Lithium battery separator performance directly determines battery safety, cycle life and charge-discharge efficiency — a qualified separator must not only allow lithium ions to pass smoothly like a “precision sieve”, but also resist puncture and high temperatures like a “solid barrier”. Performance testing is the core link to verify separator quality, among which air permeability, mechanical properties and thickness uniformity are known as the “three lifelines” of separator quality.
Based on industry standards, this article will detail the testing principles, operation methods and core instruments of these three key performances, providing a standardized and actionable testing plan for separator quality control in research and production.
1. Air Permeability Testing: Quantify Lithium Ion “Transport Efficiency”
The microporous structure of the separator is the “natural channel” for lithium ion migration. Air permeability directly reflects the smoothness of the channel. Its core evaluation index is “air permeability” — the time required for a unit volume of air to pass through the separator under a specific pressure. The shorter the time, the better the air permeability and the higher the lithium ion transport efficiency.
1. Testing Standards and Principles
Referring to international industry standards, the core of the test is to measure the time required for 100 mL of air to pass through the specified test area of the separator under a constant pressure of 1.21 kPa. The unit can be expressed as s/100mL or μm/(Pa・s) to meet the characterization needs of different scenarios.
2. Testing Process and Key Points
Sample Preparation: Select flat, defect-free separators and cut them into 3 representative samples to avoid wrinkles and damage affecting test results;
Instrument Operation: Place the sample in the center of the test chamber of the air permeability tester, the instrument clamps the sample pneumatically, and automatically controls the pressure and air flow;
Efficient Testing: Adopt a three-chamber independent test design, the total test time can be shortened to 1 minute in fast mode, balancing efficiency and accuracy.
3. Recommended Instrument and Advantages
It is recommended to use the C190H Air Permeability Tester, whose core advantages are:
Precise Control: Adopt automatic gas volume measurement method, with volume accuracy up to 1% and pressure control accuracy better than 0.01kPa;
Flexible Adaptation: The pressure can be freely set within the range of 0~2 kPa, supporting multi-unit display and compatible with different industry standards;
Easy Operation: No manual intervention is required, the instrument automatically completes the test and data recording, reducing human error.
2. Mechanical Performance Testing: “Physical Strength” to Resist Risks
Separators need to withstand stretching, friction and puncture during battery assembly and charge-discharge cycles. Insufficient mechanical performance can easily lead to separator damage and short circuits. The core test indicators include tensile strength, elongation at break and puncture strength.
1. Tensile Strength and Elongation at Break: Measure “Toughness and Ductility”
Testing Standards: Refer to international industry standards;
Testing Process:
Cut separator samples according to standard sizes to avoid burrs, wrinkles and other defects;
Clamp the sample in the upper and lower fixtures of the tensile testing machine, and set the initial distance of the fixtures to 100±5 mm;
Stretch at a speed of 250±10 mm/min. After the sample breaks, the instrument automatically calculates the tensile strength (maximum tensile force per unit area) and elongation at break (ratio of elongation at break to initial length).
2. Puncture Strength: “Safety Key” to Resist Lithium Dendrites
Lithium dendrites are the main hidden danger of battery short circuits. Puncture strength directly reflects the ability of the separator to resist puncture by lithium dendrites, defined as the puncture resistance value per unit thickness of the separator.
Testing Standards: Follow international industry standards;
Testing Process:
Flatly clamp the separator sample in the center of the fixed fixture;
The puncture needle punctures the sample at a rate of 100±10 mm/min;
Measure the thickness at four points around the puncture hole and take the average value, then calculate the puncture strength through the formula.
3. Recommended Instrument and Advantages
It is recommended to use the C610 Series Intelligent Electronic Tensile Testing Machine, whose core features are:
High Precision: Force accuracy is better than level 0.5, equipped with sensors of different ranges to adapt to the testing of separators of different thicknesses and materials;
Multi-Functional: Integrate multiple test programs such as stretching, puncture and peeling, one machine can meet the full-dimensional detection of mechanical properties;
Stability: Adopt servo operation system and precision ball screw positioning technology, with stable operation and accurate positioning, supporting overload protection and automatic return.
3. Thickness Uniformity Testing: “Basic Premise” to Ensure Consistent Performance
The thickness uniformity of the separator directly affects the overall consistency of air permeability and mechanical properties — excessive thickness deviation will lead to local poor ion transport and insufficient strength, becoming an “invisible hidden danger” of battery performance fluctuations.
1. Testing Standards and Evaluation Indicators
Referring to international industry standards, uniformity is characterized by “thickness deviation”, and the core indicators include:
Thickness Average Deviation: The difference between the measured average thickness and the nominal thickness;
Thickness Upper Deviation/Lower Deviation: The difference between the measured maximum/minimum thickness and the average value.
2. Testing Process and Key Details
Sample Selection: Select representative separator samples without wrinkles or damage;
Measuring Point Distribution: Select 15 test points at equal distances along the machine direction (MD) and transverse direction (TD) of the separator to ensure coverage of the entire sample area;
Thickness Measurement: Use a high-precision thickness gauge to measure the thickness of each measuring point, calculate the average value and various deviations, and the results must meet the requirements of industry standards.
3. Recommended Instrument
It is recommended to use the C640 Thickness Gauge, which has high resolution and stability, can accurately capture small thickness differences, and provide reliable data support for uniformity evaluation.
For more in-depth research on lithium battery separator performance testing and quality control, you can refer to the research published by the Journal of Power Sources. Our previous articles on lithium battery separator selection guide and ceramic coated separator safety upgrade further elaborate on battery material performance and modification technologies. For detailed industry standards and separator testing specifications, refer to the report released by the Institute of Electrical and Electronics Engineers (IEEE).