Is there a limit to the possible altitude for electric jets?
That electric propulsion needs no oxygen is of little help for flying high. The composition of the atmosphere does not change too much with altitude, so it is the low atmospheric density which limits maximum altitude. We had this discussion before, and indeed solar-electric propulsion helped to reach record altitudes: The AeroVironment Helios set a record at 29,524 m which will be hard to break.

Supersonic speed helps to fly high - while subsonic designs run into the coffin corner, supersonic flight helps to maintain high dynamic pressure even at altitude. However, the energy requirements for doing so will require an energy density of the batteries which is several magnitudes above what is currently possible. If you only want to go fast, the current limit with electrical propulsion and already rather generous assumptions is somewhere around Mach 2.

Flying higher does not reduce the thrust requirement since the same lift has to be generated. The best lift to drag ratio is achieved in subsonic flight. Going supersonic means crossing a drag peak, but beyond that the maximum possible lift to drag ratio is well below that at subsonic speed and decreases again with increasing Mach number.

Mr. Musk is clearly out of his competence when he claims that supersonic electric airplanes are viable, even with the progress in battery technology to be expected in the next few decades.