Local electronic structure of olivine phases of Li_xFePO ₄

Changes in the local electronic structure at atoms around Li sites in the olivine phase of LiFePO₄ were studied during delithiation. Electron energy loss spectrometry was used for measuring shifts and intensities of the near-edge structure at the K-edge of O and at the L-edges of P and Fe. Electronic structure calculations were performed on these materials with a plane-wave pseudopotential code and with an atomic multiplet code with crystal fields. It is found that both Fe and O atoms accommodate some of the charge around the Li⁺ ion, evidently in a hybridized Fe-O state. The O 2p levels appear to be fully occupied at the composition LiFePO₄. With delithiation, however, these states are partially emptied, suggestive of a more covalent bonding to the oxygen atom in FePO₄ as compared to LiFePO₄. The same behavior is found for the white lines at the Fe L_2,3-edges, which also undergo a shift in energy upon delithiation. A charge transfer of up to 0.48 electrons is found at the Fe atoms, as determined from white line intensity variations after delithiation, while the remaining charge is compensated by O atoms. No changes are evident at the P L _2,3-edges.