American Chemical Society, ACS Applied Materials and Interfaces, 7(7), p. 4089-4099, 2015
DOI: 10.1021/am5078655
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Layer-structured materials such as graphite (LiCy) or Lix(Co,Ni,Mn)O2 are important electrode materials in current battery research that still relies on insertion materials. This is due to their excellent ability to reversibly accommodate small alkali ions such as Li+ and Na+. Despite of these applications behind, microscopic information on Li ion self-diffusion in transition metal sulfides are relatively rare. Here, we used 7Li nuclear magnetic resonance (NMR) spectroscopy to study translational Li ion diffusion in the model system hexagonal (2H) LixNbS2 (x = 0.3, 0.7 and 1) by means of variable-temperature NMR relaxometry. 7Li spin-lattice relaxation rates and 7Li NMR spectra were used to determine Li jump rates and activation barriers as a function of Li content. Hereby, NMR spin-lattice relaxation rates recorded with the spin-lock technique offered the possibility to study Li ion dynamics on both the short-range and long-range length scale. Dynamic parameters, such as jump rates and activation barriers, could be extracted from complete diffusion-induced rate peaks, which are obtained when the relaxation rate is plotted vs reciprocal temperature. The peak maximum of the three samples studied shifts towards higher temperatures with increasing Li content x in 2H-LixNbS2 revealing a decrease of Li diffusivity with increasing x. Information on the dimensionality of the diffusion process was experimentally obtained by frequency dependent Rρ measurements carried out at T = 444 K, that is in the high-temperature regime of the rate peaks. A slight, but measurable frequency-dependence within this limit is found for all samples; it is in agreement with predictions from relaxation models developed to approximate low-dimensional (2D) jump diffusion; thus, Li diffusivity within the van der Waals gap governs the NMR rates recorded.