Poly(ethylene oxide)-based solid polymer electrolytes (PEO-SPEs) are regarded as promising alternatives to liquid electrolyte in batteries due to their improved safety and good compatibility with lithium-metal anode. However, the decomposition of PEO matrix at high voltage leads to capacity degradation, hindering its further deployment in high voltage all-solid-state lithium-metal batteries (ASSLMBs). Herein, we studied the failure mechanism of PEO-SPEs with high-capacity Li-rich layered cathode and reported a strategy of using an Al₂O₃ coating to improve electrochemical performance. The anion redox of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LR114) generates reactive oxygen species, causing the terminal hydrogen of PEO to dissociate into H⁺, which combines with bis(trifluoromethanesulfonyl)imide (TFSI⁻) to form HTFSI. HTFSI initiates the further autocatalytic decomposition of PEO, which induces the dissolution of transition metals and formation of the spinel-like phase on the surface of LR114. By integrating Al₂O₃ protective layer on cathodes, it adsorbs the TFSI⁻/bis(fluorosulfonyl)imide (FSI⁻) anions preferentially, leading to the formation of a LiF-rich cathode–electrolyte interphase (CEI), which in turn inhibits the decomposition of PEO. The obtained Li-In|PEO|Al₂O₃@LR114 ASSLMBs exhibit better cycling performance with a capacity retention of 93.5% after 100 cycles at 0.2 C. This study demonstrates the potential of interfacial engineering to control the chemical composition of electrode–electrolyte interphase in high voltage ASSLMBs.