Unraveling Ultrafast Li-Ion Dynamics in the Solid Electrolyte LiTi₂(PS₄)₃ by NMR down to Cryogenic Temperatures

Self-diffusion processes of small atoms or ions play a crucial role in many areas of research. The unique crystal structure of LiTi₂(PS₄)₃ (LTPS) presents a variety of energetically inequivalent diffusion pathways for small Li⁺ charge carriers and has resulted in one of the highest Li⁺ diffusion coefficients. Investigating these pathways individually at the atomic scale poses significant challenges, especially for probing jump processes. In this study, we utilized nuclear spin relaxation techniques down to cryogenic temperatures (10 K) to reveal unprecedented details about both long-range and short-range Li⁺ dynamics. The temperature-dependent ⁷Li NMR spin-lattice relaxation (SLR) rate exhibits a series of diffusion-induced peaks, allowing the extraction of activation energies and jump rates. Due to the exceptionally fast localized Li⁺ exchange processes in LTPS, temperatures as low as 50 K are required to freeze Li⁺ dynamics, on the SLR time scale, entirely within the ring-like cages of the LTPS structure.