The Arduino has an on-board regulator, supply 11V to the barrel plug.
A lithium battery will not have any difficulty sourcing enough current
for small motors - in fact you might want to consider adding an inline fuse.
With 11V to the Arduino there is 6V to be dropped by the regulator so
it may get quite hot - if the other 5V peripherals draw a lot of current
you may need a separate 7805 regulator on a heatsink (or a DC-DC
converter).
You don't mention a motor controller - are you using one? What
voltage are the motors?