You’ve put you’re finger on it - Li3N is going to have to be prepared, and that’s going to cost not just money, but also energy.
So while the reaction of CO2 and LiN3 may be exothermic (heat-releasing), the overall reaction of (something) + ammonica (to give Li3N) + CO2 is probably not. And it’s not too tough to calculate what the overall energy balance would be - it all depends on what they react to get LiN3.
Kind of neat, but I wouldn’t start investing in CO2 futures quite yet.
Re: post 12 - every “LiN3” should = “Li3N”.
Damned chemical dyslexia!
By the way, Li3N will be very water sensitive, so much so that I expect it will react spontaneously with moisture in the atmosphere, which will further complicate any real use of this system.