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-I-2 batteries based on a single conversion reaction (I-2/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(+)/I-2/2I(-) redox couple conversion with four-electron transfer for Zn-I-2 batteries. Notably, the nucleophilic niacinamide (NA) ligand suppressed the hydrolysis of electrophilic I+ by forming stable [I(NA)(2)](+) species that guaranteed the reversible I-2/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-I-2 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-I-2 batteries.