Batteries are always a problem for phones.
For most products you can use a "standard" cell profile like 18650 (18 diameter by 65mm long) which gives a reasonable power density but this would be too thick for a modern cellphone. So you are stuck with pouch/prismatic cells which come in lots of different shapes and sizes all customised to a specific end product.
Hence their issue with batteries, users expect a thin device without a big power bulge on the back.
Biting the hand that feeds IT
