Revealing the Degradation Mechanism of LiMn xFe1- xPO4 by the Single-Particle Electrochemistry Method.

The commercial application of LiMn xFe1- 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 LiMn xFe1- 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 LiMn xFe1- xPO4 for the first time. Compared with a conventionally prepared cathode, the SP LiMn xFe1- xPO4 cathode shows prominent capacity-fading with cycle numbers, which can be attributed to the formation of the MnF2 nanocrystals on the surface of LiMn xFe1- xPO4 because of the reaction between F- and dissolved Mn2+ at the interface between the electrolyte and LiMn xFe1- xPO4. The different electrochemical behaviors can be ascribed to LiMn xFe1- 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 LiMn xFe1- 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.