A method of preparing carbon-coated manganous oxide, the method including: (1) preparing a liquid polyacrylonitrile (LPAN) solution as a carbon source, stirring the LPAN solution at between 100 and 200° C. to yield a cyclized polyacrylonitrile solution; (2) heating the cyclized LPAN solution at between 200 and 300° C. to yield a thermally-oxidized polyacrylonitrile (OPAN) having a ladder structure; (3) mixing the thermally-oxidized polyacrylonitrile with a manganese compound, to yield a mixture, and adding a solvent to the mixture, uniformly mixing, to yield a polyacrylonitrile coated manganese compound; (4) drying the polyacrylonitrile coated manganese compound until the solvent is evaporated completely and the polyacrylonitrile coated on the manganese compound is crosslinked to form a solid, to yield a carbonized precursor coated manganese compound; and (5) calcining the carbonized precursor coated manganese compound in the presence of an inert gas flow, to yield a carbon-coated manganous oxide.