Meaning
A polymer membrane material acts as a physical barrier between the positive and negative electrodes in a battery while allowing the free passage of ions. Manufacturers produce microporous polyolefin through either a dry process involving stretching or a wet process involving phase separation. The material is typically made from polyethylene or polypropylene and must remain chemically stable in the presence of aggressive electrolytes.
Pore Structure
Scanning electron microscopy reveals a network of interconnected voids that provide the necessary pathways for ionic conduction. These pores are small enough in microporous polyolefin to prevent the migration of active material particles or the growth of metallic dendrites. Control over the pore size and the distribution of the voids is essential for achieving high power density.
Thermal Safety
One advantage of this material is its ability to act as a thermal fuse by melting and closing its pores if the internal temperature of the cell rises too high. The shutdown mechanism effectively stops the flow of ions through the microporous polyolefin and prevents the battery from entering a state of thermal runaway. Multilayer separators often combine different types of polymers to provide a range of shutdown and melting temperatures.
Ceramic coatings are frequently applied to the surface to enhance the structural integrity at elevated temperatures even after the base polymer has softened. This added layer prevents the electrodes from touching if the internal temperature continues to climb.
Mechanical Property
Tensile strength and puncture resistance determine how well the membrane survives the high-speed winding and assembly processes used in mass production. Thinness is also a priority for microporous polyolefin to maximize the amount of active material that can fit into the cell. Advances in coating technologies have allowed for the addition of ceramic layers to improve the heat resistance of the base polymer.
