Revealing the Degradation Mechanism of LiMn_xFe_1–xPO₄ by the Single-Particle Electrochemistry Method

The commercial application of LiMnxFe1–xPO4 materials has always been a great challenge because of their unsatisfactory structure stability during cycling and the safety issue. Herein, single-particle (SP) electrodes, where aggregated LiMnxFe1–xPO4 is dispersed into SPs so they can distribute homogeneously in the carbon-nanotube networks, have been prepared and characterized to probe the degradation mechanism of LiMnxFe1–xPO4 for the first time. Compared with a conventionally prepared cathode, the SP LiMnxFe1–xPO4 cathode shows prominent capacity-fading with cycle numbers, which can be attributed to the formation of the MnF2 nanocrystals on the surface of LiMnxFe1–xPO4 because of the reaction between F– and dissolved Mn2+ at the interface between the electrolyte and LiMnxFe1–xPO4. The different electrochemical behaviors can be ascribed to LiMnxFe1–xPO4 SPs surface reconstruction with MnF2 nucleation and growth by the interfacial reactions. In addition, by applying a thin protecting layer of Al2O3 on the surface of LiMnxFe1–xPO4, the interfacial side reactions can be suppressed. This work demonstrates that the SP method is a powerful tool to extract the information of interfacial reactions, which sometimes appear to be negligible compared with bulk reactions.