7Li NMR study on Li+ ionic diffusion and phase transition in LixCoO2

The temperature dependence of the spin-lattice relaxation time, T, and the line width of the Li-7 nucleus were measured in delithiated LixCoO2 (x=0.6, 0.8, 1.0). Two different relaxation behaviors were observed in the temperature dependence of T-l(-1) in a x=0.8 sample. These would have arisen from inequivalent Li sites in two coexisting phases; an original hexagonal (HEX-I) and a modified hexagonal (HEX-H) phase in the x=0.8 sample. We analyzed using a phenomenological non Debye-type relaxation model. Motional narrowing in the line width was observed in each sample, the result revealing that Li+ ions begin to move at low temperature in samples with less Li content. It was found that the activation energy associating with Li+ ion hopping in the HEX-II phase is smaller than that in the HEX-I phase. These results show that the HEX-II phase produced in the Li deintercalation process would be suitable for Li+ ionic diffusion in multi-phase LixCoO2, and it is expected that this would enable fast ionic diffusion. Li+ ionic diffusion related to phase transition is discussed from Li-7 NMR results. (c) 2006 Elsevier B.V. All rights reserved.