I. Characteristics of Lithium-ion Battery Separators
The main performance requirements for lithium-ion battery separators include four major performance indicators: thickness uniformity, mechanical properties (including tensile strength and puncture resistance), air permeability, and physicochemical properties (including wettability, chemical stability, thermal stability, and safety).
II. 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.
III. Classification of Lithium-ion Battery Separators
Lithium battery separators can be classified into several types based on their different physical and chemical properties, including woven membranes, nonwoven 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 battery separators in the early stages of lithium battery research and development. Although in recent years, research has been conducted on preparing lithium battery separators using other materials, such as using polyvinylidene fluoride (PVDF) as the bulk polymer in a phase inversion method, and studying cellulose composite membranes as lithium battery separator materials, etc.
