I wrote a warp drive evolution article years ago, and speculated that the Lithium used on the Constitution Class was a nano-tubule lattice work engineered on an incredibly small scale to form the same electromagnetic "tunnels" in the crystal for matter/antimatter to permiate.
Cheaper, easier to produce, Lithium being highly abundant and maybe better in being engineered from scratch rather than the randomness of a natural crystal.
But that over time, as we see in TOS, they burnt out a lot faster or cracked a lot more easily. Essentially an experiment into a manufactered replacement for Dilithium that never really worked out.