A method for preparing a lithium metal phosphor oxide having a structure of the following Formula I, the method comprising: mixing an iron salt solution and a phosphate solution in a reactor, wherein the reactor includes an agitator in form of a rotatable packed bed positioned in a sealed chamber and equipped on a vertically positioned rotation shaft, the packed bed having a cylindrical shape and including one mesh layer; applying a shearing force to the mixed solution in the reactor during the mixing by the agitator, wherein the packed bed is rotated on a rotation axis and whereby a flow condition having a Reynolds number of 2,000 to 200,000 is formed in the reactor by the shearing force to form a suspension containing amorphous nano-sized iron phosphate precipitate particles; obtaining the amorphous nano-sized iron phosphate particles from the suspension; aging the nano-sized amorphous iron phosphate precipitate particles under a condition where crystalline iron phosphate particles are formed and, optionally, mixing with a metal salt before crystallization to obtain metal-doped crystalline iron phosphate particles, wherein the metal salt is selected from metals of the group consisting of Ni, Co, Mn, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, and Mg; and mixing the crystalline iron phosphate particles, respectively metal-doped crystalline iron phosphate particles with a lithium raw material selected from the group consisting of LiOH, Li 2 CO 3, LiCI, and mixtures thereof and performing firing of the mixture: Formula I LiM n FePO 4 where M is selected from the group consisting of Ni, Co, Mn, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, and Mg, and n is in a range of 0 to 1. mixing an iron salt solution and a phosphate solution in a reactor, wherein the reactor includes an agitator in form of a rotatable packed bed positioned in a sealed chamber and equipped on a vertically positioned rotation shaft, the packed bed having a cylindrical shape and including one mesh layer; applying a shearing force to the mixed solution in the reactor during the mixing by the agitator, wherein the packed bed is rotated on a rotation axis and whereby a flow condition having a Reynolds number of 2,000 to 200,000 is formed in the reactor by the shearing force to form a suspension containing amorphous nano-sized iron phosphate precipitate particles; obtaining the amorphous nano-sized iron phosphate particles from the suspension; aging the nano-sized amorphous iron phosphate precipitate particles under a condition where crystalline iron phosphate particles are formed and, optionally, mixing with a metal salt before crystallization to obtain metal-doped crystalline iron phosphate particles, wherein the metal salt is selected from metals of the group consisting of Ni, Co, Mn, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, and Mg; and mixing the crystalline iron phosphate particles, respectively metal-doped crystalline iron phosphate particles with a lithium raw material selected from the group consisting of LiOH, Li 2 CO 3, LiCI, and mixtures thereof and performing firing of the mixture: Formula I LiM n FePO 4 where M is selected from the group consisting of Ni, Co, Mn, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, and Mg, and n is in a range of 0 to 1.