Gameboy micro battery has right volt value and the mAh value is twice as big as the original one has.
The size of the GB Micro battery is almost just a same as Chipod nanos have.
Or you can use "two" batteries, but the another one has changeable 9v-battery and you get 3h more playing time. You have to do it yourself:
http://www.instructables.com/id/EO7VZ73NWXEP286ZV3/