Lithium battery - packaging technology

1. The significance and purpose of packaging

There is a dynamic electrochemical reaction inside the lithium-ion battery, which is sensitive to moisture and oxygen. 

The organic solvents present inside the battery cells, such as electrolytes, 

react rapidly with lithium salts in the electrolyte when exposed to water and oxygen, generating large amounts of HF, 

which affects the electrochemical performance of the battery cells (such as capacity and cycle life).

The significance and purpose of the soft-pack lithium-ion battery packaging is to use 

a high barrier soft packaging material to completely isolate the interior of the battery from the outside, 

so that the interior is in a vacuum, oxygen-free, and water-free environment.

2. Packaging materials and packaging design

 Aluminum-plastic film structure

 First, the outer layer: generally a nylon layer, which functions to protect the middle layer, reduce scratches and dirt contamination, 

ensure a good appearance of the battery, prevent the penetration of air, especially oxygen, maintain the internal environment of the battery cell, 

and ensure that the packaging aluminum foil has good deformation ability.

Sometimes PET is used instead of nylon to have better chemical resistance, but this can lead to a decrease in the depth of the aluminum-plastic film.

Second, the middle layer: it has a certain thickness and strength, which can prevent water vapor penetration and external damage to the battery cells. 

The most mainstream material is aluminum foil;

III. Inner layer: mainly used for packaging, insulation, and preventing the Al layer from contacting the electrolyte, mainly using PP layer materials;

Fourth, decorative or special protective layer: In order to improve the appearance of the battery, sometimes a matte layer is added to the PET/nylon, 

but this will lead to a significant increase in the price of the aluminum-plastic film.

What is the principle of aluminum-plastic film packaging?Generally, it is fused together by mechanical high temperature.

The two layers of PP are opposite to each other, and by applying a certain amount of heat to the Nylon layer, 

the heat is transferred to the PP through the Al layer. 

The two layers of PP film are fused together under the combined action of a certain temperature, pressure, and time.

Packaging process

 Top sealing: mainly involves loading JR into the Pocket, folding and aligning the aluminum foil, fine-tuning the Tab position, 

feeding and aligning the cells, and then performing heat sealing.

The top sealing process is the most difficult process to control in the entire packaging process, with the main difficulties including:

 1) Alignment of packaging aluminum foil (cutting and folding);

 2) Control of TAB position (cell width, center distance, and edge distance);

 3) Positioning of the feeding of the electric core (the electric core is not sealed);

 4) Structural design of the heat-sealed head (width of the head, Stopper, depth of the head groove);

 5) Optimization of heat sealing process conditions (time, temperature, pressure, etc.).

P-side sealing: It is the process of packaging the completed top-sealed cells, 

with process control mainly focusing on cell positioning and optimization of heat sealing process conditions.

P-angle sealing: The purpose is to prevent damage to the corner of the sealed aluminum foil at the fold.

The requirement for corner sealing is simply to fuse the two layers of aluminum foil packaging together to prevent stress concentration caused by bending.

Process control mainly includes the optimization of cell positioning and heat sealing process conditions.

P vacuum packaging: The purpose is to seal the air bag after liquid injection to prevent electrolyte leakage and the entry of water vapor.

The requirement of vacuum packaging is only sealing, 

and the process control mainly focuses on the optimization of cell positioning and heat sealing process conditions.

PP secondary packaging (Degassing): It is a process of exhausting and sealing the formed cells.

Compared to side sealing, it is required to perform air bag puncturing and packaging under vacuum conditions.

 3. Confirm the packaging parameters and effects

 1. Temperature (melting point of PP and outer nylon, head structure, and heat dissipation)

2. Time (heat transfer and PP fusion)

3. Pressure (PP bonding, affecting melt adhesive and heat transfer)

4. Vacuum degree (mainly vacuum standing and degassing process)

Effect confirmation

 1. Parallelism inspection of packaging (based on the ok parallelism of the stopper, perform packaging, 

tear open the packaging surface to observe the milky white appearance of the foil melt adhesive, and seamlessly seal any defective areas);

2. Tensile test (including the position of the tab and the side seal) Unsealed area inspection 

(visual inspection, including the internal and external unsealed areas of the top seal, side seal, and bottom seal, and checking the internal PP condition);

3. Sealant exposure inspection (visual inspection, mainly for the negative electrode tab);

4. Resistance test (check the positive and negative electrode lugs, the negative electrode lug and the Al layer in the pocket after top sealing, 

and check the negative electrode and the Al layer in the pocket after degassing);

5. Layered inspection (final edge sealing after degassing).