Lithium Tank

This rather large container stores 1000 units of lithium.

Lithium can be consumed by reactors to breed tritium. Lithium however is not required to generate power, nor does breeding tritium consume or create power. To use the lithium to make tritium you must enable the "breed tritium" option on the reactor. This can be enabled or disabled at anytime outside of the VAB on an active reactor. Some reactors come from the VAB with a supply of lithium included.

Obtaining Lithium

This part is useful to resupply certain modes of fusion reactors that are running low on tritium, as tritium is a fuel source in some modes. It can also be used as a fuel source mode for electrically powered engines such as plasma engines. This fuel mode can be set at any time outside of the VAB.

Outside of the VAB, lithium can be obtained in certain places provided that you have an ISRU Refinery (and power to run it) on your landed ship. It is most commonly found in oceans, however extraction rates are extremely low.

Fuel Flow

Lithium containers use the all vessel flow logic similar to monopropellant. Supply can be consumed from anywhere on the ship. You can transfer it manually by alt-clicking as you do to move fuel around.