Usually divided into 3 layers: 1. The inner layer: usually use polyethylene / polypropylene etc material, for sealing. 2. The middle layer:Not only prevent the Battery outside(water vapor infiltration), but also preventing the leakage of internal electrolyte. 3. The outer layer, usually use high melting point polyester or nylon material which has a strong mechanical properties, to prevent external damage to the Battery, protect the Battery