinetically favorable cathode interfaces in ASSBs.