6Li and 7Li magic-angle spinning nuclear magnetic resonance and in situ X-ray diffraction studies of the charging and discharging of LixMn2O4 at 4 V

Li-6 and Li-7 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and in situ X-ray diffraction (XRD) have been used to study lithium manganate cathode materials (LixMn2O4, 0 < x less than or equal to 1) during and following charging and discharging. Only one major local environment is observed by 6Li MAS NMR from 0 to 1 50% charging from lithium in the tetrahedral sites of the spinel structure, the resonance shifting by no more than 8 ppm in this range (from its original position at ca. 520 ppm). When the cell is charged above 50%, a new resonance is observed at approximate to 645 ppm, due to Li+ in a spinel local environment with nearby manganese ions in oxidation states close to +4 (e.g., in a composition such as Li0.1Mn2O4). The two resonances at approximate to 520 and 645 ppm are both observed in the range 70 to <80 and 80 to 90% for the samples annealed at 850 and 650 degrees C, respectively. These coexistences occur over the same ranges as the two-phase coexistence, as observed by XRD during the first charging cycle. Resonances were observed at 830 and 930 ppm,,for the samples annealed at 650 and 850 degrees C. respectively, at greater than or equal to 90% charging. These same resonances were seen after multiple charging cycles and are assigned to defect spinels with high manganese oxidation states. Additional resonances are observed for electrodes with high carbon contents at -2 to -13 ppm, from nonspinel phases, which grow in intensity with the number of charging cycles. The presence of the defect spinels and the additional impurity phases contribute to the reduced capacity of these materials. Three-phase behavior is observed by in situ XRD during the first discharging cycle. These results are discussed in terms of previous in situ XRD charging/discharging results. (C) 2000 The Electrochemical Society. S0013-4651(99)05-031-4. All rights reserved.