Direction-Dependent Internal Damage Characteristic And Early Diagnosis Of Prismatic Li-Ion Batteries Under Mechanical Abuse

Recent technical advances of lithium-ion batteries(LIBs)and goals of hitting carbon emission peak,achieving carbon neutrality are indispensable in prosperous development of electric vehicles(EVs).Demand for LIBs in electric vehicles(EVs)has exploded and the electrification of transportation has promised to decouple market growth from environmental degradation.However,application of LIB still suffers some technical bottlenecks,among which a very important issue is how to detect the mechanical deformation probably inducing short circuit and even subsequent fire or explosion.This thesis investigates mechanical-electrical-thermal responses of large-format prismatic Li-Ion battery under multi-direction mechanical abuse and establishes a relationship between battery impedance change and internal structure damage.Results disclosed in the thesis can provide support for modeling and protection of batteries under external mechanical loadings and shed light on diagnosis of the internal damage of impacted batteries at the very early stage.An experimental study on mechanical-electrical-thermal responses of large-format prismatic LIB cells under quasi-static and dynamic mechanical indentation is conducted.The prismatic cell behaves differently between out-of-plane and transverse indentation in terms of mechanical-electrical-thermal characteristics.To gain a comprehensive understanding towards the mechanically-induced ISC and the exothermic reactions,internal configurations of the prismatic cells are further examined via X-ray computed tomography.Critical internal ruptures at ISC onset are probed and relatively sliding of electrode layers causes short contact.The influence of loading direction on SOC-and ratedependence of multi-field responses is discussed,and hazardous ISC onset and elevated surface temperature are further evaluated.FEM models of battery under out-of-plane and in-plane loading are established to investigate the effects of outing shell casing.The prediction of force curve and failure behavior agrees well with the experiments,and the shell casing is found to have a significant effect on the mechanical response of battery cells.In transverse loading cases,shell casing significantly enhances the structural stiffness,and the distance between the jellyroll and the shell casing dominates the overall mechanical responses.In out-of-plane cases,the thickness of the shell casing has an obvious effect on the failure behavior of the battery.With the increase of the thickness of the shell casing,the intrusion depth corresponding to the failure of the battery becomes larger.In addition to the case in which batteries undergo obvious deformation and even failure under severe mechanical loadings,the monitoring and diagnosis of the battery with minor damage at the early state has also received widespread attention.An approach to diagnose early internal mechanical damage of large-format LIB based on electrochemical impedance spectroscopy(EIS)is proposed.An abrupt increase of impedance is observed in each loading direction at an indentation depth apparently smaller than the depth corresponding to occurrence of internal short circuit(ISC).Post-mortem inspection indicates the intra-layer fracture of the active coating caused by micro-buckling of electrodes is the recipe for the increase of impedance.The impedance change is substantially irreversible beyond the critical indentation depth and the level of the irreversibly is dependent on the extent of local deformation.This work reveals the underlying risk in the scenarios of bottom and side indentations that a relatively small indentation could cause dramatic change in impedance,and throws light on the early diagnosis of internal mechanical damage using EIS measurement.