5Vor 6V operating DC-DC for mobile battery?
The "suitable" battery voltage will depend on the desired battery technology, and the cell voltage that technology provides.

For Lithium Ion batteries, the cell voltage is around 3.7 volts, so you can't use that technology if you require the voltage to be between 5 and 6 volts.

NiMH batteries are about 1.2 volts per cell, so five cells will give you 6 volts.