Vacancy-driven anisotropic defect distribution in LiFePO$_{4}$*

It has been reported that iron cations occupying Li sites (Fe$_{Li})$ in LiFePO$_{4}$ are locally aggregated rather than homogeneously distributed in the lattice.$^{1}$ Here we report a combination of density-functional calculations, statistical mechanics, electron-energy-loss spectra (EELS) and show the following. There is a strong binding energy between Fe$_{Li }$and a lithium vacancy (V$_{Li})$, leading to clustering of Fe$_{Li}$ along the b-axis, as observed, corresponding to the shortest separation of the Fe$_{Li }$-V$_{Li}$ pair. EELS data find that a small fraction of Fe atoms are Fe$^{3+}$, which can be accounted for in terms V$_{Li}$-Fe$_{Li}$-V$_{Li}$ clusters formed along the b-axis.