Analysis of the thermal behavior of a large format lithium-ion battery cell in a constant-power discharge process

Abstract

Most electrochemical-thermal models for lithium-ion battery (LIB) cells simulate constant-current discharge, which makes it difficult to accurately replicate the operation of devices that actually use LIB cells. To address this issue, the 3D electrochemical-thermal model used in this study is developed to analyze the electrochemical and thermal behavior of a commercial large-format NMC622 battery cell under constant-power discharge conditions. In more detail, a multiphysics numerical model capable of coupled analysis of charge transport, mass transport, and energy balance phenomena occurring during battery operation has been developed. Through this, a foundation is established for easily identifying electrochemical-thermal behaviors that are difficult to analyze experimentally. Based on the completed numerical model, constant-power analysis, rather than constant-current analysis, is conducted to simulate the power control process of an actual electric vehicle. Various heat generation mechanisms—including reaction heat, activation heat, electronic ohmic heat, and ionic ohmic heat—are thoroughly analyzed. The developed three-dimensional model is experimentally validated with respect to voltage, current, and temperature. Based on the validated model, the thermal behavior during discharge is analyzed. It is found that in the commercial cell considered in this study, ionic ohmic heat dominates during high-power discharge, while reversible reaction heat dominates during low-power discharge. Detailed heat analysis shows that most heat is generated in the two electrodes, with the negative electrode contributing the most. Due to reaction heat, an endothermic reaction occurs in the negative electrode at the early stage of discharge. Furthermore, the analysis of activation heat confirms that a significant activation overpotential develops in the positive electrode near the end of discharge. The localized heat generation is greater in the side surface (thickness direction) rather than on the front surface of the cell, as confirmed by simulation. Finally, the effect of cell geometry is investigated. The results show that as the electrode becomes thinner, lithium-ion transport becomes more efficient, leading to improved electrochemical and thermal performance.