Are fully electric helicopters suitable for military operations?
Battery weight is a huge negative. Liquid hydrocarbon fuels have energy densities of about 45MJ/Kg, and the significant advantage that as you use them, their weight stops needing to be transported. 

A lithium-air battery is theoretically capable of getting close to the energy density of hydrocarbon fuels, but the best that's been achieved so far in the lab is about 6MJ/Kg. The energy density of lithium-ion batteries, which are the best that's deployable at present. are more like 1MJ/kg, at the cell level, or rather worse at the pack level. If you want to drop used batteries, your operating costs go way up. 

An electric helicopter could be built, but its flight duration would be very short. Just like quadcopter drones.