When were Dilithium Crystals first mentioned in Star Trek?

Out of universe, the first mention of dilithium crystals was in the TOS episode "The Alternative Factor" (season 1, episode 20):

UHURA: Nothing yet, sir.

MASTERS: (A woman in a blue uniform) Report on the dilithium crystals, Captain.

KIRK: Yes.

MASTERS: Whatever that phenomenon was, it drained almost all of our crystals completely. It could mean trouble.

Those two very early references to "lithium crystals" have never been completely explained as far as I know. But there are (in-universe) earlier mentions of dilithium being used as an energy source, for instance in Enterprise - "Cold Front" (which takes place about 110 years before TOS):

SONSORRA: I'm a warp field theorist.

TUCKER: Oh. Well, I guess that covers the basics. Any questions?

ALIEN: How do you regulate positron flow in your dilithium matrix?

TUCKER: Good one. I'll bring up the schematic of the reactor assembly and you can see for yourself.