X-Ray Photoelectron Spectroscopy investigations of carbon-coated Li_xFePO₄ materials

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 LixFePO4 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