Hi,
Here is a link to a video that shows how to disassemble a Xiaomi Mi6. It is helpful as it shows how to access and remove the battery.
Here is an image, taken from the video at 4:21 minutes, which shows details of the battery
(click on image to enlarge for better viewing)
Here is a link to suppliers of the battery, (some are OEM), for the phone. (battery model is BM-39).
It is not a recommendation to use any of them. it is shown to give an idea of the cost of the battery. There are other suppliers online that may suit you better. Just search for BM-39 battery to get results. Since you prefer OEM batteries ensure that this is stated by the supplier.
Hopefully this is of some help
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