Changes in Electronic Structure upon Lithium Insertion into Fe2(SO4)3 and Fe2(MoO4)3 Investigated by X-ray Absorption Spectroscopy

Changes in electronic structure upon electrochemical lithium insertion into two iron compounds, namely, rhombohedral Fe2(SO4)3 with a NASICON-type structure and monoclinic Fe2(MoO4)3, were investigated using X-ray absorption spectroscopy (XAS). Fe K-edge and LIII- and L II-edge XAS revealed that the rearrangement of Fe d electrons or rehybridization of Fe d−O p bonding took place accompanied by the reduction of Fe ions upon Li insertion for both samples and that a larger change in spectra was observed in Fe2(SO4)3. In addition, the changes in the electronic structure of the polyanion units XO4 2- (X = S or Mo) after Li insertion were also investigated by O K-edge and S K-edge or Mo LIII-edge XAS. The results indicated that the electronic structure around oxygen markedly changed in Fe2(MoO4)3, while no significant change was observed in Fe2(SO4)3.