The compressor /burner behind the duct ed fan spin at high rate. The fan must keep the tip of blades below sound speed. Latest jet engines have a reducing gear from the compressor shaft to the fan ( just like turboprops).An electric motor will spin the fan ,with practically 100% efficiency...
The enthalpy of burning jet fuel is (approximately) 40 Mj/kg. But mr. Carnot puts an upper limit to conversion to "shaft energy"aka mechanical work(optimistically) to 0.20. The electric motor is 0.9 (this number measured) from e energy to shaft energy. LiIon is (available now) 0.9 Mj/Kg. Just...