If the battery is too low for the phone to charge it, you'll need an external, not-as-smart charger. But since the battery isn't removable, you'll have to take it to a repair shop to have it done - and they'll probably tell you that all they can do is replace the battery.
If you never let the battery drop below 40% (which is the minimum you should drop a lithium battery), this sort of thing won't happen.