Long-Life Zinc–Bromine Flow Batteries with Low-Temperature Operation Enabled by Hydration Modulation of Complexing Agents

Abstract Zinc–bromine flow batteries have gained prominence in distributed energy storage due to their high theoretical energy density, inherent safety advantages, and cost-effective electrolytes. However, they have poor low-temperature tolerance and unsatisfactory service life because of the phase transformation of polybromides and uncontrolled zinc deposition. Here we propose a hydration modulation strategy for bromine complexing agents to both enhance the liquid-phase retention capacity of polybromides at low temperatures and mitigate internal short circuits. Moderately hydrated complexing agents not only introduce an appropriate amount of water into the oily polybromide ionic liquid but also prevent zinc from growing into the membrane. Zinc–bromine flow batteries with the target complexing agent, 1-[2-hydroxyethyl]-1-methylpyrrolidinium bromide, outperform those with traditional complexing agents in cycle life (>4000 cycles versus ∼800 cycles) at high current densities (charge: 200 mA cm–2, discharge: 80 mA cm–2) and in low-temperature adaptability (>700 h versus <10 h at 40 mA cm–2 and −20 °C). This study addresses a persistent challenge in the large-scale deployment of zinc–bromine flow batteries.