The production of lithium-ion cells

When discussing lithium batteries, it is impossible to avoid focusing on their essential component, namely, the cells of which they are composed and which enable them to deliver power. The three most widely used lithium ion cell types, each employed for a distinct kind of application, are as follows:

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Cylindrical lithium cells

As can easily be inferred, cylindrical cells are cylinder-shaped, are the most commonly used and were among the first to be mass-produced. They can have different diameters, the most common being the 1865, where the number 18 indicates the diameter (18 mm) and the number 65 indicates the length (65 mm). There are, however, other formats, such as the 2170 or, again, the one most recently adopted by Tesla, the pioneer of lithium batteries for electric cars, with its 4680 used to power the Tesla Model Y. Apart from a few car manufacturers who have made this choice, cylindrical cells are routinely used in medium-small battery packs, e.g. in micro-mobility (bikes, scooters and motor scooters), portable tools, medical devices, and so on.

Lithium pouch cells

These types of lithium cell are so called because of their bag-like shape. They have a lightweight design and, as they have no inherent robustness, special protections, such as the addition of aluminium frames, must be inserted during production of the module to give them structural robustness. They come in various sizes which can be modified according to the manufacturer’s requirements. These cells are mainly used in smartphones, drones, laptops and the automobile industry.

Prismatic lithium cells

Prismatic lithium cells have a solid rectangular casing made of aluminium or of a very strong plastic material. The internal components are layered. They come in different sizes, with a variety of formats depending on the field of application. Their individual components can reach a high capacity. Due to their structure, prismatic cells are best suited for the production of lithium batteries for the machinery and industrial vehicles industry, or the energy storage sector, all of which normally require medium-high capacities.

The benefits and drawbacks of the various cell types have already been discussed, but few people ever enquire about the lithium-ion battery cell production process and how it works.

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Although the many cell types that make up a lithium battery appear very different from one another when viewed from the outside, it is astonishing to learn how similar their interiors actually are. The different types of battery cell production and assembly will now be explored in more detail.

The process begins with combining the raw materials of which the lithium cells are composed

Prismatic

Despite what the name might suggest, the prismatic battery cell is essentially a rectangular metal box. The individual layers of the battery are either stacked like a deck of cards or wound up and then pressed flat to fit into the cell casing. Prismatic cells are particularly useful in EVs because they can be packed tightly next to each other without wasting space, giving them excellent packing efficiency.

For prismatic cells, cooling is often done through the bottom of the pack, with the metal casing serving as a heat sink, absorbing the heat and carrying it down toward the cooling element. However, tightly packed cells make it inherently more challenging to keep the batteries cool while fast charging, especially if they are large – for example, prismatic cells with energy storage capacities above 50 amp-hours (Ah) versus 4.8 Ah in a standard cylindrical cell.4 The metal case serves as a heat sink, allowing the heat to drain away from the cell.

Prismatic cells have other challenges, too. For example, because the box shape is less rigid than a cylinder, the walls of the cells must be thicker, which adds weight and cost to the cell. The top of a prismatic cell (known as the header) is also relatively complex to assemble, adding cost and time to manufacturing.

When it comes to lithium-metal battery cells, the prismatic format is suitable but with caveats. The layers of a lithium-metal cell can be stacked flat in a prismatic format. However, as the anode layers expand, something else inside the cell, such as a spring or foam, would have to be present to accommodate this expansion. Since this mechanism would sit between the layers and the metal casing, it would partially block heat transfer from the interior, limiting fast-charging performance, and taking up extra space, hurting energy density.

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