Ab initio study of lithium transition metal fluorophosphate cathodes for rechargeable batteries

Abstract

Density functional theory (DFT) calculations using plane-wave methods were performed for Li₂T^MPO₄F, LiT^MPO₄F, and T^MPO₄F (T^M = V, Mn, Fe, Co, Ni) to address their feasibility as high-voltage cathode materials (>3.5 V relative to Li metal) for Li ion batteries. We computed their structures, average open circuit voltages, and thermal stabilities for step-wise lithiation/delithation (discharge/charge) reactions. The calculations suggest that associated unit cell volume changes are sufficiently small on average that they should not be a significant detriment to the mechanical stability of the cathode. In the nickel case, the calculated volume change deviates from the series by increasing during the first delithiation step. Furthermore, the volume increases for all these materials during the second delithiation step. The relative volume expansion in the series during delithiation appears related to the degree of mixing between metal d-orbtials and ligand p-orbitals. Predicted average open circuit voltages indicate that these Li-based transition metal fluorophosphates, most of which have yet to be successfully synthesized, are worthy of experimental pursuit as high-voltage cathodes, except for the lower predicted voltage for the reaction Li₂VPO₄F to LiVPO₄F. However, examination of the relative thermal stability of possible undesirable decomposition products such as LiF suggests that in these materials Li extraction efficiency and the reversibility may be low in all but the vanadium phases.