NOVELTY - The battery (1) has a half-cell electrode chamber coupled to a current collector electrode. A polymeric lithium and sodium ion conducting separator (6) is coupled to the half-cell electrode chamber. First and second redox-active mediators of the half-cell electrode chamber are circulated through the half-cell electrode chamber into an external container, where the external container comprises active electrode material (22), and the active electrode material has redox potential between redox potentials of the first and second redox-active mediators.    USE - Organic solvent mediated redox flow battery for use in an electrical grid storage system.    ADVANTAGE - The battery achieves optimum performance if a membrane prevents mediator molecules from passing through from one side of a cell to the other side of the cell while allowing passage of ions from a supporting electrolyte. The battery improves system energy density and efficiency by assembling a flow battery system in which difference between mediator potential and lithium-insertion potential is small. The battery minimizes extra voltage required by mediation during charge or discharge, so that round-trip voltage efficiency, discharge voltage and effective energy content of the system can be maximized.    DETAILED DESCRIPTION - The external container is a cathode container (8) or an anode container (10). The half-cell electrode chamber is an anode cell chamber (4) or a cathode cell chamber (2). The current collector electrode is an anode. The first and second redox-active mediators are dissolved in non-aqueous solvent selected from a group consisting of diglyme, triglyme and tetraethyleneglycol dimethylether. An INDEPENDENT CLAIM is also included for a battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a redox flow battery.    Organic solvent mediated redox flow battery (1)    Cathode cell chamber (2)    Anode cell chamber (4)    Polymeric lithium and sodium ion conducting separator (6)    Cathode container (8)    Anode container (10)    Active electrode materials (22, 24)