They want lots of energy stored in a small space. They don’t seem to understand, that is what a bomb is. If there is a fire or an accident, the energy will be released.
With gasoline and other “fuels”, there is no oxygen, so the release is slow. With batteries, capacitors, flywheels, accumulators, etc, the energy is all there, and ready to explode. I’ve seen trucks cut in half by flywheels (old style wood chipper accident). I’ve seen small radios blown apart by tiny capacitors. I’ve seen laptops incinerated by batteries. They’re trying to squeeze a balloon — every improvement in one area will increase the problems in another.
It's handy that the Earth's atmosphere has a relatively high concentration of O2. Fuel-air bombs seem to be very effective.
I was waiting for EEstor to actually deliver a car system like on the movie “Demolition Man”, which were “not” supposed to explode. Actually, such a storage system could be reconfigured to produce a soft x-ray / ultraviolet flash, in addition to a bang. Poor mans pocket-nuke.