New Mechanisms of Phase Transition in Olivine-Type LixMn0.7Fe0.3PO4 Cathodes: a Finding on Relaxation Behavior and its Implications for Battery Performance

Abstract

Phosphates of the olivine type, LiMn_yFe_1-yPO₄ (LMFP), have garnered significant attention due to their higher energy density compared to LiFePO₄ (LFP). However, their limited cycle life and rate performance remain key obstacles to their commercialization. Therefore, elucidating the intricate phase transition mechanisms during electrochemical cycling is paramount to overcoming this bottleneck. This study investigates the relaxation behavior of Li_xMn_0.7Fe_0.3PO₄ (0≤x≤1) under various conditions revealed a remarkable memory characteristic in its crystal structure: the lattice parameters of different delithiated states return to their fully lithiated configuration upon complete relaxation. Moreover, the relaxation rate is influenced by the charging/discharging conditions and storage environment: higher rates, greater delithiation, and longer relaxation times are observed, as are extended relaxation time at lower temperatures and in the absence of electrolyte. These findings provide a rational explanation for the complexity of the phase transition mechanisms in LMFP and highlight the significant differences in phase transition and relaxation behaviors, advancing the understanding of LMFP materials.