Patent ID: 8241773

Claim:
A method for preparing an electrochemical device, comprising the steps of: charging an electrochemical device using a cathode active material having an oxygen gas generation plateau potential in a range of 4.4 to 4.8 V to an extent exceeding the oxygen gas generation plateau potential; and degassing after a first charging step, wherein the cathode active material comprises a solid solution represented by the following Formula 1: XLi(Li 1/3 M 2/3 )O 2 +YLiM′O 2 [Formula 1] wherein M is at least one element selected from the group consisting of Mn, Sn and Ti metals having an oxidation number of 4+; M′ is at least one element selected from the group consisting of Ni, Mn, Co and Cr metals; and 0<X<1 and 0<Y<1, with the proviso that X+Y=1.