Comparative analysis of dry PP diaphragm and wet PE diaphragm in lithium batteries

2025-07-09

The role of battery separators in lithium-ion batteries

Lithium-ion battery separator (Separator) is a key functional film material located between the positive and negative electrodes in the battery structure.

Lithium-ion battery separator (Separator) blocks electrons, preventing short circuits, but also allows lithium ions to travel through and support the charging and discharging process of the battery.

Its role is not only to physically isolate the positive and negative electrodes, but also directly affects the safety, ion transfer efficiency, cycle life and thermal stability of the battery.

With the rapid development of new energy vehicles, energy storage and consumer electronics industries, the requirements for lithium-ion battery performance, safety and cost continue to rise. Battery diaphragm, as one of the four core materials of the battery, plays a crucial role in ensuring the normal operation of the battery and thermal runaway protection. Currently, the mainstream diaphragm technologies in the market mainly include polypropylene (PP) dry diaphragm and ultra-high molecular weight polyethylene (UHMWPE) wet diaphragm. These two diaphragms have their own characteristics in terms of raw material, process, structure, performance and application direction, and will be systematically compared and analyzed in this paper.

Ⅰ. Differences in raw materials and process paths

Dry diaphragm mainly adopts high isotactic polypropylene as raw material, and forms microporous structure through melt extrusion, shaping and bi-directional stretching. The core of the process lies in the formation of micropores induced by stretching, and the whole process does not use solvents, which is environmentally friendly.
Wet diaphragms, on the other hand, use ultra high molecular weight polyethylene as the substrate, mixed with paraffin or liquid hydrocarbons to prepare a colloidal fluid, which is calendered into a film, then solvent extracted to remove the oil phase, and bi-directionally stretched to form a dense, microporous structure. This process is more demanding in terms of equipment and purification systems, but results in porous diaphragms with higher porosity and smaller pore sizes.

II. Microstructure and performance comparison

	Polypropylene dry diaphragm (PP)	Ultra-high polymer polyethylene wet diaphragm (PE)
Microstructure	Trapezoidal pores, fibrous arrangement, regular structure	Reticulated porous structure with smaller and denser pores
Thickness Range	16–25 μm	8–20 μm
Porosity	35–45%	40–60%
Thermal shutdown temperature	160–165 °C(PP melting point)	130–140 °C(PE melting point)
Electrolyte Wettability	Medium, needs surface treatment	Good, high liquid absorption
Mechanical strength	High transverse strength and good puncture resistance	Good flexibility, but weak laterally
Costs	relatively low	relatively high

III. Differences in application areas

Areas of application	Common Diaphragm Types
Power battery (lithium iron phosphate)	PP dry diaphragm
Power battery (lithium ternary)	PE wet diaphragm
Energy storage batteries	PP dry diaphragm-based
Consumer electronics (cell phones, laptop computers)	PE wet diaphragms are more common

IV. Representative raw material companies and main products

Lithium battery separator (LiBS) is produced by dry treatment, mainly made of ultra-high purity polypropylene.Lithium battery stretch diaphragm can be manufactured without adding any additives, so the manufacturing technology and equipment requirements are relatively low and the cost is relatively low.

Common manufacturers include Yuhwa Polypro series, Singapore TPC, Lotte Chemical. The main production grades include Yuhwa Polypro S802M, Singapore TPC FS2014 and Lotte Chemical BS-120. The most important supplier is Yuhwa Korea petrochemical.

Yuhwa Polypro S802M high-purity polypropylene

It is the main raw material for manufacturing lithium battery separator (LiBS) using dry processing in the world. It has excellent low gray performance and rigidity, effectively ensuring the safety and stability of lithium batteries.

The wet process for manufacturing lithium battery separators (LiBS), mainly realized by using ultra high molecular weight polyethylene. It can produce thinner films with higher porosity and more uniform microporous structures. Therefore, it has become one of the mainstream application technologies for high-energy density batteries.

It is mainly used in the manufacture of separators for high level power batteries and consumer batteries. Since oily substances need to be mixed into the manufacturing process to form a fluid structure, it is called a wet process. Compared with the dry process, the wet process is more expensive and requires higher technology and equipment.

Currently, the world's mainstream ultra-high molecular weight polyethylene manufacturers mainly used for the production of battery separators are Yuhwa's Hiden series and Asahi Kasei's Sunfine series. Common raw materials include Yuhwa Hiden U050 UHMWPE and Asahi Kasei Sunfine UH650 UHMWPE.

Yuhwa Hiden U050 UHMWPE

This resin material is the main raw material for the manufacturing of battery separators produced by Yuhwa. It has excellent impact resistance and superior processing performance. Compared with conventional ultra-high molecular weight polyethylene materials, it has a more convenient processing procedure. At the same time, it is cheaper than Asahi Kasei's ultra high molecular weight polyethylene, so it is welcomed by battery manufacturers from China.

Asahi Kasei UH650 UHMWPE

Asahi Kasei has been exploring the manufacturing of battery separators since the early days of lithium battery development, and now has a very complete product system to meet the needs of different manufacturers.
Sunfine UH650 UHMWPE material adopts a molecular weight design to improve the puncture resistance and impact resistance of the separator; it uses Asahi Kasei's own advanced catalytic technology to minimize metal residues, effectively reducing the risk of equipment damage and effectively improving battery safety.

V. Conclusion

Dry PP diaphragm is widely used in lithium iron phosphate batteries, energy storage systems and some low-end consumer electronics products due to its relatively low cost and stable structure.
Wet PE diaphragms are dominant in ternary systems, high rate batteries, high-end power batteries (e.g. electric vehicles) and consumer electronics due to their excellent thermal shutdown capability and electrolyte affinity.

What is LCP material

Solutions for Yellowing of Polycarbonate