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Charge Localization and Transport in Lithiated Olivine Phosphate Materials

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journal contribution
posted on 22.02.2016, 10:55 by Jianguo Yu, Kevin M. Rosso, Jun Liu
We report density functional theory (DFT) calculations for olivine-type LiTMPO4 and TMPO4 (TM = Mn, Fe, Co, Ni) structures. TM is typically characterized in terms of localized 2+ or 3+ charge in LiTMPO4 and TMPO4 structures, respectively, in which electron transport would take place by thermally activated hopping of metal-centered small polarons. We assess qualitative relationships between formal TM oxidation states and the static charges on TM and the dependence of small polaron stability on d–p rehybridization. We propose an empirical correlation between the ratio of cationic/anionic charge differences (dQTM/dQO) for lithiated and delithiated end members and the relative degree of electron localization on the cationic versus anionic sublattices. Large values of dQTM relative to dQO or a dQTM/dQO value greater than 2 appears consistent with phases that would tend toward stable small polaron formation at TM sites, such as mixed 2+/3+-like oxidation states. If the ratio of dQTM/dQO is smaller than 2 and the magnitudes of the charge differences are small, the system tends to behave more itinerant-like.

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