Earlier quoted context omitted.
That is not even comparing apples to oranges, that is comparing apples to steel. You are correct in that energy density does not matter very much for weight-insensitive generation such as grid-scale generation, but energy density matters for weight-constrained applications such as airplanes and rockets as the poster mentioned. However, assuming that the renewable generation cost curve continues to improve exponential…
What are the theoretical limits of electrical energy density?
Any given energy storage technology can store a maximum amount of energy in a fixed volume or mass. Behold one of my favorite plots: [1]
From lowest to highest energy density:
- springs, which use mechanical elastic potential energy, are kinda horrible
- capacitors, which use electric permittivity, aren't great
- next are both batteries and combusted fuels, which both use chemical reactions.
- nuclear gets us another few orders of magnitude
- finally, antimatter (E=mc^2) is a ways beyond that
Both batteries and fuels rely on the energy difference between unreacted molecules, so their theoretical energy density is the same. Well, actually, fuels are burnt to create heat which is converted to energy, and this heat->energy conversion is fundamentally thermodynamically inefficient (only ~tens of percent), whereas batteries are the same sorts of reaction but much more controlled. A sufficiently clever battery, which moves atoms around to react in the right places at the right time, is thus more efficient and thus energy-dense than fuel. However, moving atoms around like this to make a more efficient battery is much more advanced nanotech than what we currently have. But it's theoretically possible.
This is what biology does: us humans are powered by chemical storage (sugar/fat/glucose), which is used more efficiently than current batteries but without combustion. (lithium-ion is ~0.8 MJ/kg, glucose is ~16 MJ/kg, gasoline ~46 MJ/kg)