Scientists from Kyungpook National University, Chung-Ang University and the Korea Institute of Machinery and Materials jointly with their colleague from North Carolina State University in the U.S. have developed a separator for lithium-ion batteries that can withstand high temperatures and significant stretching. At a temperature of 150°C, the new material shrank by only 4% in surface area, while a conventional polyethylene separator shrank by 96%, losing its original shape almost completely. Furthermore, the new membrane can stretch by approximately 67%, which makes it a promising material for flexible batteries, among other applications.
A separator is a thin porous barrier between the positive and negative electrodes of a battery. It prevents the electrodes from touching, thereby eliminating internal short circuits, while allowing lithium ions to pass freely. Separators made of polyethylene and polypropylene, which are relatively inexpensive and chemically resistant, are widely used today. However, they can soften and shrink under intense heat, raising the risk of electrode contact. Moreover, these membranes are poorly suited to strong mechanical deformation.
The researchers decided to combine heat resistance and elasticity in a single material. To achieve that, they made a separator from multiple fibers with a core-shell structure. The scientists used heat-resistant polyimide for the outer shell and a blend of polyimide and elastic thermoplastic polyurethane for the core. The stiffer shell helps maintain the structure when heated, while the elastic core absorbs the strain during stretching. Even when the outer layer begins to deteriorate under strong deformation, the core continues to support the fiber, preventing the membrane from breaking at once.
The fibers were produced via coaxial electrospinning: two polymer solutions were simultaneously fed through a special nozzle, as a result of which one material formed the core and the other formed the shell. The finished fibers were approximately 1 micron in diameter. They were then pressed into a porous membrane roughly 16 microns thick, comparable to the separators used in modern lithium-ion batteries.
Tests showed that the new separator had an elongation resistance of about 66.6%, whereas the polyethylene sample had an elongation resistance of less than 10%. After an hour at 150°C, the surface area of the new material shrank by only 4.03%, and at 200°C, it shrank by 4.47%. For the polyethylene separator, these figures stood at 96.14% and 98.44%, respectively. Polypropylene exhibited significant deformation at high temperatures as well: its shrinkage reached 55% at 150°C and almost 95% at 200°C.
The new membrane also interacted much better with a liquid electrolyte. A drop of electrolyte completely spread over its surface in five seconds, and the amount of liquid absorbed was about ten times the separator’s own weight. Ionic conductivity at room temperature reached 6.14 mS/cm versus 1.52 mS/cm for polyethylene.
The researchers then assembled lithium-ion cells with an NCM622 cathode and a graphite anode. At 25°C, batteries with the new and polyethylene separators showed similar results: after 100 cycles, they retained 93.3% and 88.9%, respectively, of their original capacity. At higher temperatures, the difference became more noticeable. After 100 cycles at 65°C, the new
