Enhancing Zinc-Bromine Battery Durability with Biphasic Electrolytes

Abstract

Aqueous static zinc-bromine batteries (ZBB) offer high theoretical energy density and low cost due to their simplified design compared to flow batteries, making them promising for energy storage systems (ESS). However, ZBBs face self-discharge issues due to bromine (Br2) and polybromide anions (Br3 crossover from the positive electrode, resulting in low coulombic efficiency (CE) and capacity fading. Also, the thermodynamic instability of zinc metal anode in aqueous solutions leads to challenges such as hydrogen evolution reaction (HER) and zinc dendrite growth. This study proposes a biphasic electrolyte system comprising aqueous and organic solutions to address challenges at both the cathode and anode. On the cathode side, an aqueous solution containing 0.1 M tetrapropylammonium bromide (TPABr), 0.5 M ZnBr2, and 1 M Zn(CH3COO)2 was used. On the anode side, diethyl carbonate (DEC) was chosen as the organic solvent due to its immiscibility with water and compatibility with zinc ions for effective stripping/plating. The high ionic conductivity of aqueous phase facilitates bromine redox reactions, while bromine crossover was suppressed by TPABr, retaining 86% capacity after a 48-h open-circuit period. This system suppressed parasitic reactions and zinc dendrite growth, resulting in high CE and extended cycle life, with more than 1,160 cycles at approximately 100% CE. In previously reported biphasic electrolytes, zinc reactions at the anode occurred in aqueous systems while bromine reactions at the cathode occurred in organic solvents, leading to zinc metal instability. Our approach overcomes these dual challenges by reversing the organic and aqueous phases. This work paves the way for future advancements in static battery design, potentially leading to more sustainable ESS.