The ecosystem of iPhone accessories largely relies on third-parties to offer a wide variety of options, but Apple does make its own offerings as well. The company has launched new "Smart Battery Cases" for all the iPhone models launched in 2018, the Xs, Xs Max, and Xr.
The battery cases aren't a new concept - they're silicone cases that protect the phone while also charging the device. Apple's new cases are smart because the battery charge of the case is passed on to the iPhone battery indicator, so you can tell how much power you have left without any additional hassle. They also support Qi wireless charging as well as fast charging, and of course you can use any other Lightning-connected accessories with them.
As for the effects on battery life, the Xs variant promises up to 33 hours of talk time, 21 hours of internet browsing, or 25 hours of video playback. The Xs Max case delivers up to 37, 20, or 25 hours for each of those scenarios, and the Xr model offers the greatest life extension, up to 39, 22, or 27 hours. All of these estimates seem to be accounting for the battery case and the iPhone battery altogether, based on the wording on the product pages.