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Features of mechanical behavior of EV battery modules under high deformation rate
Extreme Mechanics Letters 32 (2019) 100550
Contents lists available at ScienceDirect
Extreme Mechanics Letters journal homepage: www.elsevier.com/locate/eml
Features of mechanical behavior of EV battery modules under high deformation rate✩ ∗
Sergiy Kalnaus a , , Hsin Wang b , Thomas R. Watkins b , Srdjan Simunovic a , Abhijit Sengupta c a
Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6164, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6164, USA c U.S. Department of Transportation, Washington, DC, 20590, USA b
article
info
Article history: Received 14 June 2019 Received in revised form 7 August 2019 Accepted 26 August 2019 Available online 28 August 2019 Keywords: Li-ion battery Automotive Mechanics Safety
a b s t r a c t We report on changes in material behavior of electric vehicle (EV) Li-ion battery cell modules when the loading speed increases from quasi-static to high speed (close to 1 m/s). A series of out-of-plane indentation experiments were performed on stacks of pouch cells placed in the specially designed enclosure to represent conditions in battery modules. Over one hundred of large format automotive pouch cells were tested under displacement rates differing by orders of magnitude. The details of internal failure were studied by X-ray tomography (XCT). We observe a shift in the force–displacement response from a parabolic behavior characteristic of particulate materials under compression to the response typical of fully dense materials with yield point. This shift is associated with dynamic effects in active materials which result in deep propagation of damage into the battery module, despite the fact that the internal short circuit is triggered when the displacement of the indenter is one half of that under the slow displacement rate. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Li-ion batteries have become the primary source of power in the electric vehicles (EV). The last decade has brought significant improvements in energy and power density of the battery cells combined with the reduction of their manufacturing cost. This resulted in significant penetration of the Li-ion energy storage technology within the automotive sector, with the production cost competitive with that of traditional internal combustion based powertrains. Advancements in synthesis of novel materials with high energy storage capacity for battery electrodes were accompanied by engineering solutions for battery pack protection and fire mitigation (an excellent review can be found in [1]). In Li-ion cell, the charge is transported between the electrodes via the liquid electrolyte. At the electrode surface the ionic current ✩ Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). ∗ Corresponding author. E-mail address: [email protected] (S. Kalnaus). https://doi.org/10.1016/j.eml.2019.100550 2352-4316/© 2019 Elsevier Ltd. All rights reserved.
is converted to electronic current by electrochemical reactions which supplies power to external load in case of discharge, or to the cell in case of charge cycle. The electrolyte in the cell is based on Li salt dissolved in organic solvents, which are highly flammable mixtures of ethylene carbonate and diethyl, dimethyl or ethyl–methyl carbonates [2]. The presence of these flammable materials in the system which stores large amount of energy poses safety risks. In case of spontaneous contact between the electrodes of opposite polarity, this energy is rapidly released creating instantaneous rise in temperature which can trigger thermal runaway and fire. Such contact between the electrodes can occur during excessive battery deformation due to external impact, as in the case of vehicle collision, or penetration of road debris into the battery pack. Electrodes in a battery cell are separated by a porous membrane. The membrane allows Li ion transport via electrolyte while preventing direct electronic current between the electrodes. This membrane, properly termed a separator, is therefore considered a critical component of a battery cell both in terms of performance and safety. Non-surprisingly, a significant amount of research has been dedicated to the study of separator performance, mostly in terms of mechanics and temperature-dependent behavior. Detailed studies of mechanics of battery separators have been performed under compression [3–5], tension [3,5–8], and biaxial tension [9] with corresponding strain rate and temperature sensitivity analysis. In our previous work, the criterion for
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internal short circuit in the battery module under out-of-plane indentation was formulated based on the critical first principal strain in separator [9] which resulted in a good correlation between numerical simulations and experimentally observed battery failure [10]. It should be emphasized that consideration of a stress–strain state in separator alone does not accurately represent the failure criterion with predictive capabilities. Overall deformation and strain distribution within the cell needs to be considered since about 80% to 85% (by volume) of the cell are active materials of cathode and anode which impose the displacements on separator. In addition, in order to properly assess the behavior and safety of the Li-ion battery, the cells need to be tested under conditions representing their deformation inside the battery pack. In the case of pouch cells this means that out-of-plane indentation should account for the compliance of the multiple cells in the module. In this case, cells experience long range stresses compared to indentation tests on rigid surfaces. Compared to the number of studies on single cell crush, reports on the mechanics of battery modules and packs are rather limited [11] which in part can be explained by the limited availability of EV battery modules for such tests, as well as test hazards and equipment requirements. A substantial experimental program has been undertaken by Xia et al. [12] where the crush of EV battery modules has been performed utilizing drop tower. These tests were performed at 100% state of charge (SOC) and while such experiments provide a wealth of information on fire propagation and mitigation strategies, the analysis of mechanical failure precursors to short circuit under such circumstances is hard to perform since much of the material is destroyed by fire. One important aspect of battery deformation that needs to be considered is its sensitivity to the loading rate, which can be anticipated by considering the mechanical behavior of its components. It has been long recognized that at least one component inside the battery structure, separator, is highly sensitive to strain rate [5,6]. More importantly, the behavior of separators in compression is highly influenced by poroelastic effects due to the presence of liquid electrolyte inside the separator pores [3,4]. The same effects are expected to play a role in the compressive deformation of the electrode coatings, which are porous composite materials with pores filled by the electrolyte. Such structure is expected to be highly sensitive to deformation rate and significant stiffening can occur under dynamic loading. Several studies investigated the behavior of single battery cells under high speed loading [13–16] where a pouch cell is either impacted by a sharp indenter [13,14] or subjected to high speed projectile penetration [15]. Similar experiments have been performed on cylindrical cells under lateral impact [16]. However, an investigation of rate effects of deformation of battery modules has not been reported yet. In this work, we utilize previously designed setup [10] for deformation of pouch cell stacks representative of an EV battery module to study the rate effects under lateral indentation with displacement rates ranging from nearly quasi-static 127 µm/s to intermediate speed impact close to 1 m/s. We observe a fundamental change in the material response under higher loading displacement rates. In addition to an increase in indentation force and a decrease in displacement required to trigger the internal short circuit, the shape of the load–displacement curve shifts from one characteristic to compaction of granular materials or foams to that representative of deformation of fully dense material with some ductility. In addition, impact with high speed triggered long-range deformation propagating through whole cell stack consisting of 10 pouch cells, which was not observed in low-speed tests.
2. Experiments The cells for the experiments were taken from a 2013 Ford Focus EV battery pack which was disassembled into battery modules and then into battery pouch cells as shown in Fig. 1. The module consists of three major components which become subjected to transversal loading: plastic enclosure plates, cooling plates, and pouch cells Fig. 1. The cooling plates are thin aluminum laminated sheets with micro-channels for circulation of coolant; they are placed in a module in such a way that each pouch cell has one surface in contact with the cooling plate. The cells were received fully discharged which allowed us to test them without risks of thermal runaway. The cells were 5.5 mm thick 15 Ah pouch cells with 227 × 165 mm area. The battery module in our study was represented by a stack of ten pouch cells; in this manner we focus on the mechanical behavior of cells alone without variables introduced by plastic module enclosure. If the effects of inactive components need to be considered, they can be introduced into the cell stack, as was done in [10] , where it was determined that the presence of inactive components in the module did not influence the critical displacement of the indenter at which failure occurred. In our setup (Fig. 2) the stack of cells (3) is placed under compression by sandwiching it between two plates and applying compressing force via eight screws. The top plate (5) had a circular cut to allow penetration of the indenting sphere (1) which was 50.8-mm in diameter. This compressed cell stack was in turn placed in a custom manufactured die-set, in which the load to the sphere was transferred by load train (2) pushing on the top plate of the die set. The top plate was in turn guided by four posts with linear bearings (4). The indenting sphere was positioned in place by a small concave cut in the top plate of the die-set. This ensured proper alignment and since the indenting sphere was not attached to the load train any undesired bending or torsion was eliminated. Such setup allows for highly controlled mechanical testing of pouch cell stacks representing battery modules. The cells were received in fully discharged state and therefore required a small current to bring the voltage up slightly so that the potential drop due to the short circuit could be properly registered. For this purpose the top cell in the stack was connected to the constant current source; the voltage of the same cell was recorded by the data acquisition channel together with the load and displacement of the crosshead of loading frame. The data sampling frequency varied from 10 Hz to 10,000 Hz depending on the crosshead speed. A servo-hydraulic load system was used in the experiments with high speed capability of the actuator (up to 2 m/s) and 110 kN load cell. All of the experiments were performed in displacement control with maximum displacement set at 25 mm. Analysis of the cells following the indentation was performed by 3D X-ray computed tomography (XCT). 5 cm wide strips were cut from the cells with the indented area in the middle. Tapes were used to secure the layers from separating. The indented area was preserved and taken to an X-ray Computed Tomography system, Zeiss Versa 520 X-ray, using a tungsten source running at 140 kV/64.4 A max (9 W). Two magnifications were used, 0.4X and 4X. The data was collected and reconstructed with Zeiss’ Scout and Scan software [17] with further analysis conducted on TXM3DViewer [18]. The internal layered structure of the pouch cell used in current investigation is shown in Fig. 3, where the XCT image of the cell cross-section spanning one half of the cell thickness is shown together with indication of the corresponding layers. The periodic structure of the cell consists of alternating layers of positive and negative electrodes which are represented by double-coated aluminum and copper thin metal foils. The whole structure, often
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Fig. 1. Structure of the battery module containing the three major components: (1) plastic enclosure; (2) aluminum cooling plates (placed after every second cell); (3) pouch cells.
Fig. 2. Setup for out-of-plane deformation of battery cell stack.
termed jelly-roll, is encased in a pouch, which is a metalized polymer providing a tight seal for the internal cell components. Each anode is separated from the cathode by a thin porous polymer membrane which is transparent to X-rays and cannot be detected in the image. 3. Results and discussion 3.1. Low speed deformation Multiple experiments have been performed under a low indentation speed of 127 µm/s and the corresponding force– displacement curves are shown in Fig. 4(a). The shape of the load– displacement curve is dictated by mechanics of porous materials
(represented here by electrodes) and evolution of Hertzian contact. The first part of the curve can be attributed to reduction in porosity in electrodes. With the increased displacement of indenter, the densification of electrodes directly underneath the indenter contributes to increase of the slope of the curve. The points corresponding to failure, defined as the potential drop due to contact between the electrodes, are indicated with the arrows in Fig. 4. In several instances, the potential was monitored in the top two cells in the stack and the experiments were terminated after the voltage drop was detected in the second cell. It should be noted, that the voltage drop in the two top cells occurred nearly simultaneously in some cases (Fig. 4(b)), while in other instances there was a certain delay between the failure in the top cell and the cell underneath it. This signifies a degree of variation
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Fig. 3. XCT of undeformed pouch cell showing internal structure.
Fig. 4. Slow indentation of the pouch cell stack. (a) load–displacement curves with failure points; (b) voltage drop registered as a result of cell failure.
in the timing of the internal short circuit under the same loading conditions which could be the result of the cell-to-cell variation from manufacturing and usage history. The same variability is underlined by the fact that one of the cells failed prematurely, under much smaller displacement and force than the rest of the cells (Fig. 4(a)). However, it should be emphasized that in all cases any noticeable reduction in stiffness (slope of the load– displacement curve) occurred only after the failure of the second cell in the stack. Since the primary interest in this investigation is the onset of the internal short circuit which triggers catastrophic failure in the battery pack, only the potential in the top cell was monitored in the majority of the experiments. The failure analysis by X-ray tomography is shown in Fig. 5, where the views from three orthogonal cross-sections of the top cell in the stack are shown. Details of the XCT sample preparation and definition of coordinates can be found in [10]. Substantial fracturing can be observed, with the cracks aligned with the machine direction of separator, — for anisotropic properties of dry processed separators see e.g. [9,19]. The cracks profile in the plane perpendicular to the machine direction, Fig. 5(a) consist of approximately 45◦ oriented cracks propagating through the thickness of the cell. Visually, the deformation could be detected in up to five cells in the cell stack, indicating that damage propagates through five cells, causing immediate internal short circuit in one cell. 3.2. High speed deformation A total of 14 experiments on cell stacks were conducted resulting in 140 pouch cells tested under transverse loading. The results are shown in Fig. 6(a) as the load–displacement curves corresponding to different loading rates. The black asterisk markers are placed to indicate the points of voltage drop in the battery, — manifestation of failure leading to internal short circuit. Several
trends can be observed in Fig. 6 indicating major changes in the material behavior. It can be seen that the battery cell stack stiffens under impact loading compared to the quasi-static tests; the indentation force magnitude at the cell failure point, however, is on average lower than that observed during the slow deformation. More importantly, the shape of the load–displacement curve changes from concave to convex indicating a shift in type of material behavior. The parabolic force–displacement curves (circular red markers in Fig. 6(a)) are characteristic of the deformation of a porous material which undergoes compaction and jamming. On the other hand, indentation speeds higher than 25 cm/s cause the response characteristic of a dense material under compression. It has to be emphasized that the damage due to spherical indentation propagated through the entire module containing ten cells (Fig. 6(b) shows the imprint left in the bottom cell of the stack). This was not observed in the quasi-static experiment, where the visually detectable deformation propagated only through 50% of the cell stack thickness. Such a shift in the material behavior can be explained by the influence of liquid electrolyte filling the pores of the electrodes and separator. Significant stiffening of liquid-filled separator has been observed under impact loading in [3,4]. It was determined that poroelastic effect of liquid became pronounced at the strain rates of 10−3 1/s and higher. Considering that the electrode coatings in Li-ion cell are porous structures filled with liquid electrolyte, similar treatment can be extended to the whole cell realizing that by volume, approximately 85% of the cell is represented by active material coatings. Therefore, the effective behavior of the cell stack is akin to saturated soil consolidation with consideration of pore pressure. In this case, the response to compression is governed by the rate at which the liquid is being squeezed out of the pores. This rate in turn is governed by Darcy’s law, or Brinkman’s law if the shear contribution needs to be considered. Darcy’s law states that the fluid velocity is
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Fig. 5. XCT of the top cell in the stack subjected to slow speed deformation by a rigid sphere (a) side view; (b) transverse view; (c) top view of the indented area.
Fig. 6. Change in mechanical response of the battery cell stack as a function of deformation rate. (a) force–displacement response; (b) permanent deformation in the bottom cell of the stack under high rate testing.
⃗ = −κ/µ ▽ p (with ϵ proportional to the pressure gradient as ϵ · u and κ being the porosity and permeability of the solid matrix and µ representing dynamic viscosity of the liquid). The stress state of the structure is therefore modified to represent contribution from the pore pressure using the effective stress σ˜ ij = σij + bpδij where b is Biot’s coefficient [20]. Such simplified interpretation provides physics based reasoning for stiffening of the material response in Fig. 6. The real case scenario is of course complicated by the layered structure of the cell, in which the fluid flow in porous regions of electrode coatings (‘‘cell sandwich’’) is constrained by the metal current collectors with zero permeability. Therefore, the porous fluid flow is constrained to in-plane flow which under dynamic loading creates a column of high pressure that propagates deformation through the cell stack as evident in Fig. 6(b). Finally, it should be noted that the failure (i.e. internal short circuit) occurred at much smaller displacements of the indenter into the battery cell stack as can be seen in Fig. 6(a). Both the displacement and force at failure were lower than those under quasi-static loading. On average, the cell stack shorted after high-speed penetration depth being 0.5 of that under small displacement rate loading. The values for critical displacement at failure are summarized in Fig. 7 with the results spanning several orders of magnitude of the loading speed. In addition, the initial stiffness of the cell at the onset of indentation under different speeds is summarized together with critical displacement and load in Table 1. The displacement rate for each test was calculated from the displacement-time loading curve. The results of the XCT of the top cell in the stack subjected to 0.25 m/s loading are shown in Fig. 8. The overall fracture pattern is very similar to that observed in cells after low-speed deformation (Fig. 5). A more developed through-thickness cracking
Fig. 7. Critical displacement required for short circuit as a function of indentation rate.
pattern is however observed when the cells were subjected to high speed deformation, with a network of 45◦ inclined cracks running through the cell thickness, as can be seen in Fig. 8(a). Such dense network of cracks was developed under more shallow
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S. Kalnaus, H. Wang, T.R. Watkins et al. / Extreme Mechanics Letters 32 (2019) 100550 Table 1 Experimental parameters of battery module indentation. Velocity, mm/s
Stiffness, kN/m
Critical displacement, mm
Critical load, kN
0.127 203.3 242.9 261.02 377.21 557.85 737.42
667.0 6342.7 7347.0 6392.2 7330.1 13266.0 16592.0
10.14 3.55 4.04 4.56 4.26 4.02 4.47
35.05 24.21 23.68 27.22 26.62 29.84 28.02
Fig. 8. XCT of the top cell in the stack subjected to high speed deformation by a rigid sphere (a) side view; (b) transverse view; (c) top view of the indented area.
complemented with non-destructive failure analysis. Therefore, the cells in the modules were completely discharged so that the thermal runaway events would not prevent the observations of mechanical behavior. As the outcome of this comprehensive program involving 14 Ford Focus EV battery modules we observed the following.
• The displacement of the indenter into the module required
Fig. 9. Deformation of the electrodes in bottom cell of the 10-cell module subjected to high-velocity indentation by rigid sphere.
penetration of the indenter, compared to quasi-static case (Fig. 7). In all cases of the high-speed indentation, the deformation propagated through the entire cell stack, resulting in clearly visible imprint in the bottom cell, Fig. 6(b). X-ray tomography of the bottom cell of the stack revealed no through-thickness cracking. However the deformation of the electrodes can be clearly seen in the scan of the edge of the imprint in the bottom cell (Fig. 9). While such perturbation may not result in immediate safety related events, such change in electrodes geometry may trigger delayed processes and compromise the battery safety. 4. Conclusions The effect of loading speed on mechanical response and failure of the EV battery module has been investigated by out-of-plane indentation of pouch cell stacks. The goal of the investigation was to understand rate-dependent changes in the material behavior
to trigger the failure and short circuit reduces by a factor of two when the indentation speed is 25 cm/s or higher. This indicates that even a relatively shallow impact done at high speed can induce enough damage to create fault lines in the layered structure of the battery and trigger contact between electrodes of opposite charge. • The mechanical behavior of the cell stack changes profoundly under high speed loading. When impacted at speeds higher than 25 cm/s the cell stack response stiffens significantly (the force increases by an order of magnitude) and behavior shifts from a compressible, crushable type to that characteristic of a dense material. This indicates the necessity of development of new constitutive models for mechanical abuse under high displacement rates. • A significant propagation of damage into the battery module was observed under fast impact, despite the factorof-two decrease in displacement of indenter required to induce the short circuit. The XCT of the cell farthermost from the indentation site did not reveal any cracking in the electrode structure and only some rearrangement of layers due to compression was observed. Nevertheless, the fact that a 4 mm indent can propagate deformation through an entire battery module, can result in imposing additional requirements for the battery safety. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract
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DE-AC05-00OR22725, was sponsored by the National Highway Traffic Safety Administration, USA. Rick R. Lowden is acknowledged for his assistance and expertise in setting up the experiments. Xiaoqing Zhu is acknowledged for his assistance in disassembling the modules and preparing cells. References [1] S. Arora, W. Shen, A. Kapoor, Review of mechanical design and strategic placement technique of a robust battery pack for electric vehicles, Renew. Sustain. Energy Rev. 60 (2016) 1319–1331. [2] D. Aurbach, Y. Talyosef, B. Markovsky, E. Markevich, E. Zinigrad, L. Asraf, J.S. Gnanaraj, H.-J. Kim, Design of electrolyte solutions for Li and Li-ion batteries: a review, Electrochim. Acta 20 (2–3) (2004) 247–254. [3] J. Cannarella, X. Liu, C.Z. Leng, P.D. Sinko, G.Y. Gor, C.B. Arnold, Mechanical properties of a battery separator under compression and tension, J. Electrochem. Soc. 161 (11) (2014) F3117–F3122. [4] G.Y. Gor, J. Cannarella, J.H. Prevost, C.B. Arnold, A model for the behavior of battery separators in compression at different strain/charge rates, J. Electrochem. Soc. 161 (2014) F3065–F3071. [5] S. Kalnaus, Y. Wang, J. Li, A. Kumar, J.A. Turner, Temperature and strain rate dependent behavior of polymer separator for Li-ion batteries, Extreme Mech. Lett. 20 (2018) 73–80. [6] I. Avdeev, M. Martinsen, A. Francis, Rate- and temperature-dependent material behavior of a multilayer polymer battery separator, J. Mater. Eng. Perform. 23 (2014) 315–325. [7] J. Chen, Y. Yan, T. Sun, Y. Qi, X. Li, Deformation and fracture behaviors of microporous polymer separators for lithium ion batteries, RSC Adv. 4 (2014) 14904–14914. [8] S. Kalnaus, Y. Wang, J.A. Turner, Mechanical behavior and failure mechanisms of Li-ion battery separators, J. Power Sources 348 (2017) 255–263.
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[9] S. Kalnaus, A. Kumar, Y. Wang, J. Li, S. Simunovic, J.A. Turner, P. Gorney, Strain distribution and failure mode of polymer separators for Li-ion batteries under biaxial loading, J. Power Sources 378 (2018) 139–145. [10] S. Kalnaus, H. Wang, T.R. Watkins, A. Kumar, S. Simunovic, J.A. Turner, P. Gorney, Effect of packaging and cooling plates on mechanical response and failure characteristics of automotive Li-ion battery modules, J. Power Sources 403 (2018) 20–26. [11] J. Zhu, T. Wierzbicki, W. Li, A review of safety-focused mechanical modeling of commercial lithium-ion batteries, J. Power Sources 378 (2018) 153–168. [12] Y. Xia, G. Chen, Q. Zhou, X. Shi, F. Shi, Failure behaviours of 100% SOC lithium-ion battery modules under different impact loading conditions, Eng. Fail. Anal. 82 (2017) 149–160. [13] G. Kermani, E. Sahraei, Dynamic impact response of lithium-ion batteries, constitutive properties and failure model, RSC Adv. 9 (2019) 2464. [14] T. Kirsters, E. Sahraei, T. Wierzbicki, Dynamic impact tests on lithium-ion cells, Int. J. Impact Eng. 108 (2017) 205–216. [15] Y. Chen, S. Santhanagopalan, V. Babu, Y. Ding, Dynamic mechanical behavior of lithium-ion pouch cells subjected to high-velocity impact, Compos. Struct. 218 (2019) 50–59. [16] I. Avdeev, M. Gilaki, Structural analysis and experimental characterization of cylindrical lithium-ion batery cells subject to lateral impact, J. Power Sources 271 (2014) 382–391. [17] Scout and Scan Control System, v.11.1.5534.17006, Carl Zeiss X-ray Microscopy, Pleasanton, CA 94588, USA. [18] TXM3DViewer 1.2.9, Carl Zeiss X-ray Microscopy, Pleasanton, CA 94588, USA. [19] X. Zhang, E. Sahraei, K. Wang, Li-ion battery separators, mechanical integrity and failure mechanisms leading to soft and hard internal shorts, Sci. Rep. 6 (2016) 32578. [20] M.A. Biot, General theory of three-dimensional consolidation, J. Appl. Phys. 12 (1941) 155–164.
Contents lists available at ScienceDirect
Extreme Mechanics Letters journal homepage: www.elsevier.com/locate/eml
Features of mechanical behavior of EV battery modules under high deformation rate✩ ∗
Sergiy Kalnaus a , , Hsin Wang b , Thomas R. Watkins b , Srdjan Simunovic a , Abhijit Sengupta c a
Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6164, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6164, USA c U.S. Department of Transportation, Washington, DC, 20590, USA b
article
info
Article history: Received 14 June 2019 Received in revised form 7 August 2019 Accepted 26 August 2019 Available online 28 August 2019 Keywords: Li-ion battery Automotive Mechanics Safety
a b s t r a c t We report on changes in material behavior of electric vehicle (EV) Li-ion battery cell modules when the loading speed increases from quasi-static to high speed (close to 1 m/s). A series of out-of-plane indentation experiments were performed on stacks of pouch cells placed in the specially designed enclosure to represent conditions in battery modules. Over one hundred of large format automotive pouch cells were tested under displacement rates differing by orders of magnitude. The details of internal failure were studied by X-ray tomography (XCT). We observe a shift in the force–displacement response from a parabolic behavior characteristic of particulate materials under compression to the response typical of fully dense materials with yield point. This shift is associated with dynamic effects in active materials which result in deep propagation of damage into the battery module, despite the fact that the internal short circuit is triggered when the displacement of the indenter is one half of that under the slow displacement rate. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Li-ion batteries have become the primary source of power in the electric vehicles (EV). The last decade has brought significant improvements in energy and power density of the battery cells combined with the reduction of their manufacturing cost. This resulted in significant penetration of the Li-ion energy storage technology within the automotive sector, with the production cost competitive with that of traditional internal combustion based powertrains. Advancements in synthesis of novel materials with high energy storage capacity for battery electrodes were accompanied by engineering solutions for battery pack protection and fire mitigation (an excellent review can be found in [1]). In Li-ion cell, the charge is transported between the electrodes via the liquid electrolyte. At the electrode surface the ionic current ✩ Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). ∗ Corresponding author. E-mail address: [email protected] (S. Kalnaus). https://doi.org/10.1016/j.eml.2019.100550 2352-4316/© 2019 Elsevier Ltd. All rights reserved.
is converted to electronic current by electrochemical reactions which supplies power to external load in case of discharge, or to the cell in case of charge cycle. The electrolyte in the cell is based on Li salt dissolved in organic solvents, which are highly flammable mixtures of ethylene carbonate and diethyl, dimethyl or ethyl–methyl carbonates [2]. The presence of these flammable materials in the system which stores large amount of energy poses safety risks. In case of spontaneous contact between the electrodes of opposite polarity, this energy is rapidly released creating instantaneous rise in temperature which can trigger thermal runaway and fire. Such contact between the electrodes can occur during excessive battery deformation due to external impact, as in the case of vehicle collision, or penetration of road debris into the battery pack. Electrodes in a battery cell are separated by a porous membrane. The membrane allows Li ion transport via electrolyte while preventing direct electronic current between the electrodes. This membrane, properly termed a separator, is therefore considered a critical component of a battery cell both in terms of performance and safety. Non-surprisingly, a significant amount of research has been dedicated to the study of separator performance, mostly in terms of mechanics and temperature-dependent behavior. Detailed studies of mechanics of battery separators have been performed under compression [3–5], tension [3,5–8], and biaxial tension [9] with corresponding strain rate and temperature sensitivity analysis. In our previous work, the criterion for
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internal short circuit in the battery module under out-of-plane indentation was formulated based on the critical first principal strain in separator [9] which resulted in a good correlation between numerical simulations and experimentally observed battery failure [10]. It should be emphasized that consideration of a stress–strain state in separator alone does not accurately represent the failure criterion with predictive capabilities. Overall deformation and strain distribution within the cell needs to be considered since about 80% to 85% (by volume) of the cell are active materials of cathode and anode which impose the displacements on separator. In addition, in order to properly assess the behavior and safety of the Li-ion battery, the cells need to be tested under conditions representing their deformation inside the battery pack. In the case of pouch cells this means that out-of-plane indentation should account for the compliance of the multiple cells in the module. In this case, cells experience long range stresses compared to indentation tests on rigid surfaces. Compared to the number of studies on single cell crush, reports on the mechanics of battery modules and packs are rather limited [11] which in part can be explained by the limited availability of EV battery modules for such tests, as well as test hazards and equipment requirements. A substantial experimental program has been undertaken by Xia et al. [12] where the crush of EV battery modules has been performed utilizing drop tower. These tests were performed at 100% state of charge (SOC) and while such experiments provide a wealth of information on fire propagation and mitigation strategies, the analysis of mechanical failure precursors to short circuit under such circumstances is hard to perform since much of the material is destroyed by fire. One important aspect of battery deformation that needs to be considered is its sensitivity to the loading rate, which can be anticipated by considering the mechanical behavior of its components. It has been long recognized that at least one component inside the battery structure, separator, is highly sensitive to strain rate [5,6]. More importantly, the behavior of separators in compression is highly influenced by poroelastic effects due to the presence of liquid electrolyte inside the separator pores [3,4]. The same effects are expected to play a role in the compressive deformation of the electrode coatings, which are porous composite materials with pores filled by the electrolyte. Such structure is expected to be highly sensitive to deformation rate and significant stiffening can occur under dynamic loading. Several studies investigated the behavior of single battery cells under high speed loading [13–16] where a pouch cell is either impacted by a sharp indenter [13,14] or subjected to high speed projectile penetration [15]. Similar experiments have been performed on cylindrical cells under lateral impact [16]. However, an investigation of rate effects of deformation of battery modules has not been reported yet. In this work, we utilize previously designed setup [10] for deformation of pouch cell stacks representative of an EV battery module to study the rate effects under lateral indentation with displacement rates ranging from nearly quasi-static 127 µm/s to intermediate speed impact close to 1 m/s. We observe a fundamental change in the material response under higher loading displacement rates. In addition to an increase in indentation force and a decrease in displacement required to trigger the internal short circuit, the shape of the load–displacement curve shifts from one characteristic to compaction of granular materials or foams to that representative of deformation of fully dense material with some ductility. In addition, impact with high speed triggered long-range deformation propagating through whole cell stack consisting of 10 pouch cells, which was not observed in low-speed tests.
2. Experiments The cells for the experiments were taken from a 2013 Ford Focus EV battery pack which was disassembled into battery modules and then into battery pouch cells as shown in Fig. 1. The module consists of three major components which become subjected to transversal loading: plastic enclosure plates, cooling plates, and pouch cells Fig. 1. The cooling plates are thin aluminum laminated sheets with micro-channels for circulation of coolant; they are placed in a module in such a way that each pouch cell has one surface in contact with the cooling plate. The cells were received fully discharged which allowed us to test them without risks of thermal runaway. The cells were 5.5 mm thick 15 Ah pouch cells with 227 × 165 mm area. The battery module in our study was represented by a stack of ten pouch cells; in this manner we focus on the mechanical behavior of cells alone without variables introduced by plastic module enclosure. If the effects of inactive components need to be considered, they can be introduced into the cell stack, as was done in [10] , where it was determined that the presence of inactive components in the module did not influence the critical displacement of the indenter at which failure occurred. In our setup (Fig. 2) the stack of cells (3) is placed under compression by sandwiching it between two plates and applying compressing force via eight screws. The top plate (5) had a circular cut to allow penetration of the indenting sphere (1) which was 50.8-mm in diameter. This compressed cell stack was in turn placed in a custom manufactured die-set, in which the load to the sphere was transferred by load train (2) pushing on the top plate of the die set. The top plate was in turn guided by four posts with linear bearings (4). The indenting sphere was positioned in place by a small concave cut in the top plate of the die-set. This ensured proper alignment and since the indenting sphere was not attached to the load train any undesired bending or torsion was eliminated. Such setup allows for highly controlled mechanical testing of pouch cell stacks representing battery modules. The cells were received in fully discharged state and therefore required a small current to bring the voltage up slightly so that the potential drop due to the short circuit could be properly registered. For this purpose the top cell in the stack was connected to the constant current source; the voltage of the same cell was recorded by the data acquisition channel together with the load and displacement of the crosshead of loading frame. The data sampling frequency varied from 10 Hz to 10,000 Hz depending on the crosshead speed. A servo-hydraulic load system was used in the experiments with high speed capability of the actuator (up to 2 m/s) and 110 kN load cell. All of the experiments were performed in displacement control with maximum displacement set at 25 mm. Analysis of the cells following the indentation was performed by 3D X-ray computed tomography (XCT). 5 cm wide strips were cut from the cells with the indented area in the middle. Tapes were used to secure the layers from separating. The indented area was preserved and taken to an X-ray Computed Tomography system, Zeiss Versa 520 X-ray, using a tungsten source running at 140 kV/64.4 A max (9 W). Two magnifications were used, 0.4X and 4X. The data was collected and reconstructed with Zeiss’ Scout and Scan software [17] with further analysis conducted on TXM3DViewer [18]. The internal layered structure of the pouch cell used in current investigation is shown in Fig. 3, where the XCT image of the cell cross-section spanning one half of the cell thickness is shown together with indication of the corresponding layers. The periodic structure of the cell consists of alternating layers of positive and negative electrodes which are represented by double-coated aluminum and copper thin metal foils. The whole structure, often
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Fig. 1. Structure of the battery module containing the three major components: (1) plastic enclosure; (2) aluminum cooling plates (placed after every second cell); (3) pouch cells.
Fig. 2. Setup for out-of-plane deformation of battery cell stack.
termed jelly-roll, is encased in a pouch, which is a metalized polymer providing a tight seal for the internal cell components. Each anode is separated from the cathode by a thin porous polymer membrane which is transparent to X-rays and cannot be detected in the image. 3. Results and discussion 3.1. Low speed deformation Multiple experiments have been performed under a low indentation speed of 127 µm/s and the corresponding force– displacement curves are shown in Fig. 4(a). The shape of the load– displacement curve is dictated by mechanics of porous materials
(represented here by electrodes) and evolution of Hertzian contact. The first part of the curve can be attributed to reduction in porosity in electrodes. With the increased displacement of indenter, the densification of electrodes directly underneath the indenter contributes to increase of the slope of the curve. The points corresponding to failure, defined as the potential drop due to contact between the electrodes, are indicated with the arrows in Fig. 4. In several instances, the potential was monitored in the top two cells in the stack and the experiments were terminated after the voltage drop was detected in the second cell. It should be noted, that the voltage drop in the two top cells occurred nearly simultaneously in some cases (Fig. 4(b)), while in other instances there was a certain delay between the failure in the top cell and the cell underneath it. This signifies a degree of variation
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Fig. 3. XCT of undeformed pouch cell showing internal structure.
Fig. 4. Slow indentation of the pouch cell stack. (a) load–displacement curves with failure points; (b) voltage drop registered as a result of cell failure.
in the timing of the internal short circuit under the same loading conditions which could be the result of the cell-to-cell variation from manufacturing and usage history. The same variability is underlined by the fact that one of the cells failed prematurely, under much smaller displacement and force than the rest of the cells (Fig. 4(a)). However, it should be emphasized that in all cases any noticeable reduction in stiffness (slope of the load– displacement curve) occurred only after the failure of the second cell in the stack. Since the primary interest in this investigation is the onset of the internal short circuit which triggers catastrophic failure in the battery pack, only the potential in the top cell was monitored in the majority of the experiments. The failure analysis by X-ray tomography is shown in Fig. 5, where the views from three orthogonal cross-sections of the top cell in the stack are shown. Details of the XCT sample preparation and definition of coordinates can be found in [10]. Substantial fracturing can be observed, with the cracks aligned with the machine direction of separator, — for anisotropic properties of dry processed separators see e.g. [9,19]. The cracks profile in the plane perpendicular to the machine direction, Fig. 5(a) consist of approximately 45◦ oriented cracks propagating through the thickness of the cell. Visually, the deformation could be detected in up to five cells in the cell stack, indicating that damage propagates through five cells, causing immediate internal short circuit in one cell. 3.2. High speed deformation A total of 14 experiments on cell stacks were conducted resulting in 140 pouch cells tested under transverse loading. The results are shown in Fig. 6(a) as the load–displacement curves corresponding to different loading rates. The black asterisk markers are placed to indicate the points of voltage drop in the battery, — manifestation of failure leading to internal short circuit. Several
trends can be observed in Fig. 6 indicating major changes in the material behavior. It can be seen that the battery cell stack stiffens under impact loading compared to the quasi-static tests; the indentation force magnitude at the cell failure point, however, is on average lower than that observed during the slow deformation. More importantly, the shape of the load–displacement curve changes from concave to convex indicating a shift in type of material behavior. The parabolic force–displacement curves (circular red markers in Fig. 6(a)) are characteristic of the deformation of a porous material which undergoes compaction and jamming. On the other hand, indentation speeds higher than 25 cm/s cause the response characteristic of a dense material under compression. It has to be emphasized that the damage due to spherical indentation propagated through the entire module containing ten cells (Fig. 6(b) shows the imprint left in the bottom cell of the stack). This was not observed in the quasi-static experiment, where the visually detectable deformation propagated only through 50% of the cell stack thickness. Such a shift in the material behavior can be explained by the influence of liquid electrolyte filling the pores of the electrodes and separator. Significant stiffening of liquid-filled separator has been observed under impact loading in [3,4]. It was determined that poroelastic effect of liquid became pronounced at the strain rates of 10−3 1/s and higher. Considering that the electrode coatings in Li-ion cell are porous structures filled with liquid electrolyte, similar treatment can be extended to the whole cell realizing that by volume, approximately 85% of the cell is represented by active material coatings. Therefore, the effective behavior of the cell stack is akin to saturated soil consolidation with consideration of pore pressure. In this case, the response to compression is governed by the rate at which the liquid is being squeezed out of the pores. This rate in turn is governed by Darcy’s law, or Brinkman’s law if the shear contribution needs to be considered. Darcy’s law states that the fluid velocity is
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Fig. 5. XCT of the top cell in the stack subjected to slow speed deformation by a rigid sphere (a) side view; (b) transverse view; (c) top view of the indented area.
Fig. 6. Change in mechanical response of the battery cell stack as a function of deformation rate. (a) force–displacement response; (b) permanent deformation in the bottom cell of the stack under high rate testing.
⃗ = −κ/µ ▽ p (with ϵ proportional to the pressure gradient as ϵ · u and κ being the porosity and permeability of the solid matrix and µ representing dynamic viscosity of the liquid). The stress state of the structure is therefore modified to represent contribution from the pore pressure using the effective stress σ˜ ij = σij + bpδij where b is Biot’s coefficient [20]. Such simplified interpretation provides physics based reasoning for stiffening of the material response in Fig. 6. The real case scenario is of course complicated by the layered structure of the cell, in which the fluid flow in porous regions of electrode coatings (‘‘cell sandwich’’) is constrained by the metal current collectors with zero permeability. Therefore, the porous fluid flow is constrained to in-plane flow which under dynamic loading creates a column of high pressure that propagates deformation through the cell stack as evident in Fig. 6(b). Finally, it should be noted that the failure (i.e. internal short circuit) occurred at much smaller displacements of the indenter into the battery cell stack as can be seen in Fig. 6(a). Both the displacement and force at failure were lower than those under quasi-static loading. On average, the cell stack shorted after high-speed penetration depth being 0.5 of that under small displacement rate loading. The values for critical displacement at failure are summarized in Fig. 7 with the results spanning several orders of magnitude of the loading speed. In addition, the initial stiffness of the cell at the onset of indentation under different speeds is summarized together with critical displacement and load in Table 1. The displacement rate for each test was calculated from the displacement-time loading curve. The results of the XCT of the top cell in the stack subjected to 0.25 m/s loading are shown in Fig. 8. The overall fracture pattern is very similar to that observed in cells after low-speed deformation (Fig. 5). A more developed through-thickness cracking
Fig. 7. Critical displacement required for short circuit as a function of indentation rate.
pattern is however observed when the cells were subjected to high speed deformation, with a network of 45◦ inclined cracks running through the cell thickness, as can be seen in Fig. 8(a). Such dense network of cracks was developed under more shallow
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S. Kalnaus, H. Wang, T.R. Watkins et al. / Extreme Mechanics Letters 32 (2019) 100550 Table 1 Experimental parameters of battery module indentation. Velocity, mm/s
Stiffness, kN/m
Critical displacement, mm
Critical load, kN
0.127 203.3 242.9 261.02 377.21 557.85 737.42
667.0 6342.7 7347.0 6392.2 7330.1 13266.0 16592.0
10.14 3.55 4.04 4.56 4.26 4.02 4.47
35.05 24.21 23.68 27.22 26.62 29.84 28.02
Fig. 8. XCT of the top cell in the stack subjected to high speed deformation by a rigid sphere (a) side view; (b) transverse view; (c) top view of the indented area.
complemented with non-destructive failure analysis. Therefore, the cells in the modules were completely discharged so that the thermal runaway events would not prevent the observations of mechanical behavior. As the outcome of this comprehensive program involving 14 Ford Focus EV battery modules we observed the following.
• The displacement of the indenter into the module required
Fig. 9. Deformation of the electrodes in bottom cell of the 10-cell module subjected to high-velocity indentation by rigid sphere.
penetration of the indenter, compared to quasi-static case (Fig. 7). In all cases of the high-speed indentation, the deformation propagated through the entire cell stack, resulting in clearly visible imprint in the bottom cell, Fig. 6(b). X-ray tomography of the bottom cell of the stack revealed no through-thickness cracking. However the deformation of the electrodes can be clearly seen in the scan of the edge of the imprint in the bottom cell (Fig. 9). While such perturbation may not result in immediate safety related events, such change in electrodes geometry may trigger delayed processes and compromise the battery safety. 4. Conclusions The effect of loading speed on mechanical response and failure of the EV battery module has been investigated by out-of-plane indentation of pouch cell stacks. The goal of the investigation was to understand rate-dependent changes in the material behavior
to trigger the failure and short circuit reduces by a factor of two when the indentation speed is 25 cm/s or higher. This indicates that even a relatively shallow impact done at high speed can induce enough damage to create fault lines in the layered structure of the battery and trigger contact between electrodes of opposite charge. • The mechanical behavior of the cell stack changes profoundly under high speed loading. When impacted at speeds higher than 25 cm/s the cell stack response stiffens significantly (the force increases by an order of magnitude) and behavior shifts from a compressible, crushable type to that characteristic of a dense material. This indicates the necessity of development of new constitutive models for mechanical abuse under high displacement rates. • A significant propagation of damage into the battery module was observed under fast impact, despite the factorof-two decrease in displacement of indenter required to induce the short circuit. The XCT of the cell farthermost from the indentation site did not reveal any cracking in the electrode structure and only some rearrangement of layers due to compression was observed. Nevertheless, the fact that a 4 mm indent can propagate deformation through an entire battery module, can result in imposing additional requirements for the battery safety. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract
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DE-AC05-00OR22725, was sponsored by the National Highway Traffic Safety Administration, USA. Rick R. Lowden is acknowledged for his assistance and expertise in setting up the experiments. Xiaoqing Zhu is acknowledged for his assistance in disassembling the modules and preparing cells. References [1] S. Arora, W. Shen, A. Kapoor, Review of mechanical design and strategic placement technique of a robust battery pack for electric vehicles, Renew. Sustain. Energy Rev. 60 (2016) 1319–1331. [2] D. Aurbach, Y. Talyosef, B. Markovsky, E. Markevich, E. Zinigrad, L. Asraf, J.S. Gnanaraj, H.-J. Kim, Design of electrolyte solutions for Li and Li-ion batteries: a review, Electrochim. Acta 20 (2–3) (2004) 247–254. [3] J. Cannarella, X. Liu, C.Z. Leng, P.D. Sinko, G.Y. Gor, C.B. Arnold, Mechanical properties of a battery separator under compression and tension, J. Electrochem. Soc. 161 (11) (2014) F3117–F3122. [4] G.Y. Gor, J. Cannarella, J.H. Prevost, C.B. Arnold, A model for the behavior of battery separators in compression at different strain/charge rates, J. Electrochem. Soc. 161 (2014) F3065–F3071. [5] S. Kalnaus, Y. Wang, J. Li, A. Kumar, J.A. Turner, Temperature and strain rate dependent behavior of polymer separator for Li-ion batteries, Extreme Mech. Lett. 20 (2018) 73–80. [6] I. Avdeev, M. Martinsen, A. Francis, Rate- and temperature-dependent material behavior of a multilayer polymer battery separator, J. Mater. Eng. Perform. 23 (2014) 315–325. [7] J. Chen, Y. Yan, T. Sun, Y. Qi, X. Li, Deformation and fracture behaviors of microporous polymer separators for lithium ion batteries, RSC Adv. 4 (2014) 14904–14914. [8] S. Kalnaus, Y. Wang, J.A. Turner, Mechanical behavior and failure mechanisms of Li-ion battery separators, J. Power Sources 348 (2017) 255–263.
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