Observation of Preferential Cation Doping on the Surface of LiFePO₄ Particles and Its Effect on Properties

This study investigates a series of cation dopants for LiFePO4 that are reported to be beneficial for rate performance. A solid-state synthesis has been used to be consistent with a common processing method used in prior doping investigations and in commercial processes for this compound. Increased ratios of Fe3+/Fe2+ oxidation on the particle surfaces have been determined using X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (sXAS) for the doped LiFePO4, for all chosen dopants and valences (<, = or >2+), while the cores remain much closer to the characteristics of the pristine (undoped) material. These results indicate that the dopants are predominantly pushed to the particle surfaces during phase formation, even when the dopants are added at the initial fine mixing of precursors. The differential distribution of dopants between the cores and surface layers of the particles results in conductivity improvements and reduction of polarizations in electrochemical impedance spectroscopy measurements of the doped LiFePO4.