Fast charging of batteries: electrolyte, binder, and separator!

1、 Electrolyte: New type of carp salt LiFSI usage is expected to increase under fast charging system
The new type of carp salt LiFSI increases conductivity and adapts to fast charging systems, with the potential for increased usage. Under fast charging conditions, there are higher requirements for the ion conductivity and thermal stability of the electrolyte. In the conventional electrolyte of ethylene carbonate vinegar/methyl ethyl carbonate vinegar (EC/EMC), the electrolyte containing LiESI has higher conductivity than the electrolyte containing other lithium salts (LiFSI>LiPF6>LITFSI>LiCIO4 LiBF4), and its fluorine content is lower, making it more environmentally friendly. Therefore, LiFSI is considered the most promising lithium salt to replace LiPF6, especially in the field of fast charging batteries.

2、 Adhesive: Silicon based negative electrode drives PAA adhesive to increase permeability
The easy expansion of silicon-based negative electrodes presents a development opportunity for the new adhesive PAA. Lithium battery binder is one of the important components of lithium-ion battery electrode sheets. Its main function is to connect the electrode active material, conductive agent, and electrode current collector, so that the electrode active material, conductive agent, and current collector have overall connectivity, thereby reducing the impedance of the electrode. It is a technically advanced additional material in lithium-ion battery materials.

There are various types of binders for lithium batteries, divided into oil-based and water-based types. Oily typical binders include PVDF water-based binders such as CMC+SBR and PAA. Currently, PVDF has the widest application range, accounting for 54% in China in 2020, while other binders account for 46%. However, PVDF is an oily binder, and its solvent is harmful to the environment. PVDF also contains fluorine, which easily reacts with lithium embedded graphite and other materials. CMC+SBR has limited adhesion, and the tendency of silicon-based negative electrodes to expand provides opportunities for the development of new binders, such as PAA. PAA has strong adhesion, as its side chains contain more functional groups that can form hydrogen bonds with the light groups present in the active material, connecting the negative active material with the current collector. PAA can improve the cycling performance of silicon-based negative electrodes by forming a coating layer similar to SEI film with silicon.

The localization of adhesives is accelerating. In 2021, companies from the United States, Japan, and Europe held over 90% of the market share, with major manufacturers including Est é e Land from the United States, Norion from the Netherlands, BASF from Germany, Rayon Corporation from Japan, A&L Corporation, JSR Corporation, and Daicel; Domestic adhesive companies started relatively late, but in recent years, technological accumulation has led to continuous optimization of product performance, as well as continuous improvement of supporting raw materials and equipment, gradually achieving domestic substitution. Major domestic enterprises include Gudile, Shenzhen Yanyi, Jingrui Electric Materials, Songbai Chemical, Jinbang Electric Power, and Chongqing Leehom. In addition, in terms of PAA, the domestic technology iteration is relatively fast, and the main enterprises include Innolux (Putailai Holdings) Shenzhen Research Institute, Blue Ocean Blackstone, and Huitian New Materials.
3、 Diaphragm: The performance requirements of fast charging drive the demand for organic coatings to increase
The membrane coating technology comprehensively improves the performance of the membrane and effectively enhances the safety of the battery. Due to the relatively low thermal deformation temperature of polyethylene and polypropylene, the main raw materials for separators, severe thermal shrinkage occurs when the temperature is too high, resulting in short circuits between the positive and negative electrodes of the battery. Therefore, there is a risk of battery combustion or explosion in the harsh environment of high vibration and high temperature for new energy vehicle batteries. In order to improve the thermal stability of separators and comprehensively enhance their comprehensive performance, coating technology is gradually being applied in the production of lithium batteries.
The membrane coating technology not only significantly improves the thermal stability of the membrane by coating the coating material on the surface of the membrane, but also increases the tensile strength and breathability rate, thereby improving the safety of the battery. It is an effective measure to ensure the safety of new energy vehicles. At present, coating materials are mainly divided into the following types: 1) Inorganic materials such as Al2O3, boehmite, etc.: improve high temperature resistance, good liquid absorption and retention ability. Reduce thermal shrinkage rate, effectively improve cycling performance and safety performance. For example, in the thermal stability test of boehmite coating on polyethylene base film, when the temperature is heated to 170 degrees, the membrane has undergone significant deformation, and the coating film has almost no shrinkage. 2) Organic materials such as PVDF, PET, cellulose, aramid, etc. have high adhesion and excellent liquid absorption and retention abilities, reducing internal resistance and improving electrochemical performance. From the comparison before and after coating PVDF on polyethylene film, it can be seen that the polyethylene film presents a typical dendritic microporous structure of wet membrane. After surface coating with PVDF organic particles, a layer of PVDF coating is attached to the polyethylene film, forming a large number of micropores and improving the electrolyte retention rate.

The performance requirements of fast charging are driving the demand for organic coatings to increase. The performance of the separator affects the diffusion rate of carp ions, the retention of electrolyte, the internal resistance of the system, and the composition of the battery interface structure, thereby affecting the capacity, lifespan, rate performance, etc. of the battery. In the fast charging system, a comparison was made between organic coating materials and inorganic coating materials, and it was found that organic materials were significantly superior to inorganic materials in terms of pore size, electrolyte wettability, and stability.
Among them, polyolefin membranes have good mechanical strength and electrochemical stability, uniform pore structure, and suitable thermal sealing performance. However, due to the material's inherent hydrophobicity and low surface energy, the electrolyte has poor wettability, and excessive temperature can cause severe shrinkage. The pore size of polyolefin membrane is 100nm X 400nm; PET and cellulose membranes are composed of interwoven and stacked fibers with a diameter of 0.2-2um; Aramid membrane is composed of fibers with a diameter of 2-4um interwoven and stacked, with a large pore size that provides a channel for rapid ion migration. In addition, aramid is a polar material with excellent temperature resistance and high breaking temperature, which improves the safety performance of the battery. It has good wettability with the electrolyte, which is conducive to the absorption of the electrolyte. Therefore, the capacity decay is also slow. The application of high electrolyte wettability and liquid absorption rate separators in lithium-ion batteries can improve the rate performance and cycling performance of the battery.