You definitely want lMR (lithium Manganese Rechargeable) batteries in a mech or in a mod. Li-ion (Lithium Ion) batteries don't respond well to over current conditions and tend to heat, swell, and spew fire and hot liquids on the user when subjected to high current levels over time.
AW, Panasonic, Sanyo, and Sony are good brands to use. Make sure they are advertised as high drain lMR batteries and you will be safer.