Three different-sized lithium-ion cells for hybrid electric vehicles nominal capacity 5, 8, and 17 Ah, were fabricated using lithium-rich mangnese spinel Li 1.1 Mn 1.9 O 4 ͑LM͒ as a positive active material and meso carbon fiber ͑MCF͒ 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.Recently, there has been growing concern that CO 2 , a by-product of burning fossil fuels, has a major effect on global warming. As a countermeasure against the problem, hybrid electric vehicles ͑HEVs͒ are currently considered the most viable alternative propulsion system and aggressive development work is being carried out in the automotive field. 1 A lithium-ion battery ͑LIB͒ is expected and has been developed to be an attractive alternative storage system for HEVs because of its high energy and power density. 2-6 Small size LIBs have been widely used as a power source for various portable electronic devices. However, large-size LIBs are not yet commercially available because the effect of the scale-up of this kind of battery on its safety has not yet been fully estimated. For this reason, many researchers have carried out thermal stability studies on electrode and electrolyte materials for LIB and safety tests of small-or large-size LIBs. 2,7-31 However, there is no report about overcharge tests at high rate ͑more than 10C rate͒ for HEV application. We have investigated the safety performance of our cylindrical large-size LIBs with manganese spinel and reported that the LIBs have good safety performance against overcharge, heat, and nail penetrate tests. 30,31 Moreover, we investigated the influence of electrode active materials on safety performance and concluded that the combination of manganese spinel ͑LM͒ and mesocarbon fiber ͑MCF͒ is expected to have good safety performance. 32 In this paper, in order to assess the safety performance of the LM/MCF cells, we made different-sized cylindrical cells ͑nominal capacity 5, 8, and 17 Ah͒ with LM for positive and MCF for negative and carried out overcharge tests with 5, 16, 100, and 200 A current.
ExperimentalCells for safety test.- Figure 1 shows a basic structure of the test cells. The cells are cylindrical. In the case of cell A ͑5 Ah͒, the diameter is 40 mm and the length is 200 mm. In the case of cell B ͑8 Ah͒ and cell C ͑17 Ah͒, the diameter is 50 mm and the length is 200 and 300 mm, respectively. The lithium-ion cells were fabricated using lithium-rich LM Li 1.1 Mn 1.9 O 4 ͑Mitsui Kinzoku͒ as a positive active material and MCF ͑Petca͒ as a negative active material. They had a safety pressure-release vent ͑safety vent͒ that opens when internal cel...
