If we use energy to accelerate a vehicle, a lot of that energy is lost to drag and friction, especially in an electric vehicle. Why can't we harvest some of that energy using an alternator and some sort of fan/turbine, to assist in recharging the batteries?
Your suggestion comes close to being a perpetual motion machine, though it's not quite. The idea of using the power of the car's battery to recharge the battery via the slipstream is the sort of stuff perpetual motion machines are made of, and they never work. Putting a fan or windmill in the car's slipstream would reduce the car's aerodynamic efficiency and therefore compel it to use more electricity to achieve the same performance, so there would be no net gain and in fact a very slight loss of energy. The installation of a slipstream windmill would therefore be counterproductive.
Electric vehicles already use regenerative braking which is an energy recovery mechanism that slows a vehicle by using the vehicles electric motor as a generator to convert much of the kinetic energy that would otherwise be lost as friction heat in conventional braking systems, into forms that may be used for immediate use or stored for use as needed.
Driving a fan/turbine with the air flow created by motion of the car will increase air drag and ultimately have the car using more energy (electric or fossil fuel) with the fan/turbine than without the fan turbine.
That said, it is possible to use wind to generate and store electrical energy with a turbine on a car when it is stationary, though I doubt it would be practical. That's basically the same thing as land based wind turbines on wind farms.
Hope this helps.