The wet-process diaphragm can be divided into a base film and a coated diaphragm. The coated diaphragm is a wet-process diaphragm that is coated on the basis of the base film. The base film is limited by the low melting point of PE and has poor thermal stability. In order to improve the performance of the base film, the coated diaphragm is formed by coating the surface with inorganic materials, heat-resistant polymer materials or a mixture of the two, which can greatly optimize the performance of the diaphragm. Common coating materials include inorganic coating materials such as alumina and boehmite; organic coating materials such as PVDF and aramid fiber; and various composite coating materials.

Since coating can significantly improve the performance of the diaphragm and largely solve the shortcomings of the thermal stability of the wet diaphragm, the wet coating process has become the development trend of the diaphragm industry. On the one hand, the coating material reduces thermal shrinkage of the separator. For example, after heat treatment at 145°C for 30 minutes, the difference in heat shrinkage between the blank base film and the separator coated with 6μm alumina can exceed 50%.

The high temperature resistance of polyolefin separators is poor, and the temperature of lithium batteries continues to rise, and the separators will go through three stages of shrinkage, closed cells, and melting in sequence. Since the closed cell temperature is lower than the melting temperature, in theory, the ion exchange between the electrodes can be stopped when the separator is closed, so that the battery loses its function, and at the same time prevents the battery temperature from rising further, avoiding the short circuit of the battery caused by the melting of the separator. However, due to the small temperature difference between the melting membrane rupture temperature and the closed cell temperature of the polyolefin film (for example, the closed cell temperature of the PE separator is about 135°C, and the membrane rupture temperature is about 147°C), the waste heat generated after the closed cell will still make the separator As the temperature continues to rise, there is a possibility that the diaphragm may rupture and an accident may occur.

After the coating treatment, the performance of the separator is significantly improved, which can keep the original shape of the separator after reaching the melting temperature of polyolefin, prevent short circuit, and improve the safety of the battery; on the other hand, the coating material also greatly improves the performance of the separator. The anti-puncture ability further improves the safety of the battery. In addition, since the coating material has a larger pore size than the base film, it can maintain higher wettability with the electrolyte, thereby reducing the internal resistance of the battery and increasing the discharge power of the battery. With the gradual maturity of coating processes such as ceramics, PVDF, and aramid fibers, wet-coated separators gradually occupy a leading position in the industry due to their better performance and higher safety advantages.