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), Li-7 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 similar to 8 ppm observed in the Li-7 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.