From what I've seen, the AA batteries with built-in USB-C charging port are generally lithium-ion. I think they tend to include the port because they can't be c

From what I've seen, the AA batteries with built-in USB-C charging port are generally lithium-ion. I think they tend to include the port because they can't be charged in standard NiMH chargers. Also, because Li-ion cells are > 1.5 volts, they consist of a single cell plus a voltage converter. Since they already need integrated electronics, there's less sacrifice to make the voltage step-down bidirectional to integrate charging. You can get Li-ion AA batteries without the integrated charging though; just means you need a charger made for them.