Further thoughts on energy density:
- Jet Fuel energy density: ~12,000 Wh/kg 
- Batteries energy density: ~500 Wh/kg 
- Electric Motors efficiency: ~85-90%
- Jet engine efficiency ~30-40% 

So batteries at 1,000-2,000 Wh/kg starts to be competitive with Jet Fuel, and would really start to surpass it at 5000 Wh/kg

creating and destroying; built flying cars & autonomous vehicles, now building /faust, more here: /july, web: july.rocks

fuels cells or possibly direct hydrogen turbines seem like a possible way forward as well, since hydrogen is between 30,000-40,000 Wh/kg depending on the source you use

though the energy density by volume isn't quite as good because the compressed gas isn't liquid

for most applications the problem with renewable hydrogen is that you spend energy generating it through electrolysis (otherwise it's extracted as an oil and gas byproduct) and adds infra complexity which makes it uneconomical

but airplanes are a case where it might make sense imo