The Study On The Variation Of Lithium-ion Cells For Electric Vehicles

Nowdays,the lithium-ion cells（LICs）have been applied as the most extensive and stable power source in electric vehicles（EVs）.The safety,energy density,life and stability of LICs have been improved as the development of EVs.However,its performance has great gap compared to internal combustion engine.As we known,the variation of LICs is critical factor to affect these characteristics.So,the improvement of variation for LICs is one of the most necessary methods to strengthen the performance of lithium-ion batteries（LIBs）.For LICs,in the Spatial dimension,it has multiple processes,and been controlled by many factors in the production phase.It has some difficulty to identify the key control factors,parameters and its value;in the time dimension,the variation keeps changes in the whole life periods.Under these two dimension,the subject of variation belongs to an engineering and academic problem.To analyze and solve this question,we adopt engineering methods:know the problem,analyze problem,solve the problem temporarily,and solve it fundamentally.We study blow questions in this thesis.1.Study the critical parameters which effect the variation in the whole life period with the growth model of solid electrolyte interface.We get the most critical parameter:the self discharge rate-kOCA,and verify this result through experimental test（chapter 4,section 4.1）.2.Study the current distribution in the parallel and series connected LICs,and combine with the above conclusion,we find the best grouping method:capacity sorting and self discharge rate.Which not only improve the varation but also decrease the cost in the production（chapter 4,section 4.2）.3.Differitiate the thermodynamic and kinetic factors which effect the variation of LICs based on the electrochemical impedance spectroscopy（EIS）theory.We connect thesed factors with the design parameters and process parameters of electrodes,and get that the source of the variation is the design of the electrode and its production process.We study the specific contribution of positive and negative electrode through two electrode method（chapter 5）.4.Study the effect of primary material and critical parameters of electrodes on the variation of LICs through EIS model.It includes agglomerate EIS theory and porous electrode EIS theory.The main parameters involve the conductivity of electrolty（σc）,the pariticle radius of aggregate(R_sp),the radius of primary particle(R_sp)and the current density of charge transfer reaction（i₀）.In the end.we conclude their sequence at different temperature（chapter 6）.We study the variation of LICs based on the engineering experience compare with other references.In this paper,we consider the variation problem in the whole cycle life（chapter 1 give the specific example）,and substitute the aging process with self discharge rate,which save the time for sorting.We believe that this conclusion has positive effect on the development of LICs industry.