Yeah, they never intended the chemistry for the extreme southwestern US environment. The production design included an aging process that was supposed to minimize initial degradation but it wasn’t enough without active cooling, even in a pouch design.
Around mid 2014 a chemistry change was made that was intended to alleviate some of the issues, and a fair number of US packs were replaced under warranty.
Through design changes for the 64kWh packs for the newer models, they were insisting active cooling still wasn’t needed, so out they went, still sealed up without any cooling system, but I haven’t looked them up to see how well they’ve been faring since they dumped the production to a Chinese company.
Uh, I may or may not have been involved…
Oof, sorry to hear that. And yeah I think folks in the Leaf subreddit were calling them “crocodile” cells/packs, and I’m completely blanking on the internal name, only vaguely remembering one translation as ‘high heat’ which was incorporated into the warranty packs and the cell upgrades in 2015 for the 40kWh “HC1” version.
And yeah, originals were 24kWh and there’s no getting around that being just for short commutes. I’m assuming at this point on the newest models, that they’re beyond the 63kWh, but those did have a different pack design. The 40s would absolutely physically fit on an older leaf, but the battery controller wouldn’t be compatible with the computer without 3rd party changes, though I’m sure people have done that and probably more by now.