Cathode active material for lithium ion secondary battery

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 (p) having at least one metal element selected from the group consisting of Al, Y, Ga, In, La, Pr, Nd, Gd, Dy, Er and Yb and which does not contain an anion (n) having at least one non-metal element selected from the group consisting of S and B,
• a second contact step of contacting said powder of the lithium-containing composite oxide, and a second aqueous solution which contains said anion (n) (provided that the second aqueous solution may or may not contain said cation (p)), 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)+(number of moles of said cation contained in said second aqueous solution×valency of said cation)) is less than 1.