Experimental and Modeling Study of Electrochemical and Thermal Behavior of Lithium-ion Batteries

Abstract

This research work starts with experimental investigations of the heat generation characteristics and the effectiveness of passive cooling of commercially available LiFePO4 batteries (7.25mm×160mm×227mm, 19.5 Ah) using different cooling materials. The specific heat capacity and the entropy coefficient of the battery are experimentally measured. The heat generation rate of the battery at 1-4C current rates are also determined using three different methods: 1) the heat absorption calculated from the temperature increase of cooling water; 2) the energy loss calculated from the difference between the operating voltage and open circuit voltage and 3) the energy loss during a charge-discharge cycle calculated using the over-potential between charging and discharging. Results show that the heat generation rate estimated from heat absorbed by the water can be underestimated by up to 42.8% because of the temperature gradient within the battery and on the surface. The effectiveness of different passive cooling materials is compared at discharge current rates of 1-3C. The average increase of the battery surface temperature is 22.5, 17.1, 7.6, 7.2 and 6.3ºC at 3C using air, aluminum foam, octadecane, water with aluminum foam and water. Water, octadecane and water with aluminum foam could always maintain lower average temperature and temperature gradient on the battery surface. In addition to that, this research also presents a simplified electrochemical-thermal coupled battery model to predict the electrochemical and thermal behavior of a lithium ion battery. The model predicts the current density distribution, electrolyte concentration, overpotential, temperature distribution and total heat generated by the battery at 1C, 2C and 3C current rates. The electrolyte concentration in the battery changes with the change in the applied current rate, which results in the change of the current density distribution throughout the electrodes. It is also observed that the internal resistance of the battery contributes significantly to the total heat generation and cannot be neglected. Higher increase in the battery surface temperature was observed using air as the cooling material compared to water. The model overpredicts the battery surface temperature by an average of 14.4% and 33.1% using air and the water as the cooling material because of the losses that occur during the experimental measurements.