Positive electrode active material for secondary battery, and secondary battery

A positive electrode active material for secondary batteries, comprising

• a lithium metal composite oxide having a crystal structure assignable to space group Fm-3m,
• wherein the lithium metal composite oxide contains Li, a first cationic element M1 different from Li, and a second cationic element M2 different from Li and the first cationic element M1,
• Mn accounts for 50 at % or more of the first cationic element M1,
• a content of the second cationic element M2 in the lithium metal composite oxide is 10 ppm to 1000 ppm in terms of mass,
• the number mLi of atoms of Li contained in the lithium metal composite oxide and the number mM1 of atoms of the first cationic element contained in the lithium metal composite oxide satisfy 1.2≤mLi/mM1≤2.0, and
• a crystallite size of the lithium metal composite oxide is in a range of 1 nm to 1000 nm,
• or
• in an X-ray diffraction (XRD) profile of the lithium metal composite oxide using CuKα radiation, a full width at half maximum of a diffraction peak attributed to (200) plane is in a range of 0.10 to 2.5° in terms of 2θ.