7Li NMR study on Li + ionic diffusion and phase transition in Li xCoO 2

Abstract

The temperature dependence of the spin-lattice relaxation time, T 1 and the line width of the 7Li nucleus were measured in delithiated Li x CoO 2 ( x = 0.6, 0.8, 1.0). Two different relaxation behaviors were observed in the temperature dependence of T 1 − 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-II) 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 Li x CoO 2, and it is expected that this would enable fast ionic diffusion. Li + ionic diffusion related to phase transition is discussed from 7Li NMR results.