What you want is possible, but it opens a truly huge can of worms regarding battery management.
As a start, I'd suggest using two 18650 Lipo or li-ion batteries and a step down converter, opposed to NI-CD. These will give you 3.7v each cell.
Now for the can of worms, you must ensure the Pi never gets more than 5.2-5.25v ever.
You will also need to load balance the batteries so they discharge evenly.
You will need a way to force the Pi to elegantly shutdown once critical battery levels are reached - you can NOT just pull the power.
You need a way to detect when that critical point is reached (a non-trivial task)
The motors will require sufficient power, make sure the max current draw of all the motors combined, plus your Pi and other devices connected, do not exceed the battery's output limit. I suggest running them off their own batteries.
With the proper parts and assembly, your device might tun for a couple of hours at most.
Stephen Hawking, rest in peace.
8 January 1942 – 14 March 2018