A method of preparing a positive electrode active material, the method comprising:
- mixing an aqueous inorganic acid solution with a nickel-containing lithium transition metal oxide containing nickel in an amount of 60 mol % or more based on a total number of moles of transition metals excluding lithium to prepare a suspension having a pH of 8 to 12;
- forming a lithium-containing inorganic compound by reacting the lithium transition metal oxide with inorganic acid included in the aqueous inorganic acid solution;
- and
- drying and heat-treating the suspension to form a lithium-containing inorganic compound layer on a surface of the lithium transition metal oxide.