Due to the characteristics of the material itself, wet and dry diaphragms have their own advantages and disadvantages:
The wet-process diaphragm is light and thin, not easy to tear, but the melting point of PE is 135℃, which is less safe than the dry-process diaphragm. In addition to the different raw materials and production processes, the overall cost is higher than that of the dry-process diaphragm; It has better high-pressure resistance and oxidation resistance, but it is thicker than wet-process separators and is prone to longitudinal tearing, which requires higher process requirements for battery companies.
With the rapid expansion of ternary power batteries, especially high-nickel power batteries, the need for diaphragms with the advantages of high safety, strong functionality, and low cost is imminent, which has become the direction of joint efforts of domestic diaphragm companies. The "three-layer composite membrane" developed by the international diaphragm giant Celgard may be able to inspire some domestic counterparts.
The middle layer of Celgard's "three-layer composite film" is polyethylene, which has a melting point of about 132°C. When the battery is out of control and heats up, the polyethylene will melt, closing the pores and cutting off the circuit, which can form safety protection for the battery; the outer two layers are polypropylene, It has oxidation resistance and the melting point is above 165℃. Even if the battery temperature rises slightly, the physical shape of the membrane will not be affected, providing a second layer of safety protection for the battery.
In addition to high safety, Celgard President Shi Lie publicly stated that the dry process of the company's "three-layer composite membrane" products is greener and environmentally friendly than wet membranes.
Wet process requires chemical solvents and solutions. Although manufacturers are working hard to recover them, some will evaporate into the air in the end, which is harmful to people and the environment.
Shi Lie disagrees with the view that some people in China believe that the application of wet-process separators in the field of power batteries is broader. In his opinion, at the beginning of the rise of new energy vehicles, dry-process diaphragms dominated because wet-process products had relatively poor oxygen resistance and large lateral shrinkage; they would melt when the temperature reached 130°C, which was very limited; in terms of strength, The wet-process diaphragm drops by more than 50% after a period of use, while the three-layer composite film hardly changes, and its life span is longer than that of the wet-process polyethylene diaphragm.
From an objective point of view, after the wet-process diaphragm is coated, it will effectively avoid the "defects" in the inherent characteristics of the material, but this will increase the cost.
At present, most domestic ternary power battery systems use "wet method + coating". Based on cost considerations, dry-method separators with progressively improved performance have been used in large quantities for lithium iron phosphate systems, and they have also begun to be applied to ternary systems. The leading enterprises of the meta-system also use dry-process diaphragms.
At present, the proportion of high-nickel power batteries in the world is not high. Whether "wet method + coating" can simultaneously meet the requirements of high safety, strong functionality, and low-cost separators, it remains to be further verified.
Shi Lie said that the three-layer composite membrane is the only separator that can be used in high-energy batteries without coating, and the direct use of the base membrane will be more competitive than the polyethylene ceramic separator.
"Different customers or different battery designs sometimes require us to coat a layer of ceramics, so the price will be more expensive." Shi Lie added that Celgard has many successful cases in Japan, South Korea and some Chinese battery manufacturers. In the past many years, the market has given very good feedback on CELGARD's "three-layer composite membrane", especially in electric vehicles and energy storage batteries, and there has never been a safety accident, and the reliability is very guaranteed. We welcome more battery manufacturers to make more attempts and cooperation.
It is understood that Celgard is the "originator" of the global dry-process diaphragm technology. It has a history of 30 to 40 years in the diaphragm industry and is now a subsidiary of Asahi Kasei. Most of China's dry process diaphragm companies have followed Celgard's dry process, but what Chinese manufacturers cannot replicate at present is the "three-layer composite membrane" developed and imported by Celgard.
"We started to study three-layer membranes as early as a few years ago, but there was no matching production equipment at the time, and one of the raw materials needed to be imported from abroad." A senior executive of a domestic dry-process diaphragm company said that most of the domestic double-layer membranes are produced. Because not be thin'.
However, with the continuous improvement of raw materials, production equipment, and process technology, some domestic dry-process diaphragm companies are trying to develop and launch three-layer composite membranes.
