Local Electronic Structure of Olivine Phases of Li_xFePO_4

Abstract

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 LiFePO_4. With delithiation, however, these states are partially emptied, suggestive of a more covalent bonding to the oxygen atom in FePO_4 as compared to LiFePO_4. 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.

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