Charge Localization and Transport in Lithiated Olivine Phosphate Materials

We report density functional theory (DFT) calculations for olivine-type LiT M PO 4 and T M PO 4 (T M = Mn, Fe, Co, Ni) structures. T M is typically characterized in terms of localized 2+ or 3+ charge in LiT M PO 4 and T M PO 4 structures, respectively, in which electron transport would take place by thermally activated hopping of metal-centered small polarons. We assess qualitative relationships between formal T M oxidation states and the static charges on T M and the dependence of small polaron stability on d–p rehybridization. We propose an empirical correlation between the ratio of cationic/anionic charge differences ( dQ T M / dQ O ) for lithiated and delithiated end members and the relative degree of electron localization on the cationic versus anionic sublattices. Large values of dQ T M relative to dQ O or a dQ T M / dQ O value greater than 2 appears consistent with phases that would tend toward stable small polaron formation at T M sites, such as mixed 2+/3+-like oxidation states. If the ratio of dQ T M / dQ O is smaller than 2 and the magnitudes of the charge differences are small, the system tends to behave more itinerant-like.