The separator is an isolation and protective material inside lithium batteries, placed between the positive and negative electrodes to separate the two poles and avoid direct contact and short circuit. At the same time, the separator itself has a large number of micropores, allowing lithium ions to freely pass through, making it an important substrate that balances safety and electrochemical performance. At present, the mainstream substrate is polyolefin film, which is divided into two categories: single-layer and composite modified.
The basic membrane is extruded and stretched from resin raw materials to form a film. The stretching process forms interconnected micropores inside the film, and the pore size, porosity, and air permeability of the micropores are the core indicators. The porosity is too high, and the mechanical strength of the diaphragm is low; If the porosity is too low, the ion conduction resistance increases, the internal resistance of the battery increases, and the high current discharge performance deteriorates. The diaphragm also needs to have a suitable thickness, the thinner it is, the more beneficial it is to improve the energy density of the battery cell, but the puncture resistance will decrease accordingly.
Simple polyolefin membranes have limited heat resistance, and many power and energy storage scenarios use ceramic coated membranes. Coating a layer of inorganic ceramic coating on the surface of the base film can enhance its heat-resistant dimensional stability, prevent significant shrinkage at high temperatures, and reduce the risk of thermal runaway; Some separators will also undergo adhesive layer modification. During the hot pressing of the battery cell, the adhesive layer can slightly adhere to the electrode piece, improving the structural stability of the battery cell.
The entire process of diaphragm processing should control defects such as thickness uniformity, pinholes, black spots, scratches, etc. During the production of winding and laminating, the control of diaphragm tension is very important. Excessive tension can easily thin and break the diaphragm, while insufficient tension can cause wrinkles and deviation, resulting in local contact risks between the polarizer and the diaphragm. The alignment accuracy of the diaphragm and pole piece is also a key control item in the battery cell manufacturing process.
