Battery Diaphragm Air Permeability Test

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The separator is an important raw material used in making lithium-ion batteries. Its main role is to keep the positive and negative electrodes apart and serve as a path for ions to move between them. The microporous structure on the separator is exactly the key path for these ions, and its breathability directly affects the battery's performance.

With the global energy structure rapidly shifting toward green and low-carbon solutions, lithium batteries, as the core power source for new energy vehicles, energy storage systems, and consumer electronics, are drawing a lot of attention for their performance and safety. Inside these batteries, the battery separator acts like a 'heart valve'—it not only keeps the positive and negative electrodes apart to prevent short circuits, but its breathability also directly affects lithium-ion transfer efficiency, internal battery pressure regulation, and the risk of thermal runaway. At the same time, in the field of hydrogen fuel cells, the gas permeability of proton exchange membranes directly determines the battery’s efficiency and lifespan. So, scientifically, accurately, and efficiently testing the breathability of these key membrane materials has become a core part of quality control in the lithium and hydrogen energy industries.

Method of Diaphragm Air Permeability Test:

Use a breathability measurement process, which involves testing the air permeability of the battery separator. The process includes: fixing the battery separator, applying air pressure to one side, measuring the pressure drop and the time it takes, and detecting the separator's breathability. When the lithium battery separator is fixed, a certain air pressure is applied to one side. Because of the micropores in the separator, the pressure gradually decreases until it equals atmospheric pressure. By comparing the time it takes for the pressure to drop from the initial level to the final level, we can determine the separator's breathability. By comparing the pressure drop times of different separators, we can evaluate their breathability performance. The shorter the pressure drop time, the better the breathability. The whole test is simple, quick, and provides data that directly reflects the separator's breathability, making it very practical.

The microporous structure of the separator provides a "natural passage" for lithium ions to move, but how smoothly they flow is influenced by several factors, including the thickness of the separator, pore size, pore size distribution, porosity, tortuosity, and coating modification processes. We generally use "air permeability" to describe the breathability of the separator, which indirectly reflects how easily lithium ions can pass through it.

Air permeability testing is a simple and quick experiment using an air permeability tester. According to standard, three representative, flat, and defect-free pieces of separator material are cut to a suitable size and placed in the center of the instrument’s test chamber. Parameters like pressure are set, and the instrument automatically measures the time it takes for 100 mL of air to pass through the tested area of the separator sample under a pressure of 1.21 kPa. The test uses the C190H air permeability tester, which has a testing range of 30–1500 s/100 mL and supports multiple units like s/100 mL and µm/(Pa·s), meeting the requirements of different standards.

Test principle:

Under certain temperature and humidity, keep a fixed gas pressure difference on both sides of the sample. By measuring the change in gas pressure on the low-pressure side of the sample, parameters like gas permeability can be calculated.

Test Specimen:

From the roll, cut along MD (machine direction), ≥150 mm apart for sampling positions

Sample size: 100 mm × 100 mm if width ≥ 100 mm; if width < 100 mm → 100 mm × full width

Number of specimens: typically ≥3 pieces per sampling position; often 3 positions × 3 pieces for uniformity mapping

Test equipment required:

Recommend UnitedTest Gas Permeability Tester - Gas Permeation Analyzer;

Key test parameters:

Parameter	Value
Differential pressure	1.21 kPa ± 0.01 kPa (Gurley mode)
Test area	φ28.6 mm → 642 mm² (6.45 cm²)
Volume	100 mL
Pressure sensor accuracy	±0.01 kPa (electronic versions)
Time resolution	0.1 s
Clamping	Pneumatic, uniform force (avoids side leakage)

Related Standard:

ISO 5636-5 Paper and board.Determination of air permeance (medium range).Part 5: Gurley method

SJ/T 10171.9 Measurement methods for performances of separators of alkaline batteries — Determination of air permeability through separator (medium range) — Schopper method.

GB/T 36363 Polyolefin separator for lithium-ion battery.

Why This Test Matters for Battery Separators:

Ion transport proxy — Gurley correlates inversely with Li⁺ transport resistance. Lower Gurley → lower cell IR → better rate performance.

Safety window — Too low Gurley → large pores / high porosity → risk of Li dendrite penetration → internal short. Too high → high IR → heat at high C-rate.

Shut-down behavior — PE separators are designed to close pores at ~130 °C; Gurley before/after heating is a key QC check.

Uniformity — CV% of Gurley across a roll is often tighter spec than absolute value. Inconsistent Gurley → cell-to-cell capacity variance.

Ceramic coating effect — Ceramic-coated separators typically show higher Gurley (200–400 s/100 mL) vs. bare PE (~150–250), because coating partially blocks surface pores — must be balanced vs. wettability.

Typical benchmark (ceramic-coated Li-ion separator): Gurley ≈ 200–400 s/100 mL

Gas Permeability Tester is a multi-functional lab instrument dedicated to gas barrier performance testing, gas transmission rate (GTR). Conform to ISO 15105-1, ISO 2556, ASTM D1434, Widely adopted as standard QC testing equipment for flexible packaging, plastic manufacturing and new energy sectors.

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