Take a look here: XM-L2 Vf graph
The XM-L2 needs much lower voltage than a fully charged cell to power on, if you connect them directly you will end up driving the LED to very high current and possibly blow it up if not heatsinked correctly.
Take a look here: XM-L2 Vf graph
The XM-L2 needs much lower voltage than a fully charged cell to power on, if you connect them directly you will end up driving the LED to very high current and possibly blow it up if not heatsinked correctly.