As the “guardian of ion channels” for lithium batteries, the core mission of the lithium battery separator is to separate the positive and negative electrodes to prevent short circuits, and at the same time build a smooth channel for lithium ion migration through the internal tortuous and connected micropores. Its performance directly determines the battery’s capacity, cycle life and safety boundary. High-performance separators need to balance multiple indicators such as thickness uniformity, mechanical strength, air permeability, and thermal stability, and the realization of these characteristics is inseparable from precise process control and scientific parameter design.
Starting from the core functional mechanism, this article will explain the mainstream preparation processes, key characterization parameters, inventory new separator technologies and the global industrial pattern, providing comprehensive reference for scientific research and production.
1. Core Mechanism: How Does the Separator Balance “Isolation” and “Conduction”?
The working logic of the separator seems simple, but it is actually intricate:
Isolation Function: Physically separate the positive and negative electrode plates, prevent direct passage of electrons, and avoid short circuits from the root;
Conduction Function: A large number of internal micropores form ion transport channels, allowing lithium ions to migrate freely between the positive and negative electrodes, build an internal conductive loop of the battery, and form a complete current with the external circuit.
A common misunderstanding needs to be corrected: the statement that “the separator allows ions to pass but blocks electrons” is inaccurate. The internal conduction of the battery relies on ion migration, and electrons are only transmitted through the external circuit. The core function of the separator is physical isolation rather than selectively “screening” ions and electrons.
2. Mainstream Preparation Processes: Core Differences Between Dry and Wet Methods
Micropore preparation technology is the core of separator production. According to the pore formation mechanism, it can be divided into two major processes: dry method and wet method. Each has its own focus in terms of process, structure and performance:
1. Dry Process: Environmentally Friendly “Stretching Pore Formation” Technology
Polyolefin resin is made into a crystalline film through extrusion and film blowing, and then stretched at high temperature to form a microporous structure. No solvent is used in the production process, and the environmental protection is outstanding.
Uniaxial Stretching Process: The prepared film has oblong and through micropores, excellent conductivity, longitudinal strength much higher than transverse, and extremely small transverse thermal shrinkage;
Biaxial Stretching Process: Add β-crystal modifier to PP, use the difference in phase density to form micropores through crystal transformation. The film has balanced longitudinal and transverse strength and more uniform pore size distribution.
2. Wet Process: High-Precision “Phase Separation Pore Formation” Technology
Also known as phase separation method or thermally induced phase separation method, polyolefin resin is heated, melted and mixed with low molecular weight substances such as liquid hydrocarbons and paraffin, then cooled for phase separation and pressed to form a microporous film.
Advantages: Complex three-dimensional structure, high micropore tortuosity, and better pore size precision and uniformity than dry separators;
Disadvantages: Poor environmental protection due to the use of solvents, poor thermal stability, and more complex process flow.
According to the different stretching methods, the wet process can be divided into biaxial synchronous stretching (consistent longitudinal and transverse performance) and biaxial asynchronous stretching (first longitudinal then transverse, with large differences in longitudinal and transverse performance).
3. Key Characterization Parameters: Understand the “13 Core Indicators” of Separator Performance
The various performances of the separator are defined by a series of key parameters, and each parameter directly or indirectly affects the battery performance. The following is an analysis of the 13 most core characterization indicators:
ParameterDefinition & Typical ValueUnitCore ImpactThicknessCommon: 16, 18, 20, 25, 30μm; consumer batteries tend to be thinner, power batteries prefer thicker specificationsμmThe thicker the thickness, the higher the safety and puncture strength, but the lower the capacity and higher the internal resistanceAir Permeability (Gurley Number)Time for a certain volume of gas to pass through a unit area of separator, typical value: 200~800s/100mls/100mlFor the same type of separator, the larger the value, the higher the battery internal resistance (no direct comparability between different types of separators)Liquid Absorption RateWeight of electrolyte absorbed by the separator, no uniform standard; more absorption at the same thickness means better wettabilityg/m²Affects internal resistance and capacity; poor wettability will block ion transportPore SizeWet separator: 0.01~0.1μm; dry separator: 0.1~0.3μmμmToo large pore size is prone to short circuit; too small increases ion transport resistancePuncture StrengthAbility to resist puncture by sharp objects, wet typical value: > 300g/20μmg / thicknessResists electrode burrs and lithium dendrite puncture, reducing short circuit riskTensile StrengthWet MD/TD>90MPa; dry TD>150MPa, MD>5MPaMPaAffects deformation risk during manufacturing; insufficient strength easily leads to separator damageThermal Shrinkage Rate90℃ for 2h: wet MD<5.0%, TD<3.0%; dry MD<3.0%, TD<1.0%%Reflects high-temperature dimensional stability; excessive shrinkage easily leads to electrode exposure and short circuitShutdown/Breakdown TemperaturePE film: shutdown 128~135℃, breakdown > 145℃; PP film: shutdown 150~166℃℃Shutdown temperature is the safety protection threshold; breakdown temperature is the ultimate safety boundaryPorosityTypical value: 40~60%, reflecting the number of internal micropores%Too low porosity increases internal resistance; too high may affect mechanical strengthChemical StabilityRequires no reaction with electrolyte; polyolefin materials (PE/PP) all meet inert requirements-Ensures long-term use of the separator without failure and avoids electrolyte contaminationStatic ElectricityExcessive surface static electricity leads to dust absorption and difficulty in lamination alignment-Affects production efficiency and may cause process defectsCurvatureArc formed after slitting; excessive curvature leads to misalignment in lamination and spiral winding-Causes risk of electrode exposure and short circuitAreal DensityWeight per unit area, typical value: 8~12g/m²g/m²Increased areal density is usually accompanied by decreased porosity and air permeability
4. New Separator Technologies: Breaking the Performance Boundaries of Traditional Polyolefins
To meet higher safety and performance requirements, new separator technologies are constantly emerging in the industry, mainly including the following four categories:
1. Multi-Layer Composite Separator
Represented by PP/PE two-layer or PP/PE/PP three-layer composite structure, it combines the advantages of PP’s good mechanical performance and high melting temperature with PE’s flexibility and low shutdown temperature, greatly improving safety performance; however, polyolefin materials have poor affinity for electrolytes, and the short-circuit area is easy to expand in case of short circuit.
2. Organic/Inorganic Composite Separator
Coat inorganic particles such as Al₂O₃ and SiO₂ on the surface of polyolefin film or non-woven fabric, which has both the flexibility of organic materials and the low thermal conductivity and high stability of inorganic materials. It can absorb trace moisture in the electrolyte, extend battery life, and improve high-power charge and discharge performance.
3. Nanofiber Coated Separator
Coat nanoscale fibers such as PVDF on the surface of the base film, which not only improves high-temperature shrinkage resistance, but also enhances electrolyte affinity and electrode compatibility, and significantly improves liquid absorption and adhesion.
4. Electrospun Separator
Nanofiber films are formed by atomizing polymer solutions through electric fields, which have large specific surface area, high porosity (50%~60%) and small pore size; however, the mechanical performance is poor, and the strength needs to be enhanced through coaxial spinning, composite modification and other methods.
5. Global Industrial Pattern: Inventory of Mainstream Manufacturers
At present, the global separator market has formed a multi-regional competitive pattern, and mainstream enterprises have their own technical characteristics:
Representatives of Dry Process: Celgard (USA), UBE (Japan), etc.;
Representatives of Wet Process: Asahi Kasei (Japan), Tonen (Japan), Mitsui Chemicals (Japan), SK (South Korea), Entek (USA), etc.;
New Separator Technologies: Degussa (Germany, Separion separator), Hitachi Maxell (Japan), Oji Paper (Japan), etc. have in-depth layouts in organic/inorganic composite and cellulose-based separators.
Conclusion
The performance optimization of lithium battery separators is a process of “multi-parameter balance”. Dry and wet processes have their own applicable scenarios, and key characterization parameters directly determine the safety, capacity and life of the battery. With technological iteration, new separators such as multi-layer composite, organic/inorganic composite, and electrospinning continue to break the performance boundaries of traditional polyolefins, providing core support for high-energy density and high-safety lithium batteries.
For researchers, it is necessary to focus on parameter synergy optimization and new material research and development; for production enterprises, it is necessary to select suitable processes and parameters according to application scenarios (consumer electronics/power batteries/energy storage batteries), balancing performance, cost and environmental protection. In the future, separator technology will develop towards thinner, safer and more compatible with high-voltage systems, becoming an important support for the upgrading of the new energy industry.
For more in-depth research on lithium battery separator mechanisms, process optimization and new technology development, you can refer to the research published by the Journal of Power Sources. Our previous articles on ceramic separator materials and processes and CCS & PCS separator coatings further elaborate on the development of battery separator technologies. For detailed industry standards and characterization parameter specifications, refer to the report released by the Institute of Electrical and Electronics Engineers (IEEE).