Comment by adrian_b

Sea water is plentiful, but iodine is less that 60 milligrams per ton of water.

If the production of such batteries would require thousands of tons of iodine per year, that would require the processing of billions of tons of sea water per year, from which the iodine would be removed.

Moreover, I do not know the current prices, because in recent years the metal exchanges have become more and more secretive, but some years ago iodine was about 6 times more expensive than lithium, so if a large amount of it were required for such a battery it could raise its price.

Hopefully the amount of iodine used in such batteries would be low, because the amount of needed iodine is proportional with the power of the battery, but it should not depend on the amount of energy stored in the battery (because iodine is an intermediary in the electron flow, it determines the maximum current during charge and discharge, but it is not an endpoint for electrons, so it does not determine the amount of stored energy).