Numerical investigation on the polarization and thermal characteristics of LiFePO4-based batteries during charging process

Abstract

• An electrochemical-thermal couple model is developed for pouch lithium-ion batteries. • Effects of charge rate on polarization and heat generation are numerically studied. • Sudden rises in total polarization are mainly linked to solid-phase diffusion polarization. • Over 85% of the heat in the cell core under 8C charging comes from irreversible heat. • Charge energy efficiency of the cell at different current levels is further analyzed. High-current charging exacerbates internal polarization and abnormal heat generation, stymieing the development of fast-charging technology for lithium-ion batteries. Based on battery disassembly and charge test experiments, an electrochemical-thermal coupling model was developed and validated in this paper. The variation patterns of various polarization and heat generation of pouch-type lithium iron phosphate (LFP) batteries at different charge rates and their influence mechanisms were investigated numerically. Results indicate that the changes in charge voltage profiles are strongly linked to total cell polarization, and excessive charge current aggravates internal polarization, with the average polarization voltage reaching 109.2 mV at 8C, which is 471.7% higher than at 1C. The polarization in the electrodes dominates the cell polarization, especially at the negative electrode. Activation polarization accounted for the largest share of electrode polarization, the change in polarization profiles in the initial and final stages depended mainly on the solid-phase diffusion polarization. Besides, the large cell temperature difference at high charge currents is attributed to the order of magnitude difference in heat generation rates between the positive tab and the cell core. The cell temperature rise is primarily due to heat production in the cell core, where the proportion of irreversible heat increases nonlinearly with the charge rate, exceeding 85% of the total heat generation at 8C. The charge energy efficiency reduces gradually with increasing charge rate and is only 0.948 at 8C. These findings will help to develop appropriate charging protection strategy for better battery operation and life.