An Ion Exchange Membrane-Free, Ultrastable Zinc-Iodine Battery Enabled by Functionalized Graphene Electrodes

Abstract

Global warming requires a shift in the energy economy towards renewable energy sources, such as solar and wind energy, to meet the ever-increasing worldwide energy demand while having lesser negative impact on the environment. The integration of such sustainable energy resources into the existing infrastructure, however, is challenging due to their fluctuations and intermittences requiring the development of efficient (intermediate) energy storage systems. The ultimate goal for these systems pertains to high performance, durability, safety, facile scalability, cost, and environmental-friendliness. The aqueous rechargeable zinc-iodine (Zn-I2) battery is promising due to the high theoretical capacities of Zn (820 mAh gZn -1) and I2 (211 mA h gI2 -1) along with the very high solubilities in aqueous media. Other advantages include the abundance of the electrode materials and the safety of the aqueous electrolyte. The high energy density of aqueous zinc-based batteries is a result of the multi-electron redox reactions and the low electrochemical potential of Zn (-0.763 V vs. RHE) in mildly acidic electrolytes such as in a Zn-I2 battery. In addition, compared to other alkali metals such as lithium, sodium, and potassium, metallic Zn is relatively stable in an aqueous environment over a wide temperature range.Several challenges need to be addressed for Zn-I2 batteries to be competitive; namely, self-discharge, sluggish kinetics, low practical energy density, as well as the dendrite formation on the Zn anode. In this work, we design a high-performance Zn-I2 battery with an unusual long-term stability which is based on a novel design of electrodes and electrolyte. In detail, a three-dimensional functionalized graphene cathode facilitates the iodide redox reactions and also immobilizes dissolved polyiodides; thus, suppressing the detrimental shuttling effect. Furthermore, we design a composite anode made of Zn coated with a film of reduced graphene oxide. This modification significantly enhances the performance and stabilizes the anode during repeated Zn stripping/plating preventing dendrite formation. The electrolyte is formulated in a way that, along with the graphene-based cathode, allows for the utilization of an economical glass fiber separator instead of the commonly employed ion-exchange membranes, which are expensive and have relatively high ohmic resistances. Our novel Zn-I2 battery design 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 most comparable aqueous batteries.