Enabling I₃^–/I₂ Redox Couple toward High-Voltage Zn-Polyiodide Batteries by the Iodide–π Conjugation Effect

Distinct from the conventional I 3 – /I – redox couple (1.299 V), the I 3 – /I 2 redox couple (1.552 V) can enhance the output voltage and achieve higher energy density, which exhibits great development potential. However, the sluggish solid–liquid reaction rate, high conversion energy barrier, and high polyiodide solubility in aqueous electrolytes together hinder its development, especially at a low N/P ratio. Herein, we introduce an approach to achieve fast liquid–liquid reaction kinetics and a lower conversion barrier for high valence iodine electrochemistry of I 3 – /I 2, by coupling chemical liquefaction (MPII ionic liquid) and chelating catalyst (triazine-based poly(ionic liquid), PIL-tri). The MPII can spontaneously react with solid I 2 to generate liquid MPII 3, increasing reaction contact sites and accelerating reaction kinetics. Besides, PIL-tri significantly lowers the conversion barrier from I 3 – to I 2 and restricts the triiodide shuttling by distinctive iodide–π (I–π) conjugation with an I 3 – electron cloud. Such a synergistic effect kinetically and thermodynamically ensures a high valence I 3 – /I 2 redox couple. Consequently, PIL-tri@GP Zn-polyiodide batteries demonstrate a high output voltage (1.47 V), long cycling (800 cycles), and high-areal-capacity twice that of graphite paper (1.2 V) at a harsh N/P ratio (1). Meanwhile, they exhibited a polarity-switchable characteristic that maintained stable cyclability of 300 cycles when the anode and cathode were reversed every 50 cycles.