Three-layer composite lithium battery diaphragm

The three-layer composite lithium battery diaphragm comprises a polyurethane base material layer and a polyamide material layer, transition layers are arranged on the two sides of the polyurethane base material layer, the polyamide material layer is located on the sides, away from the polyurethane base material layer, of the transition layers, and a plurality of first internal micropores are evenly formed in the polyurethane base material layer in the thickness direction. And a plurality of second internal micropores are uniformly formed in the transition layer along the thickness direction. Compared with a polyethylene base material layer and a polyoxyethylene outer layer in the prior art, the polyurethane base material layer and the polyamide material layer have high chemical stability (strong oxidation corrosion resistance), and meanwhile, the first internal micropore, the second internal micropore and the third internal micropore which are communicated are arranged, so that the positive electrode and the