In the structure of lithium batteries, the separator is one of the key internal components. The performance of the lithium-ion battery separator determines the battery's interface structure, internal resistance, and other characteristics, directly affecting the battery's capacity, cycle life, and safety performance. A high-performance separator plays an important role in improving the overall performance of the battery.
I. Performance of Lithium-ion Battery Separators
The separator in a lithium-ion battery is located between the positive and negative electrodes. Its main function is to separate the active materials of the two electrodes, preventing short circuits caused by contact. Additionally, it maintains the necessary electrolyte and forms channels for ion movement during electrochemical reactions. The separator material is non-conductive, and different types of batteries use different separators. For lithium-ion batteries, since the electrolyte is an organic solvent system, the separator must possess the following properties.
1. Within the battery system, it must have good chemical stability, and the materials used must be resistant to organic solvents.
2. High mechanical strength and long service life.
3. The ionic conductivity of organic electrolytes is lower than that of aqueous solutions. In order to reduce resistance, the electrode area must be as large as possible, therefore the diaphragm must be very thin.
4. When the battery system malfunctions and the temperature rises, to prevent danger, the thermoplastic separator melts and the micropores close at the rapid heat generation temperature (120-140℃), turning into an insulator to prevent electrolyte from passing through, thereby achieving the purpose of cutting off the current.
5. From the perspective of lithium batteries, they must be able to be fully impregnated by organic electrolytes and maintain a high degree of impregnation during repeated charging and discharging.
The separator materials commonly used in batteries are generally microporous membranes made of cellulose or woven fabrics and synthetic resins. Lithium-ion batteries generally use high-strength, thin-film polyolefin-based porous membranes. Commonly used separators include polypropylene (PP) and polyethylene (PE) microporous membranes, as well as copolymers of propylene and ethylene, and polyethylene homopolymers.
II. Classification of Lithium-ion Battery Separators
Based on their different physical and chemical properties, lithium-ion battery separator materials can be classified into several categories, including woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, separator paper, and rolled membranes. Polyolefin materials possess excellent mechanical properties, chemical stability, and relatively low cost; therefore, polyolefin microporous membranes such as polyethylene and polypropylene were used as lithium-ion battery separators in the early stages of lithium-ion battery research and development. Although recent studies have explored the use of other materials to prepare lithium-ion battery separators, such as using polyvinylidene fluoride (PVDF) as the bulk polymer via phase inversion and researching cellulose composite membranes as lithium-ion battery separator materials, commercially available lithium-ion battery separator materials still primarily utilize polyethylene and polypropylene microporous membranes.
