Large-format Lithium-ion Cells Research Articles

Characterization of commercial thermal barrier materials to prevent thermal runaway propagation in large format lithium-ion cells

In recent years, electric vehicles (EVs) have been rapidly introduced as an environmental measure. It is said, however, one of the challenges to accelerate their spread rests in how to increase their driving range. Recent safety regulations for EVs have been stringent in the wake of the potential risk of thermal runaway of battery packs. The battery pack is expected to withstand intense ignition during the event of thermal runaway while measures to suppress cell-to-cell thermal runaway propagation are also being considered as a mandatory safety standard. Thermal barrier materials are viable system-level solutions to mitigate cell-to-cell thermal runaway propagation via electrical, thermal, and fire insulation to meet regulatory safety needs. Among the various researched thermal barriers, only a few materials are commercially viable. Hence, this paper explores different commercially available thermal insulating materials that can be accommodated in battery packs. The materials selected combine key characteristics such as flame resistance, heat insulation, and thickness. Subsequently, we experimentally investigate the thermal and fire-risk properties of the selected thermal barrier materials via flame torch tests and compare their performances. Among the tested materials, a potential material has been used to further validate its performance in mitigating thermal runaway propagation in battery module/pack level overheating tests. Additionally, the comprehensive results of single-configuration thermal barrier characterization tests aided in combining different thermal barrier materials, which displayed better performance than single-configuration materials.

Investigating the Thermal Runaway Behaviour of Fresh and Aged Large Prismatic Lithium-Ion Cells in Overtemperature Experiments

When using lithium-ion batteries (LiBs) with nickel-rich cathodes, safety issues such as thermal runaway (TR) propagation must be considered. To design safe LiBs, effective countermeasures against TR propagation must be developed. For this purpose, knowledge about the TR behaviour, especially the TR onset temperature, of fresh and aged lithium-ion cells is required. Therefore, the aim of this work is to investigate the TR behaviour of several fresh and aged lithium-ion cells with different cathode chemistries in overtemperature tests using an oven test setup to determine changes in the TR behaviour due to cyclisation. For the investigation of the TR behaviour of large format lithium-ion cells, the oven test setup turns out to be an effective alternative to the accelerating rate calorimetry test. Analysing the results shows that the initial TR temperature after cyclisation for one cell chemistry is significantly decreased due to the failure of one cell component. In addition, from a series of tests and the measured TR onset temperatures it can be deduced that an underlying probability distribution should be taken into account when designing safe LiBs.

Evaluation of Large-Format Lithium-Ion Cell Thermal Runaway Response Triggered by Nail Penetration using Novel Fractional Thermal Runaway Calorimetry and Gas Collection Methodology

To simultaneously optimize the battery design, reduce risk, and maintain safety margin, it is important to design from the ground up based on test determined cell-specific thermal runaway behavior as a function of heat output and analysis of the expelled gases. These data will inform the analytical models used for design optimization. Here we analyze the thermal runaway behavior of the 134 A-h GS Yuasa Li-ion cell (LSE134) using a novel large format fractional thermal runaway calorimeter and gas collection methodology. Results indicate an average total thermal runaway energy yield of 2.86 MJ, or 1.6 times the stored electrochemical energy; this follows an assertion commonly found in literature that energy yield scales linearly with capacity. The average fractional energy distribution was 2% through the cell body, 53% through the electrode winding, and 45% through the ejecta material and gases. Lot-to-lot variability in heat output was also identified. Additionally, it was found that an average of 416.6 SL of gas was generated which is approximately 3.1 l A-h−1. The exhaust gas was determined to be a mixture of carbon dioxide, methane, ethane, oxygen, hydrogen, and other short chain hydrocarbons. Carbon dioxide was the largest component by volume with a range of 41% to 52% followed by hydrogen which ranged from 28% to 41%. Larger cells appear to result in strong ejecta flow driven events with higher fractions of the total energy delivered via the flow as compared to smaller format Li-ion cells (e.g. 18650 and 21700).

State of health estimation of cycle aged large format lithium-ion cells based on partial charging

Incremental capacity analysis (ICA) is a widely used method for assessing Li-ion battery state of health (SOH). The ICA method is typically applied to low C-rate charging or discharging, which is seldom feasible in online SOH estimation in real applications. In this paper the applicability of the ICA method to higher C-rates is investigated with C/3, 1C and 2C charging data from cycle aged large format commercial LiFePO4/graphite cells, LiNiMnCoO2/graphite cells, and cells with Li2TiO3 anode. Alongside the ICA method, a less studied integrated voltage (IV) method is investigated with the same data set. The results show that both the ICA method and the IV method can be used for battery SOH estimation utilizing partial charging data. The results for the ICA method indicate an accuracy of 0.9–2.1% (RMSE) with C/3 charging for all studied cells, and corresponding accuracy also for some of the cell types studied with 1C and 2C charging currents. The accuracy of the IV method does not suffer from increasing charging current as heavily as the accuracy of the ICA method, and the SOH estimation error remains below 2.0% (RMSE) with all studied cell types up to 2C charging.

Temperature and Lithium Concentration Gradient Caused Inhomogeneous Plating in Large-format Lithium-ion Cells

Although lithium plating is one of the most-investigated degradation mechanisms occurring in lithium-ion batteries, many degradation studies still view inhomogeneous lithium plating as an artifact. It is often caused by mechanical stress, gas formation and encapsulation, or temperature and current gradients on account of the cell size. In this work, large-format lithium-ion cells are cycled at conditions harmless on the material level, but cause heavy lithium plating in the center of the commercial cell. Failure analysis requires visual inspection and further material evaluation to identify the reason for malfunction. A combination of temperature gradient and current density gradient evoke a lithium concentration gradient, which equalizes by a lateral lithium-ion flow. The inhomogeneous distribution of lithium again distorts the operating potential window, resulting in local violation of the critical lithium plating condition. Thus, our study shows that inhomogeneity in a large-format lithium-ion cell has to be taken into account when designing the operating window. In addition, accelerated testing of large-format lithium-ion cells raises degradation mechanisms, which do not appear during regular usage.

Passive thermal management systems employing hydrogel for the large-format lithium-ion cell: A systematic study

The large-format lithium-ion cells are growing in popularity as high-energy-density power sources in mobile applications, which calls for efficient and compact passive thermal management systems. Hydrogel, capable of absorbing and holding extremely large amounts of water, emerges as a new type of heat sink material. Herein, we presented a systematic study on the passive thermal management systems employing hydrogel for the 20 Ah large-format lithium-ion cell with emphasis on regulating the temperature homogeneity and temperature spike. Four types of configurations were designed and experimentally explored to achieve optimal cooling performance. It showed that introducing heat conducting plate between hydrogel and battery surface can effectively improve the temperature homogeneity, and the highest temperature and largest temperature difference were controlled to be only 40.5 °C and 2.5 °C even under a high discharge rate of 4C, which were about 2.5 °C and 2.3 °C lower than the pure hydrogel system. In addition, heat dissipation fins and copper foam were also employed to further accelerate the heat transfer process within the hydrogel. It showed that the fin-hydrogel system held the best performance and delivered a maximum surface temperature and a largest temperature difference of 32.6 °C and 1.4 °C over eight 3C/1C discharge/charge cycles.

Efficient thermal management of the large-format pouch lithium-ion cell via the boiling-cooling system operated with intermittent flow

To manage the undesirable temperature spike and significant temperature gradient developed on the large-format lithium-ion cell, we propose and experimentally demonstrate a boiling-cooling thermal management system using NOVEC 7000 as coolant for a large-format 20-Ah LiFePO4 lithium-ion cell. It shows that the boiling-cooling system delivers excellent capabilities in reducing maximum temperature and improving temperature uniformity under the static mode even for the 4C discharge process. To enable the boiling-cooling system a stable cooling performance during cyclic operations, the flow mode is explored. The maximum temperature is successfully controlled over cycles but a large temperature difference between regions near cell bottom and tabs is observed, especially at high flow rates. To resolve issues of the static and flow modes, the intermittent flow mode is finally conceived for the boiling-cooling system, with which the temperature spike and maximum temperature difference are controlled to be less than 36 °C and 2 °C at 2C operation. Such an impressive cooling performance, combined with the substantially low pumping work, suggests that efficient thermal management of large-format lithium-ion cells can be achieved by the boiling cooling system operated with an intermittent flow mode, which takes full advantage of the large latent heat absorbed and the high heat transfer coefficient during the boiling process.

Insight into the inhomogeneous capacity distribution characteristic of LiFePO4 cathode in large-format lithium ion cell

The inhomogeneous capacity degradation distribution of large-format LiFePO4-based lithium ion cell after 2500 charge-discharge cycles is analyzed via the selected regions of the cathodes using half-cell charge-discharge test, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy analysis. The results showed that the initial specific charge capacities demonstrate an obvious position-dependent characteristic, indicating inhomogeneous reactions occurring on the electrode. The low initial charge capacities near the tab location can be recovered after discharging, indicating the capacity degradation is from the active lithium loss, rather than the crystal structure destruction. The X-ray photoelectron spectroscopy and high resolution transmission electron microscopy results further verified these deduction and the X-ray absorption spectroscopy analysis results indicate that the difference on the structure change of LiFePO4 is mainly at the surface, rather than in the bulk.

Thermal conductivity inside prismatic lithium-ion cells with dependencies on temperature and external compression pressure

Abstract Significant temperature increases and gradients inside lithium-ion battery cells are highly likely during extreme fast charging and discharging, with adverse effect on aging and safety. To assess the temperature inside the cell, the thermal conductivity of the electrode-separator composite must be determined precisely. Furthermore, the dependency of the thermal conductivity on battery states like the cell temperature or composite layer pressure has to be considered. In this work, these dependencies are investigated for a large-format lithium-ion cell with a flat-wound jelly roll and prismatic aluminum hardcase with a Nickel Manganese Cobalt (NMC) cathode and graphite anode. A precise thermal conductivity test bench with defined compression boundaries for the battery cell is constructed in compliance with the guarded heater principle. Validation tests with a stainless steel reference material prove a high accuracy and robustness against changes in compression pressure. In addition, a novel method to obtain the thermal conductivity of the aluminum cell-case is introduced. According to the measurement result, the thermal conductivity increases by 11.9% when the compression pressure rises by 37.1 kPa, which is explained by thermal contact resistances. Compared to the absolute value at 20 ∘C, the thermal conductivity decreases with increasing temperature by more than -1% per K.

Post-mortem analysis of calendar aged large-format lithium-ion cells: Investigation of the solid electrolyte interphase

Although the growth of the solid electrolyte interphase is considered one of the most important degradation phenomena of lithium-ion cells, the mechanism is not yet fully understood. In this work, we present a detailed post-mortem analysis of calendar aged large-format graphite/Li(Ni1/3Mn1/3Co1/3)O2-based lithium-ion cells. X-ray photoelectron spectroscopy depth profiling reveals a distinct coherence of the growth of the solid electrolyte interphase with the phases of the lithiated graphite. Since the graphite phases are in direct correlation with the state of charge and the anode potential, the thickness of the SEI resulting from calendar aging is determined by the storage state of charge. The composition of the SEI has been analyzed as mainly organic near to the electrolyte and more inorganic towards the carbon active material. The same dependency as of the state of charge on the SEI thickness is found for the capacity retention and for the amount of irreversibly lost lithium. Additionally, gas is formed during the aging period and trapped in between the electrodes, leading to associated inhomogeneous lithium plating.

Dynamic impact tests to characterize the crashworthiness of large-format lithium-ion cells

In order to gain a better understanding of the crashworthiness of lithium-ion cells, a test-setup for dynamic impact and crush tests has been designed. An experimental study was carried out using prismatic automotive cells comprising increasing levels of manufacturing quality and specific energy made of lithium nickel manganese cobalt oxide as the cathode active material. Different loading scenarios and more than four orders of magnitude of deformation rates were applied to specimens at fully charged state. The presented work describes the test program, the experimental setup and an objective evaluation method, which finally allows for a detailed summary of the observed mechanical behavior. A distinct strain-rate dependence of hardening, failure parameters, and compressibility of the cells is found. No significant dependence on cell type and state of charge could be observed. The results constitute essential new insights into the material behavior of EV battery cells during a crash event.

Thermal Runaway of Large Format Pouch Lithium Ion Cell with Internal Temperature Detection Using Extended Volume Accelerating Rate Calorimetry

Extended volume accelerating rate calorimetry(EV-ARC) has been widely used to judge the thermal runaway(TR) behaviors of large format lithium ion cells in adiabatic atmosphere[1].Generally, when temperature reaches the onset temperature(approximately 90℃)，self heating of the cell begins due to the solid electrolyte interface(SEI) decomposition. And other exothermic chemical reactions start when temperature reaches their onset temperature. These reactions will heat the cell and at last lead to thermal runaway of the cell. The onset temperature, triggering temperature and maximum temperature are important factors to judge hazardous during TR. When testing TR of pouch cells using EV-ARC, the thermal couple is always placed between two side by side cells[2]. However, temperature detected between two cells is not the real temperature inside battery. In this work, the thermal couple is inserted into the cell directly to detect the inside temperature of cell(see Figure1). The capacities of fresh cell and operated cell are almost the same. Then, the EV-ARC tests of both cells are conducted and compared. In the result, the onset temperature of both cells are almost the same. The triggering temperature of cell with thermal couple inside is about twenty degree higher than that detected between two cells. And the maximum temperature of cell with thermal couple inside is also dozens of degree higher than that with traditional test method. Figure 1

(Invited) Safe Design of GS Yuasa Large Format Lithium Ion Cells for Industrial Applications

Since the 1990’s, GS Yuasa has developed and produced large-format Li ion cells for a variety of applications including satellites, rockets, electrified vehicles, trains, AGVs, energy storage systems, UPS, submarines, and others. Through these activities, safer Lithium-ion cell designs have evolved not only through the careful selection of electrode chemistry, but also cell shape, case materials and cell orientation within the module or battery pack. Lithium iron phosphate, for example, is stable and often considered a “safe” material for use in the positive electrodes of lithium-ion cells; however, there have been many lithium iron phosphate cell fires in the field. Fires may be caused by leakage of a flammable electrolyte in proximity to an ignition source. GS Yuasa considers the combination of using a metal cell case with laser-welded seal and a terminals-up orientation of cells in the module or battery pack to drastically decrease the risk of electrolyte leakage. Figure 1 shows the typical cell design of our large lithium ion cells for electrified vehicles and other industrial applications such as trains, AGVs, energy storage systems and UPS. The metal case with laser-welded seal offers high reliability with respect to the prevention of electrolyte leakage. This approach to cell design and orientation locates the more sensitive potential leak paths such as the rupture disk, terminal seal and case welds in the top of the cell and away from free electrolyte. In this presentation, we will detail GS Yuasa’s cell and battery systems and discuss our field experiences. Figure 1

Effects of Non-Uniform Temperature Distribution on Degradation of Lithium-Ion Cells

Lithium-ion cells degrade much faster at temperatures higher than 25 °C mainly due to accelerated solid-electrolyte interface (SEI) growth in anode and degeneration of cathode [1, 2]. It is also known that temperature distributions in lithium-ion cells are not uniform [3] due to very low thermal conductivity of battery materials. The non-uniform temperature distributions could cause non-uniform degradation, thus exacerbating the thermal degradation problem, especially for large-format lithium-ion cells during high power operation. The effects of non-uniform temperature distributions on degradation of lithium-ion cells will be investigated and progress will be presented.

Gas Monitoring and Sampling of Large-Format Lithium-Ion Cells

Lithium-ion batteries are a ubiquitous energy storage technology utilized in many military, consumer, and industrial applications. Energy density, high working voltage, and long cycle life are just a few of the advantages offered by lithium-ion batteries; however, safety of lithium-ion batteries continues to pose a challenge. Many of the benefits of lithium-ion batteries are offset by the steps taken to prevent catastrophic failure, especially in safety-critical applications. To fully realize the possibilities of lithium-ion battery deployment, an improved understanding of failure mechanisms is necessary. When lithium-ion cells are operated outside of their stable working limits (i.e. voltage, rate, temperature), they can fail catastrophically. This catastrophic failure is termed thermal runaway, and stems from a combination of gas generation mechanisms and heat generation within the cell. If sufficient gas builds up within the cell, the cell can vent gases and electrolyte into the surrounding atmosphere, either through a machined vent or due to rupture of the casing. In the worst case scenario, the released vent products can ignite and propagate to adjacent cells or systems. While gas generation mechanisms are generally understood [1-6], the majority of research focuses on gases released after the cell fails. Studying real-time gas generation in lithium-ion cells opens up avenues for developing prognostic models to enable failure avoidance strategies. In our work, we have developed a methodology for monitoring internal cell pressure in large-format (>30 Ah) cylindrical lithium-ion cells. The cells are punctured to enable direct measurement of the gas pressure without impacting cell performance. Furthermore, sampling of the gas prior to catastrophic failure is possible. Gas generation is induced by overcharging the cell at a constant C-rate (C/2) until failure occurs inside of an abusive test box designed for handling large battery failure events. Figure 1 illustrates the pressure rise in the cell throughout an overcharge event. The cell is initially pressurized due to gas generation during the life cycle of the cell, and remains constant for the preliminary portion of overcharge. Once the cell’s voltage peaks and begins to decrease, the pressure starts to rise quickly. The pressure continues to rise until a gas sample is collected just prior to failure. This process has been repeated for multiple cells with similar results, demonstrating a repeatable experimental setup that can shed insight into the evolution of gas within a large-format commercial lithium-ion cell. This has practical applications for developing failure mitigation strategies and analyzing design through physics-based battery modeling. This paper will detail the experimental setup, in-situ measurements of gas pressure during overcharge, and preliminary gas analysis.

Effects of Non-Uniform Temperature on in-Situ Current Distribution and Non-Uniform State of Charge Measurements for LiFePO4 and LiNiMnCoO2 Cells

Understanding internal state non-uniformity that occurs across the electrodes in large-format Lithium-ion batteries (LIBs), and among parallel-connected cells, is a critical part of the cell/module design process [1]. The reduction of non-uniform current distributions is critical as this leads to non-uniform utilization, which effectively reduces energy density, and can eventually cause non-uniform degradation, leading to earlier cell/module failure. Several studies, performed particularly at high charge/discharge rates (i.e., above 1C-rate), show single cell temperature differences greater than 20°C, for large-format Lithium-ion cells [2]. As engineers maximize the energy density of Lithium-ion cells, and push them to extreme limits (e.g., Tesla Supercharging), the problems of non-uniform currents and temperatures will continue to be exacerbated. Therefore, high-rate battery systems generally require the integration of a battery thermal management system (BTMS). Coupling the optimization of cell/module, and BTMS designs, to efficiently minimize thermal non-uniformity and packaging requirements, requires spatially-resolved coupled thermal-electrical-electrochemical models. However, little published research exists regarding the measurement of spatially distributed effects in Lithium-ion batteries and the experimental validation of spatially distributed models. Under uniform thermal conditions, the in-situ internal current/State of Charge (SOC) distributions within a segmented-cathode (LiFePO4) cell have been measured by Zhang et al. [3]. Yang et al. studied the non-uniform SOC buildup between two 18650 LiFePO4 cells connected in parallel, where each cell was held at a different temperature [4]. Klein et. al. studied the non-uniform temperature effects on the bulk cell resistance under constant current pulses and dynamic current charge-depleting cases for a LiNiMnCo pouch cell [5]. This study presents the non-uniform temperature effects on in-situ current/SOC distributions among modules of five parallel-connected cells. Two commercially available 18650 cell chemistries were used; LiFePO4/Graphite (LFP) and LiNiMnCo/Graphite (NMC). For each chemistry, the five cells were connected in parallel with a current measuring shunt in series with each cell. The test system is illustrated in Figure 1a, and the module is in Figure 1b. Average module temperatures of 15, 25, and 35°C were used with temperature differences across the module being controlled to 0, 5, 10, and 20°C. Direct-current pulse tests were performed at 20, 50, and 80% SOC using 10sec pulses at constant C/5, C/2, 1C, and 2C-rates. Additionally, C/5, C/2, and 1C-rate constant current full-capacity discharges were performed under the same thermal conditions to observe the effects of non-uniform SOC among the cells. The pulse results indicate that the LFP cell resistance has a lower temperature sensitivity than the NMC, and therefore experiences less current non-uniformity for the same amount of temperature non-uniformity. Larger current non-uniformity is observed at lower average temperatures, where the resistance is more temperature sensitive. The current distribution develops quickly (i.e., <2sec) and remains fairly constant during the 10sec pulse. However, the constant current full discharge results are quite different. Figure 1c-e plots the measured normalized current for the left/middle/right (cold/mean/hot) LFP cells for the average module temperature case of 25°C and the C/5, C/2, and 1C-rates, respectively. The instantaneous current distributions measured at the same average module SOC as the pulse tests are more uniform in the full capacity discharge tests. Therefore, the amount of maximum non-uniform SOC is lower than would be expected from extrapolating the pulse test current distribution results. Additionally, the NMC unit, which had a larger current distribution in the pulse testing compared to LFP, has a substantially lower current-distribution than the LFP unit for the full-capacity discharges. Finally, using the measured SOC profiles for each cell, a tolerable amount of temperature uniformity for each chemistry is interpolated from the results. Figures 1f and 1g present these results for two maximum tolerable SOC non-uniformities of 2% and 5%. The NMC chemistry provides a higher tolerance to temperature non-uniformity for a given SOC non-uniformity tolerance, as compared to the LFP chemistry. The results of this study are aimed at aiding both the validation of spatially resolved models, and to quantify the thermal non-uniformity targets for these common commercially available Lithium-ion cell chemistries. [1] T. Waldmann, G.Bisle, B.-I. Hogg, S. Stumpp, M.A. Danzer, M. Kasper, P. Axmann, M. Wohlfarht-Mehrens, Jounal of The Electrochemical Society 162 (6) (2015) A921-A927 [2] C. Veth, D. Dragicevic, C. Merten, Journal of Power Sources 267 (2014) 760-769 [3] G.Zhang, C.E. Shaffer, C.-Y. Wang, C.D. Rahn, Journal of The Electrochemical Society 160 (4) (2013) A610-A615. [4] N. Yang, X. Zhang, B. Shang, G. Li, Journal of Power Sources 306 (2016) 733-741. [5] M. Klein, S. Tong, J. Park, Applied Energy 165 (2016) 639-647. Figure 1

Inhomogeneities in Large Format Lithium Ion Cells: A Study by Battery Modelling Approach

The performance of lithium ion battery cells is influenced besides the cell chemistry, also by microscopic and macroscopic parameters. In short format cells, the macroscopic variables such as cell geometry, aspect ratio, material thickness, tab configuration etc. do not significantly affect current, voltage, state of charge (SOC) and temperature distribution. However, they cannot be neglected as the cell size increases, especially in large format cells, where the distributions becomes non homogeneous. These inhomogeneities adversely affect the cell’s performance and a long exposure leads to localized aging. This paper presents a spatially resolved modelling technique in Matlab/Simulink to evaluate cell inhomogeneity. It compares current, voltage and SOC distribution of 8 Ah and 75 Ah cells and how the distribution changes from a fresh cell to its end of life (EOL).

Inhomogeneities in Large Format Lithium Ion Cells: A Study by Battery Modelling Approach

The performance of lithium ion battery cells is influenced by microscopic and macroscopic parameters. The microscopic parameters are generally responsible for the electrochemical, thermal, safety and lifetime performance of a battery. These parameters are cell chemistry, particle size, diffusion coefficient, equilibrium potential, reaction rate, thermodynamic parameters, thermal and electrical conductivity, heat capacity of active materials and different components of a cell etc. The macroscopic parameters are cell form, aspect ratio, thickness of active materials current collector and separator, tab size and its location. They are used in the cell design process and are optimized to enhance the performance and lifetime of a given cell. When considering upscaling the cell size, the macroscopic parameters attain significance in determining the overall performance of large format cells, which may be largely ignored in short format cells. Increase in the cell size has certain advantages such as, increased cell capacity, fewer connections to the battery pack (parallel connection), low assembly cost, high weight/volume ratio and high reliability of the components of a battery pack. But on the other hand it increases the cell cost, increased safety risk and difficult thermal management etc. But the main disadvantages in terms of performance and lifetime are related to the inhomogeneities occurring in the cell due to increase in cell size. The inhomogeneities can be listed as temperature, current density and state of the charge (SOC) distribution. The current distribution in the cell is affected by tab configuration, current collector thickness and cell aspect ratio. Recent studies show that increasing the cell size increases inhomogeneous current density distribution. Despite the optimized design, the current flow near the tabs is constricted and the location near cell tabs experience higher current density compared to surfaces far away from the tabs. The current distribution influences the local heat generation and introduces temperature inhomogeneity, especially near cell tabs. It also introduces SOC inhomogeneity as SOC in the integral of current over time. These inhomogeneities are related to one another and will have a cascading effect on the overall cell performance. A long term exposure to these inhomogeneities leads to localized aging of the cell. The localized aging creates stronger and weaker regions in cell and further accelerating the aging and leads to premature end of life (EOL) of the cell. With the existing methodologies it is difficult to evaluate cell inhomogeneity as it is completely a localized process. However, recently some experimental and modelling techniques are able to evaluate cell inhomogeneity. These methods can be classified into two as, direct measurement such as, spatial temperature measurements both inside and on the cell surface, multiple or segmented electrode measurements, in-situ and ex-situ diffraction or neutron imaging tools and the second classification is modelling and simulation techniques, which is the focus of this study. This paper shows two modelling techniques, first with spatially distributed battery model developed in Matlab/Simulink and second with Multiphysics based modelling in COMSOL. Both models are able to evaluate inhomogeneity in a large format cell. This work also compares these modelling techniques with respect to complexity, computation and accuracy of the simulation results. Figure 1

Multi-Dimensional Modeling of the Influence of Cell Design on Temperature, Displacement and Stress Inhomogeneity in Large-Format Lithium-Ion Cells

In this paper, an efficient multi-dimensional approach to model mechanical effects in lithium-ion cells is presented. Using this modeling approach, we introduce mechanical criteria to rate different cell designs, complementary to commonly used criteria such as homogeneity of temperature or current density distribution. The additionally considered mechanical criteria are the displacement distribution on the cell due to the superposition of intercalation induced volume changes and thermal expansion, and the stresses in the electrodes’ active material particles. Two cell designs are compared comprising identical electrode stack dimensions with differing tab alignments. The model shows that the variation in local cell displacement is small due to the overlay of thermal and intercalation displacement, which act in opposite directions during discharge. Looking into the stress formation in the active material particles, we found that the tab pattern can influence the maximum stress and the gradients in stress distribution significantly, especially for the positive electrode particles. Our study shows the interaction of the electrochemical, electrical, thermal, and mechanical behavior of large-format pouch cells, representing a step toward a better understanding of the complex processes in lithium-ion cells and enabling the multi-disciplinary design optimization considering mechanical effects in an early stage of cell development.

Quantifying the Impact of Overdischarge on Large Format Lithium-Ion Cells

Lithium-ion batteries can help enable the Navy and Marine Corps to meet their energy storage needs; however, safety is of paramount concern. Overdischarge, followed by recharge, is a failure mechanism in lithium-ion batteries that can be associated with long-term storage, poor battery module design, and internal cell defects. Overdischarge is known to cause copper dissolution, SEI layer degradation, and gas generation. By itself, overdischarge does not result in catastrophic thermal runaway; however, if a battery is recharged and used following an overdischarge event, the chances of thermal runaway increase. This paper describes an on-going investigation into the link between depth and duration of overdischarge, quantitative changes to the cell as a result of overdischarge, and performance and safety characteristics in lithium-ion batteries. Through the use of x-ray photoelectron spectroscopy (XPS), we measure copper dissolution in both an overdischarged cell and a healthy cell.