Designing ternary hydrated eutectic electrolyte capable of four-electron conversion for advanced Zn-I₂ full batteries

Currently, the energy density and output voltage of Zn-I₂ batteries based on a single conversion reaction (I₂/I⁻) are still far from satisfactory, thus seriously hindering its rapid development. Herein, we design a new class of ternary hydrated eutectic electrolytes that can enable 2I⁺/I₂/2I⁻ redox couple conversion with four-electron transfer for Zn-I₂ batteries. Notably, the nucleophilic niacinamide (NA) ligand suppressed the hydrolysis of electrophilic I⁺ by forming stable [I(NA)₂]⁺ species that guaranteed the reversible I₂/I⁺ conversion. Moreover, the four-coordinated solvation shell of Zn²⁺-oriented dendrite-free Zn plating/stripping had a high average Coulombic efficiency of 97%. Such synergistic advantages resulting from the anode and cathode sides deliver outstanding performances for Zn-I₂ full batteries in terms of specific capacity (412 mA h g⁻¹), gravimetric energy density (404 W h kg⁻¹), and cycling stability. This exploration of multivalent Zn-halogens chemistries will open up broad prospects for practical applications of Zn-I₂ batteries.