How are lithium cells produced? All about mixing, assembly and the filling process

The other two construction techniques, however, result in a single sheet that may be rolled onto itself in a variety of ways.

Z-folding process

The folding method known as Z-folding sees the anodes and cathodes cut into sheets, while the separator remains continuous. In this case, the anode and cathode sheets are first cut and then inserted into the separator, a continuous roll which keeps the two electrodes separate by means of a Z-folding process.

Rolling process

The rolling process consists of rolling four sheets of material together, first stacked on top of each other (anode sheet + separator + cathode sheet + separator), and then rolled onto a cylindrical or ovoid base to give the typical shape of the prismatic or cylindrical cell case.

As has already been illustrated, the assembly methods may be different, but the battery cell composition remains the same. In the images of the various assembly processes can be seen how the anode has a basic brown colour, as the coating is deposited on a thin layer of copper. Then comes the plastic or ceramic separator, and finally, the cathode, which is grey in colour, since it is deposited on a layer of aluminium.

What are the pros and cons of the different cell assembly methods?

At the time of writing, no one technology prevails over the others in lithium ion cell production. Each manufacturer has a favoured methodology and each will argue the merits of its choice according to the end use of the lithium battery taken as a whole.

What can be said, however, is that the stacking method makes good use of the space on a rectangular cell (pouch or prismatic). This is because, positioned in this way, the sheets manage to fill all spaces perfectly, thus increasing the active area. Nevertheless, the stacking method carries two types of risk:

1. if the sheets are slightly out of alignment, the separator in particular may be unable to avoid contact at the edges of the two electrodes, leading to short circuits;
2.  even if a laser is used, sheet cutting might still result in imperfections or could damage the edges, all of which leads to yet another difficult-to-control factor in cell quality.

In contrast, rolling or folding systems give a better guarantee of separation between components, since there are no gaps in the separator. The disadvantage, in this case, lies at the bending or curving point, which is subjected to greater mechanical stress and thus runs a greater risk of rupture, causing the two electrodes to short-circuit.

Furthermore, in the rolling method, the end result is a more or less oval-shaped roll, meaning that the corners of the rectangular cell contain no material and hence that less use will be made of the inner volume. For this very reason, even large cells are often made by placing several smaller rolls parallel to each other inside the cell.

What does the structure of a lithium cell look like after assembly?

[Fig. 1]*

The above image gives a clear example of the internal lithium battery cell composition, before it is placed into its containment case and used in modules to create the complete lithium battery pack. In this specific case, one is dealing with a prismatic cell, where two “small packs” are held in place by a central ribbon (yellow).

In the centre can be seen the separator (white), while to the right and left are the cathode (copper) and anode (aluminium), which will then be connected to the external terminals of the cell by means of ultrasonic welding.

Once assembled, the cell is placed in a case and filled with the electrolyte

Once assembled, the various materials of lithium cell composition are placed in a containment case, with the two outer positive and negative electrodes welded to the inner tabs and a central filler hole where the electrolyte will be injected. It will be recalled that the electrolyte has the basic function of passing the lithium ions from cathode to anode.

The animation shows the various stages of cell filling, an operation that at first glance seems simple, but which in fact is a particularly delicate and costly process requiring numerous steps. A liquid containing lithium salts is inserted into the central hole. The stack gradually absorbs it by means of capillary action. Where liquid is actually needed is in the middle of the cell, between the two electrodes and the separator.

Filling is an extremely slow process, since, in order to allow the liquid to be absorbed without leaking, the cell needs to be filled at various times. Suffice to say that the final filling is done after an aging period of several days. Prior to the final sealing, a filling level control phase is carried out by weighing the cell. This step is essential and absolutely necessary, as once the cell is completely sealed the hole will be closed and no longer accessible.

Lithium cell assembly is only one piece of the puzzle

Correct assembly of the cells that make up a lithium battery, whether such cells are prismatic, cylindrical or of the pouch type, is therefore an extremely complex operation, requiring long machinery set-ups as well as the utmost precision at every stage, an essential element to ensure safety, quality and reliability over time. Because of the regular and stringent inspections for contaminants and the need for uninterrupted operations in cleanrooms, this process requires huge investments as well as expansive facilities.

After the choice of the most suitable lithium chemistry, correct cell assembly is without doubt the first step (of a long series) required to achieve complete efficiency of a lithium battery. It is a complex chain of actions that ranges from the creation of the module to the implementation of state-of-the-art control electronics with advanced BMS, and culminating in the development of battery packs with increasingly advanced architectures.

Each electrical application has its own unique specifications, and in order to ensure proper operation it is crucial to strike the right balance between all of these components which, like pieces of a jigsaw puzzle, must be expertly positioned to create the product’s identity, something that can only be ensured by a skilled and consolidated manufacturer.