Combination of Electrochemical-Calorimetric Studies on Cylindrical Lithium Ion Cells and Thermal Modelling By Comsol Multiphysics Software

In this study, commercial cylindrical 18650 lithium ion cells with LiMn₂O₄ (LMO) cathodes were tested under isoperibolic and adiabatic conditions in an accelerating rate calorimeter (ES-ARC, Thermal Hazard technology with integrated battery cycler) to investigate the heat effects during cycling in more details. These experimental results were compared with simulations by FEM-based thermal modeling using COMSOL Multiphysics software.

The isoperibolic investigations were performed at specific temperatures in the range from 25 to 60 °C. The results show that the applied environmental temperature did not largely influence the battery thermal behavior. Generally, an overall exothermic behavior for discharging half cycles and an overall endothermic behavior for charging half cycles was observed. At 1 C rate the maximum temperature increase over three cycles was 4 °C almost independent of the environmental temperature. Additionally, the heat capacity and calorimeter constant were measured after calibration using cylindrical dummy cells made of AlMgSi0.5 (EN AW-6060 F22) with the same dimensions as the cells. By integrating over the heat dissipation rate and the enthalpy accumulation rate the total generated heat was determined ion dependence of discharge C-rate. Tests under adiabatic conditions, i.e. under negligible heat loss, more accurately simulate the actual operating environment if several cells are put in a battery pack and the neighboring cells hinder or prevent the heat transfer to the ambient. The cells were studied in the adiabatic mode at starting temperatures between 20 °C and 40 °C. The cell temperature was largely increasing at 1 C rate over three cycles by more than 40 °C rate before reaching the safety limit temperature of 75 °C.

For the modeling of the cells the existing lithium ion battery model in COSMOL Multiphysics was combined with the coupled thermal and electrochemical 2D-model proposed in [1]. As input data the available experimental cell data, cycling parameters and literature data have been used.  A good qualitative agreement between the experimental and the simulation results was found both for isoperibolic and adiabatic conditions.

The next step will be the assignment of the heat effects found both in simulation and experiments to electrochemical processes and the different cell components. This could be done by separating the heat effects into reversible and irreversible parts and fitting their ratio in the simulation to the experiment. An improved understanding of the heat effects will help to improve the thermal management of the cells and prevent from thermal runaway.

[1]        L. Cai, R.E. White, Journal of Power Sources 196 (2011) 5985-5989.