There is a lithium-ion battery that is around the same size as would fill the battery compartment in a 3AAA light, and the voltage would be around the same. However, as stated above, battery holders of the type use by LED Lenser often have both the positive and negative contacts at the head end of the battery holder, whereas the li-ion cell will have the positive contact at the head end, and the negative contact at the tail end. If this is the case with your own battery holder, then the li-ion cell wouldn't work - certainly without modifying the light in any case.