LiFePO4 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 LiFePO4 // graphite cell after 200 cycles at RT and 60 degrees C was followed using X-Ray Photoelectron Spectroscopy (XPS). The results have revealed a lack of reversibility of the Fe3+/Fe2+ 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. (C) 2012 The Electrochemical Society. [DOI: 10.1149/ 2.024204jes] All rights reserved.