Irreversible phase transition between LiFePO₄ and FePO₄ during high-rate charge-discharge reaction by operando X-ray diffraction

LiFePO₄ is a practically used cathode material for lithium-ion batteries due to a high theoretical capacity, high cycle capability and the high-rate performance. The metastable Li_xFePO₄ (L_xFP) phase with an intermediate composition appears in the non-equilibrium state at high-rate condition. However, the formation process of the metastable L_xFP phase and its impact to the electrochemical property are still unclear. In order to elucidate these points, we directly observed the phase transition behavior by applying operando XRD during 10C charge-discharge. L_xFP phase does not form in charge reaction but preferentially forms in discharge reaction. The phase transition from L_xFP to Li-rich phase is less likely to proceed in the end of discharge reaction. The asymmetric phase transition between LiFePO₄ and FePO₄ results in decreasing the discharge capacity and increasing the irreversible capacity at high-rate conditions.