How is a pouch battery made?

Pouch battery, in fact, is the use of aluminum plastic film as a packaging material of the battery. Relatively speaking, the packaging of lithium-ion battery is divided into two categories, one is the pouch cell, one is the metal shell cell. The metal shell cell includes steel shell and shell case, cylinder and square and so on.

The advantages of pouch battery

Pouch battery packaging materials and structure make it has a series of advantages：

Good safety performance, the pouch battery adopts aluminum plastic film package in structure.When a security problem occurs,pouch battery will generally blow open, rather than like steel shell or aluminum shell cell explosion;

Light weight, the weight of pouch battery is 40% lighter than steel shell lithium battery of the same capacity, and 20% lighter than aluminum shell lithium battery ;

Small internal resistance,the internal resistance of pouch battery is small, can greatly reduce the self-discharge;

Long cycle life, the cycle life of the pouch battery is longer, 100 times of cycle attenuation is 4% ~ 7% less than that of the aluminum shell;

Flexible design, shape variable arbitrary shape, pouch battery can be thinner, and can also be customized according to customer needs.

The application of pouch battery

The application of pouch battery in the power battery field accounted for more than square and cylinder. With the introduction of new subsidy policies for new energy vehicles, the system energy density of batteries has become an important assessment index.

The capacity of ternary pouch battery is 10~15% higher than the steel shell battery of the same size specification, and 5~10% higher than the aluminum shell battery, and the weight is lighter than the steel shell battery and aluminum shell battery of the same capacity specification.

Therefore, the subsidy new policy is more favorable for ternary pouch battery.

In view of the advantages of pouch batteries, industry experts expect that with the development of battery technology routes, the penetration rate of pouch batteries in the new energy vehicle market will continue to improve, and the proportion of pouch batteries in various types of batteries is expected to exceed 50% in the future.

The pack method of pouch battery

The material used in the shell determines the way they are packaged. Only pouch batteries have aluminum plastic film,and need to use thermal packaging;

Metal shell batteries are generally sealed by laser welding. Aluminum plastic film generally has three layers, nylon layer is to ensure the appearance of aluminum plastic film, reduce the damage to the shell.

It ensure that before manufacturing into lithium ion battery, the film will not be deformed. It can prevent air, especially oxygen penetration, maintain the internal environment of the cell, and ensure that the packaging aluminum foil has good deformation ability.

Al layer is composed of a layer of metal Al, which is used to prevent water infiltration and maintain the internal environment of the cell with a certain thickness and strength to prevent external damage to the cell.

Lithium ion battery is very afraid of water, generally requires that the water content of the pole piece is at the PPM level, so the packaging film must be able to block the infiltration of water vapor.

Nylon is not waterproof and does not provide protection. At room temperature, metal Al reacts with oxygen in the air to form a dense oxide film, which prevents water vapor from penetrating and protects the interior of the cell.

The Al layer also provides the plasticity of the washhole during the molding of the aluminum-plastic film.

PP is short for polypropylene, a material that melts at temperatures of more than 100 ℃ and has viscosity. Therefore, the thermal packaging of the battery mainly depends on the PP layer melting and bonding together under the action of head heating, and then the head is removed, and the cooling solidified bonding. 

PP will not be dissolved or swollen by the organic solvent in the cell. It effectively prevent the internal electrolyte contact with the Al layer, avoid the corrosion of the Al layer.

Aluminum plastic film looks very simple, but in practice, it is not so easy to combine three layers of material evenly and firmly together. Now the quality of aluminum plastic film is basically imported from Japan.

Some important packing process of pouch battery

Pole forming

Die cutting is usually used for pole forming, and laser cutting or die cutting is used for lug. Pole stacking method, laminated with zigzag pole and winding type pole, cut a piece of pole, through stacking glue into a coil core, into the washed aluminum plastic film pit. Next we talk about the pit.

Pit

The pouch cell can be designed into different sizes according to the needs of customers. When the shape size is well designed, it is necessary to open the corresponding mold to make the aluminum plastic film forming.

The forming process is also known as the pit. As the name implies, it is to use the forming mold in the case of heating, in the aluminum plastic film out of a hole to accommodate the core.

After punching and cutting the aluminum plastic film, it is generally called Pocket bag. In general, when the cell is thin, the single hole is chosen, and when the cell is thick, the double hole is chosen.

If the deformation of the aluminum-plastic film is too large to break through the deformation limit of the aluminum-plastic film, it will lead to rupture. Sometimes a hole is punched in the air bag area and an air bag is added.

The air bag can be increased according to the need. The air bag is mainly used to collect the gas in the process of formation.

Place the rolled core into the washed hole and fold in half along the yellow line.The coil core is put into the pit and needs to be encapsulated after several processes.

Top sealing and side sealing

After putting the coil core into the pit, the whole aluminum film can be put into the fixture, and the top sealing and side sealing are carried out in the top side sealing machine.

When packaged, the two heads have a certain temperature(generally about 180℃), when pressed on the aluminum plastic film, the PP layer of the aluminum plastic film melted and then bonded together.

The top seal is to seal the lug, which depends on a small part of the lug – lug glue to complete. lug glue can melt and bond when heated. During packaging, PP in lug glue and PP layer of aluminum plastic film melt and bond, forming an effective packaging structure.

Liquid injection and pre-seal

After the top sealing and side sealing of the pouch cell, it is necessary to do X-ray inspection of the alignment of the coil core, and then go into the drying room to remove moisture.

Let it stand in a dry room after some time, enter the process of liquid injection and pre-seal(some factories will add coated process in liquid injection process, avoid the battery electrolyte corrosion aluminum-plastic film).

Through the above introduction we know that after the top and side sealing of batteries is completed, only one opening is left on the side of the air bag, and the opening is used for filler.

A pre-seal, is required immediately after the injection. After the pre-seal is completed, the inside of the cell is completely isolated from the external environment.

Static

After the liquid injection and pre-seal is completed, first of all, the cell needs to be static. According to the different process,it will be divided into high temperature static and normal temperature static, static purpose is to let the injected electrolyte fully infiltrate the pole piece.

The cell can then be turned into. The main function of formation is to activate the battery and form SEI membrane. In formation, due to the characteristics of the pouch battery, some lithium-ion battery manufacturers will reshape the battery, that is, the use of fixture to fix the battery into.

Secondary seal

Because gas will be generated in the formation process, the gas production varies according to different materials, so the air bag is increased according to the demand to expand the volume of the gas. We need to pull out the gas and then carry out the secondary seal.

Some businesses here have evolved into two operations: degassing and secondary seal, followed by an airbag cutting and folding process, and a few have pre-degas, formation, degas and secondary seal processes because of large batteries and large gas production.

When secondary sealing, the air bag is first stabbed by the guillotine and vacuumized at the same time, so that the gas in the air bag and a small part of the electrolyte will be pulled out.

Then immediately packaging in secondary sealing area, to ensure the air tightness of the cell. Finally cut off the air bag of the sealed cell , and then cutting and folding.

That is cut the edge of pre-seal and secondary seal to the appropriate width, and then folded, to ensure that the width of the cell does not exceed the standard.

Post-process

After folding the edge of the battery, it is put on the cabinet to sub-capacity. After that, the cell with qualified battery capacity will enter the post-process, including inspection appearance, weight, adhesive, voltage detection, internal resistance detection, Hi-pot detection, pole lug conversion welding and so on.

After that, it’s time to PACK, and a pouch battery is done.