An important energy storage method that is being given very little consideration is gas compression. The kinetic energy of the vehicle can drive an air pump or compressor that would compress air into a tank specially designed for pressures and heat of compressed air. Later the energy stored as compressed air can be released through a regulated nozzle to convert to electrical or mechanical energy.
The air pump would be connected mechanically directly to the drive train or the wheels or axle to have the braking effect. The storage process is not efficient as much heat is generated with the compression. However, the energy can be stored in the compression tank for a very long time with little loss. The capacity for energy storage is only limited by the volume of the storage tank and the maximum design pressure.
Special consideration has to be made for the lubricating oil used for the air pump/compressor. It has to be non-combustible other wise it will ignite with the oxygen in the heated compressed air. This would rule out most hydrocarbon based oils.