Step-up DC-DC converter ( 4.2V to 12V)

I would put batteries together (4 x 1 or 4 x 2 cells for more current etc etc)for 14.4 volts and use a good low loss voltage regulator to supply the voltage you wish at say max of 2.5 - 3 amps. (I realise you only need 2 amps, but put in some safety here!) This would allow you to use a greater part of the batteries capacity. Fully discharged you would still have 13.6 volts.

Looking up on the web shows that the battery works from between 3.6 volts per cell, fully charged, to about 3.4 volts discharged (the web did not help me much on this last point!).

What was also there to see is that this cell type is very sensitive to bad charging, over discharging, high temperatures, low temperatures and can, if dented, catch fire!! Can you protect it fully, if not, go to a different cell type....

There was one manufacturer that seems to have the fire part under control, at least on his web site!!