Also whats the difference between using the Ni vs Li
batteries?
Do you mean the difference between NiMH and Li-Ion?
The number one difference is safety. NiMH batteries don't burst into flames spewing super heated gasses and electrolyte if shorted or over-charged. They can still get burning hot though. Another big difference is you need 4 NiMH cells in series versus one Li-Ion cell individually to obtain the voltage required for an atomizer.
Here's some specifics;
Nominal voltage for a Li-Ion cell is 3.7V. Nominal voltage for a NiMH cell is 1.2V
Comparing AA size cells (that would be 14500 for Li-Ion), charge capacity is around 2200mAh for a NiMH cell and 750mAh for a Li-Ion cell
In terms of power, that's 2640mWh for the NiMH cell and 2775mWh for the Li-Ion cell.
Internal cell resistance for a AA NiMH is around 150mΩ. Internal cell resistance for an ICR 14500 is around 50mΩ.
When using 4 NiMH cells in series, total battery resistance is around 600mΩ compared to 50mΩ for a Li-Ion. An extra cell is required to compensate for the much higher resistance so 4 NiMH cells are required in an e-cig mod instead of 3.
The lower cell resistance of the Li-Ion results in less wasted power and much more efficient power delivery. Even though Watt-hours are close between the two, the NiMH burns more of that up internally so, Watt-hour for Watt-hour, you get significantly better run time with the Li-Ion.