It's not a lithium ion, but you got the right idea. There's a little battery at the back, when you open a little closed tab (kinda like on the bottom of remotes) Anyways it's one of those small coin sized batteries
Mine went dead years ago, but I really couldn't care less if it knows the correct time!
You can get them usually for around $5.00 - $8.00 US - or around there 