Mixed-acid mediated electrolyte for high-energy Sn-Fe redox flow batteries with reversible capacity rebalancing

As global energy demand continues to grow, developing scalable and efficient energy storage systems with high energy density becomes increasingly critical. This study addresses these needs by exploring the use of mixed-acid electrolytes and premixed configurations in Sn-Fe redox flow batteries. We demonstrate significant improvements in solubility, electrochemical stability, and cycling durability. Specifically, incorporating sulfate ions into the electrolyte mitigates the common-ion effect, greatly increasing the solubility of ferrous and stannous chlorides and enabling higher energy densities of 24.22 Wh·L −1 at 80 mA cm −2 under room temperature. Electrochemical analyses reveals that sulfate ions positively shift the Sn 2+ /Sn redox potential by 120 mV at the negative side and also expand the operational voltage window beyond 1.44 V vs. SHE at positive side, significantly reducing undesirable side reactions such as chlorine evolution. Sulfate-rich electrolytes maintain superior stability, achieving coulombic efficiencies above 98 % under diverse operating temperatures. Additionally, a novel hydrogen-Fe 3+ rebalancing method is developed to effectively address Fe 3+ accumulation, restoring battery capacity from near depletion to around 15 Wh·L −1 . These findings underline the potential of mixed-acid premixed electrolyte systems and hydrogen-mediated rebalancing as robust strategies for advancing high-performance, scalable, and sustainable aqueous energy storage solutions.