Cation ordering in Li[Ni_xMn_xCo_(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[Ni_xMn _xCo_(1-2x)]O₂ (0.01 ≤ x ≤ 1/3) were synthesized and characterized. X-ray diffraction results confirm the presence of the layered α-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 ⁶Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and neutron pair distribution function (PDF) analysis, associated with reverse Monte Carlo (RMC) calculations. The 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 ⁶Li MAS NMR ⁶Li[Ni _0.02Mn_0.02Co_0.96]O₂ 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[Ni_1/3Mn_1/3Co _1/3]O₂ 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 "LiCoO ₂" 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 insulatorto-metal transition seen in the electrochemical profiles of these materials upon charging correlates strongly with the concentration of the "LiCoO₂" resonance.