Li+ Diffusion and its Structural Basis in the Nanocrystalline and Amorphous Forms of Two-dimensionally Ion-conducting LixTiS2

The layered fast ion conductor LixTiS2 (x ≈ 2/3) has been prepared in nanocrystalline (n-LixTiS2) and amorphous (a-LixTiS2) forms. Hence, a direct comparison of the lithium diffusion in a homogeneously and a heterogeneously disordered material with the same composition is possible. As a reference system, polycrystalline LixTiS2 (in its hexagonal modification, h-LixTiS2) was also investigated by measuring the temperature and frequency dependencies of the spin−lattice relaxation rate T1-1 of 7Li. The activation energies for individually jumping ions as obtained from the low-temperature flanks of the rate maxima are 190, 160, and 70 meV in h-,n-, and a-LixTiS2, respectively. The frequency dependence of T1 is sublinear for both disordered forms, which is explained in terms of the unified site relaxation model. 7Li nuclear magnetic resonance (NMR) spectra of n-LixTiS2 tend, contrary to those of h-LixTiS2 and a-LixTiS2, to decompose into broad and narrow central line components. The relative intensity of the narrow component, which is attributed to fast moving Li ions in the interfacial regions, is temperature dependent and reaches 50% at 360 K. A schematic model for the ionic conduction process in two-dimensional nanocrystalline fast ionic conductors is proposed, introducing grain surface pathways as the dominant track for mobile ions.