Systematic First-Principles Investigation of Mixed Transition Metal Olivine Phosphates LiM_1‑yM′_yPO₄ (M/M′ = Mn, Fe, and Co) as Cathode Materials

Li based batteries are widely used for powering mobile electronic equipment and are considered as highly promising power sources for electrical propulsion. Recent developments in the area of rechargeable lithium ion batteries for electronic devices and transportation have stimulated extensive studies of cathode materials which are the limiting factor in terms of voltage and energy density. In the current work we present a systematic computational study of the geometry, electronic structure, and electrochemical potential for olivines Li_xM_1‑yM′_yPO₄ (M/M′ = Mn, Fe, Co; x = 0.00, 1.00, x = y; y = 0.00, 0.25, 0.50, 0.75, and 1.00). We find that changes in cell volume as a function of transition metal composition may largely be ascribed to changes in the atomic volumes of the oxygen atoms, which modulate the electron charge distribution. Moreover, there is considerable charge transfer from lithium to the transition metal ions and oxygen atoms upon lithiation for all systems studied. The calculated cell potentials are in good agreement with experiment for all systems, and show systematic shifts in redox potential with varying transition metal composition. We also correlate between the highest occupied molecular orbital (HOMO) energies of model transition metal complexes and the redox potentials of the pristine LiMPO₄ materials. Furthermore, we estimate the delithiation energy of LiCoPO₄ and LiFePO₄. We find that fully delithiated LiCoPO₄ is highly unstable in agreement with the experimental observation that LiCoPO₄ cannot be fully delithiated electrochemically.