Sepators in square lithium - ion batteries are thin, porous membranes that play a vital role in maintaining the integrity and safety of the battery. They physically separate the anode and the cathode, preventing short - circuits while allowing the passage of lithium ions.

Polyolefin - based separators are widely used in commercial lithium - ion batteries. Polyethylene (PE) and polypropylene (PP) are common materials for these separators. They have good chemical stability and mechanical strength. The pores in polyolefin separators are carefully engineered to be small enough to prevent the passage of electrons (thus avoiding short - circuits) but large enough to allow the diffusion of lithium ions. However, polyolefin separators have some limitations. They have a relatively low melting point, which can pose a safety risk at high temperatures.

Ceramic - coated separators are an improvement over traditional polyolefin separators. A ceramic layer, such as alumina (Al₂O₃) or silica (SiO₂), is coated onto the polyolefin substrate. The ceramic coating enhances the thermal stability of the separator. It can withstand higher temperatures without melting or shrinking, reducing the risk of thermal runaway in the battery. Additionally, the ceramic layer can improve the wettability of the separator with the electrolyte, facilitating the diffusion of lithium ions.

Non - woven separators made of materials like aramid fibers are also being explored. Aramid non - woven separators offer high mechanical strength and good chemical resistance. They can provide better puncture resistance compared to some other separator materials, which is important for preventing internal short - circuits caused by electrode protrusions.

The performance of the separator has a significant impact on the overall performance and safety of square lithium - ion batteries. A high - quality separator can improve the battery's cycle life, charge - discharge efficiency, and safety. As the demand for safer and higher - performance lithium - ion batteries increases, the development of advanced separator materials and designs is a key area of research.