Technical Papers and Presentations

The reaction current density is an important process parameter of lithium-ion battery, significantly influencing its electrochemical performance. In this study, aimed at the discharge process of lithium-ion power battery, an electrochemical-thermal model was established to analyze the distribution of the reaction current density at various parts of the cathode and its evolution with the time of discharge, and to probe into the causes of distribution and evolution. Introduction In recent years, the new energy vehicle market has developed rapidly; thus, to satisfy the needs for the Li-ion battery with higher single capacity and specific energy, the battery manufacturers are encouraged to improve their electrode design. Model Development

Fig 1. Schematic diagram of LiFePO4 a. Electrochemical kinetics b. Mass conservation c. Electronic charge conservation d. Energy conservation

Results and discussion

Fig 2. Distribution of reaction current densities in the positive electrode direction at different periods during 3C discharge rate. Fig. 2 shows the distribution of reaction current density in the direction of the positive electrode at 3C discharge rate. In the initial stage of discharge, the peak value of the current density appeared in the electrode zone near the separator, indicating that the rate of electrode reaction in this zone was greater than that in the other sites.

Fig 3. Change in liquid conductivity ratio with the variation in depths of discharge during different discharge rates. The conductivity barely changed; however, at the end of 3C and 5C discharge processes, the conductivity increased respectively by 13.1% and 22.9% compared to the initial value.

Fig 4. Depths of discharge at various positions of electrodes at the end of 3C discharge in different porosity and hickness As shown in Fig. 4, when the electrode thickness was 52.5 μm (0.75L), the depths of discharge of the active substances at various positions were distributed uniformly and the value was approximately one, indicating that such a thickness was almost similar to the “critical thickness”, at this condition, the smaller volume fraction cannot exert a significant influence on the uniformity of the distribution of depths of discharge. Conclusion In this study, an electrochemical-thermal model was established to analyze the distribution of the reaction current density at various parts of the electrode and its evolution with the time of discharge, and probed into the causes of distribution and evolution.