There are two processes for making lithium-ion battery separators: wet process and dry process.
The key characteristics of wet processing are high cost, large investment, high equipment requirements, long construction and commissioning cycles, and significant energy consumption during production, as well as the use of organic solvents. However, wet processing allows for better control over pore size, distribution, and porosity, so it is generally used in the manufacture of high-end thin films.
The raw material for dry-process separators is generally polypropylene (PP), while that for wet-process separators is generally polyethylene (PE). Generally, the melting temperature of PP is around 170℃, and that of PE is around 140℃. Therefore, although the separator produced by the wet-process is thinner, its lower melting temperature makes it prone to shrinkage at high temperatures, potentially causing a short circuit and compromising battery safety. Coating the separator surface with inorganic nanoparticles or high-temperature resistant organic chemicals can improve its high-temperature safety performance, effectively compensating for this deficiency in the wet-process.
The production principles of the two processes are as follows:
The dry process first involves melting, extruding, and blowing polyolefin resin to form a crystalline polymer film. This is followed by crystallization heat treatment and annealing to obtain a highly oriented film structure. Finally, the film is stretched at high temperature, and the separation of the crystalline cross-section is tested to form a porous battery separator. The dry process can also be divided into uniaxial stretching and biaxial stretching.
Traditional wet processing mainly uses phase inversion, while in recent years, TIPS (thermally induced phase separation) has become the dominant method. The principle involves mixing a crystalline polymer, a thermoplastic polymer, and a high-boiling-point small-molecule chemical diluent (such as paraffin oil) to form a homogeneous solution at high temperature. The solution temperature is then lowered, causing solid-liquid or liquid-liquid phase separation. After extraction and removal of the small-molecule chemical diluent, a porous membrane of thermoplastic and crystalline polymers is formed.
