is there a physics limit to the density of energy
the physics limit to the density of energy is literally a black hole. it compresses the maximum amount of mass (energy) into a space. but that's technologically useless since you can't extract the energy out of it on-demand.
The densest ways of storing energy that are technologically useful are:
- batteries (Na-Ion batteries: 0.2 kWh/kg)
- oil/carbon-based fuels (bread: 5 kWh/kg)
- uranium (pure uranium: 24 * 10^6 kWh/kg)
There's also speculative technologies like antimatter (24 * 10^9 kWh/kg) which aren't available today.