6Li magic angle spinning nuclear magnetic resonance study of the cathode materials Li1+αMn2-αO4-δ -: The effect of local structure on the electrochemical properties

The nature of the local environments in a lithium-excess lithium manganese spinel (Li1.05Mn1.95O4) and two spinel samples synthesized under conditions reported to create oxygen vacancies were investigated by using lithium magic angle spinning nuclear magnetic resonance (MAS NMR). The latter two samples were prepared by cooling Li1.05Mn1.95O4 in an N-2 atmosphere from 750degreesC and by quenching LiMn2O4 from high temperature (905degreesC), quenching resulting in a tetragonal distortion of the spinel unit cell. Additional Li-6 resonances, in addition to the resonance from the normal spinel tetrahedral local environment (at approximately 520 ppm) were observed for Li1.05Mn1.95O4, due to lithium in tetrahedral sites near Mn4+ ions (540-650 ppm), and lithium in the octahedral site (2260 ppm). Lithium doping on the octahedral site results in the formation of electron holes (i.e., Mn4+ ions) that appear to be localized near the Li-dopant and not randomly distributed throughout the lattice. Resonances at lower frequency of the main spinel peak are seen for the nominally oxygen-deficient materials due to tetrahedral lithium ions containing Mn3+ ions (i.e., localized electrons) in their local coordination sphere. No evidence for the Jahn-Teller-phase transition associated with charge ordering is seen for Li1.05Mn1.95O4. In contrast, both the reduced materials show NMR spectra characteristic with charge ordering, at low temperatures. The sample prepared by quenching, which contains the highest concentration of Mn3+ defects, shows a lower charge ordering temperature, the ordering process presumably being frustrated by the presence of random defects throughout the lattice. The lithium NMR resonances due to cations in the tetrahedral sites of Li1.05Mn1.95O4 gradually shift to higher frequency during charging, consistent with a gradual increase in manganese oxidation state. Lithium ions in the octahedral sites are not deintercalated. In contrast, three discrete resonances are observed for the spinel prepared in nitrogen as the charging proceeds, consistent with three different local environments. The absence of any long range ordering of the lithium ions in the lithium-excess material is ascribed to presence of the univalent Li cations, randomly distributed on the three-dimensional tunnel structure formed from the MnO6 octahedra. (C) 2001 The Electrochemical Society.