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

摘要

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 alpha-NaFeO2-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 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_(0.02)Mn_(0.02)Co_(0.96)]O2 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 Li[Ni_(1/3)Mn_(1/3)Co_(1/3)]O2 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"LiCoO2"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"LiCoO2 resonance.