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

Abstract

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.