Li_0.6[Li_0.2Sn_0.8S₂]-a layered lithium superionic conductor

One of the key challenges of energy research is finding solid electrolytes with high lithium conductivities comparable to those of liquid electrolytes. In this context, developing new structural families of potential Li⁺ ion conductors and identifying structural descriptors for fast Li⁺ ion conduction to occur is key to expand the scope of viable Li⁺ ion conductors. Here, we report that the layered material Li_0.6[Li_0.2Sn_0.8S₂] shows a Li⁺ ion conductivity comparable to the currently best lithium superionic conductors (LISICONs). Li_0.6[Li_0.2Sn_0.8S₂] is composed of layers comprising edge-sharing Li/SnS₆ octahedra, interleaved with both tetrahedrally and octahedrally coordinated Li⁺ ions. Pulsed field gradient (PFG) NMR studies on powder samples show intragrain (bulk) diffusion coefficients D_NMR on the order of 10^-11 m² s^-1 at room temperature, which corresponds to a conductivity σ_NMR of 9.3 × 10^-3 S cm^-1 assuming the Nernst-Einstein equation, thus putting Li_0.6[Li_0.2Sn_0.8S₂] en par with the best Li solid electrolytes reported to date. This is in agreement with impedance spectroscopy on powder pellets, showing a conductivity of 1.5 × 10^-2 S cm^-1. Direct current galvanostatic polarization/depolarization measurements on such samples show negligible electronic contributions (less than 10^-9 S cm^-1) but indicate significant contact resistance (d.c. conductivity in a reversible cell is 1.2 × 10^-4 S cm^-1 at 298 K). Our results suggest that the partial occupation of interlayer Li⁺ positions in this layered material is beneficial for its transport properties, which together with tetrahedrally coordinated Li sites provides facile Li⁺ ion diffusion pathways in the intergallery space between the covalent Sn(Li)S₂ layers. This work therefore points to a generic design principle for new layered Li⁺ ion conductors based on the controlled depletion of Li⁺ ions in the interlayer space.