Lithium ion dynamics and transport in the halide-rich argyrodite Li5.5PS4.5Cl1.5: Influence of heat treatment on cooperativity, heterogeneity and subdiffusion

We combine 7Li NMR relaxometry and diffusometry with electrochemical impedance spectroscopy to unravel the mechanisms for the dynamics and transport of lithium ions in the lithium-deficient and halide-rich argyrodite Li5.5PS4.5Cl1.5. In particular, we determine the effects of heat treatment on the cooperativity, heterogeneity, and subdiffusion of lithium ion motion. We find that heat treatment results in an enhancement of the dc conductivity by a factor of six to a high room-temperature value of σdc=14.9 mScm−1, whereas the change of the 7Li NMR self-diffusion coefficients D is considerably smaller. Accordingly, heat-treated Li5.5PS4.5Cl1.5 shows a very small Haven ration of HR=0.13 indicative of a high cooperativity of lithium ion dynamics. Moreover, after heat treatment, the collective correlation factor fI becomes very small, which is related to a strongly reduced relevance of subdiffusive lithium ion dynamics. However, heat treatment does not affect the activation energies, which are in the range Ea=0.34−0.40 eV for the dc conductivity σdc, the diffusion coefficient D and also for the jump correlation time τ. 7Li NMR field-cycling relaxometry allows for a characterization of the lithium ion jumps based on a frequency-dependent dynamical susceptibility. We find that the susceptibility peak has a strongly asymmetric shape with a hardly broadened low-frequency flank and a strongly broadened high-frequency flank, reflecting a characteristic heterogeneity of the lithium ion dynamics, which derives from the specific cage-like arrangement of the lithium sites and the resulting difference in the rates of intra-cage and inter-cage jumps. Considering further the anion disorder in the crystal lattice, we propose that heat treatment facilitates cooperative inter-cage jumps, suppressing localized subdiffusive motion and enabling long-range ion transport along percolating pathways.