Comparison of 3D and 2D Li diffusion in cubic and hexagonal Li x TiS 2

a study by solid state 7 Li NMR methods

Abstract

Li diffusion in spinel-type structured cubic and hexagonal Li_xTiS₂ with x = 0.6 and 0.7, respectively, was investigated by solid state NMR spectroscopy. In layered hexagonal Li_xTiS₂, Li diffusion is known to be confined to the van-der-Waals gap between the TiS₂ planes, i. e., it is two-dimensional (2D). In cubic Li_xTiS₂, however, Li diffusion takes place in the three-dimensional (3D) interstitial space formed by the TiS₆-polyhedra. Both polymorphs form a unique pair for studying the effects of dimensionality on Li diffusion within a host structure of the same chemical composition. ⁷Li spin-alignment echo and spin-lattice relaxation NMR methods were applied to measure Li jump rates. Additionally, solid echo NMR spectroscopy was used to estimate the Li diffusivity in cubic Li_0.6TiS₂. The activation energies for Li jumps in the two materials are very similar. Nevertheless, Li diffusion in the 3D host is slower than in the layered modification by a factor of about ten.