Cathode active material for lithium ion secondary battery and lithium ion secondary battery

The present invention provides a positive electrode active material for a lithium-ion secondary cell having exceptional lithium ion conductivity, the positive electrode active material being such that the absorption of moisture from the atmosphere can be suppressed. This positive electrode active material for a lithium-ion secondary cell has powder particles that include a specific composite oxide, and an organic silicon compound affixed to the surface of the powder particles, the organic functional groups of the organic silicon compound including at least one selected from the group consisting of C2-10 alkyl groups and C6-14 aryl groups.