Enhanced Cycling Stability of Composite Polymer Electrolyte-Based Solid-State Lithium Batteries Enabled by Single-Crystal NCM811 Cathodes

Polyethylene oxide (PEO)-based composite polymer electrolytes have attracted considerable research attention for application in solid-state lithium batteries owing to their excellent mechanical flexibility and facile processability. Nevertheless, the narrow electrochemical stability window of these polymer electrolytes severely restricts their operational compatibility with high-voltage cathodes. This work investigates the failure mechanism of solid-state lithium batteries employing PEO-based composite polymer electrolytes paired with the Ni-rich cathode LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811). Electrochemical analysis reveals that while PEO-based composite polymer electrolytes decomposition initiates above 4.0 V (vs Li + /Li), the resulting degradation products retain sufficient ionic conduction capability. However, when cycled to higher cutoff potentials of 4.3 V, systematic characterization identifies structural decay and microcrack generation within the polycrystalline NCM811 cathode particles as the predominant failure mode. The incorporation of LiNbO 3 coated single-crystal NCM811 cathodes into PEO-based solid-state battery systems effectively suppresses cathode-electrolyte interfacial reactions and enhance the structural stability of NCM811 during prolonged cycling. Electrochemical measurements revealed that LiNbO 3 -coated single-crystal NCM811 battery delivered an initial capacity of 171.3 mAh g −1 , with capacity retention of 80% after 100 cycles and 62.3% after 200 cycles. The elucidated degradation mechanisms in this work provide fundamental guidelines for cathode material design and electrolyte formulation optimization for PEO-based solid-state batteries.