Provided is a lithium-conductive solid-state electrolyte material that comprises a sulfide compound of a composition that does not deviate substantially from a formula of Li9S3N. The compound's conductivity is greater than about 1×10−7 S/cm at about 25° C. and is in contact with a negative electroactive material. Also provided is an electrochemical cell that includes an anode layer, a cathode layer, and the electrolyte layer between the anode and cathode layers. In an example, the material's activation energy can be no greater than about 0.52 eV at about 25° C.