Electronic structure of chemically-prepared LixMn2O4 determined by Mn X-ray absorption and emission spectroscopies

We have performed Mn K-edge X-ray Absorption and Mn L-edge emission spectro scopies on LiMn2O4, its chemically delithiated and lithiated derivatives (l ambda -MnO2 and Li2Mn2O4, respectively), and two Mn3+ spinel model compound s. These experiments were undertaken to understand the associated changes i n atomic and electronic structure occurring when LiMn2O4 is used in a recha rgeable lithium cell. Subtle changes In the Mn K-edge X-ray absorption near edge structure (XANES) occur upon delithiation that are consistent with li terature reports of the oxidation of Mn3+ to Mn4+, retention of the cubic p hase, and contraction of the spinel lattice. Conversely, when LiMn2O4 is li thiated, the XANES changes dramatically due to the concurrent transformatio n from a cubic to a tetragonal spinel. The spectrum is different from XANES of tetragonal Mn3+ spinels possessing approximately the same degree of tet ragonal distortion as Li2Mn2O4. This spectral difference is attributed to t he inserted Li+ imparting an increased degree of covalency within the Li2Mn 2O4 structure resulting in a Is --> 4p + LMCT (ligand to metal charge trans fer) shakedown. This increase in covalency was confirmed through Mn L-edge X-ray Emission Spectroscopy measurements. The increased degree of covalency provides insight into the lower Li+ diffusion coefficients reported in the literature and the electronic conduction mechanism for LixMn2O4 when x > 1 .