By Stéphane Melançon on August 26, 2024
Batteries & EVs
Introduced in 1995, pouch cells have always presented a unique design, where the battery is enclosed in a soft plastic film instead of a rigid casing like cylindrical and prismatic cells.
Because the soft pouch provides no protection, cells are enclosed in an external structure called a battery module. It is designed to protect the cells, manage heat, and connect them electrically.
Since modules can only accommodate a limited number of pouch cells, multiple modules are needed. In the final battery pack assembly, all modules are connected electrically to one another.
Pouch cells and the structures that enclose them have evolved over the years. Here is how they have evolved since 1995 and where they are headed.
Table of Contents
The Pouch Cell Design: Pros and Cons
Pouch cells offer an interesting mix of advantages and disadvantages.
On the bright side, they have a light design with no solid casing. This allows them to reach a higher energy density and to be used in compact devices that require thin designs, such as laptops and cellphones.
They also provide better heat extraction due to their high surface-to-volume ratio (i.e., they have a large surface area compared to their volume). This makes them ideal for high-power applications with demanding heat management requirements.
For automotive OEMs, pouch cells are advantageous because their dimensions can be adjusted to meet their specific requirements. Regardless of the final shape, the cells can simply be sealed with ultrasounds. With other types of batteries, customizations are not so simple. With cylindrical cells for example, dimensions need to be carefully optimized for maximum performance, as with standard formats like the 18650, the 21700, or the 4680.
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On the downside, pouch cells do not have the strength to resist the pressure of gases released during charge cycles. The resulting pressure can damage the cells or cause them to age non-uniformly, leading to lower performance. As a result, an external structure (module, separator plates, springs, compression mechanism, etc.) is needed to counteract pressure changes. |
A swollen pouch (source) |
Because of their soft structure, pouch cells cannot be used for cell-to-pack designs where cells contribute to the vehicle’s structure. This limits their relevance in current EV technological trends, as an increasing number of manufacturers are betting on structural battery packs to diminish weight and increase range.
Pouch Cell Formats
The Original Design
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The first pouch cells were designed with both electrical connections on top. The current collectors inside the pouch are connected to the tabs that exit the cell. |
The original pouch design |
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To extract heat, some designs use thick aluminum sheets inserted between cells. These sheets carry heat to a cooling plate placed at the bottom. The aluminum sheets, better known as heat transfer plates, are L-shaped to increase the contact area with the cooling plate. These plates add weight to the final battery assembly. They are bonded to the cooling plate using thermal interface materials. |
Heat transfer plates are inserted between each pouch. They are connected to a cooling plate at the bottom (source). |
The Long Pouch
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The long pouch was developed to make more efficient electrical connections. The most important change is that the positive and negative electrical connections are split on two sides. This allows for larger current collectors inside the cells and, as a result, less electrical resistance. Because the tabs are on different sides, the height can be reduced to better accommodate the automotive underfloor. |
The long pouch design (source) |
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With its rectangular shape, the long pouch has an increased contact area with the cooling plate, which improves thermal transfer. With this design, manufacturers started adding thermal interface materials at the bottom of the cells (where most of the heat can be extracted) and used the module to wrap the cells instead of adding plates in between cells. |
Another layer of thermal interface materials is added at the bottom of the module to improve cooling. |
The Improved Long Pouch
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The most recent iteration on the long pouch made changes that improved cooling and simplified assembly. With this design, the electrical connections are closer to the cooling plate. This makes heat extraction more efficient. |
The improved long pouch design |
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The battery module that encloses the cells is also open at the bottom, allowing the cells to be in direct contact with the cooling plate. Because the casing is open, only one layer of thermal interface materials is needed at the bottom of the cells instead of two. |
The module is open at the bottom, so pouch cells are in direct contact with the cooling plate. |
The “Metal-Free” Pouch
Pouch cells constantly undergo research and development. One of the most recent innovations is Sakuu’s metal-free battery cell, which presents a battery with no electrical connections and hence no metal.
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A polymer film holds the components together but, unlike other types of pouch cells, this film is only used to seal the edges of the cell. This leaves an opening on both surfaces and allows the cells to be electrically connected by simply stacking them. Because there are no tabs, aluminum and copper external connections are not needed. This helps reduce the weight of the battery, eliminates a source of electrical resistance, and reduces raw material costs. |
Sakuu’s pouch design with no tabs (source) |
This new type of pouch has the potential to completely change battery pack designs. As a reference, Tesla’s 4680 cylindrical cells have an energy density of 272 Wh/kg, whereas Sakuu’s pouch cells provide a much higher energy density with 354 Wh/kg. These are significant results that have the potential to improve the driving range of electric vehicles.
Still, many questions remain. These cells have yet to be integrated into a full battery pack.
What will be the total energy density of the battery pack? Tesla’s 4680 cells can be used structurally in EVs, whereas Sakuu’s cells will need stronger structures that will add more weight.
The real comparison will be possible when Sakuu’s batteries are integrated in a complete assembly.
Applications for Electric Vehicles
In the current market, pouch cells are vastly overshadowed by cylindrical cells and prismatic cells. Most manufacturers use or plan to use cylindrical cells because they offer a middle ground between cost efficiency and energy density. Prismatic cells are also very popular in China because of how well they integrate with LFP batteries, the most popular battery chemistry in China.
That being said, pouch cells are still chosen for their ability to meet high power surges and manage heat efficiently. Here are examples of applications where they are currently used:
- Early electric car models, including the Chevy Volt, Chevy Bolt, and Nissan Leaf
- Hybrid vehicles that only need low autonomy
- Electric race cars that need better heat extraction
- Military vehicles that have ultra-high-power and thermal management needs
The Future of the Pouch
Despite the recent decline in popularity of pouch cells, innovations like Sakuu’s metal-free design have the potential to renew interest, completely change how pouch cells are assembled, improve the range of electric vehicles, and diminish their cost.
As these technologies continue to develop and integrate into full battery packs, the true impact of these innovations will become clearer.
