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 spectroscopies on LiMn 2 O 4, its chemically delithiated and lithiated derivatives (λ-MnO 2 and Li 2 Mn 2 O 4, respectively), and two Mn 3+ spinel model compounds. These experiments were undertaken to understand the associated changes in atomic and electronic structure occurring when LiMn 2 O 4 is used in a rechargeable lithium cell. Subtle changes in the Mn K-edge X-ray absorption near edge structure (XANES) occur upon delithiation that are consistent with literature reports of the oxidation of Mn 3+ to Mn 4+, retention of the cubic phase, and contraction of the spinel lattice. Conversely, when LiMn 2 O 4 is lithiated, the XANES changes dramatically due to the concurrent transformation from a cubic to a tetragonal spinel. The spectrum is different from XANES of tetragonal Mn 3+ spinels possessing approximately the same degree of tetragonal distortion as Li 2 Mn 2 O 4 . This spectral difference is attributed to the inserted Li + imparting an increased degree of covalency within the Li 2 Mn 2 O 4 structure resulting in a 1s → 4p + LMCT (ligand to metal charge transfer) 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 Li x Mn 2 O 4 when x > 1.