and we put it inside Cryogenic storage dewar (thermos) that energy does not go outside we can store any amount of energy and we can get it back, when we need.
I think it is true because the adiabatic process is reversible.
Of course, in real life, we will lose something, let say 10%, because we cannot get ideal thermoisolation, we will have friction, but it is still should be very efficient.
Why is it not in use instead of lithium batteries etc? Or my consideration is wrong? we can put small tube and piston with electric generator/motor as AA battery size and that it.