6 good quality NiMH AA cells will give 7.5V or so, and support high current (I said good quality).
2 LiPo cells give 7.4V (and definitely support very high current). LiPo cells have issues though (need protection circuit, proper charger, should not be charged unattended).
When you said "3.5mah" did you mean 3.5Ah ? If so then that would imply NiMH C-cells which go up to about 4 to 5Ah.