Why does Tesla use cylindrical cells instead of prismatic?
 

I just watched a 40 minute Gigafactory tour video and was surprised to see that the new Tesla will use 2170 cylindrical cells rather than prismatic.

Why is the advantage of cylindrical cells vs prismatic?

I would think they would cost more due to increased use of materials (smaller individual capacities and more material used in the case). The increased casing material would also increase weight. Finally, there would be a lot of empty space with all the voids left by the cylinders not being able to stack flush with each other, so the energy density would be reduced.

When I bought a 20 Ah LiFePO4 battery, the best price I could find was in prismatic cells, with cylindrical being a bit more expensive.

So, what's the advantage? Will prismatic cells die out, with cylindrical being the way of the future?

My initial thoughts are cooling and strength of the casing. Cylinders are very strong under pressure and the voids allow for even flow of a cooling medium.

Simon

I too was thinking it was being done for cooling.

Perhaps because cylindrical cells are easier to construct with automated machinery? They're essentially just a long strip of foil wound into a cylinder. Here's a link with a basic explanation: Lithium Battery Manufacturing

Per kWh the battery packs in a Tesla weigh almost half of that in a Nissan leaf, they also water cool then and have really impressive discharge rates.

How does Tesla accomplish that? Why are the Leaf batteries so heavy?

Prismatic cells, especially the ones Toyota use, need to be restrained mechanically to prevent distortion. This mechanical bracing takes space and adds weight too. The larger cells also need heavier bus bars as interconnections whereas the smaller cylindrical cells have much smaller interconnects except where the strings aggregate. The prismatic cells form a large block that does not have the ability to dissipate heat from the center cells except by conduction into the adjacent cells.

Sometimes, older ideas work better. Prismatic cells are far better in low discharge scenarios where space is at a premium, hence their prevalence in the mobile phone space.

Simon

It'd seem easy enough to build packs with rows of prismatic cells separated by enough space for air or liquid cooling. Or perhaps even heat pipes sandwiched between layers of cells...