Aging Mechanisms of LiFePO₄// Graphite Cells Studied by XPS: Redox Reaction and Electrode/Electrolyte Interfaces

LiFePO 4 has monopolized a lot of research efforts over the past decade because it stands as an attractive positive electrode material for the next generation of Li-ion batteries to EVs/HEVs. However, issues concerning aging mechanisms need to be solved before large scale industrial production. In order to get more insight in such mechanisms, the evolution of both electrodes of a LiFePO 4 // graphite cell after 200 cycles at RT and 60°C was followed using X-Ray Photoelectron Spectroscopy (XPS). The results have revealed a lack of reversibility of the Fe 3+ /Fe 2+ redox reaction increasing upon cycling and at high temperature. In depth analyses of interfacial layers on both electrodes allowed us to further elucidate the origin of the detrimental loss of active lithium with the formation of small amounts of lithiated species either deposited or partly dissolved in the electrolyte.