Ionic conductivity of lithium in spinel-type Li4/3Ti5/3O4–LiMg1/2Ti3/2O4 solid-solution system

Spinel type Mg-doped lithium titanium oxides of Li(4 − 2x)MgxTi(5 − x) / 3O4 has been investigated in terms of the relationship between the crystal structure and the ionic conductivity by both computational and experimental techniques. Reduction of the energy barrier for Li+ hopping was indicated by the computations based on the first-principles density functional theory (DFT) and long-range Coulombic interactions by doping Mg2+ into tetrahedral sites of spinels, including increase of ionic conductivity of Li+. However, the experimental measurements for activation energies and ionic conductivities of Li+ were opposite to the expectation from computational study. One of the reasons for the discrepancy was neglecting defect formation energy in the spinel Li4/3Ti5/3O4 in the computations, which was supported by computations assuming Schottky-like Li vacancy formation. In addition, severe increase of activation energy was indicated by doping Mg2+ ions at the composition range, x > 0.4 in experiments. The anomalous increase of activation energy was discussed by adopting percolation theory.