Positive electrode 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 material

A method of preparing a positive electrode material, the method comprising:

• preparing a first positive electrode active material represented by Formula 1 by mixing and sintering a cobalt oxide, a lithium-containing raw material, and a doping element M1-containing raw material;
• preparing a second positive electrode active material represented by Formula 2 by solid-phase mixing and sintering a nickel oxide, a cobalt oxide, a manganese oxide, a doping element M2-containing raw material, and a lithium-containing raw material;
• and
• mixing the first positive electrode active material and the second positive electrode active material,
• wherein the second positive electrode active material has an electrical conductivity of 0.1 μS/cm to 150 μS/cm, which is measured after the second positive electrode active material is prepared in a form of a pellet by compressing the second positive electrode active material at a rolling load of 400 kgf to 2,000 kgf, and
• wherein the operation starting voltage of the second positive electrode active material is 3.75 V or more:
• LiCo1-aM1aO2 [Formula 1]
• LiNibCocMndM2eO2 [Formula 2]
• wherein, in Formula 1, M1 comprises at least one selected from the group consisting of aluminum (Al), titanium (Ti), magnesium (Mg), and zirconium (Zr), and 0≤a≤0.2, and
• wherein, in Formula 2, M2 comprises at least one selected from the group consisting of Al, Ti, Mg, Zr, yttrium (Y), strontium (Sr), and boron (B), and 0<b≤0.6, 0<c≤0.35, 0<d≤0.35, and 0<e≤0.1.