Three NiMH D cells will also provide a more constant discharge voltage (important for hotwire incandescents and direct driven LEDs ) and more current than pretty much anything short of a nanoparticle LiFePO4. Of course, nanoparticle LiFePO4 has similar constant voltage discharge characteristics to NiMH, but capacity is low. Also, regardless of what kind of Li-Ion cell you use, I would think that, with just one cell, balance would be off - the light might be lighter, but it will be head-heavy.