High-conversion-efficiency and stable six-electron Zn–I2 batteries enabled by organic iodide/thiazole-linked covalent organic frameworks

Abstract

Six-electron I⁻/I⁵⁺ redox chemistry gives a promising platform to achieve high-capacity Zn–I₂ batteries, but faces limited conversion efficiency and instability of IO₃⁻ species. Here, we design a thiazole-linked covalent organic framework (TZ-COF) hosted organic trimethylsulfonium iodide (C₃H₉IS/TZ-COFs) electrode in a 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 6e⁻ 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 6e⁻ iodine conversion gives C₃H₉IS/TZ-COFs electrodes record high capacity (1296 mA h g⁻¹) and energy density (1464 W h kg⁻¹), and superior cycling stability (1200 cycles). These findings constitute a major advance in the design of iodine redox chemistry towards state-of-the-art Zn–I₂ batteries.