In the flashlight community, there is a company called nitecore that makes such a charger. The "nitecore f2"
It is literally a liion battery charger that also has 2 USB outputs so you use your liion cells as a power bank.
Sounded like a great idea so I got one but honestly, I don't like that:
1. You need a rubber band to secure the cells. They include 2 think rubber bands for this purpose but this method feels really cheap for a finished product.
2. There is no way to power off the "power bank" without removing the battery (or batteries).
Something like this with a better method of securing the cells (with qc or pd please?) I'd definitely be interested.