Degradation Studies on Lithium Iron Phosphate - Graphite Cells. The Effect of Dissimilar Charging – Discharging Temperatures

Abstract

This work presents a systematic evaluation of the effect of dissimilar charging / discharging temperatures on the long-term performance of lithium iron phosphate / graphite based cells by using multi-factor analysis of variance. Specifically, the degradation of prototype pouch cells is presented in a range of charging and discharging temperatures from -20°C to +30°C, counting a total of 10 temperature combinations. In this manner, not only the effect of charging and discharging temperatures was analyzed, but also the correlations between them.Fitting of the data showed a quadratic relationship of degradation rate with charging temperature, a linear relationship with discharging temperature and a correlation between charging and discharging temperature. Cycling at the charge/discharge temperatures (+30°C, -5°C) produced the highest degradation rate, whereas cycling in the range from -20°C to +15°C, in various charge/discharge temperature combinations, created almost no degradation. It was also found that when Tc≅15°C the degradation rate is independent of Td. When Tc < +15°C, the higher degradation occurs at higher Td and at Tc > +15°C lower degradation occurs at higher Td.