Research On The Charge Distribution And Heat Transfer Characteristics Of The Electrode Of Lithium-ion Batteries Based On The Fractional Order Model

As a key component of electric vehicles,the power performance of lithium-ion batteries directly affects the promotion of electric vehicles.The state of charge(SOC)characterizes the remaining power of the battery,which is of great significance to the capacity monitoring of the battery management system(BMS).The performance of electrodes decreases with the charge and discharge cycles due to the complex aging mechanisms inside the batteries.Research on the electrode aging can be used to prevent possible faults in the power systems.The electrochemical characteristics of the battery can be considered as an electrochemical system with strong non-linearity.The state of the battery is difficult to measure directly.Lithium-ion batteries generate a lot of heat during the charge and discharge process,causing the battery temperature to rise.The uneven temperature distribution will affect the performance and cycle life of the battery.Therefore,it is of great signifificance to study the heat transfer characteristics of lithium-ion batteries.This dissertation uses the fractional derivative order to estimate the state of charge and electrode aging of the battery,and uses the fractional derivative order to reveal the fractal distribution of the charge during the life cycle of batteries.The heat conduction of the battery includes various forms of heat transfer.The traditional heat conduction model is not suitable for lithium-ion batteries.In this dissertation,a fractional heat conduction model is used to study the heat transfer characteristics of lithium-ion batteries.The main research contents of this dissertation are as follows:(1)The fractional equivalent circuit model of the battery is established.The electrode of lithium-ion battery is composed of porous materials,which have the self-similarity in microstructure.When the electrochemical reaction inside the battery reaches a steady state,the charge distribution on the electrode has the fractal characteristics.The equivalent circuit model is usually used to simulate the dynamic and static electrical characteristics of the battery.However,the integral order equivalent circuit model is difficult to characterize the fractal properties of the charge distribution of the electrode.Based on the correlation between the fractional derivative and fractal theory,it is more appropriate to use the fractional equivalent circuit model to simulate the electrical characteristics of the battery.(2)This dissertation proposes the hypothesis that the fractional derivative order of the fractional order model is related to the SOC,aging degree and charge density of the battery.The fractal distribution of charge is affected by the variation of electrode performance during charging and discharging.The SOC,aging degree and charge density of the battery will cause the charge distribution to gradually deviate from the original fractal distribution.Therefore,the fractional derivative order in the equivalent circuit model changes with the SOC,aging degree,and charge density of the battery.If the fractional derivative order is related to the SOC,aging degree and charge density,then the fractional derivative order can be used as an indicator of the SOC,aging degree and charge density of the battery.At the same time,the fractional derivative order can also reflect the change process of the charge distribution.This dissertation proves that the fractional derivative order is related to the SOC,aging degree and charge density of the battery.(3)A new SOC estimation method based on the fractional order model is proposed.A test platform was established to collect the transient discharge data at different SOCs.The transient discharge data set is taken into the fractional order model for identification.The identifification results showed that there is a stable monotonic relationship between the fractional derivative order and the SOC.The fractional derivative order is used to estimate the SOC.Moreover,an iterative method based on a fractional order model is proposed to improve the accuracy of SOC estimation.(4)The relationship between the electrode aging and fractional derivative order is determined,and the fractional derivative order is used as an indicator of battery aging.The transient discharge dataset of fully charged lithium-ion batteries is taken as the input signal of the identifification system of the fractional order model.The identifification results show that there is a stable monotonous relationship between the fractional derivative order and the cycle number of batteries.Using the fractional derivative order as an indicator for electrode aging can quickly evaluate the degradation level of electrodes without destroying the battery.Since the charge density on the porous electrode is difficult to be detected by conventional methods.A visual fractional derivative order is used to characterize the charge distribution of the electrode and reveal the influence of the physical properties of the electrode on the charge density.More importantly,it is found that the capacity recovery effect is closely related to the change of the charge density,and the fractional derivative order is used to explain the capacity recovery.(5)The heat conduction model of the battery is established based on the fractional calculus theory.Then,the temperature characteristic test was carried out to collect the temperature of the battery in various operating environments.Finally,the simulation effect of fractional heat conduction model was analyzed and the thermal stress distribution of the battery was calculated.The results show that the accuracy of fractional heat conduction model is higher than that of traditional heat conduction model.The fractional order model is more suitable for the analysis of heat transfer characteristics in porous electrodes.Different from the traditional estimation method,the SOC and aging degree estimation method proposed in this dissertation is a new exploration of estimating the battery state by establishing the relationship between the fractional derivative order and the internal charge distribution mechanism of the battery.In addition,this dissertation uses the fractional order heat transfer model to extend the scope of application of the integer order model.The fractional order model in this dissertation is more suitable for the heat transfer process in the porous electrode,and can improve the simulation accuracy of the temperature field of the battery.