Transition-Metal Mixing and Redox Potentials in Lix(M1-yM′y)PO4 (M, M′ = Mn, Fe, Ni) Olivine Materials from First-Principles Calculations

The performance of olivine cathode materials can be improved using core/shell structures such as LiMnPO4/LiFePO4 and LiMnPO4/LiNiPO4. We use density functional theory to calculate the energetics, phase stability, and voltages of transition-metal mixing for a series of olivine phosphate materials. For LiMni(1-y)Fe(y)PO(4), LiFe1-yNiyPO4, and LiMn1-yNiyPO4, we find phase-separating tendencies with (mean-field) maximum miscibility gap temperatures of 120, 320, and 760 K respectively. At room temperature, we find that Mn is completely miscible in LiFePO4, whereas Mn solubility in LiNiPO4 is just 0.3%. Therefore, we suggest that core/shell LiMnPO4/LiNiPO4 particles could be more effective at containing Mn in the particle core and limiting Mn dissolution into the electrolyte relative to LiMnPO4/LiFePO4 particles. We calculate shifts in redox potentials for dilute transition metals, M, substituted into LixM'PO4 host materials. Unmixed LixMnPO4 exhibits a redox potential of 4.0 V, but we find that dilute Mn in a LiNiPO4 shell exhibits a redox potential of 4.3 V and therefore remains redox inactive at lower cathode potentials. We find that strain plays a large role in the redox potentials of some mixed systems (LixMn1-yFeyPO4) but not others (LixMn1-yNiyPO4).