Well there are no 5V Li batts (they are all 3V and 3.7V), so you have a few options...
1) You can use a big 3.7V (like an 18650) and a step-up DC converter.
2) you can use two 3V in series to get 6V, then use a 5V regulator to step it down.
3) I haven't seen anyone doing this, but you may be...