Preparation and Characterization of Multi-Layer Polypropylene Separator for Li-Ion Battery

Abstract

Li-ion batteries have been widely used as power sources for various applications such as portable electronics, electric storage, and electric vehicles. The Li-ion battery consists of an anode, cathode, separator membrane and electrolyte. Among the components, the membrane is the key component of the Li-ion battery. It prevents contact between the anode and cathode and increases electrical performance. To perform its functions effectively, the separator used in a Li ion battery requires high mechanical strength, thermal stability and a suitable and uniform porosity. In this study, a microporous membrane was prepared as a Li ion cell separator by a dry process consisting of extrusion, annealing and stretching. Multi-layer PP films were prepared using a 3-way extruder with a t-die at 230 degrees C, and then the films were annealed and stretched in a biaxial drawing machine at 110 degrees C, to investigate their lamella structure. Stretching ratio and stretching speed were controlled as the stretching conditions. DSC was used to measure the melting point and heat of fusion of the sample at a heating rate of 30 degrees C/min under nitrogen gas purge. Following stretching, we determined the non-isothermal crystallization behaviors of the samples. Crystallinity (X-c) was calculated using Delta H-o and Delta H. Delta H-o is the enthalpy of a single crystal (PP: 208 J/g), and Delta H is the calculated enthalpy for the entire sample. The morphology of the membranes' surfaces were measured by FE-SEM. We compared and analyzed the porous structures of the stretched PP films' surfaces.