The major difference is that the operating potential of the lithium-iodine cell indicates its' cell reaction is more thermodynamically favorable.
The Zinc-Mercury Cell reaction is; Zn + HgO = ZnO + Hg. It also has long storage life and stable voltage(1.35V).
The major difference between the zinc-mercury cell and the lithium-iodine cell is that during the initial cell operation, each reaction is thermodynamically favorable, but the larger operating potential of the lithium-iodine cell indicates that its cell reaction is more thermodynamically favorable.
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