Engineering eutectic network for regulating the stability of polyiodides towards high rate and long cycling zinc-iodine batteries

Despite aqueous Zn-iodine batteries have gained enormous research interests, their development is restricted by disproportionation and diffusion of polyiodides. Herein, for the first time, we propose a distinct electrolyte engineering strategy for regulating the performance of Zn-iodine batteries. A hydrated eutectic electrolyte composed of zinc tetrafluoroborate hydrate (ZBF) and succinonitrile (SN) forms the unique SN[sbnd]H₂O eutectic network, which breaks the H₂O[sbnd]H₂O hydrogen network, strengthens O[sbnd]H bond in H₂O and constrains the freedom of water molecules. Consequently, the disproportionation and diffusion of polyiodides are suppressed. The investigation for the solvation structure based on the spectroscopy results and molecular dynamics simulations demonstrate the relatively weak solvation effect of the organic ligand SN with Zn²⁺, which enables superior ion conductivity and high transference number. As a result, the as-assembled Zn-iodine battery based on optimized eutectic electrolyte delivers a high capacity of 202 mAh g⁻¹ at 0.5 A g⁻¹ with 99.5% coulombic efficiency, superior rate performance (capacity retention of 60% at 20 A g⁻¹) and outstanding cycling stability (over 10,000 cycles). Benefiting from its anti-freezing ability, the hydrated eutectic electrolyte enables Zn-iodine battery to work stably at a low-temperature of -20°C. The paradigm provides an encouraging strategy to construct the stable aqueous Zn-halogen batteries through electrolyte engineering.