Operando Monitoring of Stress Evolution in LiMn_xFe_1-xPO₄ Cathodes: Deciphering the Origin of Abnormal Discharge Plateau

Lithium manganese iron phosphate (LiMn_xFe_1xPO₄, LMFP) is regarded as a promising cathode material for lithium-ion batteries, owing to its higher operating voltage than LiFePO₄ 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.