Everytime you charge your phone, you complete a charging cycle (basically you erode your anode a bit). But when you keep it at 85%, the cycle isn't completed. Although modern batteries have quite a good separation layer from electron bombardments from Cathode so you won't have to worry about your battery dying before its age. Keeping your battery between 15-85% doesn't necessarily increase the battery life, it's just Lithium Ion batteries reach peak efficiency at this range and the wear is minimized. But that shouldn't add much difference in battery life in anything made after 2013. My Note 4 had solid battery life (4+hours screen on time after 4 years of use) and I used to charge that sucker to a full 100%.
