From [Memory Alpha][1]: Constitution-class starships continued to use crystallized lithium until the mid-2260s, before switching to dilithium.

The article is pretty detailed, and if you're looking for more specific information, I'd take a look. [1]: http://memory-alpha.org/wiki/Dilithium

From Memory Alpha: Constitution-class starships continued to use crystallized lithium until the mid-2260s, before switching to dilithium.

The article is pretty detailed, and if you're looking for more specific information, I'd take a look.