From Hidden Frontier
Trilithium is a synthetic compound that is an explosive of tremendous power. It is produced as a byproduct of the matter/antimatter reactions that occur in a warp core using dilithium as a reaction moderator.
The Breen used sub-methane cooling units that used trilithium, along with kelbonite and borite compounds in their attempts to terraform Ba'ku. (ANG 1.03 - "Dreamers", HF 2.06 - "To The Stars" and HF 4.06 - "Entanglement Part 1")
In 2378, Sha'kev coerced Lieutenant Commander Robin Lefler to drain trilithium resin (its unprocessed state) from the warp core of the USS Maverick. (HF 2.04 - "Great Starship Robbery") This was later used to construct an explosive device that destroyed the first Grey Research Facility. (HF 3.09 - "Hell's Gate, Part 1")