In the structure of a lithium-ion battery, the separator is one of the key internal components. For lithium-ion batteries, since the electrolyte is an organic solvent system, separator materials that are resistant to organic solvents are required. Generally, high-strength thin-film polyolefin porous membranes are used.
I. Performance of Lithium-ion Battery Separator Products
Since the performance of the separator in a lithium-ion battery determines the key characteristics of the battery, such as capacity, cycle performance, and charge/discharge current density, the separator must have appropriate thickness, ion permeability, pore size and porosity, as well as sufficient chemical stability, thermal stability and mechanical stability.
Lithium-ion battery separators have a large number of tortuous and interconnected micropores, which can ensure that electrolyte ions can pass freely to form a charging and discharging circuit. When the battery is overcharged or the temperature rises, the separator separates the positive and negative electrodes of the battery through its pore-closing function to prevent them from directly contacting each other and short-circuiting, thereby blocking current conduction and preventing the battery from overheating or even exploding.
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 other materials have been used to prepare lithium-ion battery separators, such as the phase inversion method used by F. Boudin et al. in 1999 to prepare lithium-ion battery separators using polyvinylidene fluoride (PVDF) as the bulk polymer, and the cellulose composite membranes studied by Kuriyaash Isao et al. as lithium-ion battery separator materials, other materials have also been used.
However, to date, commercially available lithium-ion battery separator materials still primarily use polyethylene and polypropylene microporous membranes. Solid and gel electrolytes are beginning to be used as a special component, simultaneously functioning as both electrolyte and battery separator, representing an emerging technological approach.
