Electronic Structure of Chemically-Prepared Li_xMn₂O₄ Determined by Mn X-ray Absorption and Emission Spectroscopies

Abstract

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 f 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.

Characterizations on the structural and electronic properties of thermal lithiated Li0.33MnO2

“…A stronger signal of "A" in Li 0.92 MnO 2 could be attributed to the more severely distorted [MnO 6 ] octahedron, because the distortion facilities the LMCT and hence the shakedown process. In addition, it was reported that the distortion of the [MnO 6 ] octahedron also facilitates the feature "A" shifting towards lower energies [20]. In this study, the energy positions of "A" were determined at 6550 eV and 6548 eV for the as-prepared Li 0.33 MnO 2 and lithiated Li 0.92 MnO 2 , respectively, which is concordant to the increase of the [MnO 6 ] distortion with Li intercalation.…”

Jahn–Teller distortion and electronic structure of LiMn2O4

“…However, the experiment observed semiconducting characteristics of the LiMn 2 O 4 compound is not in agreement with recent first principles calculated electronic structures [10][11][12][13], which are shown to be metallic, and Mn ions in LiMn 2 O 4 are Mn 3.5+ . This is also contradictory to the X-ray absorption spectroscopy and neutron diffraction results [14][15][16]. Those experimental results strongly support that Mn ions have mixed valence state of Mn 3+ and Mn 4+ , and even the charge ordering of Mn 3+ and Mn 4+ is roughly determined [16].…”

Spectroscopic studies of the structural properties of Ni substituted spinel LiMn2O4

“…The XANES main peak (marked by ''C'') is assigned as the dipole allowed 1s Y 4p transition. The energy position of the peak ''C'' for the non-substituted LiMn 2 O 4 was 6561.2 eV, which is the identical value of that of stoichiometric LiMn 2 O 4 [26]. The peak ''C'' shifted towards 6561.…”