Apple released new iPhone battery cases for the XS, XS Mac and XR models. These cases feature a sizable battery as part of the back of the case, giving you extra hours of battery life for those that need it. They cost $129.
Unlike third-party cases, these integrate with iOS so you see the battery charge in Notification Center. They also work with wireless charging pads. Apple also offers similar cases for older iPhones, but never released one for the iPhone X.