PROCESS FOR PRODUCING CATHODE ACTIVE MATERIAL, CATHODE ACTIVE MATERIAL, POSITIVE ELECTRODE, AND LITHIUM ION SECONDARY BATTERY

To provide a process for producing a cathode active material capable of obtaining a lithium ion secondary battery which has a high discharge capacity and a high initial efficiency, a cathode active material, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery. A process for producing a cathode active material, which comprises a mixing step of mixing a lithium compound, an alkali metal compound other than Li, and a transition metal-containing compound containing at least Ni and Mn to obtain a mixture, a step of firing the mixture at a temperature of from 900 to 1,100° C. to obtain a first lithium-containing composite oxide containing the alkali metal other than Li, and a step of removing the alkali metal other than Li from the first lithium-containing composite oxide to obtain a second lithium-containing composite oxide represented by the following formula: aLi(LiMn)O. (1−a)LiMO wherein 0