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.

The incorporation of inorganic fillers into poly(ethyleneoxide)(PEO)-based solid polymer electrolytes (SPEs) is well known as a low-cost and effective method to improve their mechanical and electrochemical properties. Porous zeolitic imidazolate framework-8 (ZIF-8) is firstly used as the filler for PEO-based SPEs in this work. Due to the introduction of ZIF-8, an ionic conductivity of 2.2 × 10^-5 S/cm (30 °C) is achieved for the composite SPE, which is one order of magnitude higher than that of the pure PEO. ZIF-8 also accounts for the broader electrochemical stability window and lithium ion transference number (0.36 at 60 °C) of the composite SPE. Moreover, the improved mechanism of ZIF-8 to the composite SPE is investigated by zeta potential and Fourier transform infrared spectrograph characterizations. The stability at the composite SPE/lithium interface is greatly enhanced. The LiFePO₄||Li cells using the composite SPE exhibit high capacity and excellent cycling performance at 60 °C, i.e., 85% capacity retention with 111 mA·h/g capacity retained after 350 cycles at 0.5 C. In comparison, the cells using the pure PEO show fast capacity decay to 74 mA·h/g maintaining only 68% capacity. These results indicate that the PEO-based SPEs with ZIF-8 are of great promise for the application in solid-state lithium metal batteries.