Sparse Electrochemical Impedance Spectroscopy Of Li-ion Battery And Its Application In State Of Charge Estimation

As Lithium-ion batteries occupy a more and more important position in the field of power grid energy storage,the research on Lithium-ion batteries is also deepening.Electrochemical Impedance Spectroscopy(EIS)plays a great role in the research of battery,but its measurement time is long,which limits its popularization and application under actual working conditions.Therefore,the concept of Sparse Electrochemical Impedance Spectroscopy which can be measured quickly is proposed in this paper.In addition,since State of Charge(SOC)is one of the most important battery state information,accurate SOC estimation is an important guarantee for reliable management of energy storage system.Therefore,this paper takes the SOC estimation of Lithium iron phosphate battery(LiFePO4)as an example to reflect the advantages of Sparse Electrochemical Impedance Spectroscopy in practical application.The main work is as follows:(1)The working characteristics of Lithium iron phosphate battery were analyzed,and the Electrochemical Impedance Spectroscopy of the experimental battery was measured.According to the understanding of the working principle and characteristic parameters of Lithium iron phosphate battery,the battery charge and discharge experiment is designed and implemented,and the charge and discharge characteristics of the battery are analyzed;The experimental steps of battery EIS measurement were designed,and the EIS of the experimental battery under different SOC at 25℃ was measured to provide data support for subsequent research.(2)The concept of Sparse Electrochemical Impedance Spectroscopy is proposed,its composition and advantages are analyzed,and its equivalent circuit model is constructed.By understanding the basic concept of EIS and thinking about the practical application of EIS,aiming at the problem that EIS measurement time is long and difficult to be popularized and applied,the concept of Sparse Electrochemical Impedance Spectroscopy is proposed.According to the relationship between the characteristic value and frequency of EIS,the characteristic frequency points of Sparse Electrochemical Impedance Spectroscopy are determined,and its advantage of time-consuming measurement is proved.Then,through the analysis of EIS characteristics and equivalent circuit model,a suitable equivalent circuit model is constructed to fit the EIS of Lithium iron phosphate battery.Finally,ZSimpWin software is used to fit the actual measured impedance spectrum data,and the results show that the model has good fitting effect.(3)The parameters of the equivalent circuit model are identified by Sparse Electrochemical Impedance Spectroscopy,and then the complete EIS is reproduced.Because the Sparse Electrochemical Impedance Spectroscopy contains less impedance values,the commonly used equivalent circuit model parameter identification method is not applicable,and the relevant algorithm needs to be specially designed.Therefore,in this paper,Particle Swarm Optimization(PSO)is used to identify the parameters of the equivalent circuit model,and the algorithm is written with the help of Python language.Finally,the parameters of the Sparse Electrochemical Impedance Spectroscopy equivalent circuit model of the experimental battery are identified,and the complete EIS is reproduced according to the parameter identification results.The results show that the errors meet the accuracy requirements,which verifies the effectiveness of the proposed method.(4)Taking the SOC estimation of Lithium iron phosphate battery as an example,the advantage of Sparse Electrochemical Impedance Spectroscopy in practical application is proved.Firstly,the equivalent circuit model parameters of Sparse Electrochemical Impedance Spectroscopy are analyzed,the relationship between each parameter and battery SOC is obtained,and the route of estimating SOC by electric double-layer capacitance Y1 in constant phase angle element is determined.Since Y1-SOC curve cannot be fitted directly by polynomial,this paper divides the curve into three segments and obtains the fitting function of each segment.In the process of using this method to estimate SOC,the Y1 parameters belonging to different segments are separated by the positive and negative changes of Y1 parameter slope and Support Vector Machine(SVM),and then substituted into the fitting function of the corresponding segment to obtain the final SOC estimation value.Finally,the SOC estimation based on Sparse Electrochemical Impedance Spectroscopy is verified.The results show that the error between SOC estimation and actual value meets the accuracy requirements,which verifies the effectiveness of the SOC estimation method in this paper.At the same time,due to the short measurement time of Sparse Electrochemical Impedance Spectroscopy,the case of SOC estimation further reflects the advantages of sparse impedance spectrum in practical application.