Introduction to diaphragm manufacturing process

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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❶Dry Process （ Dry Process ）

1 Principle : Micro-holes are formed in a polymer film by physical stretching .

②Material : 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 .

④Characteristics :

💬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.

 

❷Wet Process （ Wet Process ）

1 Principle : TIPS （ Thermotropic Inverse Phase Separation ） is used to form uniform microporous .

②Material : 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 .

④Characteristics :

💬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 .

②Process :

1. Wet coating: The paste is coated and dried.
2. Dry coating: The powder directly heats the film.

③Function :

💬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

Select Tools PP PE

【 Porosity 】 30% - 40% 0% - 60%

【 Thickness 】 Thicker (16-40 μm) Thinner (5-20 μm)

【 Cost 】 Low High （ solvent recovery 30 % of cost ）

This is a great deal. 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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❶Combination of dry and wet method : such as dry base film wet coating , taking into account the cost and performance .

❷New materials : develop heat-resistant diaphragms such as polyimide （ PI ） and cellulose .

❸Thin / High Security : Increase energy density while reducing the risk of thermal runaway .