The role and technical requirements of 21700 battery pack diaphragms

A brief analysis of the development of 21700 battery pack diaphragm technology. In the structure of 21700 battery packs, the diaphragm is one of the key inner components. The diaphragm is one of the key components of 21700 battery packs. The quality of the diaphragm directly affects the battery capacity, charge and discharge cycle life, flame retardant and explosion-proof safety performance and other indicators.

The role of 21700 battery pack diaphragms

The main function of the diaphragm is to separate the positive and negative electrodes of the battery to prevent the two electrodes from contacting and short-circuiting. In addition, it also has the function of allowing electrolyte ions to pass through. The diaphragm material is non-conductive, and its physical and chemical properties have a great influence on the performance of the battery. Different types of batteries use different diaphragms.

Technical requirements for 21700 battery pack diaphragms

1. It has electronic insulation to ensure mechanical isolation of positive and negative electrodes.

2. It has a certain pore size and porosity to ensure low resistance and high ionic conductivity, and has good permeability to lithium ions.

3. Since the solvent of the electrolyte is a highly polar organic compound, the diaphragm must be resistant to electrolyte corrosion and have sufficient chemical and electrochemical stability.

4. Good wettability to electrolyte and sufficient liquid absorption and moisture retention capacity.

5. Sufficient mechanical properties, including puncture strength, tensile strength, etc., but the thickness is as small as possible.

6. Good spatial stability and flatness.

7. Good thermal stability and automatic shutdown protection performance.

Current status of technical development of 21700 battery pack diaphragms

21700 battery pack diaphragms are generally divided into dry process and wet process according to the process. Therefore, diaphragm products are also divided into dry diaphragms and wet diaphragms. From the perspective of process technology progress, at present, my country's 21700 battery pack diaphragms have made major breakthroughs in dry process, with world-class manufacturing level, and wet diaphragm process technology has been continuously improved. In recent years, production capacity and output have steadily increased.

From the current market application, from the perspective of the choice of global 21700 battery companies, dry diaphragms and wet diaphragms coexist. However, in recent years, with the increase in the proportion of ternary lithium batteries, the proportion of wet diaphragm production has also steadily increased. The latest data shows that in 2017, the domestic wet-process diaphragm production reached 806 million square meters, accounting for 56%, exceeding the share of dry-process diaphragms. In view of the respective characteristics of dry-process diaphragms and wet-process diaphragms, the demand for wet-process diaphragms will gradually expand in the future, but it will not completely replace dry-process diaphragms in a short time.

New materials for 21700 battery pack diaphragms, new changes

In the field of technological development, traditional polyolefin diaphragms can no longer meet the current needs of 21700 battery packs. High porosity, high thermal resistance, high melting point, high strength, and good wettability to electrolytes are the future development direction of 21700 battery packs.

In response to the development needs of 21700 battery pack technology, researchers have developed various new 21700 battery diaphragm materials based on traditional polyolefin diaphragms. Non-woven diaphragms use non-woven methods to orient or randomly arrange fibers to form a fiber mesh structure, and then use chemical or physical methods to reinforce the film to make it have good air permeability and liquid absorption rate.

Natural materials and synthetic materials have been widely used in the preparation of non-woven membranes. Natural materials mainly include cellulose and its derivatives, and synthetic materials include polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyamide (PA), polyimide (PI), aramid (meta-aramid, PMIA; para-aramid PPTA), etc.

21700 battery separator.jpg

21700 battery pack separator

The new separator material has greatly improved the safety performance and structural stability of 21700 battery packs, and also has an impact on the cycle life of 21700 battery packs. With the development of science and technology, domestic separator technology has become more and more developed. I believe that the overall distance of domestic separators to enter the world's first echelon is not far away.

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