Insights of the voltage decay mechanisms of LiMn_0.7Fe_0.3PO₄ cathodes over cycling

Olivine phosphates (LiMn_xFe_1-xPO₄) are regarded as promising cathode candidates for Li-ion batteries (LIBs) with high energy density and low cost. However, voltage decay significantly compromises the energy density and cycle life. Understanding the mechanism of electrochemical performance degradation in cathodes is crucial to developing cathodes for Li-ion batteries. Here, taking LiMn_0.7Fe_0.3PO₄ as the model cathode, the mechanisms of electrochemical performance degradation in different operating voltage windows are comparatively investigated from the aspect of crystal structure change. We found that different charge and discharge cutoff voltages had the greatest impact on the Mn³⁺/Mn²⁺ plateau, and the loss of capacity contributed by the Mn³⁺/Mn²⁺ plateau did not correspond to the loss of overall capacity, and the deteriorated kinetics of Li⁺ diffusion in the Mn³⁺/Mn²⁺ plateau were the reason for the voltage decay. Quantitative analyses combining the X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques reveal that cycling at a larger operating voltage range leads to bulk structure degradation, especially lattice expansion in the b-axis direction, and becomes the main reason for the voltage decay. This work demonstrates providing good references to improve the energy density and cycling performance of LiMn_xFe_1-xPO₄.