Whats the difference between lithobid (lithium carbonate) and lithium ?

Lithobid (lithium carbonate) vs lithium: Aalthough comprised of the same medication, lithobid (lithium carbonate) is time-released; whereby lithium is not. Both are equally edfective, but the time-released doesn't seem to produce as many GI side effects. Lothobid is typically taken twice/day, hence the "bid" in the name. Lithium is generally taken three times/day (lithotid?).