Electrode and electrolyte regulation to promote coulombic efficiency and cycling stability of aqueous zinc-iodine batteries.

[Display omitted] • A zinc-iodine battery with high coulombic efficiency and stability is proposed. • The polyiodide is confined at cathode by doping on polyaniline chains. • The complex in electrolyte is regulated to eliminate free iodide anions. • It delivers 99.2% coulombic efficiency with 2 mAh cm−2 capacity at 6 mA cm−2. • A stable capacity retention of 99.9% is achieved after 1000 cycles. Aqueous zinc-iodine batteries are promising electrochemical energy storage systems due to the high safety and low cost. The application of zinc halide solution as the electrolyte allows the dual-plating mechanism on both electrodes, i.e. the redox reactions of Zn2+/Zn and I 2 /I- at the anode and cathode, respectively. These solid–liquid conversion processes guarantee excellent reaction kinetics. However, soluble polyiodide (I 3 -, I 5 -, etc.) are formed at the cathode either during the oxidation of I- or from the reaction between I- and I 2. The dissolution of polyiodide in electrolytes causes rapid loss of charged products, leading to poor coulombic efficiency and fast self-discharge. Herein, we apply the synergistic regulation of electrode and electrolyte to confine the charged products. The conducting polymer polyaniline (PANI) is used as the polyiodide binder. It contains positively charged nitrogen sites, allowing the doping and effective binding of polyiodide anions through electrostatic attraction. At the same time, the complex in zinc halide electrolytes is regulated to eliminate free iodide anions and prevent the reaction with I 2 to form more polyiodide. The optimized zinc-iodine aqueous battery delivers excellent rate capability thanks to the facile solid–liquid reactions as well as the high electrical conductivity of PANI. More importantly, it achieves a high coulombic efficiency of 99.2% with the capacity of 2 mAh cm−2 at 6 mA cm−2, and an excellent capacity retention of 99.9% after 1000 cycles is realized upon long-term cycling. The work proposes a potential pathway to realize stable energy storage in aqueous zinc-halogen batteries. [ABSTRACT FROM AUTHOR]