It is easy to say lithium-ion batteries. Behind a term used more and more often lie many technologies and just as many technical solutions, some of them very different. In any case, batteries consist of one fundamental element: the cell, which can also be of various types.

The most common - if only because they have been used the longest and are adopted by Tesla on almost all its cars - are the cylindrical cells.

On the outside, they do not differ much from the classic alkaline batteries that we use every day to power the TV remote control or children's electronic games. On the inside, however, they are very different in terms of the materials used and the complexity of their construction because they have to guarantee durability and performance on a very different level.

The basics

A lithium ion cell consists of a number of main elements: the anode (negative electrode), the cathode (positive electrode), the separator and the electrolyte. These are the components that enable the cell to store energy and release it to the outside world.

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Anode, cathode and separator are assembled in overlapping sheets

  • Anode: the negative electrode is generally made of graphite but silicon anodes are also becoming established
  • Cathode: the positive electrode is made of a lithium metal oxide that may contain varying amounts of nickel, manganese and cobalt. Recently, different chemistries are being studied, the most popular being lithium-iron-phosphate. 
  • Separator: made of plastic material, often polyethylene or polypropylene.  
  • Electrolyte: is a liquid that allows the passage of lithium ions between cathode and anode. It uses solvents that make it highly flammable. Lately, aqueous solutions are being experimented with, which are more resistant to high temperatures, and also semi-solid and solid electrolytes, which also improve the performance of the cell itself.
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Cylindrical cells produced by Panasonic

A complex structure

In a cylindrical cell, the electrodes are made in overlapping sheets separated, as can be guessed, by the separator, a thin sheet of the electrodes between them. The sheets arranged one on top of the other are rolled on themselves to form a spiral, which is called a Jelly Roll, and are placed inside the outer housing and immersed in the electrolyte, which allows the passage of lithium ions between the two electrodes.

Lo schema di una cella agli ioni di litio

While electrodes, separator and electrolyte are the elements that allow the effective passage of the lithium ions in one direction and in the other according to the charging and discharging phases, there are other components in the cylindrical cells.

The outer casing, which is made of aluminium or sometimes very strong plastic, but also the negative terminal, a copper disc or other conductor that closes the cell at the bottom and is in contact with the anode, and the positive terminal, another disc, this time usually made of aluminium, that closes the cell at the top and is in contact with the cathode.

There are also safety valves in the upper part that allow better management of pressure or temperature increases in the event of a malfunction.

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The 4680 cylindrical cells produced by Tesla at the Austin Gigafactory

The advantages of cylindrical cells

Compared to other cell types, cylindrical cells offer a number of advantages. First, being the most widely used, they are more mature than other architectures, which makes them particularly reliable. Furthermore, due to their shape, they do not deform easily, making the battery interior a less stressed environment.

As they are generally smaller than cells of other formats and have a large surface area in relation to the total volume occupied, they also manage to dissipate heat well, being able to work better even under high-stress conditions.

Gallery: BMW Cell Manufacturing Competence Centre