A cover assembly for a Lithium-ion battery container to improve the durability of the battery. A high pressure injection molded polymer is combined with flanges disposed on the current collecting posts of the battery to provide an improved seal for the battery cover, especially at the interface of the cover and the current collecting posts. The injection molded polymer is fused to the flanges, current collecting posts and container cover to result in a diffusion path of increased length for gases and/or liquids which are known to enter or escape from battery containers. Additional polymer layers can be applied to the assembly to further improve the seal.