I have a 2005 Kia

the only way your key wouldn't work is if the tumbler is damaged. The de-icer you are speaking of is for ICED UP tumblers. Since your ignition is not open to the elements, it is not frozen. The grease inside may be thick, but would not prevent your key from entering.
Do not put any type of liquid in the ignition because it is an electrical switch. You can try a dry liquid graphite, however it is my opinion that the tumbler is broken/worn and needs to be replaced (this is usually from an overloaded keychain - causing the tumblers to wear and break)

Try Warming up your Key Before you enter in to the Ignition.

Anytime you CANNOT get your In Ing Tumbeler Lock The BEST thing to DO is to REPLACE The Lock Assembly and call it a Day is Other Methods Have a SMALL Probability of Working ON THE SPOT but what about when it REOCURES At A Location 40 miles away from Home?? Point being if it is DAMAGED there is NO MAGIC FIX Replace the Ignition Lock switch and Have A Cumfy Winter.
Hope This Makes Sense to you and Stay Warm, RejakWilson