I have wondered about this, generally...energy density is a Good Thing, and so is rapid charge/discharge, but in the event of a mechanical failure or deformation, where does the energy go?
Most capacitors have a self-discharge rate much higher than batteries - but good, cheap ultracapacitors might be very useful for “burst” power or storing energy from braking, and so on.
I had to start getting real interested in this stuff about 18 years ago when our shop moved into an office building on the next to top floor where the top floor had a Faraday Cage construction throughout (for testing Space Shuttle parts). The Sun toked up a bit, spewed out all sorts of stuff, and sure as shooting every day how many ever minutes it took after Sunup for extra charge to hit that building, that's when my computer system cabling would pick up "static".
Oh, yeah, just ordinary static ~ but in a DEC system that's all it took to destroy connectivity. We tried all sorts of shielding ~ then we finally realized that the electric charge buildup on the Faraday Cage surface was actually a FLOW, and it was discharging into the metal pipes that served the heat exchanger for the AC in the computer room.
The solution was "pray for rain".