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

Abstract

We combine 7Li field-cycling relaxometry with 7Li 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. 7Li field-cycling relaxometry, which provides access to the spin–lattice relaxation time T1 over broad frequency and temperature ranges, allows us to determine the activation energy distribution g(Ea) of the local jumps in a model-free way. We find that g(Ea) has a Gaussian form and is characterized by the mean energy Em = 0.40 eV and the standard deviation σ = 0.08 eV. 7Li field-gradient studies indicate free diffusion and yield self-diffusion coefficients of D ∼ 10–12 m2/s at 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(Ea) 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.