Influence of Aliovalent Cation Substitution and Mechanical Compression on Li-Ion Conductivity and Diffusivity in Argyrodite Solid Electrolytes

All-solid-state batteries employing sulfide superionic conductors demand new electrolyte materials with excellent Li-ion transport properties. We report on dual-modified superionic conductors in the Li-argyrodite family. In these materials prepared by a rapid synthesis method, aliovalent doping of the Li+ site with Ca2+ or Al3+ generates vacancies which improve Li+ diffusion and conductivity. This is confirmed by pulsed-field gradient (PFG)-NMR and impedance spectroscopy. The “super Cl-rich” material with overall composition Li5.35Ca0.1PS4.5Cl1.55 exhibits a superionic room temperature conductivity of 10.2 mS·cm–1 in the cold-pressed state and an exceptional diffusivity of 1.21 × 10–11 m2/s. The presence of the aliovalent dopant on the Li sites in the cubic structure is supported by 7Li magic-angle spinning NMR and Rietveld refinement of neutron powder diffraction data. Importantly, we also probed the impact of mechanical modification on grain boundary diffusion in these sulfide electrolytes using PFG-NMR. Detection of a clear difference in activation energies between the powder and pellet-pressed versions indicates that the efficacy of particle-scale, materials engineering modifications of fast-diffusing solid electrolytes can be interrogated with the use of PFG-NMR. Our studies also show that analysis over a wide range of temperatures is often necessary for fitting PFG-NMR Arrhenius plots in order to be able to compare macroscopic measurements with transport coefficients indirectly extrapolated from microscopic measurements, such as NMR relaxometry techniques.