Ford currently has no plans for a heavy-duty Lightning to join the F-250 and F-350 series.
Ford has officially ruled out making a heavy-duty electric pickup truck, at least for now, as it focuses on starting production of the F-150 Lightning, the electric Transit van and the vehicle (or vehicles) that it is developing on Volkswagen’s MEB platform. We may see a fully-electric Super Duty Lightning in the future at some point, it was not ruled out, but its creation is not a priority right now.

However, it will surely happen at some point in the not too distant future as more and more types of vehicles will be switched from ICE to EV. When Kumar Galhotra, president, Americas and International Markets Group was asked about an electric Super Duty, he said that

At the moment, we do not have any plans to go into heavy duty with battery-electric vehicles.