Positive active material for secondary battery of improved rate capability

PURPOSE: A positive electrode active material is provided to improve rate performance by minimizing a separating amount of oxygen from a crystal structure of Li2MnO3 at a high voltage of 4.3-4.6 V and to increase safety of a secondary battery by minimizing the gas generation due to a side reaction with electrolyte liquid. CONSTITUTION: A positive electrode active material contains excessive lithium in a cation layer of a transition metal to reduce a separating amount of oxygen from a crystal structure at a high voltage of 4.3-4.6 V and is represented by chemical formula 1: (1-x)Li(Li_aM'_bM_(1-a-b))O_2* xLi_2M''O_3. In chemical formula 1, 0 < x < 0.3, 0 < a < 0.2, 0 Δ b Δ 0.2, M is one or two more atom selected from Ni or Mn, M' is a transition metal except Ni and Mn, M'' is one or more atom selected from Mn, Sn or Ti.