Large lithium-ion cells with more than 1200 W/kg power were fabricated using Li-1.1(Ni0.025Ti0.025)Mn1.85O4 (LM) and LiNi0.78Co0.19Al0.03MxO2 (M: Na, Sr, Ba, x=0.0005-0.1) (LN) as a positive active material, and graphitized carbon (KS25) and meso carbon fiber (MCF) as a negative active material. Overcharge tests at 100 A were carried out to estimate a safety level. From the overcharge test, the safety level of the cells are arranged Cell C (LN/MCF)> Cell A(LM/KS25)> Cell B(LN/KS25). Thermal stability of the active materials are arranged LM > LN as positive, and MCF > KS25 as negative from differential thermal analysis of each material with electrolyte. The safety level of the cells can be explained by the thermal stability of the each active material. (c) 2006 The Electrochemical Society.