Lack of oxygen to burn fuel for the engines isn't the only thing that keeps civilian aircraft generally below 50,000 feet. There's a few other things.
For subsonic aircraft, the higher they go, the smaller the "coffin corner" gets. As air gets thinner, the speed of the aircraft where it will stall goes up (in terms of True Airspeed. In terms of Indicated Airspeed it basically stays the same which is why coffin corner is a thing). At the same time, the speed of the air moving around the wings gets faster and faster. This is because the aircraft has to move faster and faster through the air to move the same amount of molecules of air over the wings to create enough lift to hold the aircraft up. At a certain point, the stall speed of the aircraft equals the speed at which air moves over the wings at supersonic speeds. The aircraft stalls while simultaneously overspeeding. Bad things ensue. It is possible to design around this (see U-2), but for civilian aircraft, there's little benefit compared to flying at more typical flight altitudes.
The other issue is in regards to the length of time the pilots will stay conscious in the even of cabin depressurization. For aircraft that fly at high altitudes, the autopilot will have an emergency descent mode in the event that cabin altitude rises above a certain point. This is less of an issue than the first one, though.
In the end, this means that for an electric aircraft to fly higher and faster, it would end up needing to fly at supersonic speeds, and thus deal with the increased energy requirements of that, as well as the regulatory hurdles.
This doesn't mean we won't eventually see electric airliners, but they probably won't have a much different flight profile than today's aircraft.
For subsonic aircraft, the higher they go, the smaller the "coffin corner" gets. As air gets thinner, the speed of the aircraft where it will stall goes up (in terms of True Airspeed. In terms of Indicated Airspeed it basically stays the same which is why coffin corner is a thing). At the same time, the speed of the air moving around the wings gets faster and faster. This is because the aircraft has to move faster and faster through the air to move the same amount of molecules of air over the wings to create enough lift to hold the aircraft up. At a certain point, the stall speed of the aircraft equals the speed at which air moves over the wings at supersonic speeds. The aircraft stalls while simultaneously overspeeding. Bad things ensue. It is possible to design around this (see U-2), but for civilian aircraft, there's little benefit compared to flying at more typical flight altitudes.
The other issue is in regards to the length of time the pilots will stay conscious in the even of cabin depressurization. For aircraft that fly at high altitudes, the autopilot will have an emergency descent mode in the event that cabin altitude rises above a certain point. This is less of an issue than the first one, though.
In the end, this means that for an electric aircraft to fly higher and faster, it would end up needing to fly at supersonic speeds, and thus deal with the increased energy requirements of that, as well as the regulatory hurdles.
This doesn't mean we won't eventually see electric airliners, but they probably won't have a much different flight profile than today's aircraft.