Simplifying Electrothermal Dynamics of 15-Ah Prismatic Li-Ion Batteries

A simply coupled electrochemical and thermal model is validated and presented for a 15 Ah large format prismatic lithium iron phosphate cell. Understanding the thermal response of lithium-ion cells is essential because it would allow the batteries to be operated more efficiently. Two important issues regarding the thermal response are "cold start" and "thermal runaway."[1] Use of a validated simulation can aid in enhancing the performance and safety of these batteries.

To minimize the number of essential parameters, the simplest models are carefully chosen to depict the battery responses. At least two physical models are necessary for describing an electrochemical system: an electrochemical model and a local energy balance. For the electrochemical model, we select the Newman and Tobias model, which provides an analytic solution with an assumption that the ion concentration gradient is negligible.[2] For the energy balance [3,4], three different heat sources are necessary to match our experimental data: (1) Joule heating of the cell materials (simple IR heating), (2) an additional Joule heating generated by the interfacial resistance between electrode and electrolyte, and (3) the reaction heating from reversible entropy change. Joule heating is always exothermic; however, the reversible heat depends on the electrochemical reaction and can be endothermic or exothermic.

Even though the minimal physics are chosen for simplicity, the number of physicochemical parameters in the completed model was more than ten; however, the number could be decreased to five after rigorous dimensional analysis. Therefore, not only do these parameters suggest important factors for the battery response, but they also help to control the battery in efficient ways, since this test shows that many parameters have the same effect in the electrical/thermal response. Comparison to target experimental data characteristics of the battery (Fig. 1) allows iterations of the dimensionless parameters to increase accuracy through multidimensional Newton-Raphson method. Finalized parameters will guide simulations to help control the battery in efficient ways.

[1] T. M. Bandhauer, S. Garimella, and T. F. Fuller, Journal of The Electrochemical Society 158, R1 (2011).

[2] J. S. Newman and C. W. Tobias, 109(12), 1183 (1962).

[3] D. Bernardi, E. Pawlikowski, and J. Newman, J. Electrochem. Soc. 132, 5 (1985).

[4] J. Newman and K. E. Thomas-Alyea, Electrochemical Systems, 3rd ed. Wiley, 2004.