TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Battery: The redox flow battery further comprises a positive electrode cell comprising a positive electrode, a negative electrode cell comprising a negative electrode, a positive electrolytic solution tank that is fluid connected to the positive electrode cell, and a negative electrolytic solution tank that is fluid connected to the negative electrode cell. The metal-ligand coordination complexes are reversibly oxidized and reduced.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The ligand contained in the metal-ligand coordination complexes comprises at least one selected from 2,2'-bipyridine, terpyridyl, ethylenediamine, propylenediamine, 1,10-phenanthroline, N-heterocyclic carbene (NHC) (2,6-bis(methylimidazole-2-ylidene)pyridine) or acetylacetonate (preferably iron-2,2'-bipyridine, iron-1,10-phenanthroline, an iron-acetylacetonate coordination complex, vanadium-2,2'-bipyridine, vanadium-1,10-phenanthroline, or vanadium-acetylacetonate). The counter anions of the metal-ligand coordination complexes are BF4-, PF6-, CF3SO3-, or (CF3SO2)2N-. The support electrolyte is at least one electrolyte selected from LiBF4, LiPF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, tetraethylammonium tetrafluoroborate (TEABF4) or tetrabutylammonium tertrafluoroborate (TBABF4). The solvent is a non-aqueous solvent (preferably diethyl carbonate, dimethyl carbonate, acetonitrile, gamma -butyrolactone (GBL), propylene carbonate (PC), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), fluoroethylene carbonate, or N,N-dimethyl acetamide.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The ion exchange membrane is an anion exchange membrane, and is prepared by amination performed by introducing a chloromethyl group to a copolymer of styrene-divinylbenzene as a base, an anion exchange membrane prepared by substituting a copolymer of vinylpyridine-divinylbenzene with quaternary pyridium, or an anion exchange membrane prepared by amination performed by introducing a chloromethyl group to an aromatic polysulfone copolymer as a base. SPECIFIC COMPOUNDS - Six compounds i.e. Fe2+(1,10-Phenanthroline (Phen))3(BF4)2 (Ia), Fe2+(1,1'-bipyridine(byp))3(BF4)2, Fe2+(2,2':6,2''-terpyridine)2(BF4)2, V3+(Phen)3(BF4)2, V3+(bpy)3(BF4)2 are disclosed as the metal-ligand coordination complex.    EXAMPLE - No suitable example is given.