Operando ultrasonic monitoring of lithium-ion battery temperature and behaviour at different cycling rates and under drive cycle conditions

Effective diagnostic techniques for Li-ion batteries are vital to ensure that they operate in the required voltage and temperature window to prevent premature degradation and failure. Ultrasonic analysis has been gaining significant attention as a low cost, fast, non-destructive, operando technique for assessing the state-of-charge and state-of-health of Li-ion batteries. Thus far, the majority of studies have focused on a single C-rate at relatively low charge and discharge currents, and as such the relationship between the changing acoustic signal and C-rate is not well understood. In this work, the effect of cell temperature on the acoustic signal is studied and shown to have a strong correlation with the signal's time-of-flight. This correlation allows for the cell temperature to be inferred using ultrasound and to compensate for these effects to accurately predict the state-of-charge regardless of the C-rate at which the cell is being cycled. Ultrasonic state-of-charge monitoring of a cell during a drive cycle illustrates the suitability of this technique to be applied in real-world situations, an important step in the implementation of this technique in battery management systems with the potential to improve pack safety, performance, and efficiency

Identifying Defects in Li-Ion Cells Using Ultrasound Acoustic Measurements

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

Evaluation of combustion properties of vent gases from Li-ion batteries

Fire incidents involving Li-ion batteries is an increasing concern as the use of battery electric vehicles is increasing. Abuse conditions such as heating can result in ejection of flammable and toxic gases, presenting a health risk and risk of explosion or fire. The purpose of the present work is to increase the understanding of combustion of gas mixtures vented from Li-ion batteries. The investigation uses a new merged kinetic mechanism including hydrocarbons, hydrogen, carbon oxides, carbonates and fluorinated compounds. Seven typical Li-ion vent gas mixtures were selected based on published studies, and ignition and laminar flames were simulated. Modeling reveal a large variation in laminar burning velocity, flame temperature and heat release. Determining factors for laminar flames are the relative content of the carbonates and hydrogen gas, and the inert carbon dioxide. Gases from highly charged battery cells have the shortest ignition time at high temperatures and the fastest laminar burning velocity. The results can be used as input in computational fluid dynamics or safety engineering modeling. In addition, the versatile kinetic model can be used for fundamental studies of the combustion process and for generation of combustion characteristics such as laminar burning velocities for other vent gas mixtures

Fire Tests on E-vehicle Battery Cells and Packs

<div><p><b>Objective:</b> 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.</p><p><b>Method:</b> 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.</p><p>Heat release rate (HRR) data and concentrations of toxic gases were acquired by oxygen consumption calorimetry and Fourier transform infrared spectroscopy (FTIR), respectively.</p><p><b>Results:</b> 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.</p><p><b>Conclusions:</b> 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.</p></div

Thermal fault detection by changes in electrical behaviour in lithium-ion cells

With this paper a method to detect faults of lithium-ion cells during operation is first presented and later validated by experiment. Since every cell fault will increase the cell temperature towards its process until thermal runaway the method uses the temperature-dependent change of the cell impedance as fault feature. Using a 46 Ah pouch cell the model was parameterised by electrochemical impedance spectroscopy and then validated during dynamic load. For this purpose the Worldwide harmonised Light vehicles Test Procedure (WLTP) was chosen. The presence of a fault was simulated by heating the cell once uniformly and once locally and the progression of the chosen fault feature analysed. For both test cases the method proposed was able to detect the present heat source before the thermal runaway was triggered and venting or voltage drop were observed

Future Material Developments for Electric Vehicle Battery Cells Answering Growing Demands from an End-User Perspective

Nowadays, batteries for electric vehicles are expected to have a high energy density, allow fast charging and maintain long cycle life, while providing affordable traction, and complying with stringent safety and environmental standards. Extensive research on novel materials at cell level is hence needed for the continuous improvement of the batteries coupled towards achieving these requirements. This article firstly delves into future developments in electric vehicles from a technology perspective, and the perspective of changing end-user demands. After these end-user needs are defined, their translation into future battery requirements is described. A detailed review of expected material developments follows, to address these dynamic and changing needs. Developments on anodes, cathodes, electrolyte and cell level will be discussed. Finally, a special section will discuss the safety aspects with these increasing end-user demands and how to overcome these issues