Phase separation in Li x FePO 4 induced by correlation effects

We report on a significant failure of the local density approximation (LDA) and the generalized gradient approximation (GGA) to reproduce the phase stability and thermodynamics of mixed-valence LixFePO4 compounds. Experimentally, LixFePO4 compositions (0less than or equal toxless than or equal to1) are known to be unstable and phase separate into LiFePO4 and FePO4. However, first-principles calculations with LDA/GGA yield energetically favorable intermediate compounds and hence no phase separation. This qualitative failure of LDA/GGA seems to have its origin in the LDA/GGA self-interaction which delocalizes charge over the mixed-valence Fe ions, and is corrected by explicitly considering correlation effects in this material. This is demonstrated with LDA+U calculations which correctly predict phase separation in LixFePO4 for U-Jgreater than or similar to3.5 eV. The origin of the destabilization of intermediate compounds is identified as electron localization and charge ordering at different iron sites. Introduction of correlation also yields more accurate electrochemical reaction energies between FePO4/LixFePO4 and Li/Li+ electrodes.