These LED flashlights are powered by CR123A lithium batteries. The advantages of the CR123A is they offer twice the voltage of AA or AAA batteries (3 volts each vs. 1.5 volts), tend to last longer and have a 10-year shelf life! Higher voltage = higher lumen potential.
The disadvantage of CR123A batteries is they are more expensive to buy (not necessarily to operate) and somewhat harder to find. That said, CR123A batteries are the most common type of lithium battery as they are used in some digital camera's and can be found in the great majority of stores that sell common alkaline batteries.