Progress and challenges in zinc-bromine batteries (ZBBs): A path towards safety and mitigation of high-performance systems

The global push to decarbonise the energy system and interest in electrical energy storage have grown dramatically as a result of the fast expansion of intermittent renewable energy sources like solar and wind. To meet rising demand for green electricity, dependable, affordable and safe energy storage systems (ESSs) have been developed. Among these, electrochemical devices are particularly well-suited due to their compact size, efficient power and energy combination, flexible location options, and scalable installation and construction. Zinc‑bromine batteries (ZBBs), first patented in 1885 and later established as hybrid flow batteries by Exxon, Gould, and NASA in the 1970s, offer a high theoretical energy density (440 Wh/kg) and high cell voltage (1.8 V). These batteries have been placed and made available for purchase by numerous companies over the past few decades, thanks to their modular design, which facilitates scaling to multi-kW levels. Although lithium-ion batteries currently dominate the market for grid-scale ESSs, they face several drawbacks, including low power density, high replacement and maintenance costs, and fire hazards from organic electrolytes. ZBBs, with their water-based electrolytes, offer a safer alternative with a longer lifecycle and higher energy density, making them a promising choice for grid-scale applications and emergency power sources. This review highlights the evolution of ZBBs over the last 40 years, focusing on their scientific research and commercial development. We compare ZBBs with other energy storage technologies, noting their advantages such as lower material costs, higher safety, and better energy density. We discuss the technical challenges associated with ZBBs, including dendrite formation, corrosion, and side reactions, and explore various solutions involving advanced materials like carbon nanotubes, Graphene, and mesoporous carbon. Recent developments in electrolyte formulations, electrode designs, and flowless ZBB configurations are also covered. This comprehensive overview aims to provide insights into the current state and future transition to renewable energy sources.