MAS NMR Study of the Metastable Solid Solution Found in the LiFePO₄/FePO₄ System

^6,7Li and ³¹P NMR experiments were conducted on a series of single- or two-phase samples in the LiFePO₄−FePO₄ system with different overall lithium contents, and containing the two end-members and/or two metastable solid solution phases, Li_0.6FePO₄ or Li_0.34FePO₄. These experiments were carried out at different temperatures in order to search for vacancy/charge ordering and ion/electron mobility in the metastable phases. Evidence for Li⁺−Fe²⁺ interactions was observed for both Li_0.6FePO₄ and Li_0.34FePO₄. The strength of this interaction leads to the formation of LiFePO₄-like clusters in the latter, as shown by the room temperature data. Different motional processes are proposed to exist as the temperature is increased and various scenarios are discussed. While concerted lithium-electron hopping and/or correlations explains the data below 125 °C, evidence for some uncorrelated motion is found at higher temperatures, together with the onset of phase mixing.