There are companies that make special cases with built-in batteries. Basically, they are created for large manufacturers, like Apple or Samsung. Find something like this for a less famous brand is unrealistic. And then the craftsmen come to the rescue.
What is known
In the vastness of the Chinese Internet there was news that the user of the smartphone OnePlus 5 was able to attach the battery to 8000 mAh to its device. Initially, the device has a battery of 3300 mAh and this is enough for many, but apparently not all.
The man laid out photos with evidence that shows a large ledge on the back of OP 5. He also demonstrated the work of fast charging Dash Charge.
In the dry residue
It is clear that it is difficult to talk about the reliability of such information. But, on the other hand, it is interesting to learn about such experiments.
Read in Russian: Кастомный OnePlus 5 с аккумулятором на 8000 мАч