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 Li_5.5PS_4.5Cl_1.5. 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 ${\sigma }_{\mathrm{dc}}=14.9$ mScm⁻¹, whereas the change of the ⁷Li NMR self-diffusion coefficients $D$ is considerably smaller. Accordingly, heat-treated Li_5.5PS_4.5Cl_1.5 shows a very small Haven ration of $H_R=0.13$ indicative of a high cooperativity of lithium ion dynamics. Moreover, after heat treatment, the collective correlation factor $f_I$ 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 $E_a=0.34-0.40$ eV for the dc conductivity ${\sigma }_{\mathrm{dc}}$, the diffusion coefficient $D$ and also for the jump correlation time $\tau$. 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.