Tailoring Zn-ion Solvation Structures for Enhanced Durability and Efficiency in Zinc-Bromine Flow Batteries

Aqueous zinc-bromine flow batteries (ZBFBs) are among the most appealing technologies for large-scale stationary energy storage due to their scalability, cost-effectiveness, safety and sustainability. However, their long-term durability is challenged by issues like the hydrogen evolution reaction (HER) and dendritic zinc electroplating. Herein, we address these challenges by reshaping the Zn2+ ion solvation structures in zinc bromide (ZnBr2) aqueous electrolytes using a robust hydrogen bond acceptor as a co-solvent additive. Our findings highlight the critical role of interactions within the first and second Zn2+ solvation shells in determining electrochemical performance. By selectively incorporating a low volume percentage of organic additive into the second coordination shell, we achieve effective proton capture, electrolyte pH stabilisation during the Zn0 electroplating, and mitigation of ion transport resistance. This approach prevents the formation of a passivation interphase layer on the electrode surface, which typically occurs with higher additive concentrations, leading to increased interphase resistance, viscosity and cell polarization. This work opens a new avenue in modulating Zn2+ reactivity and stability through precise solvation structure design, enabling efficient and reversible Zn0/2+ cycling in aqueous electrolytes without H2 evolution. These findings pave the way for the development of commercially viable, high-performance ZBFBs for energy storage applications.