Development of Multilayer Polypropylene Separators for Lithium-Ion Batteries via an Industrial Process
Honglin Liu, Feng Yang, Ming Xiang, Ya Cao, Tong Wu
Abstract
Safety function is becoming the largest issue of lithium-ion batteries (LIBs) with the increase in the capacity and charge–discharge rate of LIBs in recent years. In this study, we successfully produce an isotactic polypropylene (iPP)/polypropylene random copolymer (PPR) + silicon dioxide (SiO2)/iPP tri-layer separator through a facile industrial method. Since the PPR core layer has compatible processability and cavitation behavior with the iPP skin layer, the multilayer coextrusion and biaxial stretching could be carried out in a continuous process. The electrochemical tests show that the iPP/PPR + SiO2/iPP separator has a lower impedance and better charge–discharge cycling properties than the commercial separators, resulting from the large pores and introduction of SiO2 in the core layer. On the other hand, it is found that the PPR + SiO2 core layer would melt and close the pores at 150 °C to increase the impedance of the tri-layer separator significantly; meanwhile, the iPP skin layer with a higher thermal stability can prevent the whole separator from shrinking. The iPP/PPR + SiO2/iPP tri-layer separator could act as a safety switch to prevent serious thermal runaway in LIBs. The fascinating performance and convenient processing method make the iPP/PPR + SiO2/iPP separator a promising application in high-performance LIBs.