Abstract: A method of operating a redox flow battery includes a step of observing a difference in relative volume between the anolyte fluid volume and the catholyte fluid volume. The ionic molality of anolyte fluid is increased if the relative volume of the anolyte fluid decreases. A redox flow battery having balanced anolyte and catholyte initial ionic molalities is also provided.

Abstract: A secondary redox flow battery having a charge capacity and an efficiency includes an anode half-cell and a cathode half-cell having a fluid-containing vessel defining a cavity in which is disposed an electrode and a catholyte. The catholyte consists of a solvent, at least two cation species, and an anionic transition metal complex. The catholyte cation species are selected from the group consisting of Group I element ions, Group II element ions and ammonium ions. The battery also includes a reservoir fluidly communicating with the cavity and a separator ionically communicating between the anode half-cell and the cathode half-cell. The battery is capable of a discharge current equal to or greater than 20 milliamperes/cm2.