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The present invention relates to nickel-manganese (Ni--Mn) binary compounds useful as an electrode material for electrochemical supercapacitors, which is one of nickel-manganese coprecipitated hydroxides having a spinel-like structure, nickel-manganese coprecipitated hydroxocarbonates having a calcite-like structure and nickel-manganese oxides having an ilmenite-like structure. The present invention also relates to a method of preparing the above nickel-manganese (Ni--Mn) binary compounds by chemical coprecipitation and freeze-drying. Since the nickel-manganese binary compounds according to the present invention show high electrochemical efficiency, good reversibility, excellent specific capacity per unit area, a low capacity fade rate and improved cycle life, they can be effectively used as an electrode material for electrochemical supercapacitors.