Tetrabutylammonium bromide incorporated hydrated deep eutectic solvents: Simultaneously addressing anode stability and cathode efficiency in zinc-bromine batteries

Stationary zinc–bromine batteries are promising next-generation energy solution. However, their commercialization has been challenged by the instability of Zn metal at the anode and the cross-diffusion of redox-active bromine at the cathode. In this work, we present a strategy to concurrently address the issues at both electrodes by utilizing a hydrated deep eutectic solvent (HDES) electrolyte incorporating tetrabutylammonium bromide (TBABr). TBABr plays a pivotal dual role: it stabilizes Zn metal by creating a protective layer at the anode and minimizes bromine cross-diffusion by forming solid complexes with Br₃^– species at the cathode. Given the challenge posed by the low solubility of TBABr for its effective utilization, we systematically fine-tuned the HDES composition by adjusting the Zn-to-halide ratio, varying the water content, and incorporating ethylene glycol. This electrolyte enables the Zn anode to cycle for 3300 h at 0.5 mA/cm² and 0.5 mAh/cm² in a symmetric pouch-cell setup. In a full-cell configuration, the cell exhibited a record-high specific capacity of 297 mAh/g while maintaining stability over 11,500 cycles with 99.9 % coulombic efficiency.