Do I have to pay for a new battery?

Yes you do, unless you purchased your iPhone less than 12 months ago and Battery Health is at 80% or less, which is highly unlikely. To start a battery repair, you can use this link --> iPhone Battery Repair & Replacement - Apple Support

Check here.

iPhone Battery Repair & Replacement - Apple Support

Do I have to pay for a new battery?

Maybe not, if Apple decides that the battery is defective and your iPhone is still covered by the AppleCare warranty at the time.