Development of a high power lithium secondary battery for hybrid electric vehicles

A lithium secondary battery (Type II cell) for hybrid electric vehicles (HEV) was developed on the basis of previous battery techniques (Type I cell with amorphous carbon/Li1 + xMn2O4). It used an improved cathode material and more advanced electrolyte. Cell performances of the Type II cell were evaluated and compared with a Type I cell of the same cell size, i.e. 40ϕ (diameter) ×108 mm (length). The Type II cell discharged 5.9 Ah, which was 1.5 times higher than the amount discharged by the Type I cell (3.8 Ah). The former had an input–output power of 800 W at 45% SOC (state of charge) and 25 °C, which was 1.3 times higher than that of the latter (600 W at around 40%). Moreover, the former had an input–output power of more than 100 W at −30 °C, though the Type I cell output power was only 50 W. A pulse charge–discharge cycle test with 167 W input and 260 W output for a 30–70% SOC range and a storage test at 50% SOC were carried out at various temperatures for the Type II cell. Less than a 15% DCR (direct current resistance) increase was observed in the pulse mode cycle test after 450 K cycles at 50 °C. No more than a 25% DCR increase was detected after 240 days in the 50% SOC storage test even at 65 °C. The activation energies for capacity change and DCR change in the storage test were also estimated for the Type II cell.