I believe Scotty said they wer “Di-Lithium Crystals” Captain. And sheez, they were always on verge of blowing up or something.
Txchnologist, an online magazine sponsored by General Electric, talked to team member and aerospace engineering PH.D. candidate Ross Cortez, he said “The fusion fuel we’re focusing on is deuterium [a stable isotope of hydrogen] and Li6 [a stable isotope of the metal lithium] in a crystal structure.”
“That’s basically dilithium crystals we’re using,” he said.
Di-Lithium crystals were used in the later starships. The earlier ones used Lithium crystals. Unless Wiki got it wrong.
The problem with dilithium is that as it is more strongly energized the crystal structure breaks down faster. So going from warp four to warp six is hard on the crystals, but going from warp six to warp seven induces even more fracturing.