Originally, the someone holding the rank of Commander was to command a ship in battle. A Captain would oversee a small fleet of ships, often commanding the main ship in the fleet as well. A Lt. Commander simply is the equivalent of the XO aboard those ships. The name Lieutenant comes from latin, which means "in lieu of" or "in place of". As for a Lt. Cmdr commanding a carrier, it's entirely possible during off seasons or night shifts. However, at war I'd doubt that anyone less senior than a Captain or a Commodore (Vice Adm. Lower) would be in charge.