Active material for lithium ion secondary battery, and lithium ion secondary battery

An active material in the form of a solid solution for lithium ion secondary battery, the active material having a layered crystal structure and expressed by a compositional formula (1) below,

• LiyNiaCobMncMdOx (1) where M is at least one element selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba, and V;
• 1.9≦(a+b+c+d+y)≦2.1;
• 1.05≦y≦1.35;
• 0<a≦0.3;
• 0<b≦0.25;
• 0.3≦c≦0.7;
• 0≦d≦0.1;
• and 1.9≦x≦2.1,
• wherein 0.69≦Niβ/Niα≦0.85, where Niα is a Ni composition amount at a center portion of a primary particle of the active material, and Niβ is a Ni composition amount in a region extending from the active material surface in a depth direction by approximately 30 nm,
• wherein FWHM (003) in x-ray diffraction pattern/FWHM (104) in x-ray diffraction pattern ≦0.57.