[alted]

Excellent question! A sufficiently advanced battery can theoretically beat gasoline.

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)

[1] https://en.wikipedia.org/wiki/Energy_density

[fbdab103]

That is indeed a fun chart.

One thing I wanted to add is that fat (lipids) are much more energy dense than glucose. ~38 Mj/kg, though I am not sure what fraction of that the body recovers. Which makes sense, you want to maintain your long-term storage in a denser format.

[xxs]

Fuel(gasoline, carbohydrates, fats, etc. but not rocket one) does use oxygen from the atmosphere.Batteries are self contained (like rocket fuel).

So the density of chemical reactions is by definition higher.

Side note: energy density should apply to volume, not weight, but we'll - it is too common now.

[aswanson]

What makes one chemical more able to store a greater energy per unit mass than another? Wouldn't the theoretical limit be a volume of pure electrons compressed in the densest unit volume possible? Say, stored in a magnetic field?