The thermal neutron cross section isn't very big. http://www.nndc.bnl.gov/atlas/atlasvalues.html and the energy release isn't especially large. Q~7 MeV. Though, the Q value is sufficiently large that there's a good chance you'd breakup the resulting 7Li (QBU = -2.47 MeV), so that'll complicate things. Indeed, 6Li + n -> a + t is a way to breed more tritium in fusion reactors.
Then you have to get a thermal neutron source from somewhere, so you're basically running a nuclear reactor, so why don't you just use that as a power source?