A process for making a cathode active material for a lithium ion battery is described. The process includes (a) a step of synthesizing a mixed oxide of formula Li1+xTM1−xO2 at a temperature ranging from 750 to 1000° C. in an oxidizing atmosphere, where TM is a combination of two or more transition metals of Mn, Co and Ni and, optionally, at least one more metal of Ba, Al, Ti, Zr, W, Fe, Cr, K, Mo, Nb, Mg, Na and V, and x is a number ranging from zero to 0.2, (b) a step of cooling down the material obtained from step (a) to a temperature ranging from 100 to 400° C., (c) a step of adding at least one reactant of BF3, SO2 and SO3 at the temperature of 100 to 400° C., and (d) a step of cooling down to a temperature of 50° C. or below.