Known as the “third electrode” of lithium batteries, the polyolefin lithium battery separator is a core component that determines battery energy density, charge-discharge efficiency, cycle life, and safety. Currently, the mainstream lithium battery separators in the market are mainly polyolefin separators (PE/PP-based) and coated/composite separators. Among them, polyolefin separators have long dominated the market due to their mature processes, stable performance, and cost advantages.
The performance of the polyolefin lithium battery separator depends not only on scientific process design but also on the quality of upstream PP and PE raw materials. This article will systematically disassemble the mainstream preparation processes of separators (wet and dry methods), compare the characteristics and application scenarios of different types of polyolefin separators, and inventory the product advantages of 9 global core PP/PE raw material suppliers, providing comprehensive reference for process selection and raw material procurement in scientific research and industrial production.
Mainstream Separator Processes: Technical Game Between Wet and Dry Methods
The preparation processes of polyolefin separators are mainly divided into two categories: wet method (thermally induced phase separation method) and dry method (melt stretching method). The two processes have different focuses in principles, processes, and product characteristics, adapting to different battery application needs.
Wet Process: Thin and Uniform, Suitable for High-Performance Requirements
The wet process takes “thermally induced phase separation” as the core principle, producing submicron-sized microporous membranes through steps such as mixing, phase separation, stretching, and extraction of plasticizers and polyolefin resins. It is currently the mainstream technology for preparing high-performance thin separators.
Core Process: First, preprocess raw materials such as PE and pore-forming agents, then melt and plasticize them, and extrude and cast through a die to form thick sheets; then orient the molecular chains through longitudinal and transverse stretching (asynchronous and synchronous methods); then extract the pore-forming agent with a volatile solvent, and finally obtain the finished product through drying, shaping, and slitting.
Process Differences: Asynchronous stretching requires separate longitudinal and transverse stretching, with stronger adjustability; synchronous stretching orients in both longitudinal and transverse directions simultaneously, eliminating the need for separate longitudinal stretching steps. The separator has better thickness uniformity but has limitations such as slow speed and fixed longitudinal stretching ratio.
Product Advantages: The separator has good thickness uniformity, excellent physical and chemical properties, and mechanical properties. It is suitable for producing thinner single-layer PE separators, which can meet the strict requirements of high-performance batteries such as power batteries for separators.
Representative Enterprises: Asahi Kasei, Tonen, Entek, SK, W-scope, etc.
Dry Process: Diversified Adaptation, Covering Multiple Scenarios
The dry process is based on the principle of “melt stretching induced pore formation”, forming micropores through processes such as crystallization, stretching, and defect expansion of polymer melts. It is mainly divided into two sub-technologies: uniaxial stretching and biaxial stretching, with more abundant product types.
1. Dry Uniaxial Stretching
Core Process: Select low molecular weight, high fluidity PE or PP raw materials, melt and plasticize them, cast to form highly oriented, low-crystallinity cast sheets; obtain hard elastic films through heat treatment, then form micropores through cold stretching and hot stretching, and finally slit into finished products.
Product Characteristics: Uniform pore size, uniaxial orientation, relatively simple process, controllable cost, suitable for producing basic polyolefin separators.
2. Dry Biaxial Stretching
Core Principle: Utilize the β-crystal form characteristics of PP, add β-crystal form modifiers to PP raw materials, and form micropores through crystal form transformation (β-crystal → α-crystal) during the stretching process, making full use of the density difference between different phases to achieve pore formation.
Core Process: After preprocessing PP raw materials, cast to form cast sheets with high β-crystal content, first stretch longitudinally to form pores, then stretch transversely to expand pores and optimize pore size distribution, and finally perform high-temperature heat treatment to reduce thermal shrinkage and improve dimensional stability.
Product Characteristics: It balances mechanical strength and pore size uniformity, and can adapt to the basic needs of various battery scenarios.
Representative Enterprises: Xingyuan Material, Celgard, Ube Industries (uniaxial stretching); Green (biaxial stretching), etc.
Polyolefin Separator Family: Characteristics and Applications of Different Structures
According to the type and number of layers of the base material, polyolefin separators can be divided into single-layer PP, single-layer PE, PP/PE/PP three-layer and other types. Each type integrates the advantages of the corresponding base material and adapts to different battery performance requirements.
Structure TypeCore MaterialProduction MethodCore AdvantagesPotential ShortcomingsApplication ScopeSingle-layer PPPPDry MethodGood heat resistance and excellent ion permeabilityHigh shutdown temperature (>141℃), slightly slow safety shutdown responseDigital batteries, power batteriesSingle-layer PEPEDry Method, Wet MethodHigh mechanical strength, good low-temperature shutdown performance (about 130℃)Heat resistance is weaker than PPDigital batteriesThree-layer PP/PE/PPPP, PEDry MethodCombines PP’s heat resistance and PE’s shutdown characteristics, excellent mechanical strength and higher safetyRelatively poor high-temperature permeabilityDigital batteries
Upstream Core: Inventory of 9 Global PP/PE Raw Material Suppliers
Indicators such as purity, crystallinity, and molecular weight distribution of PP and PE raw materials directly affect the pore formation quality, mechanical strength, and electrochemical stability of the polyolefin lithium battery separator. The following are the product characteristics and advantages of 9 global core raw material suppliers:
1. KOREA PETROCHEMICAL
Headquartered in Seoul, with R&D and production bases in Ulsan, its core products include general-purpose polypropylene for LiBS separators.
Product Advantages: High-purity characteristics, minimizing impurity mixing through refined catalysts, excellent voltage resistance and long-term use stability; accurately controlling crystallinity and molecular weight distribution during polymerization to balance processability, pore uniformity, and physical properties of the membrane; optimizing molecular design to improve material heat resistance and durability, adapting to the long-term use needs of lithium batteries.
2. The Polyolefin Company (TPC), Singapore
A leading polyolefin manufacturer in Southeast Asia, with products covering LDPE, PP homopolymers, etc.
Product Advantages: PP homopolymer (COSMOPLENE®) is designed for diverse applications. The BOPP film grade has high-speed processability and precise thickness control, with outstanding transparency, mechanical properties, and barrier properties; LDPE (COSMOTHENE®) has excellent adhesion and sealing strength, small shrinkage, suitable for extrusion coating and other processing scenarios.
3. Celanese, USA
Headquartered in Dallas, Texas, it is the world’s largest supplier of ultra-high molecular weight polyethylene raw materials.
Product Advantages: The core product GUR® UHMW-PE (ultra-high molecular weight polyethylene) has high wear resistance, high impact resistance, surface non-stickiness, and self-lubrication, and maintains excellent mechanical properties at low temperatures; the product grades are complete, including UHMW-PE, HMW-PE, VHMW-PE, etc., adapting to different separator processing needs.
4. Mitsui Chemicals, Japan
Headquartered in Tokyo, with years of technical accumulation in the polyolefin field, its core product is HI-ZEX MILLION™ ultra-high molecular weight polyethylene powder.
Product Advantages: The average molecular weight reaches 500,000 to 6,000,000, with excellent mechanical properties and a density of only 935kg/m³, enabling product lightweight; non-absorbent, non-hygroscopic, no swelling or hydrolysis risk, no pre-heat treatment required; wear-resistant, impact-resistant, low friction coefficient, strong chemical stability, suitable for high-end applications such as battery separators.
5. NORTH HUAJIN
With multiple production bases, its core products include high rigidity and high heat resistance homopolypropylene (HJ8005), low flow homopolypropylene (F302).
Product Advantages: HJ8005 has high crystallinity, high rigidity, and excellent long-term heat resistance through catalyst adjustment and high-efficiency nucleating agent addition; F302 is produced by liquid-gas bulk polymerization process, with high purity, high isotacticity, and excellent electrical properties, adapting to the high-purity requirements of raw materials for separators.
6. CNPC Dushanzi
It has ten-million-ton crude oil processing capacity and one-million-ton ethylene production capacity. Its core products include polypropylene resin, high-density polyethylene resin, and linear low-density polyethylene resin.
Product Advantages: Relying on a complete refining and chemical industrial chain, it has stable raw material supply and strong product batch consistency, which can meet the needs of large-scale separator production and cover the raw material needs of basic polyolefin separators.
7. SINOPEC Yangzi
It has a large cluster of petrochemical plants, and its core products include low-density polyethylene, high-density polyethylene, and polypropylene resin.
Product Advantages: Polyethylene resin is odorless, non-toxic, high in electrical insulation, low in water absorption, and remains flexible at low temperatures; polypropylene resin has low density (0.90～0.91g/cm³), excellent mechanical properties and heat resistance, service temperature range of -30℃～140℃, strong chemical stability, adapting to diverse separator production scenarios.
8. SINOPEC Maoming
China’s first ten-million-ton refinery and one-million-ton ethylene plant, its core products cover various polyolefin materials such as polyethylene and polypropylene.
Product Advantages: Significant integration advantages of refining and chemical industry, able to stably supply different grades of PP and PE raw materials with high purity and stable performance, adapting to various separator preparation processes such as dry and wet methods, meeting the needs of large-scale industrial production.
9. SECCO, Shanghai
It has large ethylene cracking and downstream polyolefin plants, and its core products include LLDPE, HDPE, and polypropylene.
Product Advantages: The polyethylene plant adopts BP patent technology with an annual output of 600,000 tons, and the products have a wide range of applications; the annual output of polypropylene is 250,000 tons, which is light milky white translucent pellets, with excellent thermal performance and electrical insulation performance, small molding shrinkage, good stress crack resistance, and strong processing adaptability.
Coated Separators: The Performance Upgrade Path of Traditional Processes
With the continuous improvement of power batteries’ requirements for safety and heat resistance, simple polyolefin separators can no longer meet some high-end needs, so coated separators have emerged as the times require.
Coated separators use polyolefin, polyester, etc. as the base film, and coat functional coatings such as ceramic materials and heat-resistant polymers on the surface through dry coating, wet coating, dip coating and other processes to achieve performance upgrades:
Ceramic Coated Separators (with PP or PE microporous film as the base film): Significantly improve heat resistance and puncture resistance, and reduce thermal shrinkage;
PET-based Coated Separators: Optimize substrate performance through coating modification and expand application scenarios;
Heat-Resistant Polymer Coated Separators: Further enhance the chemical stability and high-temperature tolerance of the separator.
The core value of the coating process is to “develop strengths and avoid weaknesses”. While retaining the original advantages of the polyolefin base film, it makes up for its shortcomings such as heat resistance and puncture resistance, becoming the core choice for high-end power batteries. For more research on coated separator technology, you can refer to the research published by the Journal of Power Sources.
Conclusion: Process and Raw Material Synergy Drives Separator Technology Upgrade
The performance optimization of lithium battery separators is the result of two-way synergy between “process design” and “raw material selection”: the wet process occupies the high-performance market with the advantage of being thin and uniform, the dry process covers multiple scenarios with diverse adaptability, and the coating process becomes the performance upgrade direction of traditional separators; the indicators such as purity, crystallinity, and molecular weight distribution of PP and PE raw materials are the core basis for ensuring the process stability and product consistency of the polyolefin lithium battery separator.
For researchers and producers, it is necessary to accurately match the separator process and raw materials according to the battery’s application scenarios (digital, power, energy storage, etc.) and performance requirements: high-performance power batteries can prefer thin PE separators or ceramic coated separators prepared by wet process, combined with high-purity, high-heat resistance PP/PE raw materials; basic digital batteries can choose dry process separators to balance performance and cost.
In the future, as lithium batteries develop towards high energy density and high safety, the separator process will continue to upgrade towards “thinner, more uniform, and more high-temperature resistant”, while upstream raw materials will evolve towards “high purity, customization, and functionalization”. The in-depth synergy between process and raw materials will become the core driving force for the high-quality development of the lithium battery separator industry. Our previous article onPGZ composite separators further elaborates on the development of high-performance separator materials. For detailed industry data on polyolefin materials, refer to the report released by the Institute of Electrical and Electronics Engineers (IEEE).