[sup 6]Li Magic Angle Spinning NMR Study of the Cathode Material LiNi[sub x]Mn[sub 2−x]O[sub 4]: The Effect of Ni Doping on the Local Structure during Charging

6 Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to study the local structure of LiNi x Mn 2 − x O 4 ( 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 Mn 3 + and Mn 4 + 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 Mn 4 + ions are present, and only one major resonance, due to the lithium local environment Li ( ONi 2 + ) 3 ( OMn 4 + ) 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 / δ ( δ = 6 Li NMR shift) vs. temperature is consistent with the predominance of antiferromagnetic correlations for low doping level samples, while Ni 2 + – O – Mn 4 + antiferromagnetic correlations clearly dominate the behavior of the x = 0.5 sample, resulting in overall ferrimagnetic behavior. 6 Li MAS NMR spectra were also collected following various levels of charging. In the case of LiNi 0.1 Mn 1.9 O 4 , Li + is deintercalated from the different local sites 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 sites containing progressively more Mn 4 + ions. In contrast, for LiNi 0.5 Mn 1.5 O 4 , where the deintercalation process involves oxidation of Ni 2 + 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.