https://www.techradar.com/news/2018s-iphones-could-offer-much-better-battery-life-thanks-to-new-a12-chips

Apple's A11 chip made last year's iPhones exceptional in terms of performance, and thanks to a new chip manufacturing process, we could be seeing another huge boost in 2018, according to Cult of Mac. 

TSMC, the Taiwanese company that is exclusively manufacturing Apple's iPhone processors this year, is reportedly implementing a 7 nanometre process for 2018's devices, which should shrink the size of chips by around 30% when compared to the 10 nanometre process that was used on the iPhone 8, iPhone 8 Plus and iPhone X.