Causes and Solutions for Lithium Battery Separator Wrinkling

The lithium battery separator acts as a protective barrier between the positive and negative electrodes, conducting ions but not electrons. In the ideal state after electrolyte filling and formation, the separator should maintain complete and flat contact with the electrodes. However, when we disassemble batteries, we often find severe wrinkling of the separator. (This can also be clearly observed through the wrinkling of the negative electrode).

Below, we discuss separator wrinkling in three aspects: the hazards, the causes, and the solutions.

1. Hazards of Separator Wrinkling:

Increased Internal Resistance: The microporous structure of the separator is damaged at the wrinkled areas, hindering lithium-ion transport paths. This increases internal resistance by 15%-30%, significantly reducing charge-discharge efficiency.

Capacity Fade: Uneven electrolyte infiltration in wrinkled regions leads to decreased utilization of active materials. Experimental data from a certain NMC cathode battery showed a capacity fade rate of up to 8% per 100 cycles.

Lithium Dendrite Growth and Short Circuit Risk: Wrinkling causes localized increases in current density, raising the probability of lithium dendrite growth on the graphite anode surface and increasing the risk of short circuits.

2. Causes of Separator Wrinkling:

Material Defects: Separators with surface roughness > 0.3μm (standard Ra value 0.1-0.3μm) or tensile strength < 300MPa are more prone to wrinkling.

Process Defects: Excessive winding tension fluctuations (exceeding±3% where the standard requires±1%) lead to uneven roll tightness. Other issues include excessive oven temperature gradients (>5°C where the allowable value is≤2°C).

Unreasonable Hot Pressing Process: Improper settings for hot pressing parameters - pressure, temperature, time - result in poor electrode roll shaping.

Unreasonable Electrolyte Filling Process: The filling process typically involves (vacuum drawing - primary filling - standing - pre-charging - vacuum drawing - secondary filling). If the vacuum negative pressure is too high or the speed is too fast, it can easily cause separation between the separator and electrodes. Furthermore, gas is generated after pre-charging, and the vacuum drawing during secondary filling can remove both this gas and some electrolyte. When gas is extracted, it can form channels, also leading to visible wrinkles.

Incomplete Electrolyte Infiltration: If the electrolyte does not fully infiltrate, leaving dry areas, gaps, or bubbles between the electrodes and separator, wrinkling can easily occur during vacuum drawing.

Electrode Surface Defects: Inherent defects on the electrodes (such as protrusions, pits, etc.) can easily affect how well the separator conforms to them, leading to wrinkling.

3. Solutions for Separator Wrinkling:

The solutions are often related to the causes and include improving the fundamental properties of the separator, and enhancing the winding, hot pressing, baking, and electrolyte filling processes. Explore our battery production line solutions for optimized processes.

By extending infiltration time and appropriately increasing the infiltration temperature (e.g., infiltration at 45°C), the optimal infiltration state for the electrodes can be achieved.

Additionally, beyond process improvements, using coated separators can effectively address the wrinkling issue. For instance, separators with single-sided PVDF coating significantly improve adhesion after hot pressing, completely eliminating or reducing wrinkles. Learn more about our advanced battery material offerings, including coated separators.

TOB NEW ENERGY specializes in providing comprehensive battery line solutions, from pilot and research lines to mass production. We offer customized equipment, advanced technical support, and a full range of high-quality materials to help you optimize your battery manufacturing and research processes, addressing challenges like separator wrinkling effectively.