Improved metal-based redox flow batteries (RFBs) can utilize a metal and a divalent cation of the metal (M2+) as an active redox couple for a first electrode and electrolyte, respectively, in a first half-cell. For example, the metal can be Zn. The RFBs can also utilize a second electrolyte having I−, anions of Ix (for x≧3), or both in an aqueous solution, wherein the I− and the anions of Ix (for x≧3) compose an active redox couple in a second half-cell.