Discharge Behavior Research Of The Lithium Ion Power Battery Based On The Electrochemical-thermal Coupling Model

The research and development on the Li-ion battery of high performance of is the active demand for the progress of the new energy of automobile industry, and also an important part of the national strategic emerging industry. In this thesis, a mathematic model was based on the relationship between electrochemical and thermal coupling, and a detailed and through study on the electronic and thermal behavior of the discharging process of Li-ion battery was conducted. Also, the influence of temperature, design of materials and process on the electrochemical reaction and diffusion of the cell was discussed, and revealed the influence of the discharging rate, convection condition on the rule of temperature field distribution. What’s more, optimizing of the design of temperature distribution of the battery module was considered, which laid a foundation for the dynamic optimization design of the battery. Main research results are as follows:1) The interaction between electrochemical and thermal performance is one of characteristics of batteries. In this work, a coupling calculation between a one dimensional electrochemical and three-dimensional thermal model was implemented through parameters transferring in real time, and comprehensive and detailed information about electrochemical and temperature field distributions were obtained. The results showed that temperature would affect the voltage platform as well as the utilization of active material, and the effect of electrochemical reaction on the temperature was mainly reflected at low discharge rate.2) The electrochemical reaction rate and mass transfer process during the discharging process was studied. While discharging, the electrochemical reaction rate varied from different position on the electrode, and electrochemical polarization was formed. At the beginning of the discharging, the reaction rate close to separator was the highest, and the lowest reaction rate was found near the current collector. With the development of discharge, the location with the fastest reaction rate would move to the current collector. The difference of mass transfer rate would cause diffusion polarization, and solid phase diffusion polarization and liquid phase diffusion polarization existed in both positive and negative electrode. The two phase polarizations would both increase with the discharging. It was also found the particle size of electrode active material had a great influence on the solid-phase diffusion, while electrode thickness was one of the main factors affecting liquid diffusion polarization.3) In this work, the influence of the discharge rate and convection condition on the distribution of temperature field was summarized. It was found that improving cooling temperature could lower the average temperature during the discharge process. When the condition of heat coefficient is5W/(m2·K), the average temperatures rise of the battery cells are6.46K、17.67K、27.53K at1C,3C,5C discharge rate, respectively. If the heat coefficient increased to25W/(m2·K), the average temperatures of the battery cells reduced by2.91K,4.68K,5.62K, respectively. During the discharge process, the average temperature of the positive current collecting tab was the highest of all, the battery was the lowest, and the negative current collecting tab was moderate. It was also found the highest temperature is at the connecting area of the battery plate and the tabs. Due to the large heat resistance of the battery, adding the surface heat transfer coefficient would intensify the inconsistency of inside and outside surface temperature distribution.4) The optimization method of Battery module temperature field distribution was developed. The influence of convection heat transfer coefficient and cooling plates on the temperature field of modules were studied. It was found that the convective heat transfer coefficient could significantly affect the average temperature of the power lithium ion battery module. However, along with the rising of the convective heat transfer coefficient, the temperature uniformity of battery module decreased. When the heat coefficient was5W/(m2·K), the temperature range of module was319.36K～320.27K, and heterogeneity degree was0.45%. When the heat coefficient was100W/(m2·K), the temperature range of module was310.42K～320.15K, and heterogeneity degree was3.08%. When cooling plates were added in the battery modules, the uniformity of temperature distribution was obviously improved. The fin number of cooling plate had little influence on temperature field uniformity. When the heat coefficient was5W/(m2·K), fin number increasing from26to96, battery module temperature uniformity coefficient changed only0.1%at most, during a discharging period of720s.