Elaborating the Iodine/Polyiodide Equilibrium Effects in Nanoporous Carbon-based Battery Electrode via Extreme Mass Asymmetry in Hybrid Cells

This article discusses the conversion of electrodeposited iodine to polyiodides within the nanopores of carbon electrodes that affect the performance of iodide electrolyte-based electrochemical cells. Here, carbon electrodes have been polarized in aqueous sodium iodide electrolyte to store charge in the form of solid iodine via highly reversible reaction (2I ⁻⇌I ₂+2e ⁻). The stored iodine within the pores interacts with free iodide ions present in the bulk electrolyte via comproportionation reactions leading to polyiodide (I ₃ ⁻ and I ₅ ⁻) formations. By tuning the mass asymmetry of carbon electrodes in hybrid cells and using the in-situ Raman spectroscopy on positive battery electrode, we show the influence of iodine/polyiodides equilibrium shifts on the self-discharge and voltage rebounds during open circuit conditions. This study provides insights into the charging mechanisms of carbon electrodes for iodine-based hybrid supercapacitors and battery systems.