I know that some teams like the Energizer E2 Lithium Batteries for FLL. Additionally, according to the LEGO website, the NXT rechargeable battery (http://shop.lego.com/Product/?p=9798) uses lithium chemistry also . If you travel with your FLL robots on an airplane, you may want to read the following from the DOT.
DOT's rule on lithium batteries in air travel, which took effect on January 1, 2008, prohibits carriage of spare lithium batteries in checked baggage, such as large suitcases you hand over to the airline for handling. If you put a portable electronic device in checked baggage, you may still do so with the batteries installed in the device. In carry-on baggage, you may still carry any number of some types of lithium batteries, such as the ones used in cell phones and most laptop computers, provided you take measures to protect terminals. You may also carry up to two more powerful batteries, within the limits described here: http://safetravel.dot.gov/whats_new_main.html
Effective January 1, 2008, you may not pack spare lithium batteries in your checked baggage. You may pack spare lithium batteries in your carry-on baggage. Please see our Spare Battery Tips and How-To sections to find out how to pack spare batteries safely! If you pack a device containing batteries, secure it against activation by locking the activation switch in the "off" position, placing the device in a protective case, or by other appropriate measures.
For personal use, there is generally no restriction on the number of spare batteries allowed in carry-on baggage. This is the case for cell phone batteries, "hearing aid" button cells, and AA batteries/AAA batteries available in retail stores, as well as almost all standard laptop computer batteries.