Spectroscopic Study of Multistep Electrochemical Reactions of Iodine in Single-Walled Carbon Nanotubes

This study elucidates the detailed reaction mechanism of the multistep battery reactions occurring at iodine-encapsulated single-walled carbon nanotube (I@SWCNT) electrodes in a concentrated ZnCl2 aqueous electrolyte. A comprehensive analysis was conducted using not only electrochemical measurements but also Raman spectroscopy, fluorescence X-ray emission, and X-ray absorption near edge structure (XANES) measurements. Cyclic voltammograms and charge–discharge curves revealed the presence of three distinct redox reactions. In situ Raman observations during the charge–discharge process showed that the oxidation of iodine resulted in the elongation of polyiodide chains and the chemi-sorption of chloride ions (Cl–) from the electrolyte onto the oxidized iodine. The ICl vibration Raman peak, indicative of chloride ion adsorption, disappeared between 1.83 and 1.9 V, confirming the subsequent oxidation of chlorine to its zerovalent state. The three-step redox reactions, namely I–/I0, I0/I+, and Cl–/Cl0, were further corroborated by XANES and fluorescence X-ray emission measurements. The oxidation states of iodine were determined using iodine LIII-edge XANES, while the chemical adsorption of chloride ions and their subsequent oxidation were confirmed through fluorescence X-ray analysis. Changes in the chemical state of chlorine during oxidation were assessed using Cl K-edge XANES, with variations in the pre-edge peak intensity providing crucial insights. At 1.4 V, the emergence of a weak pre-edge peak indicated the onset of chloride ion adsorption. As the potential increased to 1.9 V, the pre-edge peak intensity grew significantly, confirming the oxidation of adsorbed chloride ions. Beyond 1.9 V, however, further increasing the potential to 2.1 V resulted in minimal changes in pre-edge peak intensity. This suggested that chlorine oxidation reactions at higher potentials occurred without retention within the nanotubes, contributing to the irreversible capacity loss.