You cannot run aircraft on electricity; liquid fuel is the only option. The military is keenly interested in being able to refuel aircraft in situations where liquid fuel shipments might be denied or unavailable for some reason.
You can't in any practical sense. Locomotives don't have a problem carrying the weight of a diesel engine plus a generator; in fact the extra weight works in their favor.
If you tried that trick with an airplane it would never get off the ground. Locomotives use this system not for efficiency but because they need the low-end torque electric motors provide. (If they only needed efficiency they'd just drive the wheels with the diesel engine directly.) Airplanes don't need torque; they need power.
You can run an airplane on batteries but only for a few minutes with modern battery technology. Practical battery-powered aircraft for military applications are not going to happen any time soon. We might see battery-powered air taxis soon but those are not even close to meeting military requirements for fighters, transports, etc.
> [For] large passenger aircraft, an improvement of the energy density by a factor 20 compared to li-ion batteries would be required
The time it takes to surpass this energy density threshold is affected by battery tech investments; which had been comparatively paltry in terms of defense spending. Trillions on batteries would've been a much better investment; with ROI.
Sadly, some folks in defense still can't understand why non-oil investments in battery tech are best for all.
There are multiple electric trainer aircraft with flight times over an hour and quite a few more in development.
Jet engines are terribly inefficient (30-50% efficient) compared to electric motors.
