Cylindrical. Prismtaic. Pouch.

Li-Ion cells are manufactured in three basic formats: cylindrical, prismatic and pouch.

Cylindrical cells are encased in metal cans and are, quite simply, cylindrical in shape.

A central mandrel is wrapped in a long strip positive and negative electrode foils with a separator in between. This is commonly known as a “jelly roll”. When cylindrical cells are used in a battery there is wasted space between each cell, although sometimes this area can be used to assist in thermal management if the application requires it.

Cylindrical cells come in various sizes, some small and are connected in battery packs by welding nickel or copper strips, large cylindrical cells may have a threaded nut or hole. They retain their shape against expansion during charging and aging.

A prismatic cell is essentially the same construction as a cylindrical cell and is also encased in a metal can, only this time the mandrel is flat.

The same two long strips of positive and negative electrode foils and separator are used. The final structure becomes prismatic due to the shape of the mandrel. A prismatic cell has a small amount of wasted space at each corner where the jelly roll has not gone to the edge. These cells can be used in a battery where space if of a premium as there is less wasted space between cells. And some applications require a flat cell due to mechanical constraints, a mobile phone being a prime example.

Prismatic cells may also be connected by nickel or copper strips (small cells), or they may use threaded bolts or holes in their case (large cells). There can be small amounts of swelling during charging and aging of a prismatic and this must be accounted for in the design of the housing for the application.

A pouch cell differs in construction, instead of two long strips of electrodes the pouch cell uses a stacked structure. The outer case looks like a thin plastic bag and that is why it is call a pouch, but it is stronger than a plastic bag

This structure optimises space even further than the prismatic cell since all stacks go to the edge of the packaging. When using a pouch cell in a battery careful consideration needs to be made in terms of structure of the battery, end plates are required to keep the cells from expanding during charge. Any expansion of the cell may allow the stack of electrodes to separate causing higher resistance points (loss of capacity) or fracture of the separator (internal short and rapid uncontrolled disassembly).

Pouch cells have tabs coming out of the end of the cell and are often soldered onto a PCB that joins them in a parallel and series array to make up a battery.

Summing up, cylindrical and prismatic are spiral wound structures of electrodes and are manufactured in essentially the same way, the difference being the shape of the mandrel the electrodes are wrapped around. Pouch cells are constructed from a stack of electrodes with separators between each layer.