CE-680A First Officer reported sliding off the taxiway during taxi from the ramp area on an ice covered taxiway. The aircraft was removed after the taxiway was treated by a deicer. There was no observed damage to aircraft or airport equipment.

Narrative

This was an empty; repositioning flight from ZZZ to ZZZ1. It was going to be my flight leg. [The] FBO parked us facing south on their Ramp; and while we prepared for the leg to ZZZ1 they parked a King Air directly behind us. The Ramp itself was icy with a Citation parked towards our left. The Captain and I had discussed the best taxi out would be straight ahead to Taxiway 1 to join Taxiway 2. All checklists were completed and I briefed the departure as well as the arrival into ZZZ1. ATIS was calling the runway 5/5/5 and the taxiways good to poor. The taxi clearance was Runway XX via [taxiways] 3; 2; 4; cross Runway XY. Due to the positioning of the aircraft and icy ramp; we headed towards Taxiway 1 and before I could say anything to Ground; they called us stating [taxiways] 1; 2; 4 to [Runway] XX was fine. In reviewing Jepp; it appears that the Ramp just turns into [Taxiway] 2 and [Taxiway] 1 is actually towards the end of Runway XZ.The Captain was taxiing slow given the known conditions. Both of us were eyes outside. It was a normal; initial 45° turn and then I witnessed the nose continue through the taxiway centerline. I called out; 'You need to turn right;' when the Captain said he was hard right on the tiller. At that point I said; You're going to slide off the end (of the taxiway);" with the reply of there's nothing I (he) can do. After stopping; we completed the checklist for shutdown with APU.I informed Ground that we believe we had slid off the taxiway. Ground contacted Airport Maintenance and I called the FBO for a tug. After discussion with the Captain; I did not see any taxi lights below the door; and therefore opened the door. The nose of the aircraft was maybe 12 inches left of concrete with the left main just over the yellow line of the taxiway. From the skid line; one could discern the nose was turned hard right with only one noticeable line across the ice; instead of two tire marks.The taxiway itself was glare ice; nil at best. [The] FBO was unable to move us until the airport came with their chemical deicer. Once that was applied; the layer of ice dissipated; and the aircraft was removed. There was no observed damage. Suggestions: Confirm with ATC all taxiways are treated."