Iodine is a strong oxidizing agent. This means there is a great tendency for iodine to gain electrons and reduce to iodide ions:
From this, we also know that it is not easy to cause iodide ions to oxidize into iodine (the reverse of the above reaction) and it will take a very strong oxidizing agent to do this.
Zinc ions are not a strong oxidizing agent. This means that the conversion of zinc ions into zinc metal does not readily occur.
So, the reaction stated in the question, trying to synthesize zinc and iodine from their ions does not occur spontaneously. It will have to be made to occur in an electrolytic process, which will require energy (from a battery in this case).