Polymerized Melamine Catalyzes Direct I^-/I₂ Conversion via ─N═N─ Motif for HighCapacity Zn-I₂ Batteries.

Abstract

To fundamentally solve low iodine conversion efficiency and severe polyiodide shuttling, a high-performance catalytic organic cathode is developed by in situ polymerizing melamine (MA) on activated carbon (denoted as pMA@AC) for high-capacity Zn-I₂ batteries. The pMA@AC cathode exhibits a high capacity of 1.6 mAh cm^-2 and an ultra-long lifespan over 20 000 cycles, benefiting the stable pouch cell with 440 mAh and 183 Wh kg^-1 after 100 cycles. In/ex situ characterizations coupled with theoretical calculations demonstrate that polymelamine (pMA) catalyzed efficient I^-/I₂ conversion, suppressing polyiodide formation and avoiding consequent shuttling for high Coulombic efficiency and enhanced lifespan. The high catalytic activity of pMA is attributed to the delocalized π electron cloud on ─N═N─ sites, which connect to electron-withdrawing triazine groups, with weaker physicochemical adsorption to I^-/I₂ than traditional carbon-based catalysts. It thus boosts the formation/desorption of resultant I₂ molecules at higher potential for enhanced cycling stability. More importantly, pMA@AC exhibits robust catalytic ability for four-electron I^-/I⁰/I⁺ chemistry with 405 mAh g^-1 and 10 000 cycles, further strengthening the potential application of such azo compounds for advanced Zn-halogen batteries.