Cathode active material for lithium ion secondary battery, and method for producing same

Provided are: a cathode material that is able to prevent leaching of a transition metal into an electrolyte of a lithium ion secondary battery without necessitating cleaning; a method for producing the cathode material; and a cell using the cathode material and having superior discharge capacity, cycling characteristics, and the like. The method for producing a cathode active material for a lithium ion secondary battery causes fluorine gas to contact a lithium-containing complex oxide containing the element Li and at least one transition metal element selected from the group consisting of Ni, Co, and Mn (however, the number of moles of the element Li is greater than 1.2 times the total number of moles of the transition metal elements).