How pouch cells are better than other cells

1. How are pouch cells made?
2. How do pouch cells work?
3. Advantages of pouch cells
4. The applications of pouch cells
5. Differences between pouch cells and prismatic cells
6. Differences between cylindrical cells and pouch cells
7. Why do pouch cells swell?
8. How do we choose pouch cells?
9. Why does Tesla not use pouch cells?
10. Summary

Pouch cells are batteries that all the components are enclosed inside a soft layer of aluminium film. Pouch cells are very common in battery industry nowadays and they also have their own advantages. These pouch cells have high energy density and store much power. Outer covering is not a hard metal case so they are light in weight.

How are pouch cells made?

Pouch cell has a layered structures. Among these layers, electrolyte is packed. While in other kinds of battery, electrolyte is different. If you want to know more about electrolyte, you can read our lithium ion battery electrolyte article.  All components are packed in a soft aluminium sheet. Following are the steps for making pouch cells:

1. Preparation of electrode sheet
2. Assembly of Lithium Ion Cell
3. Formation and sealing of the battery
4. Testing of battery

Preparation: For making pouch cells, electrode sheets or current collectors are prepared. Mixture of binding agent, active material and conductive material goes in a machine after getting vacuumed. Then, it goes into a rolling press where it is converted into a sheet. This can be cut into desired shapes. Then, using a stacking machine, electrolyte is placed between collectors. This is assembly or the last step called ‘formation’.

Gas bag: During manufacturing, there is a temporary gas bag where gases escape when solid electrolyte interface (SEI) is formed.

How do pouch cells work?

Pouch cells have a stacked structure where electrolyte is present between two current collectors. One sheet carry positive charge while the other carry negative charge. Lithium ions are present between two electrode sheet to move charges. Separator is present between the cathode and anode.

Current collectors: On both of the sides, pouch cells have current collectors. These collectors carry positive and negative charge from outside. These current collectors are made up of copper or aluminium. There is a coating of an active material, on both collectors.

Separator: Between both the collectors, there is electrolyte, consisting of lithium ions. In middle, there is a membrane that separates cathode from anode. It only allows the charged ions to pass through it.

Advantages of pouch cells

• All the components of pouch cells are packed in a small size, inside a aluminium plastic foil. This takes less space. Under the same size, the capacity of the soft pack battery is larger.
• There is no metal covering so pouch cells are light in weight.
• Aluminium plastic packaging is soft and it does not explode like steel or aluminium case, it just swells only when the battery is subject to shock or high temperature leading to battery to become unstable.
• The design of pouch cell is thin, flexible and can be made into desired shapes.
• Good electrochemical performance and long lifespan: the internal resistance of the pouch battery is small, which can greatly reduce the self-consumption of the battery. Thus, the cycle life of pouch battery is longer.

The applications of pouch cells

• Pouch cells are used in consumer, military as well as in automotive industries.
• Due to its preferable small size, pouch cells are also used in portable devices where high load of current is needed. It includes digital products and drones.
• There is not any standardised pouch cell so manufacturers make them accordingly.

Differences between pouch cells and prismatic cells

1. Prismatic cells are made up of hard and rigid outer shells of aluminium or steel whereas pouch cells have a light and soft aluminium plastic foil on them, so they are different in appearance.
2. The shape of pouch cells in comparison to that of prismatic cell, is more flexible.
3. Pouch cells are lighter and smaller in size than most prismatic cells.
4. Pouch cells are safer than prismatic cells because of its aluminum shell.
5. Prismatic cells has the problem of heat dissipation.

Differences between cylindrical cells and pouch cells

1. Physical appearance of pouch cells is different from the cylindrical cells.
2. Higher standardization of cylindrical cell production and greater flexibility in soft packs.
3. Pouch cells are lighter in weight than cylindrical cells. They also occupy less space.
4. The production process of cylindrical batteries is relatively mature, while soft-pack batteries have advantages in terms of energy density due to packaging and structure. Therefore, for the same capacity of the battery pack, the cylindrical battery requires more cells than the pouch battery.

Why do pouch cells swell?

Swelling of pouch cells occur due to excessive gas discharge. Sometimes in the process of the charging and discharging of pouch cell, gas is generated. This causes swelling of the cell. Swelling usually occurs due to fault in manufacturing. Swelling puts pressure on outer aluminium soft covering which can also break, sometimes. To avoid swelling, sometimes temporary gas bags are also attached.

The compaction density has a great influence on the graphite anode. During the cold pressing of the pole piece, a large compressive stress is generated in the graphite anode film layer. This stress is difficult to be completely released in the subsequent high temperature baking of the pole piece and leads to swelling. The swollen battery is dangerous. Once the battery is swelling, it is not recommended to use it again.

How do we choose pouch cells?

In order to choose a cell several factors are taken into consideration. These include space available, the demand of current. We choose pouch cells because they can be carried anywhere with portable devices because of their small size. Pouch cells are chosen because they are lighter and can be customised according to user’s demand.

Why does Tesla not use pouch cells?

Thermal runaway is the main reason for not using pouch cells for Tesla. It is because thermal runaway is high when large pouch cells are used. Thermal runaway is a dangerous self heating state of a cell or battery. In this case, the temperature of pouch cells is high and energy will be leaked and may cause danger. This process is uncontrollable.

Summary

Pouch cells are small and light in weight. They have all components inside a soft outer covering. Due to this they can be made in any required form. They can be customized in different shapes depending on customers’ needs. Occupying less space provides them with an edge over all other cells such as prismatic cells and cylindrical cells. They can be made to fit in any available space.

They are preferably used in portable devices, toys where cells are required in small space. They have uses in military and automotive industry also. Pouch cells are promising in automotive industry and communication field. They can swell when excessive gas is discharged so we need to avoid the factors that will cause swelling.

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