That's key. Electrical propulsion systems, like ion engines, generally work by ionizing atoms of a propellant like xenon gas, accelerating them in an electric field, and ejecting them through a jet to create thrust. Pound for pound, they are far more efficient than chemical thrusters and largely viewed as a means to deep space travel, but they also have some unique drawbacks. Chief among these, at least as it pertains to satellites, is the plume of ionized particles left in the spacecraft's wake. Some of these particles end up in contact with the exterior of the spacecraft itself and can foul things like solar panels that are critical to keeping a spacecraft operational.