I take 1500mgs/day with about two large coffees...no problems at all for me. But it's possible so don't go overboard until you see how it affects you. From the Drugs.com Interactions Checker:
"lithium ↔ Caffeine
Moderate Drug Interaction
Using caffeine together with lithium may increase the effects of caffeine. Contact your doctor if you experience nausea, vomiting, diarrhea, drowsiness, muscle weakness, tremor, lack of coordination, blurred vision, or ringing in your ears."