Ti-, Al-, and Cu-doping induced gap states in LiFePO4

We studied the changes in the electronic structure of LiFePO4 induced by Ti-, Al-, and Cu-doping. The technique used in the study was Fe 2p and O 1s X-ray absorption (XAS) spectroscopy. XAS spectra were analyzed using atomic multiplet and band structure calculations. Fe 2p XAS spectra show that the Fe ions in LiFePO4 are in a divalent Fe2+ high-spin state, and that the Fe ions remain in this Fe2+ state in the Ti-, Al-, and Cu- doped materials. O 1s XAS spectra present relatively weak transitions to mixed O 2p-Fe 3d states at threshold (the transitions to O 2p-Fe 3sp states appears at higher energies due to the larger hybridization). O 1s XAS spectra of the doped materials show additional absorption intensity just below threshold. These features correspond to transitions to doped states in the bandgap of the LiFePO4 compound. (c) 2005 The Electrochemical Society.