Performance of high capacity Li-ion pouch cells over wide range of operating temperatures and discharge rates

Abstract

Operating temperature of Lithium-ion batteries (LIBs) significantly affects their electrochemical-thermal performance, cycle life, and cost. This study presents the thermal and electrochemical performance of 20 Ah LiFePO4 cells for 8 current rates (0.2C-5C) at 8 operating temperatures (-20 degrees C to 50 degrees C). Results show that the effects of operating temperature and current rate on cell performance differ above 10 degrees C, between 10 degrees C and 0 degrees C, and at subzero temperatures. Based on the electrochemical impedance spectroscopy (EIS) measurements, significantly higher bulk and charge-transfer resistances in conjunction with the lower diffusion coefficients results in poor battery efficiency at subzero temperatures. Optimum operating condition is 50 degrees C at a rate of 0.2C, in terms of utilized power and capacity, while a considerable power loss and capacity decrease occur below 20 degrees C. Furthermore, increasing the current rate is detrimental above 0 degrees C, whereas it improves cell performance at -10 degrees C, in terms of cell capacity. Moreover, cell temperature reaches an undesirable value at 50 degrees C and 5C rate, thus a thermal management system is necessary for high capacity LiFePO4 cells at higher temperatures and/or at higher C-rates. Additionally, temperature differences on the surface of high capacity cells reach 10 degrees C below room temperature at high current rates which can lead to nonuniform material utilization, and consequently cell failures. Finally, the cycle life of 20 Ah LiFePO4 cells decreases dramatically as discharge current rate increases.