6Li magic angle spinning NMR study of the cathode material LiNixMn2-xO4 -: The effect of Ni doping on the local structure during charging

Li-6 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to study the local structure df LiNixMn2-xO4 (x = 0.05,0.1,0.3,0.5) synthesized by solid-state reactions. When the extent of doping is small (x = 0.05 and 0.1), several resonances are observed, which are assigned to lithium local environments containing different numbers of Mn3+ and Mn4+ ions. As the doping level increases and the average manganese oxidation state rises, the intensity of the resonances assigned to lithium environments containing a greater number of higher-oxidation state manganese ions increases; this results in a gradual shift of the center of mass of the spectrum to higher frequency. By x = 0.5, only Mn4+ ions are present, and only one major resonance, due to the lithium local environment Li(ONi2+)(3)(OMn4+)(9) is observed; this is consistent with Mn/Ni ordering on the octahedral sites of the spinel structure. The variable temperature NMR behavior of these samples is indicative of different magnetic behavior for the undoped and doped materials. A plot of 1/delta (delta = Li-6 NMR shift) vs. temperature is consistent with the predominance of antiferromagnetic correlations for low doping level samples, while Ni2+-O-Mn4+ antiferromagnetic correlations clearly dominate the behavior of the x = 0.5 sample, resulting in overall ferrimagnetic behavior. Li-6 MAS NMR spectra were also collected following various levels of charging. In the case of LiNi0.1Mn1.9O4, Li+ is deintercalated from the different local sires sequentially: lithium ions in the local environment containing manganese ions with an average oxidation state of +3.5 are deintercalated first, followed by lithium ions in sires containing progressively more Mn4+ ions. In contrast, for LiNi0.5Mn1.5O4, where the deintercalation process involves oxidation of Ni2+ only, no changes in the lithium local environments were observed during the charging process: the shift position remained constant during the charging cycle and only a loss of the intensity of the lithium signal was observed. (C) 2001 The Electrochemical Society. All rights reserved.