An ion exchange membrane-free, ultrastable zinc-iodine battery enabled by functionalized graphene electrodes

The aqueous rechargeable zinc-iodine battery is a promising system due to its high theoretical capacity, zinc and iodine abundance, and safety of the aqueous electrolyte. However, several challenges need to be addressed for zinc-iodine batteries to be competitive, including self-discharge, sluggish kinetics, low practical energy density, and dendrite formation on zinc anodes. Here, we realize a high-performing zinc-iodine battery with long-term stability through a novel design of electrodes and electrolytes. A three-dimensional functionalized graphene cathode facilitates the iodide redox reactions as well as immobilizes the dissolved polyiodides, suppressing the detrimental shuttling effect. Additionally, we introduce a composite anode made of zinc coated with reduced graphene oxide. This design greatly enhances the performance and stabilizes the anode during repeated zinc stripping/plating preventing dendrite formation. The electrolyte is formulated in a way that allows the use of an economical and efficient glass fiber separator instead of the commonly employed ion-exchange membranes which are expensive and have a relatively high Ohmic resistance. Our optimized zinc-iodine batteries exhibit high current efficiencies of nearly 100%, along with stable specific capacities of 257, 186, 150, 84 mAh g−1 at corresponding current densities of 1, 2, 5, 10 A g−1. Furthermore, a long-term capacity retention of 96.7% at 5 A g−1 over 2000 cycles is achieved, outperforming many similar aqueous batteries. This work also provides valuable guidance in designing electrodes for other aqueous metal-halide energy storage systems.