First-principles evaluation of the inherent stabilities of pure Li_xMPO₄ (M=Mn, Fe, Co,) and mixed binary Li_xFe_yM'_1-yPO₄ (M'=Mn, Co) olivine phosphates

The inherent stabilities of pure and mixed transition metal olivine phosphates of Li_xMPO₄ (M = Mn, Fe, Co) and various Li_xFe_yM'_1-yPO₄ (M^' = Mn, Co) compositions were evaluated as a function of the transition metal, y, and lithium content, x. In the pure compounds, Li_xMPO₄, the delithiation process is energetically more favorable for Fe than for Mn and Co, in agreement with available experimental data. The possible formation of solid solutions of partially delithiated mixed olivine phosphates was evaluated as well. The results show that the stability of the solid solution relative to the two end-phases (i.e. The fully lithiated and fully delithiated materials), depends on both the amount of lithium, x, and the transition metal composition, y. In the case of LiFePO₄ and LiMnPO₄ the phase separated material appears to be the most stable whereas for LiCoPO₄, the solid solution is most stable. Interestingly, a highly complex stability pattern emerges for the mixed olivines, and this pattern is governed by the transition metal composition and the lithiation state. In particular, for the mixed olivines we find correlation between the stability patterns and the electronic structure of the transition metals as function of the lithiation state.