There comes a time in every iPhone’s life that the battery will need to be replaced. Rechargeable batteries are great but they do not last forever and you can thank the second law of thermodynamics. Essentially, any time a battery is charged or discharged, energy is not conserved and a little bit of energy is lost which can never be recovered.
You may be tempted to go to a cheap 3rd party battery replacement service. However, you may want to reconsider. Since iOS 11.3, Apple has a ‘Battery Health’ feature to monitor your phone’s battery performance from day to day. It was introduced back in iOS 11.3. This Battery Health feature could potentially be disabled if the battery is not installed by an authorized Apple provider.
I know what you are thinking, this is just another way for Apple to make money of their products, and, you would be correct. But, the Battery Health feature is a nice addition to your phone and the amount you save with a 3rd party service provider is not worth the risk of an under performing phone or a phone dying on you at a time when you need your phone operational.