View detailed report of drug-drug and drug-food interactions for pentobarbital, lithium
www.drugs.com
As far as I know, nothing bad in terms of using the two together to CTB. From the sounds of it, using lithium with N may further increase your chances of CTBing.
Although if I'm honest, I think the results I got from my link above are just a default answer it gives to any combination of drugs that have an effect on your central nervous system. So unless if the lithium you've got is very potent (I dunno how potent of a drug lithium is), it may not aid much in CTBing.