Fact Checking
Well-Known Member
BTW., I suspect an interesting topic for the "Battery Investor Day" will be whether Tesla is going to keep the cylindrical form factor or might switch to prismatic cells - which are cheaper/faster to manufacture and have higher volumetric density.
I think the default assumption should be that they are going to stay with the 21,700 format, but the probability that they might use something else is higher than 0% - because if Tesla starts making their own cells the industry standard compatibility and outside supplier arguments become much less important, and the overall complexity and cost of raw materials to battery modules, with the physical properties of the final product, becomes the key parameter.
Another advantage that prismatic cells might have over cylindrical cells is protection against a worst-case runaway thermal cycle within the casing. For critical applications such as EV-airplanes or EV-ships, where passengers cannot simply disembark in case of a fire, and where the capital value of the vehicle is too high to allow a runaway fire within the battery pack in case of worst-case cell failure, the (much) lower mass fraction of protective casing mass versus battery content mass might be a critical factor.
Note that the non-invasive migration of their vehicle platforms to prismatic cells might be possible because they use less volume - so the changes to the battery pack would not impact the rest of the vehicle.
This might also explain the delay of the Tesla Semi, beyond their cell supply constraints: if they are certain about prismatic cells then they'd not want to roll out the Semi based on a cylindrical form factor.
In addition to prismatic cells, the pouch cell format is a possibility too.
Maxwell's battery presentation from last year mentioned prototypes in the pouch cell format:
http://www.powersourcesconference.com/Power Sources 2018 Digest/docs/3-1.pdf
"Additionally, prolonged cycle life performance of dry coated electrodes in small prototype pouch cells and large format pouch cells, ≥10Ah, is under evaluation."
"Additionally, prolonged cycle life performance of dry coated electrodes in small prototype pouch cells and large format pouch cells, ≥10Ah, is under evaluation."
Obviously the cell form factor picked by Maxwell in their R&D effort does not limit the form factors that can be used by their dry cell technology - it can be used for cylindrical cells just as much - but I think Tesla will have a really close first principles look at what battery form factor they are going to use going forward.