An ultra-thin and high-strength composite separator by high-temperature annealing for lithium-ion batteries

As a critical component of lithium-ion batteries (LIBs), separators play a pivotal role in determining their performance and safety. However, the widely use polyolefin separators in commercial LIBs have certain limitations, such as poor affinity with electrolyte and low thermal stability. In this study, we propose an ultra-thin, highstrength composite (UPE/NFS) separator to overcome these problems and enhance the overall performance of LIBs. Ultrahigh molecular weight polyethylene (UHMWPE), nano-fumed silica (NFS), and liquid paraffin (LP) are melt-blended using a twin-eccentric rotor extruder (TERE) to overcome the processing difficulties associated with UHMWPE and to ensure uniform dispersion of the NFS. Following biaxial stretching and LP removal, the UPE/NFS separator undergo high-temperature annealing at 140 degrees C to optimize its pore structure. The annealing process coarsens the fiber structure and increases the pore size of the separator, significantly improving its mechanical strength, thermal stability, and ionic conductivity. As a result, the battery exhibits better cycling stability, with a high initial capacity of 140.7 mAh g-1 and a capacity retention rate of 92 % after 150 cycles at 1 C. Therefore, considering the fabrication process, cost, and the comprehensive evaluation of battery safety and performance, the UPE/NFS separator demonstrates significant potential for practical applications.