The 12 V overcharge instability of the LiCoO2 cathode material was improved by the physical blending it with LiNi0.8Co0.1 Mn0.1O2. Even though a Li-ion cell containing a LiCoO2 cathode did not exhibit thermal runaway at 12 V at the 1 C overcharging rate, it showed thermal runaway at the 2 C overcharging rate, and the cell surface temperature reached more than 400 degrees C. However, the LiCoO2 cell containing 40, 50, and 60 wt.% LiNi0.8Co0.1Mn0.1O2 did not exhibit thermal runaway at the 2 C overcharging rate. In conclusion, 60 wt.% LiNi0.8Co0.1Mn0.1O2 in the LiCoO2 cathode showed the lowest cell surface temperature of < 90 degrees C even at a 3 C overcharging rate. (c) 2005 Elsevier B.V. All rights reserved.