FIG. 7 is an optical photograph of the active material film (3 cm×5 cm×280 μm) produced according to an exemplary embodiment of the present invention. The active material film of FIG. 7 is obtained by producing an active material bulk by molding and sintering, and then slicing the active material bulk into the film having a thickness of 280 μm using an electric saw. Specifically, molding was performed by placing a mixture in which artificial graphite:pitch were mixed at a weight ratio of 8:2 in a mold, performing compression to perform primary molding, and then subjecting the primarily molded body to second pressing with cold isostatic pressing (CIP). For sintering, the molded body produced under a nitrogen atmosphere having an oxygen concentration of 50 ppm or less was heated to 700° C. at a rate of 2° C./min, subjected to primary heat treatment at 700° C. for 60 minutes, heated again from 700° C. to 1200° C. at a rate of 3° C./min, and subjected to secondary heat treatment at 1200° C. for 60 minutes, thereby producing an active material bulk. Apparent porosities of the produced active material bulk and the active material film were substantially the same, which were 19.8%.
