| The pursuit of high energy density has enabled lithium ion batteries to be satisfied for automobile applications. However, the relationship between energy density and thermal stability is a trade-off. To provide the comprehensive research foundation and theoretical basis for the design and thermal management of lithium ion batteries(LIBs), a pouch type LIBs of Li[Ni0.7Co0.15Mn0.15]O2/graphite with high specific energy were developed for fully understanding the thermal behavior under routine use conditions. The main research contents and results are as follows:(1) Thermal behavior of the battery during discharge was mastered by measuring temperature increment, key effect factors of heat generation(overvoltage, entropic heat coefficient dU/dT), battery heat capacity, and heat transfer coefficient. A prediction model with lumped parameters was used to estimate the average temperature evolution of LIBs. The suitablity of the thermal model was validated to predict the average temperature for the large scale batteries under normal operating conditions.Results show that the temperature increment of LIBs origniates from irreversible heat and reversible heat arising from the electrochemical reaction, the irreversible heat increases significantly, almost linearly increase with the rate increasing, the reversible heat is independent of the discharge rate, while the reversible heat contributes much to the middle evolution of the temperature during discharge, especially at low discharge rate, the temperature drop in the middle evolution is dominated by reversible heat. At the ambient and nature convection condition, compared with the heat dissipation property, temperature increment of the battery is mainly due to the heat generation during discharge. Subsequently, based on the experimental data of irreversible heat, reversible heat and specific heat capacity of battery, the model with lumped parameters is used to estimate the temperature evolution at different discharge rates. The predicted results match well with the experimental results at all discharge rates. Therefore the thermal model is suitable to predict the average temperature for the large scale batteries during discharge.(2) The performance and temperature evolution of rate discharge of Li[Ni0.7Co0.15Mn0.15]O2/graphite battery before and after 853 cycle was studied. The entropic heat coefficient(i.e., dU/dT) of battery before and after cycle was determined. The characteristic of entropy change of the battery before and after cycle was mastered. By associating entropy change with the Li-intercalated state, lithium intercalation number in negative and positive electrode was calculated and compared for the battery before and after cycle, respectively. The aging mechanisms in LIBs was analyzed.Results show that the discharge capacity and discharge voltage of the battery after cycle are less than that of the battery before cycle, meanwhile the temperature increment is much higher than that of the battery before cycle during discharge. The temperature increment is mainly influenced by the increment of irreversible heat, the reversible heat of the battery before and after cycle changes little. The value of dU/dT before and after cycle is close and the trend of dU/dT with discharge capacity is basically in consistent, moreover, the entropy change of the battery is mainly influenced by negative electrode. By comparison of the variation of Li-intercalated content of positive and negative electrode before and after cycle, the maximum Li-intercalated content of negative electrode after cycle decreases obviously, which is related with the polarization increase of negative electrode during intercalation process. The causes of capacity fade and changes of thermal behavior of the battery after cycle are due to the changes of SEI, and electrode structure etc.(3) A electrochemical-thermal coupling model was developed. The discharge performance and thermal characteristics under different convection conditions were studied. The influence of discharge rate and convection condition on uniformity of distribution of temperature field was analyzed. The limiting step of the battery electrochemical process was mastered. At last, the main causes for the decline of the battery performance under the condition of forced convection and high discharge rate were investigated.Results show that the electrochemical performance is mainly influenced by the forced convection conditon in high discharge rates. With the intensity of convection increase, the attenuation trend of battery discharge performance gradually increases. The temperature distribution along the thickness of the battery is more influenced by the intensity of convection conditon. Thermal conductivity(ky) in the direction of battery thickness and discharge rate are the two key parameters, which influence the temperature distribution along the direction of battery thickness. The smaller of ky, the greater of the temperature gradient in the direction of battery thickness. And the inhomogeneity of the temperature linearly increases with increase of discharge rate. The limiting step is the diffusion process of Li+ from the center to the surface of graphite particles. Under the condition of forced convection and high discharge rate, the solid diffusion coefficient of Li+ in the graphite particles is obviously decreased with the battery temperature decrease, and the difference of solid diffusion coefficient of Li+in different position of graphite particles increases with increase of the temperature gradient in the battery. These two aspects are both the main causes for the decline of the battery performance. Polarization impedance and heat generation rate could be effectively reduced by a reasonable design of the size of graphite particle. |
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