Operando Monitoring of Stress Evolution in LiMn _x Fe _1‐x PO ₄ Cathodes: Deciphering the Origin of Abnormal Discharge Plateau

ABSTRACT Lithium manganese iron phosphate (LiMn x Fe 1x PO 4 , LMFP) is regarded as a promising cathode material for lithium‐ion batteries, owing to its higher operating voltage than LiFePO 4 and better safety features than layered oxide cathodes. Nevertheless, the emergence of an abnormal discharge voltage plateau under high‐rate conditions compromises its performance in fast‐charging applications. Further investigation of the fading mechanism and effective solution strategies is needed. In this study, an operando stress‐sensing cathode is developed, in which electrochemically induced stress originating from the lattice variation of active materials is directly transduced via an integrated optical fiber. The monitoring results reveal a distinct stress‐voltage derivative peak and a non‐linear stress evolution during discharge. These signatures confirm that the abnormal plateau originates from kinetic‐induced stress accumulation triggered by non‐equilibrium lithiation of the Mn‐rich phase. This stress buildup increases the energy barrier for Li + insertion and reduces the overall kinetics. Furthermore, by synthesizing LMFP particles with enhanced Li + transport kinetics, the stress concentration in LMFP is alleviated, and the abnormal plateau is effectively eliminated, leading to enhanced rate capability and cycling stability. This work deciphers the chemo‐mechanical origin of the voltage anomaly in LMFP by operando stress monitoring and provides a practical pathway toward high‐performance cathode design.