There are a few high performance/track cars that use lifepo4 batteries for weight distribution, but these are always located in the trunk or other areas outside the engine bay. I believe Porsche was offering this as a 'track day car' option for the last 10-15 years, same for BMW (and others I expect) - but they are hugely expensive:
I read the porsche batteries are about $4500, and BMW charges around $2700:
Lithium-ion starter battery. 70AH. 2022 BMW M3. Genuine BMW Part - 61218092860 (61-21-8-092-860)
shop.bmw.ca
Bad idea if its under the hood due to thermal management issues. Its just too hot (and too cold). Not worth the risk of lighting up your vehicle - there are literally zero aftermarket options for under the hood Lifepo4 starting batteries, so I'd take that as an indicator.
FYI - your AGM probably died early because your alternator is setup for conventional lead acid battery. 12V Lead acid batteries charge at anywhere from 13.6-14.4V whereas an AGM needs closer to 14.7V to fully charge. I drive an old Lexus GX470 with the 4.7 and the alternators are set to a low charging voltage (most people report rarely over 14.0V). People 'upgrade' to an AGM and it dies in short order the battery rarely fully charges.
In lots of cases there are ways to adjust to increase charging voltage for better compatibility with AGM. For your current battery, maybe try to run the repair cycle on a NOCO charger. I keep a jump pack with me, so I'd just run the battery till it dies.
