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. 

From http://safetravel.dot.gov/:

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.

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