In the structure of a lithium battery, the separator is one of the key internal components. The performance of the lithium battery separator determines the battery's interface structure, internal resistance, and other characteristics, directly affecting the battery's capacity, cycle life, and safety performance. A high-performance separator plays an important role in improving the overall performance of the battery.
The main function of a lithium-ion battery separator is to separate the positive and negative electrodes, preventing short circuits caused by contact between them. It also allows electrolyte ions to pass through. The separator material is non-conductive, and its physicochemical properties significantly influence battery performance. Different types of batteries use different separators. For lithium-ion batteries, since the electrolyte is an organic solvent system, separator materials resistant to organic solvents are required; generally, high-strength, thin-film polyolefin porous membranes are used.
Commercially available membrane materials are mainly polyolefin membranes, primarily made of polyethylene (PE) and polypropylene (PP). PE products are mainly produced using wet processes, while PP products are mainly produced using dry processes. The characteristics of PE and PP are as follows:
1. PP is more resistant to high temperatures, while PE is more resistant to low temperatures;
2. PP has a lower density than PE;
3. PP has a higher melting point and closed-cell temperature than PE;
4. PP products are more brittle than PE products;
5. PE is more sensitive to environmental stress.
The main battery separator materials include single-layer PP, single-layer PE, PP+ceramic coating, PE+ceramic coating, double-layer PP/PE, double-layer PP/PP, and triple-layer PP/PE/PP. The first two types are primarily used in the 3C small battery field, while the latter types are mainly used in the power lithium battery field. Among the separator materials for power lithium batteries, double-layer PP/PP separators are mainly produced by Chinese companies and used in mainland China. This is primarily because no Chinese company yet possesses the technology and capability to fabricate a double-layer composite film from PP and PE. Globally, the separators used in automotive power lithium batteries mainly consist of triple-layer PP/PE/PP, double-layer PP/PE, and PP+ceramic coating, PE+ceramic coating, and other separator materials.
Meanwhile, other new types of separator materials are emerging and beginning to be applied. However, due to limited quantity and high price, they are mainly used in the manufacturing of power lithium batteries. These products mainly include: coated polyester films (PET, Polyethylene Terephthalate), cellulose films, polyimide films (PI), polyamide films (PA), spandex or aramid films, etc. These separators have the advantages of high temperature resistance, low temperature output, long charge cycle life, and moderate mechanical strength. Overall, lithium battery separator materials are showing a clear trend of diversification.
