Transition-Metal Mixing and Redox Potentials in Li_x(M_1-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 LiMn_1-yFe_yPO₄, LiFe_1-yNi_yPO₄, and LiMn_1-yNi_yPO₄, 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 Li_xM′PO₄ host materials. Unmixed Li_xMnPO₄ 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 (Li_xMn_1-yFe_yPO₄) but not others (Li_xMn_1-yNi_yPO₄).