At present, the mainstream production process of lithium-ion battery separators mainly includes two kinds of "dry-form stretching process" and "wet-form (phase separation) process," which have characteristics and often combine with subsequent coating treatment to improve performance. Here are the specific processes and comparisons:
☛Previous review: What are the key quality characteristics of the diaphragm？
01
Mainstream production processes
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❶ Dry Process （ Dry Process ）
1 Principle : Micro-holes are formed in a polymer film by physical stretching .
② Material : mainly polypropylene （ PP ） .
3 Step :
1 . Raw material mixing : PP resin is mixed with additives such as pore-forming agents .
2 . Melt extrusion : Melt at high temperature and extrude into thick sheet （ extrude into film ） .
3 . Annealing treatment : eliminate internal stress and form a hard elastic film .
4 . Unidirectional stretching : stretching along the longitudinal direction to form slit-shaped micropores .
5 . Thermal setting : high temperature fixed pore structure to avoid shrinkage .
④ Characteristics :
💬Advantages: simple process, low cost, environmentally friendly (no solvent required).
💬Disadvantages: Poor aperture uniformity and low mechanical strength.
💬Applications: power batteries (such as lithium iron phosphate batteries), energy storage batteries.
❷ Wet Process （ Wet Process ）
1 Principle : TIPS （ Thermotropic Inverse Phase Separation ） is used to form uniform microporous .
② Material : polyethylene （ PE ） with solvent （ such as white oil , paraffin oil ） .
3 Step :
1 . Raw material blending : PE is blended with liquid hydrocarbon solvents .
2 . Melt extrusion : forming homogeneous gel film .
3 . Biaxial stretching : Microcellularity is formed by longitudinal （ MD ） and transverse （ TD ） stretching .
4 . Solvent extraction : Remove residual solvents with a volatile solvent （ e.g. dichloromethane ） .
5 . Drying and fixing : Remove the solvent and roll up .
④ Characteristics :
💬Advantages: The aperture is uniform and the porosity is high (40% -60%), suitable for high energy density batteries.
💬Cons: Cost is high, process complex, and solvent recovery needs to be treated environmentally.
💬Applications: tritium material batteries, consumer electronics batteries (e.g. mobile phones, notebooks).
02
Follow-up treatment processes
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In order to improve the membrane performance, coating treatment is usually carried out:
1 Material : Ceramic （ Al2O3 , SiO2 ） , PVDF , Aramid , etc .
② Process :
1. Wet coating: The paste is coated and dried.
2. Dry coating: The powder directly heats the film.
③ Function :
💬Enhance high temperature resistance (prevent thermal contraction).
💬Improve electrolyte infiltration.
💬Improve mechanical strength and safety.
03
Process comparison
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【 Item 】 Dry process Wet process
【 Material 】 PP PE
【 Porosity 】 30% - 40% 40% - 60%
【 Thickness 】 Thicker （ 16-40 μm ） Thinner （ 5-20 μm ）
【 Cost 】 Low High （ solvent recovery 30 % of cost ）
【 Properties 】 Low mechanical strength Excellent uniformity and closed cell properties
【 Application area 】 Power battery , energy storage , high-end consumer electronics , ternary battery
04
Trends
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❶ Combination of dry and wet method : such as dry base film + wet coating , taking into account the cost and performance .
❷ New materials : develop heat-resistant diaphragms such as polyimide （ PI ） and cellulose .
❸ Thin / High Security : Increase energy density while reducing the risk of thermal runaway .
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