Thermally stable and high electrochemical performance ultra-high molecular weight polyethylene/poly(4-methyl-1-pentene) blend film used as Li-ion battery separator

The advances in Li-ion batteries (LIBs) for energy storage continuously face major challenges like acquiring high performance, stability and safety. A separator plays major role for safety and performance of batteries through its porous structure and interaction with electrolyte. In this work, a suitable microporous structure of ultra-high molecular weight polyethylene/poly(4-methyl-1-pentene) (UHMWPE/PMP) blend film via sequential biaxial stretching is achieved by tuning PMP content. The incorporation of PMP leads to high porosity and wettability enabling a significant increase of air permeability, electrolyte uptake and ionic conductivity. A low shrinkage, of 0.7% in transverse direction and 1.6% in machine direction, is observed after exposing PM2 (the film with 7.5 wt% of PMP) at 120°C for 1 h indicating the high thermal stability. Most importantly, a cell containing PM2 presents an excellent electrochemical performance. The discharge capacity reaching 172.8 mAh/g and efficient stable cycling with 99.89% as coulombic efficiency during 100 cycles are obtained at 0.1 and 1 C-rate, respectively. Apart from existing blend separators, current work provides an alternative of using UHMWPE/PMP blend films and deepens the understanding of the role of porous structure for the design of LIBs with best performance in future.