Three different-sized lithium-ion cells for hybrid electric vehicles nominal capacity 5, 8, and 17 Ah, were fabricated using lithium-rich mangnese spinel Li1.1Mn1.9O4 (LM ) as a positive active material and meso carbon fiber (MCF ) as a negative active material. Overcharge tests at about 5, 16, 100 and 200 A current were carried out to estimate a safety level of LM/MCF cells. As a result of the overcharge test, all cells were not fired and did not burst and themal runaway of the 5 and 17 Ah cells did not occur at 1C rate. LM/MCF cells have great ability for safety under high-rate overcharge. However, thermal runaway occurred at 12C-40C rate overcharge. Thermal runaway depends strongly on the C rate at overcharge. (c) 2006 The Electrochemical Society.