Mechanical Behavior Of Lithium-ion Battery Under External Loading

As a reversible secondary battery,lithium-ion battery（LIB）has the advantages of high energy density,output efficiency and charge-discharge efficiency and so on,so LIB is one of the most promising new energy sources in the future.However,serious mechanical safety problems are esisted in LIBs and components under external loading,which affect the durability,stability and commercialization of LIB.Therefore,it is necessary to investigate the mechanical safety of LIBs and components.The mechanical safety of LIBs and components under external loading is investigated and the methods are put forward to reduce the internal short circuit（ISC）and improve the resistance exterior force property of LIB by theoretical derivation,finite element simulation and experimental investigation.The main research work is as follows:（1）The resistance exterior force property of LIB based on different shape functions are studied because theoretical models established by different shape functions have different characterizations of resistance exterior force property and triggering ISC of LIB.By taking energy conservation principle into account,the theoretical model with different shape functions is established to study the mechanical behavior triggering ISC of LIB based on the assumption of homogenized,isotropic and continuum.The relationship among the indentation force,indentation displacement and indentation region is proposed.Both of increasing compressed yield stress of jellyroll core and punch radius and decreasing flow stress of soft casing and thickness of soft casing can avoid triggering ISC of LIB.The indentation loading is reduced to flat compression when punch radius approaches infinity,and the ISC of LIB under flat compression is very difficult to be triggered.Effectiveness and application scope of different shape functions are also discussed,which are closely related to the material properties（compressed yield stress of jellyroll core and flow stress of soft casing）and geometric structures（thickness of soft casing and punch radius）.（2）The theoretical expressions between indentation force and indentation displacement based on sinusoidal shape function are established to investigate resistance exterior force and triggering ISC of LIB with different positive materials under different punch loading.From mechanical point of view,LIB has a better property to resist external force when LiMnNiCoO₂,rather than LiCoO₂ and Nanophosphate,is used as positive material.In addition,on the basis of energy conservation law,it is concluded that LIB is more likely to produce mechanical failure and ISC when normalized deformation region is greater than or equal to 0.4 and plastic strain energy ratio is greater than or equal to 1.5.（3）The strain rate effect and microbuckling behavior of two types of anisotropy separators for LIB under different strain rates are investigated.The separator has high tension strength along machine direction（MD）and good toughness along diagonal direction（DD）and transverse direction（TD）.In addition,the rotation phenomenon along DD is observed in numerical and experimental results.With the increment of strain rate,the elastic modulus and yield stress of separator increase while the failure strain decreases.The strain neutrality along MD and strain hardening along DD and TD are displayed in cold drawing phase.Based on large deformation behavior,the constitutive relationship and microbuckling model of separators with strain rate effect are established by introducing strain rate strengthening coefficient and compliance constant.The effects of strain rate strengthening coefficient and compliance constant on the mechanical properties of LIB separator are discussed.（4）The polydopamine（PDA）nanoparticles are uniformly deposited on LIB separator by immersing method to improve the electrochemical performance and enhance the mechanical strength of polypropylene（PP）and polypropylene/polyethylene/polypropylene（PP/PE/PP）separators.Thermogravimetric and differential scanning calorimeter results show that thermal stabilities of PDA modified separators are improved.The dopamine modified particles can reduce the crystallinity and increase the proportion of amorphous crystalline.So more electrolyte can be absorbed and ionic conductivity of separator is improved.The ionic conductivities of PDA modified PP(κ=0.14 m S cm^-1)and PP/PE/PP(κ=2.69 m S cm^-1)separators are 1.5 times and 6.1 times higher than that of pristine PP and PP/PE/PP separators.In addition,mechanical properties of PP（yield stresses:17.48%～100.11%;failure stresses:13.45%～82.71%;failure strains:4.08%～303.13%）and PP/PE/PP（yield stresses:11.77%～296.00%;failure stresses:12.50%～248.30%;failure strains:16.53%～32.56%）separators are increased because PDA modified method increases the strength of lamellaes and fibrils.（5）The sinusoidal pattern is introduced on the LIB shell to form the corrugated tube and then study the resistance exterior force property and energy absorption capability of the single LIB shell and the corrugauted tubes connected to the battery module under quasi-static and dynamic loading.The plastic progressive buckling theoretical models are established of corrugated tube by considering the eccentric factor,amplitude factor and strain hardening effect.The mechanism of plastic progressive buckling is also discussed.The analytical expression is deduced under quasi-static（average crushing force and normalization crushing force）and dynamic loading（average impact force and normalization impact force）,and the upper bound solution,lower bound solution and exact solution are obtained.The theoretical results are compared with experimental and numerical simulation results to verify the accuracy of present model.In addition,the parametric study shows that corrugated tube can slow down the deformation process.The average crushing load and energy absorption performance of LIB corrugated shell can be predicted by present theoretical results.