All batteries deteriorate a bit with each major charging cycle. Lithium type batteries usually get rated at several hundred charging cycles, but tend to last longer than that, depending on how you charge them. There is a lot of misinformation on the internet about charging, but basically don't wait for the charge to drop to 20% or less before charging. The more deep cycle charges you do, the quicker the battery will deteriorate. Personally I don't let my iPhone fall below 40% before charging. I typically keep my iPhones for 3 or more years, and I’ve never had one need a battery replacement. YMMV of course. Take care of the battery and it will last for years.