Researchers develop new system for high-energy-density, long-life, multi-electron transfer bromine-based flow batteries

Bromine-based flow batteries operate through the redox reaction between bromide ions and elemental bromine, offering advantages such as abundant resources, high redox potential, and good solubility. However, the substantial bromine generated during the charging process can corrode battery components, shorten cycle life, and increase system costs. Although traditional bromine complexing agents can alleviate corrosion to some extent, they often induce phase separation, compromising electrolyte homogeneity and adding complexity to the system.

In a recent study published in Nature Energy, a research team led by Prof. LI Xianfeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) developed a novel bromine-based two-electron transfer reaction system and applied it to a zinc-bromine flow battery. This study demonstrates both the proof of concept and the system-scale up of a long-life zinc-bromine flow battery.

Researchers proposed an innovative bromine two-electron transfer reaction by introducing amine compounds as bromine scavengers into the electrolyte. They discovered that the bromine (Br2) generated during the electrochemical reaction could be converted into brominated amine compounds, reducing the concentration of Br2 in the solution to an ultra-low level of approximately 7 mM. Unlike the traditional single-electron transfer reaction from bromide ions to Br2, this new reaction enables a two-electron transfer from bromide ions to brominated amine compounds, increasing the battery’s energy density. Simultaneously, the ultra-low Br2 concentration substantially reduces electrolyte corrosivity, extending battery life.

Furthermore, researchers applied this novel reaction to zinc-bromine flow batteries. Due to the extremely low Br2 concentration in the electrolyte, the long-term stable operation was achieved by assembling a single battery using a conventional non-fluorinated ion exchange membrane (SPEEK), thereby reducing battery costs. In a 5 kW system scale-up test, the battery operated stably for over 700 cycles under a current density of 40 mA cm⁻², achieving an energy efficiency above 78%. With the drastically reduced Br2 concentration, no corrosion was observed in key materials—including current collectors, electrodes, and membranes—either before or after cycling.

"Our study provides a novel approach to the design of long-life bromine-based flow batteries and lays the foundation for the further application and promotion of zinc-bromine flow batteries," said Prof. LI.

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