Positive Electrode Active Material For Lithium Secondary Battery, Method Of Preparing The Same, And Positive Electrode For Lithium Secondary Battery And Lithium Secondary Battery Which Include The Positive Electrode Active Material

Abstract

A positive electrode active material includes a lithium transition metal oxide, in which a cobalt content in the lithium transition metal oxide is less than a manganese content, wherein at least one of nickel, cobalt, and manganese in the lithium transition metal oxide has a concentration gradient that gradually changes from a center of a particle to a surface thereof, the positive electrode active material is in the form of a secondary particle formed by agglomeration of primary particles, and a ratio in which angles between c-axis directions, which are measured at at least 8 points on a surface of the positive electrode active material by TEM analysis, and a growth direction of the particle at the measuring point satisfy 85° to 95° is 60% or more.

1 . A method of preparing a positive electrode active material, the method comprising:
 preparing a first transition metal-containing solution including a nickel raw material and a second transition metal-containing solution having a transition metal concentration different from that of the first transition metal-containing solution and including the nickel raw material, a cobalt raw material, and a manganese raw material;   preparing a positive electrode active material precursor by mixing the first transition metal-containing solution and the second transition metal-containing solution while gradually changing a mixing ratio of the first transition metal-containing solution to the second transition metal-containing solution; and   mixing the positive electrode active material precursor with a lithium-containing raw material to form a mixture and sintering the mixture to synthesize the positive electrode active material,   wherein the preparing of the positive electrode active material precursor is controlled such that a pH is gradually decreased as a nickel content in the mixed solution is reduced.   
     
     
         2 . The method of  claim 1 , wherein a molar ratio of nickel:cobalt:manganese in the first transition metal-containing solution is in a range of 80 to 100:0 to 10:0 to 20. 
     
     
         3 . The method of  claim 1 , wherein a molar ratio of nickel:cobalt:manganese in the second transition metal-containing solution is in a range of 50 to 80:10 to 30:10 to 35. 
     
     
         4 . The method of  claim 1 , wherein the pH is gradually decreased within a range of 13 to 9. 
     
     
         5 . The method of  claim 1 , further comprising a doping element M 1  raw material, wherein M 1  comprises at least one selected from the group consisting of Al, Zr, Mg, Zn, Y, Fe, and Ti, during the mixing of the positive electrode active material precursor with the lithium-containing raw material.