Hmm, but would it necessarily expand in that direction? If the layers swell, wouldn’t that push out on the back? I’d bet good money the expansion direction is related to how the cells are arranged.
They expand in all directions, but much more in the direction parallel, or actually horizontal, to the plates of the cells. Since that is usually in the thickness dimension, that would be where you would want the most expansion relief. You are right, arrangement of cells would have something to do with where the expansion direction is required. But from the photos the engineers' posted on their site, there was no expansion in any direction provided. If it was in the thickness dimension, it would not matter as the battery is essentially free-floating, even though glued to the bottom. Expansion at the glued, fixed bottom would lift the unfixed top as the entire battery expanded, but only if there were room to do so. This as the engineering failure that caused the problem. . . there was no room for that lifting to take place into.