The Zephyr is using Lithium-Sulfur batteries, which have a better energy-to-weight ratio ("Currently the best Li–S batteries offer specific energies on the order of 500 W·h/kg" according to Wikipedia [1]).

These aren't commercially available though. Twice the performance of currently available automotive batteries is a major advantage for aircraft applications.

Pretty cool.

[1] https://en.wikipedia.org/wiki/Lithium%E2%80%93sulfur_battery