Mono-axial stretching-induced growth of crystalline phase of melt-processed perfluoroalkoxy alkane (PFA) films for protecting inner layer of battery pouch films

Abstract

Cast polypropylene (CPP) is employed as an inner layer of pouch films; however, CPP shows poor electrolyte, mechanical, and electrical stability, which can lead to instability during cell operation. In this study, we pioneered the use of perfluoroalkoxy alkanes (PFA) in the melt extrusion process to fabricate films based on their high melting flow index, establishing optimal conditions for producing uniform films. We further enhanced the functionality of these films by controlling the crystalline phase orientation through high-temperature uniaxial stretching, which is aimed at applications in battery pouch films. The stretched PFA films exhibited dramatically improved mechanical and insulation properties compared to traditional CPP films, 307.4% increase in tensile strength and 1,748.8% increase in breakdown strength, respectively. Based on these findings, we proposed that stretched PFA films, with their superior performance, are viable substitutes for CPP as the inner layer in battery pouch films. This research not only advances the material science of polymer films but also contributes to the development of safer and more efficient battery technologies.