Large-format Lithium-ion Cells Research Articles

Quantifying the Impact of Overdischarge on Large Format Lithium Ion Cell Safety

Lithium-ion battery safety has always been a concern; however, safety is becoming increasingly important due to the proliferation of high capacity, high power cells and modules utilized in electric vehicle, grid, aerospace, and military applications. While safety measures such as battery management algorithms, thermal management, fuses, and sensors can reduce the risk of catastrophic failure, not all of the risk can be managed while the battery is active. Self-discharge of lithium ion cells below a zero state of charge voltage, overdischarge, is one such case. Overdischarge has generally been considered more of a performance issue than a safety concern unless coupled with recharge. However, with battery-powered devices potentially left for long periods of time in storage, DoD interest in extending the shelf life of devices in the field, and proliferation of battery packs composed of large numbers of cells, there is an increased interest in determining limits of low voltage recharge and developing a methodology for determining safety limits. Furthermore, overdischarge is a class of safety risk that is inherent to the cell and cannot be prevented with passive protection devices. This talk will cover safety concerns associated with overdischarge of large format lithium-ion batteries including gas generation, copper dissolution at low voltage and precipitation upon recharge, and health of the Solid Electrolyte Interphase layer. While the gas generation behavior of lithium-ion cells has been studied extensively[1], a methodology for assessing the gas generation prior to full thermal runaway has not been developed. It has been demonstrated that oxygen can be released from the electrode structure due to a destabilization of the crystal lattice.[2] Additionally, electrolyte decomposition, typically during charging, contributes to gas buildup within the cell that can eventually cause rupturing of the casing.[3] We discuss methods for assessing the amount of gas generated at intermediate stages of cell abuse. Finally, we present recent data on the safety implications of copper dissolution in overdischarged large format cells. [1] E. Roth, C. Crafts, D. Doughty, and J. McBreen, “Advanced technology development program for lithium-ion batteries: thermal abuse performance of 18650 li-ion cells,” Sandia National Laboratory Report, SAND2004-0584, 2004 [2] W. Kong, H. Li, X. Huang, and L. Chen, “Gas evolution behaviors for several cathode materials in lithium-ion batteries,” Journal of Power Sources Vol. 142, 2005, pp. 285-291. [3] K. Kumai, H. Miyashiro, Y. Kobayashi, K. Takei, and R. Ishikawa, “Gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cell,” Journal of Power Sources Vol. 81-82, 1999, pp. 715-719.

Novel thermal management system design methodology for power lithium-ion battery

Battery packs conformed by large format lithium-ion cells are increasingly being adopted in hybrid and pure electric vehicles in order to use the energy more efficiently and for a better environmental performance. Safety and cycle life are two of the main concerns regarding this technology, which are closely related to the cell's operating behavior and temperature asymmetries in the system. Therefore, the temperature of the cells in battery packs needs to be controlled by thermal management systems (TMSs). In the present paper an improved design methodology for developing TMSs is proposed. This methodology involves the development of different mathematical models for heat generation, transmission, and dissipation and their coupling and integration in the battery pack product design methodology in order to improve the overall safety and performance. The methodology is validated by comparing simulation results with laboratory measurements on a single module of the battery pack designed at IK4-IKERLAN for a traction application. The maximum difference between model predictions and experimental temperature data is 2 °C. The models developed have shown potential for use in battery thermal management studies for EV/HEV applications since they allow for scalability with accuracy and reasonable simulation time.

Thermal characterizations of a large-format lithium ion cell focused on high current discharges

The thermal behavior of a large-format lithium ion cell has been investigated during measurements on cell and battery level. High current discharges up to 300 A are the main topic of this study. This paper demonstrates that the temperature response to high current loads provides the possibility to investigate internal cell parameters and their inhomogeneity. In order to identify thermal response caused by internal cell processes, the heat input due to contact resistances has been minimized. The differences between the thermal footprint of a cell during cell and battery measurements are being addressed. The study presented here focuses on the investigation of thermal hot and cold spots as well as temperature gradients in a 50 Ah pouch cell. Furthermore, it is demonstrated that the difference between charge and discharge can have significant influence on the thermal behavior of lithium ion cells. Moreover, the miscellaneous thermal characteristics of differently aged lithium ion cells highlight the possibility of an ex-situ non-destructive post-mortem-analysis, providing the possibility of a qualitative and quantitative characterization of inhomogeneous cell-aging. These investigations also generate excellent data for the validation and parameterization of electro-thermal cell models, predicting the distribution of temperature, current, potential, SOC and SOH inside large-format cells.

3D Electro-Thermal Modeling of Large Format Lithium Ion Cells

Thermal management of lithium ion batteries is a key-task in order to meet performance, lifetime and safety requirements for automotive application. Especially for large format lithium ion cells, which are commonly used in hybrid and full electric vehicles, a proper thermal management can extend significant lifetime and range. Therefore, thermal modeling is an integral part of battery development for traction applications in order to shorten development time and to reduce cost of development.In this contribution a 3D electro-thermal modeling approach is shown in order to predict three dimensional behavior of large format lithium ion cells up to high discharge currents. The model is able to predict reliable the distribution and inhomogeneities of inner cell parameters like temperature, power loss, state of charges, current and potential.The development of a 3D electrical model using an impedance-based cell model is presented showing its direct linking with a 3D thermal FEM model. Besides for this model also a convergence test in sub-cell resolution has to be done. Each sub-cell is treated as elementary cell linked with electrical resistances due to the current collectors. Moreover a comprehensive validation of this model is shown with data from a thermal characterization of the large lithium ion cells at different temperatures and up to 300 A discharge currents. These measurements where done with a thermal camera on cell level and with a fiber optical measurement system on battery pack level. The validation is done regarding absolute cell temperatures. In addition with this kind of models one is also able to predict temperature gradients during high current discharge in total agreement with measurement. One can also predict gradients over cell geometry of heat dissipation, state of charge, current and potential, which have crucial influence on performance and lifetime. For state of charge distribution an indirect validation is presented due to its impact on time development of temperature gradients during constant current discharges.The model is used in order to study the effect of geometrical cell design on internal cell parameters. This study is dedicated to inhomogeneities in distribution of temperature, heat dissipation, state of charge, current and voltage. Furthermore the study points out the effect of self-intensifying and damping of these gradients depending on the state of operation.It is also shown the prediction of cell gradients and local hot and cold spots during different operation state in electrical vehicles, addressing the self-intensifying of external thermal gradients due to the parallel circuit character of elementary cells inside a lithium ion cell. Especially the external gradients due to different cooling strategies are investigated.For an effective operation strategy it is essential to know during all states of operation the cold and hot spots in addition to the gradient over the cells. Due to the fact that the position of min and max temperature differs because of operation conditions and state of health it is not trivial to measure these values during operation. Nevertheless one should also know the internal cell parameters like current density to address lifetime issues like lithium-plating, which strongly depends on temperature and current density. Therefore there is also the need to run this kind of models in real time on battery management systems. Here also an approach of model order reduction is shown on system level simulation in order to reduce significant computing time.

3D Electro-Thermal Model Approach for the Prediction of Internal State Values in Large-Format Lithium Ion Cells and Its Validation

This paper presents a 3D electro-thermal model approach for large-format lithium ion cells. The model development is focused on the prediction of 3D behavior of large-format lithium ion cells up to high discharge currents. The distribution and inhomogeneity of internal state values in the cell like temperature, current, power loss, SOC as well as voltage can be predicted reliably with this kind of models. The demand for calculating these state values on battery management systems can be met with a model order reduction introduced in this paper. Moreover, the interdependency of the temperature inhomogeneities and the inhomogeneities of inner state values in the cell can be studied. These dependencies have major influence on lifetime and performance of large-format lithium ion cells during common traction applications. The paper presents the configuration of a 3D equivalent circuit model considering full cell geometry and internal electrical resistances. In order to consider 3D thermal as well as 3D electrical effects, the direct coupling of the 3D electrical model with a 3D thermal FEM model is introduced. Furthermore, in this paper a convergence study on elementary cell resolution is performed. Additionally, the model is comprehensively validated with a thermal characterization over a wide range of temperature and current up to 300 A.

In-depth safety-focused analysis of solvents used in electrolytes for large scale lithium ion batteries

To better rule out the complex fire risk related to large format lithium ion cells, a detailed and systematic evaluation, both at component and cell levels, could be an invaluable milestone. Therefore, combustion analysis was conducted for major single organic solvents and their mixtures used in lithium ion battery technology, both in oxygen rich and lean environments using a Tewarson calorimeter. Well controlled test conditions have enabled the determination of key parameters governing the fire induced hazards such as flash point, ease of ignition, heat release rate, effective heat of combustion, specific mass loss rate, as well as the assessment of fire induced toxicity. Moreover, a rule of thumb for the screening of new solvents including the safety perspective such as the Boie correlation and N-factor were introduced for predicting the heat of combustion and combustion kinetics, respectively, prior to conducting any experimental work. Fire induced toxicity of single solvents and their mixtures was also briefly examined by performing toxic gas measurements.

Modeling of Non-uniform Degradation in Large Format Lithium Ion Cells

not Available.

Evaluating LiBOB/Lactone Electrolytes in Large-Format Lithium-Ion Cells Based on Nickelate and Iron Phosphate

The electrolyte compositions based on lithium bis(oxalato)borate (LiBOB) and lactones were tested in large-format lithium-ion cells designed for hybrid electric vehicle applications using either or cathode chemistry. It was found that the impurity in LiBOB has a significant impact on the cell cycle life, especially at elevated temperatures, where the anodic decomposition of the oxalate impurities and carboxylate remnants accelerates and results in pressure to vent the cell. Recrystallization can essentially remove those detrimental impurities and eliminates the internal gassing. As compared to its counterparts, purified LiBOB imparts the cells with cycling stability at high temperatures and enhanced safety under abuses of nail penetration and overcharging; however, the more protective interfaces formed by BOB also render lower power densities and poorer performances at subzero temperatures. Considering the trade-off, LiBOB electrolytes may find an application niche with high-temperature and low-to-medium power requirements.