https://www.techradar.com/news/samsung-galaxy-x-could-have-a-curved-battery-of-up-to-6000mah

A foldable phone requires more than just a flexible screen as it can also be necessary for some of the internal components to be curved, which is probably one of the reasons we haven’t seen one yet. It sounds like Samsung is close though, as the company has apparently created a curved battery suitable for such a phone.

We say ‘suitable’, because Samsung SDI actually created a curved battery four years ago, but it only had a capacity of 210mAh. Now, though, it’s created one that can be between 3,000mAh and 6,000mAh and which is set to be used in the folding Samsung Galaxy X according to ITHome, citing industry sources.