The nice!nano is a Pro Micro replacement development board offering BLE using the nRF52840 chip. It has the same pinout as the Pro Micro meaning it will work with almost any Pro Micro keyboard. The nice!nano also has a 3.7V lithium battery charger on board as well as a software level switch to cut off power to LEDs, which can eat 1mA each even when off!
If you're a hardware designer interested to learn more about the pinout and schematic check out the Pinout and Schematic.
To run your nice!nano wirelessly, you need a battery. In specific, you need a 3.7v rechargeable lithium battery that is at least 100mAh large (you can probably get away with 80mAh). Note when choosing a battery, the recharge rate is 100mA, so a 2,000mAh battery will take 20 hours to charge. The overall recommended battery for most low power (no LEDs or extra power sinks) is the 301230 battery. This battery is 3mm thick and fits underneath a socketed nice!nano very nicely. These can be found on some vendors' stores as well as on AliExpress.
It's also highly recommended that you socket your nice!nano. It offers ease of access to the battery and makes debugging your keyboard so much easier. In short, it's a small investment to save you lots of time and possibly money in the future. Standard female machine pin sockets are recommended to go along with the 301230 battery. In most cases the battery should fit underneath the socketed nice!nano. Be weary o flipped nice!nanos with pins and other parts underneath (you don't want to puncture your battery!). You can find these on AliExpress all over the place.