Insights into the Capacity Fade of High-Voltage Lithium Cathodes in Solid Polymer Electrolyte Cells

Solid polymer electrolytes (SPEs) can enable safer and more energy-dense Li metal batteries, yet their compatibility with high-voltage cathodes remains a key challenge. In this study, we systematically investigate the degradation mechanisms of poly(ethylene oxide)-based SPEs paired with Li nickel manganese cobalt oxide (NMC) cathodes. While stable cycling is observed with low-voltage Li iron phosphate (LFP) cathodes, NMC|SPE|Li cells exhibit severe capacity fading, which becomes increasingly pronounced at higher cutoff voltages. Electrochemical and structural analyses reveal that this degradation is not primarily due to Al dissolution or bulk polymer oxidation but instead arises from increased charge-transfer resistance and morphological changes within the cathode. Rate-dependent cycling reveals that a significant portion of the lost capacity is recoverable at low current, indicating kinetic limitations, while the remaining loss is irreversible and linked to structural degradation of the cathode. Comparative experiments with liquid electrolyte-based cells at 70 °C reveal degradation features similar to those observed in SPE-based cells, indicating that the dominant failure mechanisms arise under combined thermal and electrochemical oxidation stress rather than from polymer-specific chemistry. These findings emphasize the need for thermally and interfacially robust SPE systems to support stable high-voltage operation in solid-state Li metal batteries.