The Federal Aviation Administration (FAA) is clear about this: Spare (uninstalled) lithium ion and lithium metal batteries must be carried in carry-on baggage only. When a carry-on bag is checked at the gate or at planeside, all spare lithium batteries must be removed from the bag and kept with the passenger in the aircraft cabin. The battery terminals must be protected from short circuit.
Be careful out there. A bit of tape on the terminals helps.