Lithium batteries are primary batteries that have lithium metal or lithium compounds as an anode. Depending on the design and chemical compounds used, lithium cells can produce voltages from 1.5 V to about 4 V, more than twice the voltage of an ordinary zinc-carbon battery or alkaline cell. Lithium batteries are used in many portable consumer electronic devices, Radio controlled models, and are widely used in industry.
The type most often used in Antweights but also now seen in Featherweights, is Lithium-ion polymer batteries, or more commonly lithium polymer batteries (often abbreviated Li-poly, LiPo, LIP, or LiP). The regulations of the Fighting Robot Association do not allow the use of Lithium based batteries in any robot heavier than featherweight[(http://www.fightingrobots.co.uk/rules.php)].
They are rechargeable batteries which have technologically evolved from lithium-ion batteries. Ultimately, the lithium-salt electrolyte is not held in an organic solvent as in the lithium-ion design, but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile.
LiPo batteries are now commonplace in Antweights, and have become popular due to their small size, cost and lower weight when compared to the older NiCd batteries previously used by most Antweights. They can provide enough power to run brushless motors, servos and many other widespread components.
Suppliers:
Batteries with a capacity of 200mAh or above should be more than adequate for most Antweights, high powered brushless motors may require a higher capacity battery in order to run on full power for an entire fight. Higher capacity batteries tend to be heavier however, and most people find that around 200mAh is a good compromise between high capacity and low weight.
http://www.robotwars101.org/phpBB/viewtopic.php?t=1247 - topic on RW101 discussing the best LiPo batteries.