The plastic parts used in batteries vary depending on the battery usage. Some batteries have entirely plastic formulations ranging from the electrolyte (polymeric electrolytes) to the casing. Most batteries intended for vehicle usage have higher proportions of plastic materials. This is due to the very low weight of plastics as compared to metals. Plastic incorporation in batteries increases the electrolyte efficiency if used in polymeric electrolytes.
Commercially available lithium-ion batteries have limited plastic usage. Plastic components incorporated in batteries include separators and casing components. Plastics have good shock absorbing characteristics (Polypropylene has good shock absorbing characteristics) and prevent damage to the basic cell unit from minor accidental shocks. Also, as plastics are insulators their formulations (like Polyethylene Terephthalate) are excellent at preventing short-circuit and act as separators.
Plastic parts also lower the cost of production of batteries and will eventually replace most metallic components of batteries.