Dual-coordination electrolyte additive to achieve high energy density 4-electron aqueous zinc iodine battery

Four-electron redox chemistry in aqueous zinc‑iodine batteries (4e-ZIBs) offers an exceptional energy density but also suffers from poor cycle stability and rapid self-discharge, originating from the hydrolysis of I⁺ and the crossover reaction between I⁺ and I^-. In this work, a green and multifunctional electrolyte additive, pyridine hydrochloride (py-HCl), is proposed to stabilize the I⁺ species and improve the kinetics of the four-electron conversion reaction. The py-HCl additive provides Cl^- to coordinate with I⁺ and form iodine monochloride (ICl) interhalogen, and then pyridine (py) to further coordinate with the resulting ICl and form a more stable solid complex pyridine‑iodine chloride complex (py-ICl). This py-ICl dual-coordination mechanism enables a novel liquid-solid-solid reaction pathway, allowing stable operation of the 4e-ZIBs even at a low Cl^- concentration of 1 mol L^-1. At a current density of 2 A g^-1, the 4e-ZIBs achieve a decent cycling lifetime of more than 4000 cycles, and a large average specific capacity of 449 mAh g^-1. The practicability of the electrolyte is further demonstrated by constructing a 5 × 6 cm² soft-package battery. The device achieves an initial areal capacity of 1 mAh cm^-2 and a capacity retention of 75% after 50 cycles. This dual-coordination strategy opens a new avenue for the construction of high-performance 4e-ZIBs.