A Dual Plating Battery with the Iodine/[ZnI_x (OH₂ )_4-x ]^2-x Cathode

Plating battery electrodes typically deliver higher specific capacity values than insertion or conversion electrodes because the ion charge carriers represent the sole electrode active mass, and a host electrode is unnecessary. However, reversible plating electrodes are rare for electronically insulating nonmetals. Now, a highly reversible iodine plating cathode is presented that operates on the redox couples of I₂ /[ZnI_x (OH₂ )_4-x ]^2-x in a water-in-salt electrolyte. The iodine plating cathode with the theoretical capacity of 211 mAh g^-1 plates on carbon fiber paper as the current collector, delivering a large areal capacity of 4 mAh cm^-2 . Tunable femtosecond stimulated Raman spectroscopy coupled with DFT calculations elucidate a series of [ZnI_x (OH₂ )_4-x ]^2-x superhalide ions serving as iodide vehicles in the electrolyte, which eliminates most free iodide ions, thus preventing the consequent dissolution of the cathode-plated iodine as triiodides.