Mechanical behavior of shell casing and separator of lithium-ion battery

Abstract

With the rapid growth of electric vehicle (EV) market, the mechanical safety of lithium-ion batteries has become a critical concern for car and battery manufacturers as well as the public. Lithium-ion battery cells consist of cathode, anode, separator and shell casing or aluminum plastic cover. Among them, the shell casing provides substantial strength and fracture resistance under mechanical loading, and the failure of the separator determines onset of internal short circuit of the cell. In the first part of this thesis, a plasticity and fracture model of the battery shell casing by taking the anisotropic plasticity and stress-state dependent fracture into account was developed. The shell casing model is calibrated and validated at both specimen and component levels. This shell casing model, together with homogenized jellyroll model could predict mechanical behavior of single cylindrical 18650 cell well and could serve for battery pack crash simulation purposes. Another part of this thesis is mechanical test, characterization and modeling of battery separators since the mechanical properties of separators are crucial to internal shorts of lithium-ion batteries. Mechanical properties of commercially available four typical separators that including polypropylene (PP), trilayer (PP-PE-PP), ceramic-coated and nonwoven separators were compared, such as in-plane tensile strength, out-of-plane compression strength and puncture strength. Two distinct failure modes of dry-processed separators under biaxial loading were observed in the tests and used to explain the differences in short circuit characteristics of same cells. A conservative defection-based failure criterion for predicting of onset of short from experimental data was proposed. Numerical model of separator was developed and it succeeded in predicting the response of PP separator under biaxial loading. Owing to the micro porous semi-crystalline nature of widely used PP separator, interrupted tests of PP separator under different in-plane tension including machine direction, transverse direction and diagonal direction were conducted in order to reveal deformation mechanism at the micrometer level. Through scanning electric microscopy (SEM) observation and X-ray diffraction of deformed regions from interrupted test specimens, deformation sequences of micro fibrils and lamellae blocks of PP separator are reported. Lastly, significant mechanical degradation of separator due to charge-discharge cycling was described.