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Several members of the compositional series LiNi x Mn x Co (1–2 x ) O 2 (0.01 ≤ x ≤ 1/3) were synthesized and characterized. X-ray diffraction results confirm the presence of the layered α-NaFeO 2 -type structure, while X-ray absorption near-edge spectroscopy experiments verify the presence of Ni 2+, Mn 4+, and Co 3+ . Their local environment and short-range ordering were investigated by using a combination of 6 Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and neutron pair distribution function (PDF) analysis, associated with reverse Monte Carlo (RMC) calculations. The 6 Li MAS NMR spectra of compounds with low Ni/Mn contents ( x ≤ 0.10) show several well-resolved resonances, which start to merge when the amount of Ni and Mn increases, finally forming a broad resonance at high Ni/Mn contents. Analysis of the 6 Li MAS NMR 6 LiNi 0.02 Mn 0.02 Co 0.96 O 2 spectrum, is consistent with the formation of Ni 2+ and Mn 4+ clusters within the transition-metal layers, even at these low-doping levels. The oxidation state of Ni in this high Co content sample strongly depends upon the Li/transition metal ratio of the starting materials. Neutron PDF analysis of the highest Ni/Mn content sample LiNi 1/3 Mn 1/3 Co 1/3 O 2 shows a tendency for Ni cations to be close to Mn cations in the first coordination shell; however, the Co 3+ ions are randomly distributed. Analysis of the intensity of the “LiCoO 2 ” resonance, arising from Li surrounded by Co 3+ in its first two cation coordination shells, for the whole series provides further evidence for a nonrandom distribution of the transition-metal cations. The presence of the insulator-to-metal transition seen in the electrochemical profiles of these materials upon charging correlates strongly with the concentration of the “LiCoO 2 ” resonance.
Zeng et al. (Thu,) studied this question.