7Li NMR Studies of Short-Range and Long-Range Lithium Ion Dynamics in a Heat-Treated Lithium Iodide-Doped Lithium Thiophosphate Glass Featuring High Ion Conductivity

We combine Li-7 field-cycling relaxometry with Li-7 field-gradient diffusometry to study both short-range and long-range lithium ion dynamics in a heat-treated 0.33 LiI + 0.67 Li3PS4 glass, which exhibits a high ionic conductivity of 6.5 mS/cm.Li-7 field-cycling relaxometry, which provides access to the spin-lattice relaxation time T-1 over broad frequency and temperature ranges, allows us to determine the activation energy distribution g(E-a) of the local jumps in a model-free way. We find that g(E-a) has a Gaussian form and is characterized by the mean energy E-m = 0.40 eV and the standard deviation sigma = 0.08 eV. Li-7 field-gradient studies indicate free diffusion and yield self-diffusion coefficients of D similar to 10(-12) m(2)/s at E-a ambient temperatures. Comparing our results for the short-range and long-range ion dynamics and considering previous knowledge about ionic conductivity, we show that the local jumps characterized by the Gaussian distribution g(E-a) determine the macroscopic transport. We conclude that the high ionic conductivity of the heat-treated glass does not result from the formation of minor crystalline regions during the annealing process but rather from a modification of the predominant amorphous phase.