Preparation of PP/PP-g-NPMI Separators with High Hydrophilicity and Heat Resistance for Lithium Batteries

Polypropylene-g-N-phenylmaleimide was prepared by melt grafting reaction of polypropylene and N-phenylmaleimide and was added to the polypropylene matrix in different match ratios. After that the base films were fabricated by casting of the mixture of polypropylene and polypropylene-g-N-phenylmaleimide, and then separators were obtained by stretching. Adoption of N-phenylmaleimide is a key technique to ensure separators with excellent heat resistance. It was also found that polypropylene-g-N-phenylmaleimide played an important role in improving the overall performance of those base films and separators. Compared with the polypropylene base film, the crystallinity, crystalline orientation, and elastic recovery of base films containing polypropylene-g-N-phenylmaleimide increased obviously. The prepared separators showed good pore structure and hydrophilicity. The heat shrinkage of separators measured at 130°C reduced from 18.8 to 10.6% while the content of polypropylene-g-N-phenylmaleimide increased from 0 to 10%. Coin cells assembled with separators also show higher battery capacity than the cells assembled with pure polypropylene separators.