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

A lithium secondary battery, comprising a cathode including a cathode active material layer and an anode including an anode active material layer,

• wherein:
• the cathode active material layer comprises a cathode active material formed as a secondary grain including an assembly of a plurality of monocrystalline primary grains of a lithium composite oxide having a layered rock salt structure,
• the primary grains each have an average grain size of 0.01 μm or more and 5 μm or less;
• and
• the secondary grain has:
• a (003) plane orientation degree of 60% or more;
• an average grain size of 1 μm or more and 100 μm or less;
• an aspect ratio, which is a value obtained by dividing a long axis size by a short axis size, of 1.0 or more and less than 2;
• a voidage of 3% or more and 30% or less;
• an average pore size of 0.1 μm or more and 5 μm or less;
• and
• a value obtained by dividing the average grain size of each of the primary grains by the average pore size of 0.1 or more and 5 or less.