[ben_w]

All depends on what you want. You can have “infinite” range with “no” battery — gliders are a thing, you can rise on thermals and turn the potential energy into kinetic to get wherever.

But if you want to go as fast as a chemical engine and sustain that for as long as a chemical engine, you either need at least the energy density of a chemical fuel or some way to refuel in-flight.

Fuel efficient often means “slow”. Great for cargo, not so much for passengers.

Edit:

Just to add, I’m looking forward to a future of short-range electric personal aircraft, I’m just not expecting pure electric intercontinental for anyone other than hobbyists — unless there’s a big breakthrough in density or wireless power transmission.

[Robotbeat]

The longest range conventional chemical aircraft flew around the world without landing. It's not necessary to equal that range for any commercial application. Kerosene is better than it needs to be.

Fuel efficient for aircraft doesn't necessarily mean slow, however (unlike ships). What matters is cruise lift to drag ratio. As long as you can adjust cruise altitude for peak efficiency and as long as all the flow is fully subsonic, then lift to drag ratio is mostly independent of speed (as you can increase altitude where the air is thinner to compensate).

So yeah, electric aircraft might stay at Mach 0.5 or so, but they don't need to be slow. Mach 0.5 is still much faster than any passenger high speed rail service and MUCH faster than car or bus or boat.

Pure electric intercontinental may be feasible for near-term chemistries like lithium-sulfur if you continue to push efficiencies (both structural and aerodynamic). Very long haul, like LA to Tokyo, will need lithium-air technology which is a few decades off.

[cjbprime]

Electric motor gliders should be great! You only need the battery/prop for takeoff and occasional sustaining flight and the glide ratio is ~4x better than general aviation planes.