Charge Localization and Transport in Lithiated Olivine Phosphate Materials

We report density functional theory (DFT) calculations for olivine-type (LiTPO4)-P-M and (TPO4)-P-M (T-M = Mn, Fe, Co, Ni) structures. TM is typically characterized in terms of localized 2+ or 3+ charge in (LiTPO4)-P-M and (TPO4)-P-M 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(TM)/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(TM) relative to dQ(O) or a dQ(TM)/dQ(O), value greater than 2 appears consistent with phases that would tend toward stable small polaron formation at T sites, such as mixed 2+/3+-like oxidation states. If the ratio of dQ(TM)/dQ(O) is smaller than 2 and the magnitudes of the charge differences are small, the system tends to behave more itinerant-like.