TECHNOLOGY FOCUS - ELECTRONICS - Preferred Components: The redox flow battery further comprises a catholyte solution tank and an anolyte solution tank that are fluid-connected to the positive electrode cell and the negative electrode cell, respectively.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The metal is nickel, cobalt, iron, ruthenium, zinc, manganese, yttrium, zirconium, titanium, chromium, magnesium, cerium, copper, lead or vanadium.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The aliphatic ligand comprises an aliphatic amine type or an aliphatic oxide type ligand. The aliphatic amine type ligand is ethylenediamine, propylenediamine, N,N'-bis(2-aminoethyl)propane-1,3-diamine, 1,4,8,11-tetraazacyclotetradecane, 3,7-bis(2-aminoethyl)-1,3,5,7-tetraazabicyclo(3.3.1)nonane, or 1,3,6,8,10,13,16,19-octaazabicyclo(6.6.6)eicosane. The concentration of the metal-ligand coordination compound in the solution is 0.1-3 M. The metal-ligand coordination compound containing the aliphatic ligand is 18 compounds e.g. metal complexes of formulae (a)-(j). The metal-ligand coordination compound is subject to a reversible oxidation and reduction reaction. The nonaqueous solvent comprises dimethylacetamide, diethylcarbonate, dimethylcarbonate, acetonitrile, gamma -butyrolactone (GBL), propylene carbonate (PC), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), fluoroethylene carbonate, N,N-dimethylacetamide, or adiponitrile (ADPN). The redox flow battery further comprises boron tetrafluoride (BF4-), hexafluorophosphate (PF6-), trifluoromethanesulfonic acid (CF3SO3-), or bis(trifluoromethane)sulfonimide ((CF3SO2)2N-) as a counter anion of the metal-ligand coordination compound. The supporting electrolyte comprises lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium triflate, lithium bistrifluoromethylsulfonyl imide, triethylamine tetrafluoroborate, tributhylamine tetrafluoroborate, sodium tetrafluoroborate, sodium hexafluorophosphate, trimethylsulfonylchloride, or ammonium sulfate.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The ion exchange membrane is an anion ion exchange membrane. The anion ion exchange membrane comprises an anion exchange membrane i.e. obtained by aminating a copolymer of styrenedivinylbenzene by introducing a chloromethyl group, an anion exchange membrane i.e. obtained by substituting a copolymer of vinylpyridine-divinylbenzene with quaternary pyridium, or an anion exchange membrane i.e. obtained by aminating an aromatic polysulfone copolymer by introducing a chloromethyl group.