Investigations Of Simulation Models And Control Algorithms For Applications Of Lithium-sulfur Traction Battery

With the rapid growth of passenger cars,automobile exhaust pollution has become more and more severe.The development of zero-emission electric vehicles has become an important strategy of governments to boost the future economy.In recent years,lithium-ion batteries have been widely used in electric vehicles.However,due to the problems of high cost,low energy density and poor safety,it is difficult to meet the demands of electric vehicles for long range,high safety and intelligent travel.Lithium sulfur battery owns prominent advantages of low cost,rich material reserves and environment friendly as well as the theoretical specific capacity of up to 1675 m Ah g^-1and the theoretical specific energy of 2600 Wh kg^-1.It has been widely studied and considered as one of the next generation traction batteries that can replace lithium-ion battery.However,due to the shuttle effect,poor cycle life and safety problems,lithium sulfur battery still faces great challenges in industrialization and commercialization.A large number of researches have been devoted to solve the above-mentioned problems,and great progress has been made.However,advanced technologies of engineering application and industrialization still needs to be developed vigorously for promoting the wide application of lithium-sulfur traction batteries in electric vehicles.Obviously,the investigations of simulation models and control algorithms for applications of lithium-sulfur traction battery is one of the key technologies for the development of lithium-sulfur traction.In this thesis,starting from investigation of the cathode material of lithium-sulfur battery,followed by mixing,coating,rolling,laminating and other processes,the material battery was scaled up and fabricated to the current mainstream electric vehicle pouch cell,and the key parameters were characterized and analyzed.From both scientific and technological point view,this thesis integrates the modeling and simulation technology to establish an innovative method combining academic research and industry applied technology for applications of lithium sulfur batteries tractions in comparison to the lithium-ion batteries.By its unique discharge platform,offering much more targeted,higher accuracy of calculation method of SOC.The aim for this work is to build a bridge between research in lab and practical applications in electric vehicles,thereby not only vividly showing more intuitively observation to the application performance of new battery technology in electric vehicles applications,but also guiding researchers to optimize technical attributes,set further development direction,thus greatly promote the application of new technology in electric vehicles.1.In Chapter 3 of this paper,Lotus Root-like Nano Fiber（LNF）and Porous Nano Fiber（PNF）were synthesized by electrospinning as cathode substrates of lithium-sulfur batteries,and the two materials were analyzed and studied by physical characterization and electrochemical test,the electrochemical characterizations of the button cell show that the capacity of the cell can still remain at 1263m Ah g^-1 with almost no capacity attenuation after 500 cycles at a rate of 0.2C.The material was assembled into 3.3 Ah pouch cell monomer through mixing slurry,coating,laminating and other processes,and the corresponding values of OCV and r₀ under different SOC were measured in the1.5-2.45 V voltage platform of the pouch battery by using current pulse.This chapter provides research materials and parameter value basis for the following work contents.2.Chapter 4 of this paper establishes a simple,efficient,real and reliable research method that connects basic lab research with industrial application development,which can save time and labor costs,and could study the application performance of lithium-sulfur batteries in electric vehicles more accurately and intuitively.Firstly,the battery model was established by using the equivalent circuit method,the dynamics simulation model of the EV was established by the driving force balance formula,thus,a close-loop interaction model for traction battery applied on EV was established to support the simulation verification of different EV test conditions.The accuracy and rationality of the simulation model has been verified by comparing the simulation results of the model to the measured values of a real vehicle.This model provides a method to estimate lithium-sulfur battery’s performance on electric vehicle,so as to promote the industrialization of basic lab research.3.Chapter 5 of this thesis further studies the performance of lithium-sulfur batteries in electric vehicle through the model simulation method verified in Chapter 4,and compares the application effects of lithium-sulfur batteries with NCM lithium-ion batteries,which are the most widely used in electric vehicles at present.NEDC（New European Driving Cycle）,WLTC（World-wide harmonized Light duty Test Cycle）and HWFT（Highway Fuel Economy Test）drive cycles are selected to study the performance differences between lithium-sulfur batteries and NCM batteries.A typical quick charging strategy is designed to analyze and compare the performance differences between lithium-sulfur batteries and NCM lithium-ion batteries during electric vehicle charging.The results exhibit that the current performance of lithium-sulfur batteries is capable of the application conditions of electric vehicles.However,because its internal resistance is about 10 times larger than that of the ternary lithium battery,it has a more obvious output voltage vibration and internal resistance energy loss.At the same time,the charging time of the lithium-sulfur battery is about 5 times as that of the NCM lithium-ion battery due to the charge current limited by its large internal resistance.Based on the simulation result,the application performance of lithium-sulfur batteries in electric vehicles is systematically evaluated,which can help researchers to efficiently,safely and intuitively point out the direction to optimize its performance in EV applications.4.In Chapter 6 of this paper,in order to address the obstacle for estimating the SOC of lithium-sulfur batteries based on its unique discharge platform,a new SOC calculation algorithm is established.The algorithm uses ampere-hour integration as a basic calculation mechanism,and uses open circuit voltage when the system powers up and Kalman filter amends while battery charge and discharge to change the SOC value with weight coefficient.In order to verify the rationality and accuracy of the SOC algorithm,a hardware-in-the-loop（HIL）test platform of the battery management system（BMS）is established with fast-prototype controller and real-time simulation system.The validation test result shows the open circuit voltage and Kalman filter amending algorithm have effectively improve the conventional SOC control strategy.In summary,this study establishes a research method connecting the innovativebasic battery lab research results with the application development of practical electric vehicles.Starting from the research on the cathode material of lithium-sulfur batteries,followed by fabricating a lithium-sulfur pouch cell that can be used in electric vehicles,and then identifying its performance parameters.The high-precision simulation model of traction batteries and electric vehicles are built to realize efficient,safe and intuitive analysis and research for new lithium sulfur battery’s application on electric vehicle.Furthermore,a more targeted and higher precision SOC algorithm of lithium-sulfur batteries is established considering its unique discharge platform.This thesis provides a method to bridge lab research and application development of lithium-sulfur batteries in the field of electric vehicles.