Fabrication of Composite Polyimide Separator for Lithium Ion Battery Thermal Safety Improvement

The poor thermal stability and inferior ion transport performance of polyolefin separators have been the bottlenecks restricting the development of lithium ion battery (LIB)-based high-power energy storage systems. Thus, the preparation of advanced separators is essential as the alternative of traditional polyolefin for reliable LIB-based large-scale grid energy storage. In this study, a composite polyimide-based separator doped with a lithium-conducting inorganic was prepared by electrospinning combined with in situ thermal imidization as the matrix. The effects of the inorganic and thermal treatment conditions on the structure and performance of the separator were systematically investigated. The results show that the optimized composite separator exhibits excellent comprehensive performance: (1) its thermal stability is significantly better than that of commercial polyolefin separators, with no obvious size shrinkage at high temperatures; (2) the porosity and electrolyte wettability are greatly improved, effectively promoting the uniform transport and desolvation process of lithium ions, and the ion transport performance is significantly enhanced; and (3) the mechanical properties meet the requirements for commercial batteries. In half-cell tests, the composite separator achieved excellent rate performance and cycling stability. Particularly, overcharge tests for the assembled pouch cells with Ah-level were performed, where the low overcharge temperature verified the better thermal stability of the composite separator than that of batteries assembled with traditional commercial separators. This study provides new ideas and technical support for the design and industrial application of high-safety and long-life energy storage battery separators.