Back in the day, lower volatility was given as one reason the metal cylinder could be dispensed with. This is somewhat of a red herring since there was no standard way of comparing volatility. Certainly using a flexible plastic pouch was less explosive then a sealed metal can. Either type will burn so it may be a somewhat moot point. Either way, I believe the goal was to reduce the mass, improve the packing density and keep the safety more or less the same. The end result was hopefully lower cost, easy scalability and better overall energy density at the pack level.Just nitpicking here, but a couple of things. First, I don't think the chemistry was format dependent. GM and other automakers could have gone with similar NCA chemistry as Tesla; however, that chemistry is more flammable and volatile, something they wanted to avoid for automotive use. Second, the Ultium cells would reduce the number of cells required for the Bolt EV, but only by 1/3. The cells in the 2020 Chevy Bolt EV are over 60 Ah, so you'd need two Ultium 100 Ah cells to replace three Bolt EV pouch cells (essentially 96s2p).
Also (and this is why I'm so critical of cylindrical cells for automotive use), despite each cylindrical cell being individually packaged and self-contained, the battery pack still has to serve as a structural component for the car. So the cylindrical cell casing is basically wasted mass at that point.
My bad on the math. My main point was that fewer connections are better with this line of reasoning. I actually look at Ultium as a tweak to the design that the Bolt EV already uses.
As for structure, there really are major differences. GM's approach on the Bolt EV is to largely isolate the cells as a structural element. The Bolt EV uses a very robust bottom "cover" that is built like a battleship. I believe that I saw that it weighs some 190lbs. This structure bolts to the car structure to form a very robust steel box to mount the cells in. This struck me as overkill but why not. This may or may not add as much mass as is saved by not having cylindrical cells.
Tesla definitely does use the cells as a structural element. The cells are built up as more of a matrix of cells and "glue" to form a very solid brick. These bricks are enclosed in a rather light weight housing. The reduced housing mass helps offset the mass of the cylindrical cells. It sort of looks like the overall energy density works out to be similar to pouch cells when mounted in a strong box. Obviously the differences in the details of the construction are what can help make or break the direction chosen. Is the Bolt EV overkill? Is the Tesla solution actually adequate? We have no objective tests to compare the two approaches.
As a retired power system designer, My response to the Tesla design was that it was complete rubbish that would set back the cause of the EV decades. The Tesla solution was completely counter to the direction of the entire battery industry over decades of R&D.
I continue to be amazed that this rube Goldberg contraption actually seems to work. I continue to be convinced that Tesla will need to switch to pouch cells at some point but who knows. I'm also in the "never Apple" camp. LOL