Parasitic High-Voltage Effects on the Degradation of LiMn _0.75 Fe _0.25 PO ₄ Cathodes in Li-Ion Batteries

LiMnxFe1–xPO4 (LMFP) is a promising cathode material owing to its higher energy density compared to commercialized LiFePO4 and its compatibility with layered oxides (e.g., LiNixCoyMn1–x–yO2 (NCM)) in blend cathode systems. However, when combined with NCM, LMFP is inevitably subjected to a broad range of upper cutoff voltages, under which its degradation behavior remains insufficiently understood. Here, we investigate the pivotal influence of high cutoff voltage on the degradation mechanisms of the LiMn0.75Fe0.25PO4/C (LMFP75) electrode. Through comparative analysis of cathodes cycled to 4.2 and 4.5 V cutoff voltages, we observe significantly accelerated degradation under the higher voltage condition. Electrochemical analyses, combined with structural characterization and solution nuclear magnetic resonance spectroscopy of electrolyte decomposition products, reveal that interfacial side reactions between the LMFP75 surface and the electrolyte are the primary causes of voltage-dependent degradation. These reactions lead to the loss of the conductive carbon coating and the emergence of an amorphous surface layer on the cathode material, thereby increasing the electrode’s charge-transfer resistance and promoting cell degradation. Our findings elucidate the mechanisms governing LMFP degradation under high-voltage operation and provide practical guidelines for enhancing the durability of LMFP-based batteries.