Lithium-ion Power Cells Inconsistency Analysis Model And Influencing Factors Simulation

When the lithium-ion cells are used as the driving device of the electric vehicle,the single cells need to be connected in series and parallel to meet the energy and power requirements.The difference between the single batteries will lead to the performance degradation and the life shortening of the battery pack.The problem of cells inconsistency not only increases the cost of cells manufacturing and usage,but also brings harm to the performance and safety of packed cells.The inconsistency of Li-ion cells is not only the main goal of cells quality control and screening in the manufacturing process,but also the main factor of accurate monitoring and protection of battery pack.To control the generation and development of cells inconsistency in the manufacturing stage,formation and rules of cells inconsistency should be understood,which can effectively reduce and prevent the generation of cells inconsistency,and provide theoretical reference for cells quality control and cells condition monitoring.In this study,the inconsistency of capacity and voltage caused by manufacturing process deviation is simulated by simulation.Through the simulation method to predict the cells inconsistency and analyze the influencing factors;combining with the electrochemical theory and statistical theory to analyze the formation mechanism and evolution rule of the cells inconsistency difference;according to the inconsistency characteristics of capacity and voltage,the inconsistency prediction model of capacity and voltage is established.The normal distribution statistical model and Weibull distribution statistical model are used to analysis the inconsistency characteristics of discharge capacity and discharge voltage of C/3 ratio constant current in actual manufacturing battery.The results show that the discharge capacity distribution conforms to the Normal distribution,and the voltage distribution changes with the discharge process conform to the statistical characteristics of Weibull distribution.The discharge voltage is close to the normal distribution at the initial stage of discharge,and at the end of discharge it is an extreme right distribution.Based on Newman electrochemical model,the statistical characteristics of the normal distribution of process parameters such as the area density of positive and negative electrodes,the thickness of positive and negative electrodes and the concentration of electrolyte in the actual production are used for an establishment of an analysis model of the influence of key parameters on the cells inconsistency in the manufacturing process.The Newman model is solved by the finite element method,and the inconsistency characteristics of cells discharge capacity and discharge voltage are obtained.The calculation results show that the simulation results are in good agreement with the statistical results of the actual battery,which means that the established inconsistency analysis model has good accuracy and practicability.Based on the electrochemistry theory and variance analysis model,the correlation analysis model of battery capacity and discharge voltage inconsistency and the polarization voltage inconsistency analysis model in the discharge process are established.Using the correlation analysis between battery capacity and discharge voltage inconsistency,the influence of capacity inconsistency on discharge voltage inconsistency is analyzed.The results show that the statistical characteristics of voltage inconsistency described by Weibull Model are determined by the characteristics of capacity inconsistency and discharge voltage curve,and the characteristics of voltage distribution evolve with the discharge process.The influence of open circuit voltage,polarization voltage and ohmic polarization voltage inconsistency on voltage inconsistency is analyzed by using polarization voltage analysis model.The results show that the inconsistency of discharge voltage is determined by ohmic voltage drop inconsistency at the beginning of discharge,by open circuit voltage inconsistency at the middle of discharge,by polarization voltage inconsistency at the end of discharge,and the fluctuation of area density of positive and negative electrodes lead to the change of the open circuit voltage,which makes the inconsistency between the open circuit voltage and the polarization voltage have obvious correlation.The correlation between the two is closely related to the polarization control process during discharge.Using the established inconsistency analysis model,the influence of the area density of positive and negative electrode surface,thickness of positive and negative electrode and concentration of electrolyte on the inconsistency of cells capacity and discharge voltage in the manufacturing process are studied.The statistical analysis shows that there is a strong correlation between the capacity and the area density of positive electrode,with the correlation coefficient reaching 0.9954.Using the cell theoretical capacity inconsistency analysis model and the variance analysis model,an analysis method is established to analyze the inconsistency of the area density of positive and negative electrodes,the NP ratio and the lithium insertion percent of the positive and negative polar materials from the cell capacity inconsistency.The calculation and analysis results show that the change of the area density of positive electrode induced by the manufacturing process conforms to the normal distribution model,and the cells capacity also shows the normal distribution characteristics.The research results of the influence of key factors on the inconsistency of discharge voltage in the manufacturing process are changing in different stages of discharge,and the influence of electrolyte concentration in the initial stage of discharge is dominant;the influence of positive polar density in the middle stage of discharge is dominant;the influence of negative polar density in the final stage of discharge is the dominant factors.