No abstract

Identification of the state-of-health (SoH) of Li-ion cells is a vital tool to protect operating battery packs against accelerated degradation and failure. This is becoming increasingly important as the energy and power densities demanded by batteries and the economic costs of packs increase. Here, ultrasonic time-of-flight analysis is performed to demonstrate the technique as a tool for the identification of a range of defects and SoH in Li-ion cells. Analysis of large, purpose-built defects across multiple length scales is performed in pouch cells. The technique is then demonstrated to detect a microscale defect in a commercial cell, which is validated by examining the acoustic transmission signal through the cell. The location and scale of the defects are confirmed using X-ray computed tomography, which also provides information pertaining to the layered structure of the cells. The demonstration of this technique as a methodology for obtaining direct, non-destructive, depth-resolved measurements of the condition of electrode layers highlights the potential application of acoustic methods in real-time diagnostics for SoH monitoring and manufacturing processes.

One way to support the development of new safety practices in testing and field failure situations of electric vehicles and their lithium-ion (Li-ion) traction batteries is to conduct studies simulating plausible incident scenarios. This paper focuses on risks and hazards associated with venting of gaseous species formed by thermal decomposition reactions of the electrolyte and electrode materials during thermal runaway of the cell. A test set-up for qualitative and quantitative measurements of both major and minor gas species in the vented emissions from Li-ion batteries is described. The objective of the study is to measure gas emissions in the absence of flames, since gassing can occur without subsequent fire. Test results regarding gas emission rates, total gas emission volumes, and amounts of hydrogen fluoride (HF) and CO2 formed in inert atmosphere when heating lithium iron phosphate (LFP) and lithium nickel-manganese-cobalt (NMC) dioxide/lithium manganese oxide (LMO) spinel cell stacks are presented and discussed. Important test findings include the large difference in total gas emissions from NMC/LMO cells compared to LFP, 780 L kg−1 battery cells, and 42 L kg−1 battery cells, respectively. However, there was no significant difference in the total amount of HF formed for both cell types, suggesting that LFP releases higher concentrations of HF than NMC/LMO cells.

Objective: The purpose of this study was to investigate the effects of abuse conditions, including realistic crash scenarios, on Li ion battery systems in E-vehicles in order to develop safe practices and priorities when responding to accidents involving E-vehicles.Method: External fire tests using a single burning item equipment were performed on commercial Li ion battery cells and battery packs for electric vehicle (E-vehicle) application. The 2 most common battery cell technologies were tested: Lithium iron phosphate (LFP) and mixed transition metal oxide (lithium nickel manganese cobalt oxide, NMC) cathodes against graphite anodes, respectively. The cell types investigated were "pouch" cells, with similar physical dimensions, but the NMC cells have double the electric capacity of the LFP cells due to the higher energy density of the NMC chemistry, 7 and 14 Ah, respectively.Heat release rate (HRR) data and concentrations of toxic gases were acquired by oxygen consumption calorimetry and Fourier transform infrared spectroscopy (FTIR), respectively.Results: The test results indicate that the state of charge (SOC) affects the HRR as well as the amount of toxic hydrogen fluoride (HF) gas formed during combustion. A larger number of cells increases the amount of HF formed per cell. There are significant differences in response to the fire exposure between the NMC and LFP cells in this study. The LFP cells generate a lot more HF per cell, but the overall reactivity of the NMC cells is higher. However, the total energy released by both batteries during combustion was independent of SOC, which indicates that the electric energy content of the test object contributes to the activation energy of the thermal and heat release process, whereas the chemical energy stored in the materials is the main source of thermal energy in the batteries. Conclusions:The results imply that it is difficult to draw conclusions about higher order system behavior with respect to HF emissions based on data from tests on single cells or small assemblies of cells. This applies to energy release rates as well. The present data show that mass and shielding effects between cells in multicell assemblies affect the propagation of a thermal event.