Well if Wiki says it, it must be true. There simply is no better historical future authority. ;<)
According to Star Trek Encyclopedia, lithium was originally used as the basis of power generation and warp technology. However it was soon realized that lithium is a real element with known properties. Dilithium was invented to allow for a fuel material with unknown properties. In the Star Trek: Enterprise episodes “The Augments” and “Affliction”, it is established that the NX-class Earth ships and the Klingon Bird-of-Prey in fact already used a dilithium matrix in the mid-22nd century.
Lithium in a crystal form was utilized in “lithium crystal circuits,” important components in the power-generation systems of Constitution-class starships. In order to be utilized in this fashion, lithium crystals were required to be processed in large “cracking stations,” such as the one on planet Delta Vega. (TOS: “Where No Man Has Gone Before”)
Overexpenditure of power could damage the lithium crystals, as happened to the Starship Enterprise in 2266, when chasing a Class-J starship into an asteroid belt. Forced to extend her shields around the vessel to protect it from asteroid collisions, the Enterprise burned out three of four lithium crystals, forcing the crew to supplement with battery power. The fourth crystal subsequently failed, overstressed from handling all of the ship’s power. (TOS: “Mudd’s Women”)
http://en.memory-alpha.org/wiki/Lithium_crystal