That seems to have energy density lower than even batteries though. When you vaporize LN2, you apparently need 0.2MJ/kg of heat. The amount of mechanical energ

That seems to have energy density lower than even batteries though. When you vaporize LN2, you apparently need 0.2MJ/kg of heat. The amount of mechanical energy you extract out of vaporizing that (when heat is provided by the environment) can't exceed the heat required to do so (because condensing the nitrogen back into LN2 is exothermic). Apparently current Li-ion batteries have ~250 Wh/kg = ~0.9MJ/kg energy density, which is significantly higher than the upper bound above. Am I mistaken somewhere?