Balanced-state electrolytes overcome crossover in vanadium redox flow batteries.

Flow batteries are a leading large-scale energy storage technology, valued for inherent safety and scalability. However, active species cross over the membrane-beyond intended charge carriers- resulting in rapid capacity decay and hindering further development. Traditional approaches to mitigate capacity decay focus on increasing membrane ion selectivity, but this typically compromises power density. Here, we introduce a balanced-state electrolyte strategy that departs from traditional symmetric electrolyte designs by independently tuning both concentration and valence. This approach enables precise control over transmembrane ion flux, thereby maintaining the dynamic equilibrium of active species and effectively reducing capacity decay. Vanadium flow battery tests demonstrate that this approach overcomes the traditional trade-off between proton conductivity and ion selectivity: a battery employing a 15 μm-thick Nafion membrane with balanced-state electrolytes achieves a 75.4% reduction in capacity decay rate- from 0.061% to 0.015% per cycle over 1,000 cycles -compared to a system using a 183 μm-thick Nafion membrane with traditional electrolytes. This method shows the potential to lower the capital cost of a 1 MW/4 MWh flow battery system by over 41.7%. Crucially, the balanced-state electrolyte approach circumvents existing membrane-related constraints in redox flow battery development and establishes a framework for advanced electrolyte design.