Effect of temperature and charge stand on performance of lithium-ion polymer pouch cell

Abstract

The effect of temperature in the range of −20°C to 60°C and charge stand periods of 1 day, 7 days and 15 days on the discharge capacity of lithium-ion (Li-ion) polymer cells is investigated quantitatively. Commercial 3.2Ah Li-ion polymer pouch cells with LiCoO2 cathode and graphite anode were used as test systems. The cells were charged at a constant current of 1C until the voltage reached 4.2V, then stored at different temperatures from −20°C to 60°C for charge stand periods of 1 day, 7 days and 15 days and discharged at 1C rate current until the cell voltage reached 3.0V. Although the discharge capacity of these cells at and around room temperature is reasonable, at temperatures below 0°C and above 50°C the performance is poor. At −15°C, −20°C, 40°C, 50°C and 60°C temperatures, the discharge capacities decreased with charge stand periods. The discharge capacities are not affected by the charge stand periods from −10°C to 30°C. To identify the reasons for the poor performance at sub-ambient temperatures, electrochemical impedance studies were carried out on a fresh cell. The impedance data are analyzed using an equivalent circuit and Zman fitting software and the impedance parameters are evaluated. The resistances corresponding to high frequency and low frequency of Nyquist impedance plot exhibit a strong dependence on temperatures. The kinetic parameter namely, apparent exchange current density is calculated and discussed.