Late or not, I've just joined.
Try the LFP (lithiu-ferro-phosphate) technology. It has 3.6-2.5V range but for most of the SoC (80%) it is 3.3 or 3.2 flat and that is the range where the power is available.
If you are working with lithium-ion batteries and your operating voltage is 8,2-6,2 so my guess is that there are 3cells in parallel and two in series (3P2S).
Regular lithium-ion battery has a OCV vs SOC characteristics well defined (opposite to ferrophosphate lithium ion battery which has a...