Lithium ion secondary battery and method for preparing the same

A lithium ion secondary battery and a method for preparing the same are provided. The lithium ion secondary battery includes a positive electrode having a positive active material represented by Formula (1), a negative electrode and an electrolyte, wherein the positive active material is activated by performing a first charge operation with a voltage ranging from 4.5 V to 4.7 V and the voltage is then reduced to less than 4.5 V: aLi 2 MnO 3 -(1-a)LiMO 2 (1) where M is at least one selected from the group consisting of Ni, Co and Mn, and 0