Some chinesium fence post capping solar lights I got from Amazon and installed at my mother's home in 2022 included LiFePo4 AAA batteries. I think it's just a matter of time before they're more ubiquitous in the US...

The chemistries produce a very different voltage, you need a tiny converter circuit inside each canister, which then has its own quiescent current draw. They make these, and they're good for some applications.

They do. Lithium ion has a 3.7V nominal voltage compared to the 1.5V of alkaline and 1.2V of NiMH, and so liions in the AA format are called 14500s (14mm diameter 50mm length) and 11500s in AAA format. There's also liion AAs with voltage regulators to provide 1.5V that often have a USB micro B or C port on them

There are both AA-shaped Li-ion batteries meant to be used with spacers, as well as USB-C charged AA batteries. It scares electronics-savvy people(myself included) but the market do seem to be growing.