A cathode active material for a lithium secondary cell used in a cellular phone is disclosed. The cathode active material for the lithium secondary cell and the method the same having a high capacity and a long lifetime, different from LiCoO₂ and LiMn₂O₄, Li(Ni, Co)O₂, and V-system oxide that has been researched as the active material for substituting LiCoO₂ are provided. The cathode active material for the lithium secondary cell in the next formula 1 is obtained by heating or chemically treating diadochite [Fe₂(PO₄)(SO₄)(OH).6H₂O] that is the mineral containing PO₄^3-, SO₄^2-, and OH^-.

Li_aFe_bM_c(PO₄)_x(SO₄)_y(OH)_z   (1)

In the formula, M is at least one element selected from a radical consisting of Mg, Ti, Cr, Mn, Co, Ni, Cu, Zn, Al, and Si, with 0a, c0.5, 1b2, 0.5x, y, z1.5.