A thin film coating with inorganic/organic composite materials for application on a porous separator of a rechargeable metal battery cell is disclosed. The composite material, which is comprised of ion conductive ceramic particles mixed with, or embedded within a matrix of, at least one polymer comprising at least one anionic functional group and at least one metallic cation. The composite coating layer enhances the overall electrochemical performance of rechargeable metal batteries by preventing the formation of metal dendrites on the metallic anode of a metal battery cell.