Before removing the battery, it was unsoldered from the MCU PCB. This was safety measure, so the that the battery could not be accidentally shorted during the teardown. It is recommend to unsolder the battery in any teardown where it is soldered in and not easy to remove to reduce the change of problems

The battery was pried out using a small flathead screw driver

The battery is a Li-ion 3.7V / 0.2Wh or 3.7V / 54mAh

Was not able to the the vendor of the battery. If you happen to know the battery vendor. Please leave it in the comments