These LED flashlights are powered by 18650 lithium batteries. The advantages of the 18650 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 18650 batteries is they are more expensive to buy (not necessarily to operate) and somewhat harder to find. That said, 18650 batteries are the second most common type of lithium battery used in high-end flashlights and can be very economical to when one invests in a rechargable set.