We do have one product that supports LiFePO4 lithium batteries, which are stable at low temperatures and much much safer generally, but 3.3-3.2v over the usable range instead of 4.2-3.7v for normal lithium ion batteries, so once LiFePO4 is near flat it's too low to power the mcu.  The product we sell always boosts the voltage to 4.5volts so it's near the 5v ideal power level at all times.

 The device is designed for usb/solar/dc-barrel-jack power to charge the battery (and also power the downstream device/board), so uses that incoming DC power when available or the battery (boosted) if not, always outputting 4.5v for the downstream device (the load).

Make sure to follow the learn guide, cutting one jumper and soldering another to set LiFePO4 mode.

Product Page: https://www.adafruit.com/product/6091

Learn Guide: https://learn.adafruit.com/adafruit-bq25185-usb-dc-solar-lithium-ion-polymer-charger/pinouts

You'd want some wire or connectors potentially to make life easier (from charger load pins to MCU board either wired as battery in / external power or as usb connector), and to have a receiver for the battery connector.

It's maybe a bit too complex for a beginner's first go, but it's suitable as a level progression project with WipperSnapper (no-code IOT firmware - written in Arduino with open-source contributors). 

Plus you need to "obtain" the LiFePO4 batteries and attach connectors if not already there.