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.