Cation ordering in Li[NixMnxCo(1-2x)]O2-layered cathode materials:: A nuclear magnetic resonance (NMR), pair distribution function, X-ray absorption spectroscopy, and electrochemical study

Several members of the compositional series Li[NixMnxCo(1-2x)]O-2 (0.01 <= x <= 1/3) were synthesized and characterized. X-ray diffraction results confirm the presence of the layered alpha-NaFeO2-type structure, while X-ray absorption near-edge spectroscopy experiments verify the presence of Ni2+, Mn4+, and Co3+. 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 Li-6 MAS NMR Li-6[Ni0.02Mn0.02Co0.96]O-2 spectrum, is consistent with the formation of Ni2+ and Mn4+ 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 Li[Ni1/3Mn1/3Co1/3]O-2 shows a tendency for Ni cations to be close to Mn cations in the first coordination shell; however, the Co3+ ions are randomly distributed. Analysis of the intensity of the LiCoO2 resonance, arising from Li surrounded by Co3+ 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 LiCoO2 resonance.