Howdy there OP,
Unless you have an extended warranty most damage won't be covered past one year. In other cases, most normal warranties won't explicitly cover battery expansion unless it's determined to be a result of a defect in materials or workmanship. As for what I would recommend next, you could attempt to contact Dell and see if you can get it fixed. Otherwise, if you're confident enough and have the tools, you'll need to [parts|Dell XPS|replace the battery yourself.|itemtype=batteries]
Best of luck to you!