Lithium

Lithium is a material added by various mods.

IndustrialCraft 2
Tiny Pile of Lithium Dust can be obtained by processing Nether Quartz in a Thermal Centrifuge. Despite there being an item named Fuel Rod (Lithium), it is currently unobtainable, making Lithium useless without other mods installed.

NuclearCraft
Lithium is a naturally occurring element that spawns as Lithium Ore between Y=0 and Y=32. It is used to produce many various items and alloys, including Tough Alloy and various Lithium Ion batteries. Its isotopes can be used with other light elements in various Fusion Reactor recipes.