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As a leading large-scale energy storage technology, flow batteries are highly respected for their inherent safety and scalability. However, the active material will pass through the membrane-which is beyond the scope of the expected charge carrier-causing the capacity to decay rapidly, hindering further development of the technology. Traditional approaches to mitigating capacity fade have focused on improving the ion selectivity of the membrane, but this typically sacrifices power density.

Here, we introduce an "equilibrium" electrolyte strategy. This strategy is different from the traditional symmetrical electrolyte design, which is realized by adjusting the concentration and valence state independently. This method can accurately control the transmembrane ion flux, so as to maintain the dynamic balance of the active material and effectively reduce the capacity fading.

vanadium redox flow battery tests show that this method overcomes the traditional trade-off between proton conductivity and ion selectivity: the capacity decay rate of the battery using 15 micron thick Nafion membrane with equilibrium electrolyte decreases from 0.061 per cycle to 0.015 in 1000 cycles; In contrast, the system using 183 micron thick Nafion membrane and traditional electrolyte has different performance, and the decay rate of the former is reduced by 75.4.

This method shows the potential to reduce the capital cost of a 1 MW/4 MWh flow battery system by more than 41.7. Crucially, the equilibrium electrolyte approach circumvents existing membrane-related limitations in redox flow battery development and establishes a framework for advanced electrolyte design.