NOVELTY - Preparation of lithium secondary battery involves activating a cell (C1) comprising a cathode active material (B) containing complex oxide (I) under voltage of more than uniform potential voltage region at which the structural variation of the cathode active material (A) occurs, and activating cell (C2) comprising a cathode active material (B) under voltage of less than uniform potential voltage region at which the structural variation of the cathode active material (I) occurs, and electrically connecting the cell (C1) and the cell (C2) in parallel or series.    USE - Preparation of lithium secondary battery. Uses include but are not limited to cellular phone, camcorder, notebook computer, personal computer and electric car.    ADVANTAGE - The method enables preparation of lithium secondary battery with high capacity and durability. The lithium secondary battery is operated at voltage less than the uniform potential voltage region of the cathode active material containing complex oxide. The cathode active material (B) used in the cell (C2) does not undergo its structural variation, such that the performance deterioration of the battery is prevented during battery operation.    DETAILED DESCRIPTION - Preparation of lithium secondary battery involves activating a cell (C1) comprising a cathode active material (A) containing complex oxide of formula: Li(LixMy-y'M'y')O2-zAz (I), where x is more than 0 to less than 0.5, y is more than 0.6 to less than 1.1, y' and z are 0 or more to less than 0.2, M is manganese, nickel, cobalt, iron, chromium, vanadium, copper, zinc or titanium, M' is aluminum, magnesium or boron and A is fluorine, sulfur or nitrogen, under voltage of more than uniform potential voltage region at which the structural variation of the cathode active material (A) occurs, and activating cell (C2) comprising a cathode active material (B) under voltage of less than uniform potential voltage region at which the structural variation of the cathode active material (A) occurs, and electrically connecting the cell (C1) and the cell (C2) in parallel or series.