It's a little bit of a misnomer to say that electric aviation is infeasible on lithium ion batteries, not to mention hybrid & hydrogen fuel cell alternatives. They're obviously not flying transatlantic flights at the moment, but regional, small payload flights can already be flown fully electric and there's around a dozen companies working through certification processes globally.
That depends on what kind of aviation we are talking about. An air taxi usable over 200km with 2 passengers is easy to achieve. But a minimally useful regional plane with 100+ passenger capacity is an entirely different matter, because it will be subject to the same regulations as conventional airliners. That is operational margin, winds, diversion and hold, etc. This means you probably need something like 2000km net range to be able to fly 500-1000km routes, which means you need close to 1000 Wh/kg batteries under reasonable assumptions for battery mass fraction and L/D-ratio.
