Elham Sahraei

Modelling of cracks developed in lithium-ion cells under mechanical loading

This research reports on an experimental and numerical study of material failure in the electrode assemblies (i.e. “jelly roll” and/or “electrode stack”) of lithium-ion batteries after local mechanical loading. Deformed cylindrical and pouch cells (i.e. lithium-ion polymer cells) were subjected to X-ray computed tomography (CT scanning) to detect location, size, and orientation of cracks that developed in the electrode assemblies at onset of short-circuit. An experimental program was completed to acquire properties of electrode–separator micro components of electrode assemblies in tension. This data was used for calibration of an anisotropic material model. Finite element models were developed for both cell types and a maximum strain criteria was used for element failure and deletion at short circuit. The models developed here predict the location of cracks in both pouch and cylindrical cells. The finite element models corroborated the CT scan regarding location and orientation of cracks formed in the electrode assemblies. In both pouch and cylindrical cells, cracks were found to initiate perpendicular to the transverse direction of the separator.

Li-ion Battery Separators, Mechanical Integrity and Failure Mechanisms Leading to Soft and Hard Internal Shorts.

Separator integrity is an important factor in preventing internal short circuit in lithium-ion batteries. Local penetration tests (nail or conical punch) often produce presumably sporadic results, where in exactly similar cell and test set-ups one cell goes to thermal runaway while the other shows minimal reactions. We conducted an experimental study of the separators under mechanical loading, and discovered two distinct deformation and failure mechanisms, which could explain the difference in short circuit characteristics of otherwise similar tests. Additionally, by investigation of failure modes, we provided a hypothesis about the process of formation of local “soft short circuits” in cells with undetectable failure. Finally, we proposed a criterion for predicting onset of soft short from experimental data.

Degradation of battery separators under charge–discharge cycles

Researchers have reported on the electrochemical aging of lithium-ion batteries. The mechanisms of battery capacity loss, such as consumption of electrolytes and fading of electrodes, commonly seen as fracture of coatings, have been studied intensively. The widely used polymeric separators sandwiched between cathode and anode, which do not directly contribute to the electrochemical properties of the cell, are usually taken as chemically, thermally and structurally stable materials. In this paper, the degradation of a dry processed trilayer separator due to charge–discharge cycles is investigated. It has been found that the separators that underwent higher cycles failed at lower lateral punch force and smaller deformation. Live cell tests also indicate that the deformation and force intensity at the onset of short circuit decreased for a cell after 1200 cycles compared to those for a non-cycled cell, when under lateral indentation. Different characterization methods were used to understand this charge–discharge induced mechanical aging. SEM through-thickness views of the separators show no significant pore size change, but reaction products accumulated in pores of the separator middle layer. FTIR (Fourier Transform Infrared) examination of the surfaces of those separators shows there was no apparent chemical bond change on the surface of the separator during charging and discharging process.

High strength steels, stiffness of vehicle front-end structure, and risk of injury to rear seat occupants

Previous research has shown that rear seat occupant protection has decreased over model years, and front-end stiffness is a possible factor causing this trend. In this research, the effects of a change in stiffness on protection of rear seat occupants in frontal crashes were investigated. The stiffness was adjusted by using higher strength steels (DP and TRIP), or thicker metal sheets. Finite element simulations were performed, using an LS Dyna vehicle model coupled with a MADYMO dummy. Simulation results showed that an increase in stiffness, to the extent it happened in recent model years, can increase the risk of AIS3+ head injuries from 4.8% in the original model (with a stiffness of 1,000 N/mm) to 24.2% in a modified model (with a stiffness of 2,356 N/mm). The simulations also showed an increased risk of chest injury from 9.1% in the original model to 11.8% in the modified model. Distribution of injuries from real world accident data confirms the findings of the simulations.

Adhesion strength of the cathode in lithium-ion batteries under combined tension/shear loadings

To understand the failure mechanism and establish reliable deformation tolerances for lithium-ion batteries under mechanical loading, accurate testing and modeling of individual components are indispensable. This paper is focused on one of the most common failure scenarios, which is the de-bonding between the coating material and the current collector. A new specimen is carefully designed to measure the failure strength of the coating-foil interface. The electrode is bonded to two acrylic substrates using liquid formula glue for one side and gel formula glue for the other. Compared with conventional peeling tests using double-sided tape, the major advantage of this new specimen is that it realizes conducting shear tests. Using this special specimen, the failure strength of the coating-foil interface is obtained under combined tension/shear loadings. The new method is less susceptible to the testing conditions such as loading rate. For the cathode studied in this paper, the shear strength of the coating-foil interface turns out to be almost twice its tensile strength, which emphasizes the necessity of carrying out combined tension/shear loading tests. Moreover, a combined adhesion and cohesion failure mode is observed at the failure interface, where with larger shear component, the adhesion failure becomes dominant.

Effects of Vehicle Front-End Stiffness on Rear Seat Dummies in NCAP and FMVSS208 Tests

Objective: This study is devoted to quantifying changes in mass and stiffness of vehicles tested by the National Highway Traffic Safety Administration (NHTSA) over the past 3 decades (model years 1982 to 2010) and understanding the effect of those changes on protection of rear seat occupants. Methods: A total of 1179 tests were used, and the changes in their mass and stiffness versus their model year was quantified. Additionally, data from 439 dummies tested in rear seats of NHTSAs full frontal crashes were analyzed. Dummies were divided into 3 groups based on their reference injury criteria. Multiple regressions were performed with speed, stiffness, and mass as predicting variables for head, neck, and chest injury criteria. Results: A significant increase in mass and stiffness over model year of vehicles was observed, for passenger cars as well as large platform vehicles. The result showed a significant correlation (P-value < .05) between the increase in stiffness of the vehicles and increase in head and chest injury criteria for all dummy sizes. Conclusions: These results explain that stiffness is a significant contributor to previously reported decreases in protection of rear seat occupants over model years of vehicles. Supplemental materials are available for this article. Go to the publishers online edition of Traffic Injury Prevention to view the supplemental file.