Recently, we discussed the status of lithium-ion batteries in 2020. One of the most recent developments in this field came from Tesla Battery Day with a tabless battery cell Elon Musk called a “breakthrough” in contrast to the three traditional form factors of lithium-ion batteries: cylindrical, prismatic, and pouch types.
Pouch cell (left) cylindrical cell (center), and prismatic cell (right). Image used courtesy of Eric Maiser
Each of these battery types offers a set of advantages and disadvantages. There is not a clear winner, but the battery you choose can affect the design of a product in several different ways. For example, each of these battery form factors can have a different temperature distribution and heat transfer model.
A good understanding of the battery heat generation and transfer is required to design an efficient battery thermal management (BTM) system; BTM consists of heating and cooling systems that keep the battery temperature in a desired range. In this article, we’ll take a look at the important features of each of these battery formats.
A cylindrical cell consists of sheet-like anodes, separators, and cathodes that are sandwiched, rolled up, and packed into a cylinder-shaped can. This type is one of the first mass-produced types of batteries and is still very popular.
These cells are suited for automated manufacturing. Another advantage is mechanical stability. The round shape of the battery distributes the internal pressure from side reactions over the cell circumference almost evenly. This allows the cell to tolerate a higher level of internal pressure without deformation.
When combining cylindrical cells into packs and modules, the cell’s circular cross-section does not allow us to fully utilize the available space. As a result, the packaging density of cylindrical cells is low. However, thermal management of a pack of cylindrical cells can be easier because the space cavities let the coolant easily circulate around the cells within a battery pack.
Packing principle for cylindrical cells (left) vs. the packing principle of prismatic and pouch cells (right). Image used courtesy of Roeland Bisschop et. al
The temperature distribution of a Li-ion cell at a discharge rate of 1 C and SOC=0.1 is shown below:
Image used courtesy of Dong Hyup Jeon and Seung ManBaek
As you can see, the inner parts of the cell are at a higher temperature compared to the cell surface. In this particular example, the temperature difference is not significant. However, with a high-capacity cell, the low heat transfer from the cell center to the outside can pose serious challenges.
In terms of size, cylindrical cells are usually produced in standard models. One common size is the 18650 type (18 mm diameter, 65 mm height). This type has a total mass of about 45 grams and can support a capacity of about 1.2 to 3 Ah depending on the technology employed.
Cylindrical cells are commonly found in medical instruments, laptops, e-bikes, and power tools. These batteries are also used in Tesla vehicles. Other EV manufacturers employ prismatic cells.
Li-ion prismatic cells consist of large sheets of anodes, cathodes, and separators sandwiched, rolled up, and pressed to fit into a metallic or hard-plastic housing in cubic form. The electrodes can also be assembled by layer stacking rather than jelly rolling.
Structure of a Li-ion prismatic cell. Image used courtesy of J. M. Tarascon and M. Armand
Parts of the electrode and separator sheets of a prismatic cell that are close to the container corners can experience more stress. This can damage electrode coating and lead to non-uniform distribution of the electrolyte.
When combining prismatic cells into packs, the cell box-like shape enables optimal use of the available space. However, this optimal space is achieved at the cost of more challenging thermal management. This is because there are no space cavities between the cells as there are in a pack of cylindrical cells.
Prismatic cells are manufactured with a capacity ranging from several ampere-hours targeted for laptops and cell phones to hundreds of ampere-hours designed for EV applications. Prismatic cells can be more expensive to manufacture. Besides, they can expand with use.
Although they suffer less from swelling than pouch cells, they do not perform as well as cylindrical cells in this regard.
Pouch cells do not have a rigid enclosure and use a sealed flexible foil as the cell container. This is a somewhat minimalistic approach to packaging; it reduces weight and leads to flexible cells that can easily fit the available space of a given product.
Pouch cell batteries can swell with gas during charge and discharge. Image used courtesy of Epec
The electrode and separator layers of a pouch cell are stacked rather than jelly rolled. With pouch cells, the designer should allocate enough space for the cell swelling. A swelling of as much as 8% to 10% can occur after 500 cycles.
Due to the cell’s soft construction, a support structure is required with pouch cells and the cell should not be placed near sharp edges.
Li-ion Batteries Rise in Popularity—and Innovation
Lithium-ion batteries will continue to rise in popularity, especially as electric vehicles become more ubiquitous. In fact, Reuters recently reported that Toyota and Panasonic are teaming up to mass-produce lithium-ion batteries for hybrid cars, announcing plans for a plant in Western Japan starting in 2022.
With this demand ever-rising, it’s important for engineers to familiarize themselves with the three common form factors of lithium-ion batteries—cylindrical, prismatic, and pouch—and stay up to date on new updates to Li-ion batteries—for instance, like those announced at Tesla’s Battery Day this year.
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