A new patent added to the World Intellectual Property Office (WIPO) database shows that Samsung has developed a foldable battery to power phones of the same nature. It is constructed of layers in a way that allows one of its cells to connect to another via a flexible commisure that crosses the hinge-portion of the device in question. This removes the need for 2 separate solid batteries and their potential drawbacks.
https://www.notebookcheck.net/Samsung-s-next-gen-foldables-may-have-equally-flexible-batteries.478819.0.html
For a second I thought the cells would be that flexible, which is not impossible but would severely impact density, turns out it's sort of an integrated connector that saves a little space at the expense of no longer being able to charge two cells separately like Oppo's fast charging.