Study On Electrochemical And Heat Transfer Model Of LiFePO₄Power Battery During Discharge Process

Large capacity lithium ion phosphate (LiFePO4) batteries are becoming preferred energy storage devices for electric vehicles. However, the safety problems incurred by the high heat generation rate and the non-uniformity of the battery discharge are the significant challenges faced by automakers. The key points of the battery thermal management system (BTMS) study are the mechanism of the electrochemical reaction, heat generation and electrochemical-thermal coupled characteristics of the lithium ion battery. The investigation of the interaction among battery kinetics, temperature and electric field distribution during discharge is of great significance for improving safety performance and temperature field uniformity of the battery.To study the heat generation rate of the LiFePO4lithium ion battery at various galvanostatic discharge rates, an electro-thermal mathematical model for the1D cell unit is developed where the mass, charge, and thermal transport processes are considered, as well as the electrochemical reaction phenomena. The thermal characteristics predicted by the model agree well with the experimental data. The results indicate that the reversible heat has a larger proportion of total heat generation under a lower discharge rate, while the ohmic heat is dominant under a higher discharge rate. A battery thermal management system is necessary under high discharge rate due to the higher temperature rise.The distributions of potential and reaction rates in a lithium ion power battery during discharge process have great influences on the battery thermal characteristics. A two-dimensional electrochemical-thermal model has been developed for the axial cross-section of a cylindrical LiFePO4battery. The model also includes battery current collectors. The modeling results are validated for both the electrochemical performance and thermal behavior during galvanostatic discharge process. The potential distribution on the cylindrical battery was found to have a significant effect on the distributions of its reaction rates, which therefore affect heat generation rates, and thus the distribution of the temperature within the battery.A lithium ion battery consists of numerous electrochemical cell units. Thermal and electrical behaviors of these local cell units have great influences on the battery performance and safety. To study the relationship between the cell units and the battery cell, a pseudo three-dimensional (3D) single cell model was developed for a prismatic LiFePO4battery. The model treated the battery with current collecting tabs as3D and the local cell units as1D. Both electrochemical and thermal characteristics of the battery were studied using this simplified model during discharge process. A uniformity index characterizing the SOC distributions among1D cell units was also introduced. This index was used to investigate the effects of the tab placement on the uniformity of the battery cell. The placement of the positive and the negative current collecting tabs on the prismatic battery was found to have a significant effect on the distributions of its potential and local reaction rates, which therefore affect the heat generation rates, and thus the temperature distribution within the battery.