Study On The Electrochemical Characteristics Of High Capacity Lithium Sulfur Batteries And The Reconstruction Of The Battery Model

Due to its high energy density,lithium-ion batteries are the first choice of the electric vehicle battery.However,compared to the energy density of lithium-sulfur batteries?2600 wh/kg?,the energy density of lithium-ion battery?200-250 wh/kg?obviously can't meet the demand of further development of electric vehicles.In addition to high energy density,lithium-sulfur batterier are also cost-effective and environmental-friendly,which makes it become a research hotspot for global researchers.However,due to the special electrochemical reaction mechanism?including charge and discharge process in the middle of the product through the diaphragm in the expansion of the shuttle effect of positive and negative electrode and the anode effect?,its commercialization process is limited.This paper proposes a method to improve the performance of lithium-sulfur battery by replacing common separator with g-C₃N₄ separator to solve a series of problems existing in the lithium-sulfur battery system.Through this process,the mechanism of charge and discharge process and electrochemical performance of the lithium-sulfur battery has been studied,while provided necessary theoretical basis and experimental data for subsequent lithium-sulfur battery modeling and parameter identification.The battery model is a bridge connecting the internal and external characteristics of the battery.At the same time,the battery model can supply reliable data during its operation.Therefore,the battery model is a necessary step for the application of battery into electric vehicles.In this paper,the following aspects of the electrochemical characteristics and battery model of lithium-sufhur battery has been studied:1)modification of lithium-sulfur battery.By comparing the electrochemical properties of g-C₃N₄ modified separator and common separator,the effect of g-C₃N₄ modified separator on the electrochemical reaction of the battery was studied and its mechanism was explored.Study found:the g-C₃N₄ separator is composed of orderly pieces,the tunnels between g-C₃N₄separator layers provide rapid transport channels of Li⁺,and the pyridine nitrogen on g-C₃N₄separator can tightly anchor sulfur ions producing in charge and discharge process,effectively restrain shuttle effect.The capacity of the g-C₃N₄ separator battery can reach 990 mA h g^-1under the current rate of 0.2C.After 200 cycles,its capacity remains 829 mA h g^-1,and the attenuation rate of each cycle is only 0.5%.This modified separator battery can still maintain the capacity of 400 mA h g^-1 under the current rate of 2 C,and the reversible capacity of g-C₃N₄ battery is 830m Ah g^-1 when the current rate is back to 0.2c.All kinds of electrochemical experiments in this area provide experimental data and theoretical basis for the subsequent modeling of lithium sulfur batteries.2)The equivalent model of lithium-sulfur battery was established by using the first-order RC equivalent circuit model and the second-order RC equivalent circuit model.Firstly,the parameters of lithium-sulfur batteries was obtained by directly using the impedance spectrum experiment data of each state of charge?SOC?point.The corresponding simulation platform of the first-order and second-order equivalent circuit model has beeb established in the MATLAB/Simulink to verify the working condition of lithium-sulfur batteries model in constant current.The simulation results show that the first-order RC equivalent circuit model and second-order RC equivalent circuit model both have good fitting precision.The overall error of the first order RC equivalent circuit model keeps within 2.5%,while the error of the second-order RC equivalent circuit model remains at 1%or less.The error comparison analysis shows that the second-order RC equivalent circuit model is incapable of fitting the discharge curve of lithium-sulfur battery.3)the establishment of electrochemical model of lithium-sulfur battery.Based on porous electrode theory and concentrated solution theory,a single particle model of lithium-sulfur battery is established.The single particle model is composed of the solid phase lithium ion diffusion equation,the Butler Volmer equation and the battery terminal voltage equation.The three-parameter parabola method is used to reduce the complexity of the model by simplifing the second-order partial differential equation as ordinary differential equations and algebraic equations.Then the expression of positive and negative open circuit voltage is fitted and the unknown parameters in the model are identified by using the bacteria foraging optimization algorithm.Finally,a single particle model of lithium-sulfur battery was built in Matlab/Simulink,and the parameter values obtained were brought into the single particle model to carry out the simulation experiment,and the accuracy of the model was verified under the condition of constant current condition.The experimental results show that the single particle model of lithium-sulphur battery can fit the discharge curve of lithium-sulfur battery,which has satisfactory simulation accuracy.