Effect of annealing temperature on pore formation in preparation of advanced polyethylene battery separator membranes

Polymeric separator membranes are a key component in modern lithium ion batteries, as they are placed between anode and cathode to prevent short-circuiting while at the same time allowing an efficient diffusion of Li ions. Manufacturing conditions must be finely tuned to reach the many diverse and conflicting requirements for battery separators used in modern consumer applications as well as in electric vehicles. Little has been published about these proprietary combined extrusion/biaxial stretching processes. This work presents a closer look on one of the key aspects of separator membrane formation. Extruded sheets of high density polyethylene containing a hydrocarbon solvent as plasticizer were stretched in machine direction and annealed at temperatures between 100 °C and 120 °C, i.e., in a range between the onset of melting and actual melting temperature, as deduced from differential scanning calorimetry. The formation of stacked-lamellar morphologies as seen in scanning electron microscopy required a minimum strain and was also influenced by processing temperatures. The increase of the annealing temperature led to a significant increase in crystallinity and chain orientation as revealed by texture analysis, performed using X-ray powder diffraction studies on samples after dedicated preparation. After a second stretching step in transversal direction, a clear correlation to separator membrane permeability and porosity was found, with a higher crystallinity leading to lower Gurley values, i.e. higher permeabilities. The effects of blending high density with ultra-high molecular weight polyethylene of different molecular weight onto structure and morphology were also elucidated in detail. In light of the growing market of electric vehicles, high performance and safety are the main focus during separator production. The influence of important production parameters on final membrane properties is discussed.