1. The meaning and purpose of encapsulation

There are dynamic electrochemical reactions in lithium-ion batteries, which are sensitive to water and oxygen. Organic solvents, such as electrolyte, will react with lithium salt in electrolyte to generate a large amount of HF when they meet with water and oxygen, which will affect the electrochemical performance (such as capacity and cycle life).

The significance and purpose of soft pack lithium-ion battery packaging is to use high barrier soft pack materials to completely isolate the inside and outside of the cell, so that the inside is in a vacuum, oxygen free and water free environment.

2. Packaging materials and packaging design

Aluminum plastic film structure

1、 Outer layer: generally nylon layer, which is used to protect the middle layer, reduce scratches and dirt, and ensure the good appearance of the battery, prevent the penetration of air, especially oxygen, and maintain the internal environment of the cell, and ensure that the packaging aluminum foil has good deformation capacity. Sometimes pet will replace nylon to have better chemical corrosion resistance, but this will lead to the reduction of the punching depth of the aluminum-plastic film.

2、 Intermediate layer: it has a certain thickness and strength, which can prevent water vapor penetration and external damage to the cell. The most popular is aluminum foil;

3、 Inner layer: it is mainly used for encapsulation, insulation and preventing the contact between Al layer and electrolyte. PP layer is mainly used;

4、 Decoration or special protective layer: in order to improve the appearance and luster of the battery, sometimes a matt layer is added outside the PET / nylon, but this will lead to a significant rise in the price of aluminum plastic film.

So what is the principle of aluminum plastic film packaging? In general, it is combined by mechanical high temperature.

The two layers of PP are opposite to each other. By giving a certain amount of heat to the nylon layer, the heat is transferred to PP through the Al layer, and the two layers of PP film are fused together under a certain temperature, pressure and time.

Packaging process

P top sealing: it is mainly to put Jr into pocket, fold and align the packaging aluminum foil, fine tune the tab position, charge and align the cell, and then heat seal. The top sealing process is the most difficult to control in the whole packaging process

1) Packing foil alignment (cutting, folding in half);

2) Tab position control (cell width, center distance and edge distance);

3) Cell feeding positioning (cell unsealed area);

4) Structure design of heat sealing head (head width, stopper, head groove depth);

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

P side seal: it is used to package the top sealed cell, and the process control is mainly to optimize the cell positioning and heat sealing process conditions.

P corner seal: the purpose is to prevent the corner damage of the seal at the folding place of the packaging aluminum foil. The requirement of corner seal is to fuse two layers of aluminum foil to prevent stress concentration caused by bending. The process control is mainly 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 water vapor entering. The requirement of vacuum packaging is only sealing, and the process control is mainly cell positioning and optimization of heat sealing process conditions.

Degassing is the process of venting and sealing the formed cell. Compared with the side seal, it is required to puncture and package the air bag in vacuum.

3. Package parameters and effect confirmation package parameters

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

2. Time (heat transfer and PP fusion)

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

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

Effect confirmation

1. Inspection of package parallelism (package on the basis of stopper parallelism OK, tear open the sealing surface to observe the milky white of fuel melt adhesive and seamless package defects);

2. Tensile test (including the position of polar ear and side seal) and inspection of unsealed area (visual inspection, including the inner and outer unsealed areas of top seal, side seal and bottom seal, and inspection of internal PP condition);

3. Sealant exposure inspection (visual inspection, mainly negative pole ear);

4. Resistance test (check the Al layer between the positive and negative electrode ears, the negative electrode ears and the pocket bag after sealing, and check the Al layer between the negative electrode and the pocket bag after degassing);

5. Delamination inspection (final sealing after degassing).