The VIN pad on the bottom of the XIAO is connected to the input of the voltage regulator. The regulator is an LDO regulator with a minimum differential of 250mv so you can use a LiPo battery down to ~3.5 volts. Obviously not a complete discharge but depending on your application (i.e. are you using sleep) it could last a long time. Removing the power LED (can make it last a really long time). Obviously you will need a battery charger. Incidentally this is exactly what I will be doing for a kitchen timer that will be sleeping most of the time and awakened with a button.