NOVELTY - A redox flow battery comprises: a positive electrode cell (1) containing a positive electrode (13) and a catholyte solution (11); a negative electrode cell (2) containing a negative electrode (14) and an anolyte solution (12); and an ion exchange membrane (10) interposed between the positive electrode cell and the negative electrode cell, where the catholyte solution and the anolyte solution includes solvent, supporting electrolyte, and at least one metal-ligand coordination compound that includes a ligand containing an electron withdrawing group.    USE - As redox flow battery (claimed) used in mobile phones, mobile computers, electric vehicles, and hybrid vehicles.    ADVANTAGE - The redox flow battery exhibits high energy density and excellent charge/discharge efficiency and voltage efficiencies; and has improved cycle characteristics and the voltage characteristics.    DETAILED DESCRIPTION - A redox flow battery comprises: a positive electrode cell (1) containing a positive electrode (13) and a catholyte solution (11); a negative electrode cell (2) containing a negative electrode (14) and an anolyte solution (12); and an ion exchange membrane (10) interposed between the positive electrode cell and the negative electrode cell, where the catholyte solution and the anolyte solution each include a solvent, a supporting electrolyte, and at least one metal-ligand coordination compound that includes a ligand containing an electron withdrawing group. The redox flow battery further comprises BF4-, PF6-, CF3SO3-, or (CF3SO2)2N- as a counter anion of the metal-ligand coordination compound; and catholyte tank (21) and an anolyte tank (22) that are fluid-connected to the positive electrode cell and the negative electrode cell, respectively. The anion ion exchange membrane comprises an anion exchange membrane that is either obtained by aminating a copolymer of styrene-divinylbenzene by introducing a chloromethyl group; or obtained by substituting a copolymer of vinylpyridine-divinyl benzene with quaternary pyridium; or obtained by aminating an aromatic polysulfone copolymer by introducing a chloromethyl group. INDEPENDENT CLAIMS are included for the following:    (1) a catholyte solution comprising a non-aqueous solvent, supporting electrolyte, and the metal-ligand coordination compound;    (2) a positive electrode cell comprising a positive electrode cell and the catholyte solution;    (3) an anolyte solution comprising a non-aqueous solvent, a supporting electrolyte, and the metal-ligand coordination compound; and    (4) a negative electrode cell comprising a negative electrode and the anolyte solution.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a redox flow battery.    Positive electrode cell (1)    Negative electrode cell (2)    Catholyte solution (11)    Anolyte solution (12)    Positive electrode (13)    Negative electrode (14)    Catholyte tank (21)    Anolyte tank (22)