Functionalized polymeric binders for electrolyte and electrode compositions include a polymer having a polymer backbone and functional groups. In some embodiments, a polymer includes a non-polar polymer backbone and a functional group that is 0.1 to 5 wt % of the polymer. In some embodiments, a polymer includes a polar backbone and a functional group that is 0.1 to 50% weight percent of the polymer. Also described are composites for electrolyte separators and electrodes that include argyrodite ion conductors and polar polymers.