Because of its surface sensitivity, X-ray photoelectron spectroscopy allowed us to access the mechanisms at the very surface of 100 nm diameter carbon-coated LiFePO₄ nanoparticles. A continuous evolution of the Fe³⁺/Fe²⁺ ratio was observed at the surface of the particles upon charge and discharge, in good agreement with the change in the average lithium content of Li_xFePO₄ electrode material. These results support these models considering migration of a reaction front along the a-axis inside the cristallites vs. the shrinking-core model, to describe lithium (de)intercalation in LiFePO₄. XPS analyses showed also that electrode/electrolyte interface phenomena are minimized using carbon-coated LiFePO₄ particles as positive electrode material vs LiCoO₂ for instance, despite the nanometric size of the particles.