Revealing the Degradation Mechanism of LiMn_<i>x</i>Fe_1–<i>x</i>PO₄ by the Single-Particle Electrochemistry Method

Abstract

The commercial application of LiMn _xFe_{1- x}PO₄ 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 _xFe_{1- x}PO₄ 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 _xFe_{1- x}PO₄ for the first time. Compared with a conventionally prepared cathode, the SP LiMn _xFe_{1- x}PO₄ cathode shows prominent capacity-fading with cycle numbers, which can be attributed to the formation of the MnF₂ nanocrystals on the surface of LiMn _xFe_{1- x}PO₄ because of the reaction between F^- and dissolved Mn²⁺ at the interface between the electrolyte and LiMn _xFe_{1- x}PO₄. The different electrochemical behaviors can be ascribed to LiMn _xFe_{1- x}PO₄ 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 _xFe_{1- x}PO₄, 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.