Yes, spare lithium batteries can go in your carry-on bag, but not checked luggage. Batteries that power any device can go in checked-in luggage if they are in the electronic device and the device is turned off.
Batteries can overheat and catch fire.
You will be asked to relinquish all dangerous or prohibited items found in your carry-on luggage. If you refuse:
We x-ray bags going on planes to make sure there is nothing dangerous in them. If something comes up on the x-ray that needs checking, the bag will be opened, searched and any dangerous or prohibited item will be removed.