Look for an opamp that's specially meant for low voltage applications, and has it's negative terminal as part of its input range. I've used the TLV2370 series (
https://focus.ti.com/docs/prod/folders/print/tlv2370.html) for a similar app, but it only works down to 2.7V.
BTW, if you plan on using a rechargable Lithium battery, you ought to calculate what sort of battery life you want - and how much you get at the low range of the battery's voltage. At that point the battery terminal voltage is heading south pretty quickly, and you run the risk of damaging the battery.