Cations Distribution and Valence States in Mn-Substituted Li4Ti5O12 Structure

Structure, cation distribution, Mn oxidation states, and conductivity behavior of the Mn-substituted (up to 30% of Ti ions) Li4Ti5O12 have been investigated by the combined use of X-ray powder diffraction, electron paramagnetic resonance (EPR), 7Li MAS NMR, and impedance spectroscopy techniques. The spinel structure of the lithium titanate is preserved and the lattice parameter decreases with increasing the Mn content. Mn2+ ions progressively occupy the tetrahedral site up to an approximately constant value reached for 10% Mn-substituted samples. Mn3+ ions occupy both octahedral and tetrahedral sites, with a constant value on the tetrahedral one, independent of the total Mn amount; Mn4+ ions are not detected. The Mn2+ paramagnetic ions give rise to a through-space interaction with Li+ ions of both cationic sites, as evaluated by the area, proportional to the Mn2+ ions content, of a peak at ∼8 ppm observed in the 7Li NMR spectra for the substituted samples. The obtained cation distribution and the Mn valence states satisfactorily explain the decrease of conductivity observed in the Mn-doped samples with respect to the pure Li4Ti5O12