back in my day we used to upcycle old 18650s from old laptop batteries to use for Nokia phones. Since back in the day phone batteries were basically just insert-in and held by a clip-on backplate, it was just a question of fabricating some sort of a replacement backplate, tape on some batteries, reuse the charging protection board with the connector from the original phone battery and wire it together. It easily doubled the size, looked like a prop from an amateur terrorist movie, but battery life was a week+ ;-)