Development of Multilayer Polypropylene Separators for Lithium-Ion Batteries via an Industrial Process

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 degrees 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.