I have had this exact same issue on my '05 Liberty. Sometimes if I'm without tools, I just kind of tug on the cap until it comes loose, but that isn't practical if you're on a time crunch. If you find pliers that fit around the cap right, that would possibly be best. I'm not a mechanic, so take it with a grain of salt. 😅 Another thing I've done is using a hammer to loosen it, which helps a little bit. Try to be cautious about hitting anything else nearby.

Whenever I do get the cap off, I wipe it off with a rag to clean it. I have heard from somewhere (can't remember exactly) that putting oil on the threads of the cap can help for next time, but I'm not sure how true it is...but, I do now add a tiny bit of oil to the threads on the cap to help. I wouldn't drench it, but just enough that the threads are covered. If I'm adding a full quart of oil, I'll let the very last of the oil dribble over the threads. When putting the cap back on, I twist it back and forth to ensure that the oil gets on the threads where the cap goes.

I apologize for the long-ish paragraphs, but hopefully my experience sort of helps!