Positive electrode active material for lithium ion secondary battery, method for producing the same, and lithium ion secondary battery

A method for producing a positive electrode active material for a lithium ion secondary battery which contains a lithium-nickel-manganese composite oxide configured by secondary particles with a plurality of aggregated primary particles, the method comprising:

• a mixing process of adding a mixture containing at least a nickel-manganese composite compound, a titanium compound, and a lithium compound;
• a firing process of firing the mixture in an oxidizing atmosphere having an oxygen concentration of 80 vol % or more and 100 vol % or less at 750° C. or higher and 1000° C. or lower so as to obtain the lithium-nickel-manganese composite oxide;
• a water-washing process of mixing water at a ratio of 50 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the lithium-nickel-manganese composite oxide and stirring the mixture so as to perform solid-liquid separation;
• and
• a drying process of drying the water-washed lithium-nickel-manganese composite oxide,
• wherein the nickel-manganese composite compound contains nickel (Ni), manganese (Mn), and an element M (M) that is at least one element selected from the group consisting of Co, V, Mg, Mo, Nb, Ca, Cr, Zr, Ta, and Al as metal elements, and a mole number ratio of the metal elements is represented as Ni:Mn:M=(1-x-y):x:y, provided that 0.05≤x≤0.15 and 0≤y≤0.15,
• a ratio (Li/Me) of a lithium mole number (Li) to a total mole number (Me) of nickel, manganese, the element M, and titanium contained in the mixture is 0.97 or more and 1.25 or less, and a ratio (Ti/Me) of a titanium mole number (Ti) to the total mole number (Me) is 0.01 or more and 0.05 or less, and
• a ratio of a total amount of diffraction peak intensities of most intense lines of the titanium compound to a (003) diffraction peak intensity that is the most intense line of a hexagonal layered structure in XRD measurement of the positive electrode active material is 0.2 or less.