Missing,
Like you, I have a stack of Li rechargeables lying around from other projects. May as well use them for the radar.
Note, the internal 6xAA battery bay is wired at the factory to 3S2P, so 4.5V total. They don't use a voltage regulator that I can see. This 4.5V is close to the international 5V standard going in the Micro USB slot. Labradar declined to tell me the voltage range when I asked but it's safe to say it's around 4.5-5V, and not much more.
I say 'not much more' because there's online discussion of some owners frying their radars by introducing higher than 5V. Some commercial Lithium power packs have a range of available voltages: e.g. 5V, 9V, 12V. Labradar's advice here is to only buy their external power pack, which makes sense as they want to make money. But, if you control what power goes in, it's a non-issue. I've been using a DC/DC regulator set at 5V for a couple of years now without issue.