Soft Pack Lithium Battery Package Technology
What is soft pack lithium battery?
The soft-packed battery is a name compared to the other two hard-shell batteries, cylindrical and square. Its internal composition (positive electrode, negative electrode, diaphragm, electrolyte) is not much different from square and cylindrical lithium batteries. The biggest difference is that the soft-packed battery uses aluminum-plastic composite film as the shell, while the square and cylindrical batteries use metal materials as the shell.
Soft-packed batteries are actually batteries that use aluminum-plastic packaging film as the packaging material. Relatively speaking, the packaging of lithium-ion batteries is divided into two categories, one is a soft-packed battery, and the other is a metal shell battery. Metal shell batteries also include steel shells and aluminum shells. In recent years, due to special needs, some batteries with plastic shells can also be classified as such.
Structure: Liquid lithium-ion batteries with a layer of polymer shell. The package material is aluminum-plastic composite film which is the most critical and technically difficult material in pouch Li-ion batteries.
Packaging materials have three layers:
Outer barrier– outer protective layer composed of nylon BOPA or PET
Permeable layer– intermediate layer made of aluminum foil
Inner layer–Multifunctional high barrier layer
Application of Pouch lithium battery: Consumer, military, as well as automotive industries.
Mainly used in equipment which has equipment weight requirement or compact space. Pouch batteries are suitable for conditions where the working environment is relatively stable and there are no complex road conditions to ensure the structural stability and service life of lithium battery packs.
1、The meaning of soft pack lithium battery package
There is a dynamic electrochemical reaction inside lithium-ion batteries, which are more sensitive to moisture and oxygen. Because there are organic solvent inside the battery cell, if water or oxygen come into lithium battery through leakage, they will quickly react with the lithium salt in the electrolyte and generate a large amount of HF. HF is one of strong acid, which will affect the electrochemical properties of the battery (such as capacity and cycle life).
Therefore, the meaning and purpose of packaging lithium-ion battery is to use packaging materials with strong barrier properties to completely isolate the inside of the battery from the outside. Keep the interior in a vacuum, oxygen-free, and anhydrous environment.
2、Package Materials and Design
Aluminum-plastic film structure
Outer layer: Generally nylon layer,
Its function is
- to protect the middle layer,
- to reduce scratches and dirt stains,
- to ensure that the battery has a good appearance.
- to prevent the penetration of air, especially oxygen,
- to maintain the internal environment of the battery.
- to ensure that the packaging aluminum foil has good deformation ability.
Sometimes PET is used instead of nylon to have better chemical resistance, but this will lead to a decrease in the depth of the aluminum-plastic film.
Intermediate layer: It has a certain thickness and strength,
The function: to prevent water vapor penetration and external damage to the battery cell.
The most mainstream is the use of aluminum foil.；
Inner layer: PP layer material is mainly used.
Function: It mainly plays the role of encapsulation, insulation, and preventing contact between the Al layer and the electrolyte.
Decoration or special protective layer: Sometimes a MATT layer is added to the outside of PET/nylon, but this will lead to a significant increase in the price of aluminum-plastic film.
Function: In order to improve the appearance and luster of the battery,
So How to Encapsulate Aluminum-plastic Film?
In general, it is fused together by mechanical high temperature. The two layers of PP are opposite to each other. By giving the Nylon layer a certain amount of heat, the heat is transmitted to the PP through the Al layer. And the two layers of PP film are fused together under the combined action of a certain temperature, pressure, and time.
Packaging Process Encapsulation process
- load the JR into the pocket,
- fold the aluminum foil in half and align it
- fine-tune the Tab position
- load and align the battery,
- use heat to seal it.
The top sealing process is the most difficult process to control in the entire package. The main difficulties include：
1) Align the packaging aluminum foil (cut, fold in half)；
2) Control of TAB position (cell width, center distance, margin)；
3) Battery feeding positioning (battery unsealed area)；
4) Heat sealing head structure design (head width, stopper, head groove depth)；
5) Optimization of heat sealing process conditions (time, temperature, pressure, etc.).
Side sealing: It is the encapsulation of the battery after the top sealing is completed. The process is mainly to optimize the heat sealing process conditions and cell position.
The purpose is to prevent the corners sealed in the folded corners of the packaging aluminum foil from being damaged.
The requirement for corner sealing is only to fuse the two layers of packaging aluminum foil together. The aim is to prevent stress concentration caused by bending. Surely, the process controlling is mainly to the optimization of battery cell positioning and heat sealing process conditions.
The purpose is to seal the air bag after liquid injection. The aim is to prevent electrolyte from leaking and keep water vapor outside.
The requirement of vacuum packaging is only sealing. For the same, the process controlling is mainly to the optimization of battery cell positioning and heat sealing process conditions.
Secondary encapsulation (Degassing):
It is the process of venting and sealing the formed battery cells.
Compared with the side seal, it is required to pierce and encapsulate the air bag under vacuum conditions.
Package parameters and effect confirmation package parameters
- Temperature (melting point of PP and outer nylon, head structure and heat dissipation)
- Time (heat transfer and PP fusion)
- Pressure (PP bonding, affecting melt glue and heat transfer)
- Degree of vacuum (mainly vacuum standing and degassing process)
- Check package parallelism (on the basis of stopper parallelism is OK, worker firstly start to package, secondly tear open the package surface to observe that the foil melt is milky white, and then to observe that the seamless package is defective)；
- Tensile testing (including the position of the tab and the position of the side seal
- Inspection of the unsealed area (visual inspection, including the inside and outside of the top seal, side seal, and bottom seal, check the internal PP condition)；
- Sealant exposure inspection (visual inspection, mainly negative electrode ears)；
- Resistance test (after the top side is sealed, the inspection includes the Al layer in the middle of the positive and negative tabs and in the middle of the negative tab and the pocket bag.
After degassing encapsulation, check Al layer between cathode and pocket bag
- Layered inspection (final edge banding after degassing).
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