Electronic structure, chemical bonding, and vibronic coupling in MnIV/MnIII mixed valent LixMn2O4 spinels and their effect on the dynamics of intercalated Li:: A cluster study using DFT

Density functional theory (DFT) calculations on the (Mn7O14)-O-IV and Mn-III/Mn-IV mixed valent Mn7O14- clusters are reported and used to characterize the Mn-O bonding in LixMn2O4 spinel. A recipe is proposed of how to extract from calculations on clusters electronic hopping and charge transfer (CT) energy parameters and to use them in a semiempirical (Hubbard type) Hamitonian which simultaneously accounts for electronic correlation and translational symmetry. The first application of this approach to Mn2O4 shows that, in contrast with conventional band calculations, the 3d electrons of Mn are rather localized. Vibronic coupling due to the 2t(2g)(3)2e(g)(1) configuration of Mn-III with the alpha(1g) and the Jahn-Teller(JT) active epsilon(g) modes is analyzed using DFT calculations of the mixed valent Mn7O14- cluster and is found to lead to a lengthening of the Mn-O bond and to tetragonally elongated octahedra accompanied by an appreciable stabilization energy(-0.445 eV), Vibronic coupling and electron hopping energies obtained from the DFT calculation are used to set up a simple model of small polaron for the Mn-III/Mn-IV spinels. The calculated energy barrier of electron transfer (0.34 eV, static JT-effect, dr ground state) agrees well with the one deduced from polaronic conductivity data on LiMn2O4 (0.4 eV). In the case of an energetic equivalency of the d(z2) and d(x2)-(y2) orbitals (such as LixMn2O4, x < 1, dynamic JT-effect), this model provides the possibility for electron transfer with a very low activation energy. The electronic transfer from Mn-III to Mn-IV in LixMn2O4 (0 < x much less than 1) is predicted to be fast and thus expected to assist the diffusion of Li+ through the interstitial spinel framework, The interaction of Li+ with the electrochemically active 2e(g)(Mn-III) electron does screen Li+ from large variation of the Madelung potential along its path from the octahedral 16c to the tetrahedral sa site and thus lowers the potential barrier for its diffusion through the spinel lattice.