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

Distinct from the conventional I₃^–/I^– redox couple (1.299 V), the I₃^–/I₂ 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 I₃^–/I₂, by coupling chemical liquefaction (MPII ionic liquid) and chelating catalyst (triazine-based poly(ionic liquid), PIL-tri). The MPII can spontaneously react with solid I₂ to generate liquid MPII₃, increasing reaction contact sites and accelerating reaction kinetics. Besides, PIL-tri significantly lowers the conversion barrier from I₃^– to I₂ and restricts the triiodide shuttling by distinctive iodide–π (I–π) conjugation with an I₃^– electron cloud. Such a synergistic effect kinetically and thermodynamically ensures a high valence I₃^–/I₂ 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.