High-Conversion-Efficiency and Stable Six-Electron Zn-I2 Batteries Enabled by Organic Iodide/Thiazole-Linked Covalent Organic Frameworks

Six-electron I−/I5+ redox chemistry gives a promising platform to propel high-capacity Zn-I2 batteries, but faces limited conversion efficiency and instability of IO3− species. Here we design a thiazole-linked covalent organic frameworks (TZ-COFs) hosted organic trimethylsulfonium iodide (C3H9IS/TZ-COFs) electrode in 1-methyl-3-propylimidazolium bromide (MPIBr)-containing electrolyte to stimulate I−/I0/I+/I5+ iodine conversion chemistry with better electrochemical efficiency and stability. Compared with inorganic symmetric I2 molecules, the more easily exposed I− center of polar C3H9IS combines with the oxygen in H2O to form HIO3, which initiates 6 e− I−/IO3− conversion through I+ activation of MPIBr, thus reducing the oxidation/reduction potential gap to achieve 97% iodine conversion efficiency. Meanwhile, thiazole units of TZ-COFs enable strong chemical adsorption with IO3− species to improve redox stability with high reversibility due to reduced energy barriers (−5.1 vs. −3.5 eV in activated carbon (AC) host) and upgraded conversion kinetics (activation energy: 0.21 vs. 0.38 eV in AC). Such a stable and high-efficiency 6 e− iodine conversion gives C3H9IS/TZ-COFs electrode record capacity (1296 mAh g−1) and energy density (1464 Wh kg−1), and superior cycling stability (1200 cycles). These findings constitute a major advance for the design of iodine redox chemistry towards state-of-the-art Zn-I2 batteries.