Thermal Conductivity, Heat Sources and Temperature Profiles of Li-ion Batteries

In this paper we report the thermal conductivity of several commercial and non-commercial Li-ion secondary battery electrode materials with and without electrolyte solvents. We also measure the Tafel potential, the ohmic resistance, reaction entropy and external temperature of a commercial pouch cell secondary Li-ion battery. Finally we combined all the experimentally obtained data in a thermal model and discuss the corresponding internal temperature effects. The thermal conductivity of dry electrode material was found to range from 0.07 to 0.41 W K-1 m(-1) while the electrode material soaked in electrolyte solvent ranged from 0.36 to 1.10 W K-1 m(-1). For all the different materials it was found that adding the electrolyte solvent increased the thermal conductivity by at least a factor of three. For one of the anode materials it was found that heat treatment at 3000 K increased the thermal conductivity by a factor of almost five. Measuring the electric heat sources of an air cooled commercial pouch cell battery at up to +/- 2C and the thermal conductivity of the electrode components made it possible to estimate internal temperature profiles. Combining the heat sources with tabulated convective heat transfer coefficients of air allowed us to calculate the ambient temperature profiles. At 12C charging rate (corresponding to 5 minutes complete charging) the internal temperature differences was estimated to be in the range of 4-20K, depending on the electrode thermal conductivity. The external temperature drop in air flowing at the battery surface was estimated to nearly 40K. Evaluating thermal management of batteries in the light of our measurement led to the conclusion that external cooling is more challenging than internal, though neither should be neglected.