Cessna Citation First Officer reported movement of the aircraft after being chocked on the icy ramp. Crew pulled emergency brake to stop the aircraft movement.

Narrative

The airplane had been in the hangar at the FBO when we arrived in the morning. The plane was preflight and prepared as much as possible while inside. Once the line crews pulled us out of the hangar; they pushed us back beside the hangar on the southwest side of their facility (both pilots were in the airplane as it was being moved. The batteries were turned on as they were pushing us back into position. No other power was on. Once in place; my co-pilot opened the door and the ground crew indicated we were chocked; and started pulling the tug away from the plane. My co-pilot yelled the plane was moving; and the tug operator jumped up as if he was trying to push back/hold the plane in place. I jumped back into my seat and pushed the brake pedals all the way down; the hydraulic aux pump had not been turned on. In an attempt to stop further motion; I pulled the emergency brake; and the plane stopped. Line Svc said again that the plane was chocked; and it was verified before I released the emergency brake. Line Svc stated to us that the airplane slid on the icy/snow covered ramp while chocked.There were tire tracks in the snow going about 5 feet behind the plane.The tug operator stated to me that we did not make contact with the tug; and there was no visible damage.Maintenance was contacted; and the plane was AOG to refill the emergency brake bottle.Suggestions: I don't know.