Cathode active material for lithium ion secondary battery, and process for its production

A process for producing a cathode active material for a lithium ion secondary battery, which comprises:

• a first contact step of contacting a powder of a lithium-containing composite oxide comprising lithium and a transition metal element, and a first aqueous solution which contains a cation having at least one metal element selected from the group consisting of elements in Groups 3 and 13 of the periodic table and lanthanoid elements,
• a second contact step of contacting said powder of the lithium-containing composite oxide, and a second aqueous solution which contains an anion having P and which does not contain a cation having the metal element, and
• a heating step of heating, after the first and second contact steps, the obtained treated powder of the lithium-containing composite oxide to a temperature of from 250 to 700° C., wherein
• in the entire aqueous solution having the first and second aqueous solutions put together, |(number of moles of said anion contained in said second aqueous solution×valency of said anion)|/(number of moles of said cation contained in said first aqueous solution×valency of said cation) is less than 1.