The 12V overcharge instability of the LiCoO2 cathode material was improved by the physical blending it with LiNi0.8Co0.1Mn0.1O2. Even though a Li-ion cell containing a LiCoO2 cathode did not exhibit thermal runaway at 12V at the 1C overcharging rate, it showed thermal runaway at the 2C overcharging rate, and the cell surface temperature reached more than 400°C. However, the LiCoO2 cell containing 40, 50, and 60wt.% LiNi0.8Co0.1Mn0.1O2 did not exhibit thermal runaway at the 2C overcharging rate. In conclusion, 60wt.% LiNi0.8Co0.1Mn0.1O2 in the LiCoO2 cathode showed the lowest cell surface temperature of <90°C even at a 3C overcharging rate.