What type of batteries does Milo take?

Milo contains a rechargeable lithium battery, so you never have to change batteries, just recharge with the enclosed USB-to-USB-C cable.

The battery is integrated - that means you can't remove it. Same as your phone. So the rules that apply to packing your mobile phone for airplane travel also apply to Milo. You should have no issues bringing Milos in your carry-on as long as they are powered off.