As one of the key inner layer components in the battery, the separator will have an important impact on the interface structure, internal resistance, capacity, cycle and safety performance of the battery. The excellent performance of the separator is conducive to improving the overall performance of the battery. The main functions of the diaphragm are physical barrier, electrolyte storage, ion permeation and safety. Among them, the physical barrier refers to the separation of the positive and negative electrodes of the battery to prevent the electrodes from contacting and inducing a short circuit; electrolyte storage and ion permeation refer to the fact that the pore structure inside the separator can store a certain amount of electrolyte, and use the distribution of pore size to promote the passage of ions; the safety function is that the pore structure in the separator is closed by the influence of charging or temperature, thereby blocking the conduction of current, preventing the battery from overheating or other dangers.
General battery requirements for separators mainly focus on thickness, ion permeability, pore size and porosity, chemical stability, thermal stability and mechanical stability, etc., and have the following basic requirements: it has electronic insulation to ensure good mechanical barrier between electrodes; moderate pore size and porosity, low resistance, high ionic conductivity, good lithium ion permeability; good chemical and electrochemical stability, resistance to electrolyte corrosion; it has good wettability with electrolyte and has a certain ability to absorb liquid; it is thin and has good mechanical properties, and has certain puncture strength, tensile strength, etc.; good thermal stability and automatic shutdown protection performance.
According to the different physical and chemical properties, battery separators can be divided into woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, separator paper, and laminated membranes. Among them, the advantages of polyolefin materials in mechanical properties, chemical stability and price make polyolefin microporous films widely used as battery separators.
In recent years, some other separators such as polyvinylidene fluoride (PVDF) separators, polyamide/imide separators, cellulose composite membranes, etc. have also been partially studied and used in batteries, but commercial separators still focus on single-layer or multi-layer composite separators of polyethylene and polypropylene.
In addition to the basic requirements of traditional separators, the separators in lithium-sulfur batteries should also have a certain limiting effect on soluble lithium polysulfides and have a certain impact on the stability of metal lithium anodes. Therefore, it is very necessary to carry out surface modification and modification for conventional separators, such as introducing functional groups or modified layers on the surface of the separators. Scientists skillfully combined the intermediate intercalation with the separator, and directly coated the slurry on the surface of the separator to obtain a modified separator. The use of modified separators in lithium-sulfur batteries can significantly improve the electrochemical performance of sulfur electrodes. Various carbon materials, functional polymers, ceramic materials and composite coating materials can be used for battery separator coating. In view of various problems in lithium-sulfur batteries, the targeted selection of separator coating materials, such as electronic or ionic conductors, promoting electrolyte flow, and Li2Sn adsorption materials, is helpful for the construction of high-performance lithium-sulfur batteries.