| Research on the mechanism of power battery capacity attenuation is of great significance for ensuring the efficient operation of electric vehicles,optimizing power battery performance and reducing power battery production and manufacturing costs.In view of the existing power battery capacity decay detection and analysis methods which are based on electrochemical performance and microstructure morphology,it is difficult to continuously and non-destructively detect the relationship between the positive and negative active materials of the power battery in a confined space as its working conditions change.This leads to the problem that it cannot truly reflect the capacity attenuation of power batteries.The thesis research is in line with the principle and method of the detection and analysis of the capacity attenuation of the lithium-ion power battery based on the tomographic structure image.The research work was funded by the Science and Technology Plan Project of Guangdong Province(No.2014B010106004 and No.2014B090901054).Based on the working principle of power battery and computer tomographic image detection method,the thesis takes the lithium iron phosphate power battery(battery for short)in the cycle life detection process as the object.And it carries out the research on battery capacity attenuation mechanism from three aspects: battery capacity attenuation influencing factors,capacity attenuation non-destructive detection mechanism and tomographic structure image analysis method.Its main contents include:1.In view of the main determinant of power battery capacity-positive and negative active materials are difficult to detect in a confined space,the method of battery capacity attenuation detection and analysis mechanism based on the tomographic structure image was proposed.The relationship between the battery capacity and the electrochemical effect on the positive and negative active materials was studied,and the relationship between the battery capacity and the structural parameters such as the density and volume of the positive and negative active materials was discussed.Similarly,the interaction between X-rays and battery internal materials and the detection mechanism of battery three-dimensional tomographic structure image was also studied.Battery capacity decay detection experiments based on tomographic structure morphology images were carried out.Through the electrochemical performance experiment of battery cycle life,the relationship between battery capacity,unit cycle capacity attenuation rate,capacity increase curve,rate performance,DC internal resistance and the number of cycles during cycle life detection was analyzed.Moreover the behavior of battery capacity decay on electrochemical performance was discussed.Using the tomographic image detection experiment of battery capacity attenuation,the three-dimensional tomographic structure morphology images of the battery with different cycles were obtained non-destructively.The experimental results show that the battery capacity tomographic image detection method can not only obtain non-destructively the tomographic structure image of the positive and negative active materials of the battery and the corresponding battery capacity,but also can detect the time and space distribution and space of the battery capacity.2.By analyzing the relationship between the three-dimensional tomographic structure image of the battery and its structure,a method for classification and processing of the image of the battery positive and negative tomographic structure was proposed.Aiming at the problem that the positive and negative active materials of the battery are in a confined and narrow space,and the image feature information is small and difficult to identify,the relationship between the color,shape and physical characteristics of the three-dimensional tomographic structure image of the battery and the relationship between the battery structure in time and space was analyzed.Besides the difference in the geometric structure size of the battery positive and negative electrodes was also analyzed.The relationship between the gray scale of the battery positive and negative tomographic structure and the corresponding pixels and the composition of the positive and negative materials was discussed.Furthermore we investigated the spatial and temporal distribution of battery positive and negative active materials in the tomographic structure image and the relationship between changes with the number of cycles.And according to this we can realize the classification and processing of the image of the battery positive and negative tomographic structure.The experimental results show that the tomographic image classification method of the positive and negative electrodes of the battery can accurately obtain the tomographic structure morphology images of the positive and negative active materials at the same position under different cycles.3.Research on battery capacity decay analysis method based on tomographic structure image was carried out.Aiming at the characteristics of the image signal of the battery positive and negative tomographic structure morphology not only occupying a small proportion in the image geometry space,but also having a large amount of change under the action of the electrochemical reaction force,temperature and electromagnetic field inside the battery,resulting in difficult problems of analyzing its image information,we proposed a battery tomographic structure image analysis method based on contours.In terms of time and space,the relationship between the pixel decay rate per unit cycle,the discharge capacity decay rate per unit cycle and the cycle number interval of the battery positive and negative active materials in the tomographic structure image was analyzed.In addition,the relationship between the positive and negative active materials of the battery in the image of the tomographic structure and the change of the cycle number was studied.And moreover the correlation between the strong and weak active materials of the positive and negative electrodes and their discharge capacity under different cycles was discussed.Furthermore,the relationship between the battery discharge capacity decay in a single cycle and the decay rate of the number of pixels per unit cycle of the positive and negative active materials in the tomographic structure image was analyzed.And the relationship between the positive and negative active material content of the battery and its spatial distribution with the change of the cycle number was discussed and on the basis of this we study the attenuation mechanism of the battery capacity in the cycle life detection process.The research results showed that this method can not only detect the composition and structure of the positive and negative active materials of the battery under different working conditions,but also analyze the relationship between the battery capacity attenuation and the change of the chromatographic structure.The experimental results show that this analysis method can not only analyze the relationship between the change of the chromatographic structure of the positive electrode and the negative electrode of the battery with the change of its capacity,but also obtain the decay behavior of the battery capacity in time and space,which can be used to design new power batteries and optimize Battery performance provides technical support. |
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