Redox flow batteries use a reductio-oxidation between two valence states in solution rather than changing the composition, and hence the valence states of solid material on an electrode. A flow battery consists of two volumes of solution separated by a selective membrane which allows some ions to pass but not others. The two solutions are pumped to the permeable membrane, which allows xxxx.

Flow batteries have several potential advantages over solid batteries. A key advantage, which is particularly important in transport applications, is that the battery may be re-charged simply by pumping out the uncharged solution and replacing the solution with charged solution. This eliminates potentially long recharging times, such as are encountered in electric vehicles. Replacement of the solution allows the electric car to be recharged in the same fashion in which a car is filled with fuel. Another advantage is that the capacity of the battery is determined by the volume of solution, while the power of the battery is determined by the membrane contact area between the two solutions.

The vanadium-Vanadium redox flow battery, developed at the University of New South Wales, is a particularly promising flow battery. It consists of two states of Vanadium. It has high efficiencies, with coulombic efficiencies of 97% and energy efficiencies of 87%. In addition, since both solutions (anode and cathode) in the battery use vanadium, cross contamination between the two solutions may discharge the battery, but will not cause damage the battery.