Process for producing cathode active material, cathode active material, positive electrode, and lithium ion secondary battery

A process for producing a cathode active material, comprising:

• a 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 secondary particles having a plurality of primary particles of a second lithium-containing composite oxide agglomerated represented by the following formula 1:
• aLi(Li1/3Mn2/3)O2·(1−a)LiMO2 formula 1
• wherein 0<a<1, and M is an alkali metal other than Li and an element containing at least transition metals Ni and Mn and a ratio of a molar amount of the alkali metal other than Li to a total molar amount of the transition metals is at least 0.001 and at most 0.004, and in an X-ray diffraction pattern of the lithium-containing composite oxide, a ratio of a height (H110) of a peak of (110) plane assigned to a crystal structure with space group C2/m to a height (H003) of a peak of (003) plane assigned to a crystal structure with space group R-3m (i.e. H110/H003) is at least 0.018, and an integral breadth of a peak of (110) plane assigned to a crystal structure with space group C2/m is at most 0.5 deg.