I'd like to understand if there's a theoretical bound on how efficiently you could, in principle, store energy to be used for propulsion. The important quantity to consider is energy stored per kg of battery, since a heavier battery makes it harder to move. Antimatter seems like an efficient medium for storing energy, so I have that as a baseline.

Is it possible to store energy in *any* form in a more mass-efficient way than as antimatter? I initially thought you could maybe use something like a really really fast flywheel as a battery... but since there's an equivalence between energy and (relativistic) mass, adding kinetic energy to a flywheel (or any other kinetic battery?) should add mass to the flywheel. It seems like the mass of a battery is always going to be extractable energy (kinetic energy in a flywheel, chemical potential energy in a car battery, antimatter mass in an antimatter battery, etc) plus overhead (whatever mass, heat, and chemical potential energy goes into the physical object storing the extractable energy).

If you keep adding kinetic energy to a kinetic battery, does its energy/mass ratio just approach that of really dense antimatter? Is there some way to store more energy per kilogram of battery, or does the relativistic equivalence between mass and energy put a fundamental limit on how efficiently you can store energy?