| Lithium ion batteries(LIBs)are expected as the first option for electic vehicle(EV)application due to many advantages.In the past,the performance of single battery was investigated by mang researchers all around the world.But in real applications,the module and pack were usually assembled by connecting batteries in form of different series and parallel.Therefore study on the battery performance under module/pack conditions is more practical and significant.During module/pack assembling,external force was applied in order to fasten the batteries,and the binding force has inevitable impact on the battery performance.On the other hand,the electrode has volume change during lithiation/delithiation,which has effect on swelling froce under binding conditions.Among current naterials of lithium ion batteries,anode materials usually have higher volume change during charge/discharge,the most frequently investigated materials include carbon-based graphite,hard carbon,silicon and silicon oxide et al.In contrast,cathode materials,including lithium iron phosphate,NCM ternary material and lithium manganite,have little volume change during electrochemical operation.Hence,study on the interfacial properties of above materials as well as electrode swelling force is necessary and imperative.In this thesis,we focus on the study of the following five aspects:(1)interface properties of graphite anode materials and electrode swelling force evolution;(2)interface properties of SiOx anode materials and electrode swelling force evolution;(3)the performance and stress analysis of high nickle NCM materials and electrodes;(4)the performance analysis of large pouch cells under external force;(5)stress evolution of NCM prismatic cell under scenario/profile cycle life operation.The main innovations and conclusions are as following:1.Study on the interface stability of natural graphite and the evolution of electode swelling force.As menstioned before,the volume effect of anode is more obvious compared with cathode.Herein the characteristics and performance of natural graphite with different electrolyte addtives were characterized by using different spectroscopic techniques and by electrochemical facilities.Natural graphite is cost-effective,abundant,but it has larger specific surface area,more active sites on graphite edge side,as well as larger inner stress.Selection and optimization of electrolyte additives is one effective way to improve its battery performance.LiBOB and FEC were found more effective after characterized by gassing during formation,cycle stability and coulombic efficiency,as well as morphology evaluation after cycling.Based on the combination of LiBOB+FEC,natural graphite can achieve near the same capacity retention(～80%)as artificial graphite(AG)after 800 cycles under high temperature.At the same time,the swelling force evolution of 60 Ah larger pouch battery was also measured by pressure measurement.The results showed that natural graphite has much higher swelling force than AG.Specificially,the swelling force of NG vs.AG is 7660 vs.3740 N after 1000 cycles under pre-load force 1000 N@50%SOC.Besides,anode binder with PAA can form cross-link network and provide robust adhesive,and the related swelling force of NG is 4870 N after 1000 cycles.2.Study on the material properties and the swelling force evolution of Si-based materials.60 Ah large pouch batteries were assembled with commercialized SiOx/Graphite and NCM811 cathode.Then their cycle life,swelling force increase were investgated.The results showed that the swelling force increase during the 1st cycle is 7320 N,4 times higher than that of graphite-based batteries.Cycling test under different ambient temperatures shows high temperature-dependent tendency.At 25,45 and 60℃,the cycle number is 980,850,500 cycles corresponding to 70%SOH while the maximum swelling force is to be 25107,25490,23667 N,respectively.The root cause for the swelling force increase is the SEI growth and thickening with repeated electrochemical cycles.The compression curve was applied to sorting appropriate cushion which can accommodate the swelling force.The results shows polyurethane cushion has the best compression properties which is able to reduce the swelling force by 50%.3.Study on the performance and electrode swelling force evolution of monocrystalline and polycrystalline NCM811 cathode materials.2 Ah pouch batteries were assembled with AG anode and different NCM811 cathodes.Although the monocrystalline NCM811 battery has larger swelling force change during charging/discharging,it can still deliver better cycle stability.On the contrary,polycrystalline NCM811 cathode has larger crystalline volume chage,especially when charged upper to more than 4.0 V.Because above 4.0 V,the contraction of crystalline volume is obvious,which will deteriorate cycle stability.Cross-section morphology showed that the particles have beed cracked after certain cycles,which resulted in fast capacity fading.Hence,the volume change and the resulted mechanical failure of the polycrystalline NCM811 cathode can be alleviated by decreasing the charge voltage limit of the cell.4.Analysis of the cycle stability and swelling force evolution of 60 Ah large pouch cell under different pre-load force.The batteries were designed with NCM811 as cathodes and three different anodes,which were then investigated under 1000,3000,5000 N initial pre-load forces respectively.Ther results showed that the cell under 3000 N(0.0525 MPa)pre-load force has the best performance.The capacity retentions after 1000cycles were 93.7%(AG+SBR anode),90.9%(NG+SBR anode),91.2%(NG+PAA anode)respectively.In addition,all the batteries exhibited linear increasing correlation between swelling force and cycle numbers after charging and discharging for 300cycles.Furthermore,battery with NG+PAA as electrode showed the lowest polarization after cycles.5.Based on above analyses,the stress evolution under profile lading was investigated for high power prismatic batteries.The dynamic and static force was measured by press equipment during module assembly.As the SOC raised,both the dynamic and static force increased.During cycling of typical driving profile for high power cell application,both the swelling foce and DCIR reduced after initial 3%capacity loss,which was ascribed to electrode rebound and hence fullly filled cell can.After that,linear increasing correlation was found between swelling force and capacity loss.In summary,in the framework of the national policy "Carbon peaking and Carbon neutrality",we proposed herein one design solution for high energy,low cost lithium ion battery,which is anode(NG+PAA)/Electrolyte with LiBOB+FEC adding/cathode(monocrystalline or upper voltage controlled polycrystalline NCM811)with optimzied pre-load force(0.0525 MPa).Besides,the swelling force evoluation has mathematical correlation with many aspects of battery properties.Further study on building coupled model and combined with BMS strategy can provide optimized application for EV batteries. |
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