In an aspect, an electrochemical cell comprises: a positive electrode; a negative electrode; and a solid state electrolyte in ionic communication with the positive electrode and the negative electrode; wherein: the electrolyte is characterized by formula (FX1): MPS3 (FX1); wherein M is one or more metal cations and optionally metal cation vacancies; and wherein at least one of said one or more metal cations is a divalent cation; the electrolyte is characterized by a divalent ion conductivity; and the electrolyte is electrically insulating. The solid state electrolyte is optionally not an electrocatalyst material or does not function as an electrocatalyst in the electrochemical cell during operation (e.g., charging and/or discharging) of the electrochemical cell. The solid state electrolyte is optionally not an electrode or does not function as an electrode in the electrochemical cell during operation (e.g., charging and/or discharging) of the electrochemical cell.