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Experiments and 3D detailed modeling for a pouch battery cell under impact loading
Journal of Energy Storage 27 (2020) 101016
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Experiments and 3D detailed modeling for a pouch battery cell under impact loading Pan Zhexin, Li Wei, Xia Yong
T
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State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion pouch cell Impact loading Detailed modeling Fracture Internal short circuit
Mechanical behavior of Lithium-ion batteries under dynamic impact loading is crucial in assessing and improving the crash safety of batteries. To understand the possible causes of internal short circuit (ISC) in the impacted batteries, both experimental and numerical methods are necessary. Quasi-static and dynamic tests for a type of pouch-type battery cell are performed according to a UN standard. The mechanical characterization of all the component materials in the battery is carefully carried out under different strain rate and loading conditions. Then a detailed 3D Finite Element (FE) Model of the pouch cell with all the components is established. Combining the experimental and numerical approaches, the dynamic deformation process under the UN 38.3 Impact tests and the corresponding failure mechanism of pouch cells are firstly captured. Meanwhile, the conditions for the occurrence of thermal runaway are investigated with the comparison of quasi-static and dynamic tests. It is believed that the rough fracture surface and the crushed powdery residue are responsible for the continuous internal short circuits and thermal runaway of the broken batteries. To improve the crash safety performance of the pouch cell, some safety suggestions are proposed.
1. Introduction With high demands in markets of consumer electronics and electric vehicles, the production and applications of lithium-ion pouch cell batteries come to an explosive growth. As a daily-use energy storage unit, lithium-ion batteries have received primary safety concerns. The batteries under external mechanical abuse conditions may lead to the internal short-circuit (ISC) and even fire or explosion subsequently. To provide technical basis for lithium-ion battery safety in crash events, the mechanical failure behaviors and ISC mechanism should be systematically studied. In recent years, mechanical responses of cylindrical cells, pouch cells and prismatic cells have attracted interest of quite a few research teams worldwide, and valuable experimental and simulation investigations have been carried out, as summarized in [1]. We are going to elaborate the results from battery mechanical experiments under different loading conditions to various approaches of simulations. From the perspective of battery experiment, Maleki and Howard suggested that the pinching tests of pouch cells is quite suitable to mimic the high-risk ISC situations [2], which inspires the similar indentation tests with indenters in different shape such as a hemispherical punch [3,4]. Wierzbicki et al. conducted various quasi-static tests of both cylindrical and pouch cells, and revealed the correlation between ⁎
mechanical failure and occurrence of ISC [4–8]. In the meantime, Sahraei et al. developed an effective method to equivalently characterize the mechanical properties of pouch batteries under local indentation as a homogeneous material [4,5]. Luo et al. studied the damage evolution of a pouch cell subjected to indentation by performing indentation tests at different force levels and inspection of change of constituents inside the post-mortem samples [9]. Li et al. performed comparative tests of several typical battery types and observed different failure patterns due to different indenter shapes [10]. Most researchers tested the pouch cells through the thickness direction by considering the fact that the large flat area has higher probability to be severely loaded, while Pan et al. for example investigated the mechanical behavior of pouch cells subjected to in-plane compression, and observed the local buckling, kink and shear-band deformations of cell components [11,12]. Above-mentioned researches usually focused on the quasi-static loading conditions, which is indispensable for understanding the basic mechanical response of the battery structures. However, in reality the safety issue of lithium-ion batteries sometimes has to be addressed in impact scenarios like electric vehicle crash or electronic device drop. Accordingly, the dynamic tests could be more representative. The dynamic tests of batteries are relatively difficult in terms of test setting-up and technical operation. Thereby fewer reports can be tracked in the
Corresponding author. E-mail address: [email protected] (Y. Xia).
https://doi.org/10.1016/j.est.2019.101016 Received 14 August 2019; Received in revised form 30 September 2019; Accepted 8 October 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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with electrolyte. Jellyroll is enveloped inside a pocket of aluminumplastic film. The cathode is a sandwich of an aluminum-foil current collector and two coating layers of Lithium Cobalt Oxide, while the anode is a sandwich of a copper-foil current collector and two coating layers of graphite, also shown in Fig. 1(b). The separator is made of polyethylene. All the geometrical parameters of the pouch cell are summarized in Table 1. Considering the structural complexity, any test directly upon the full battery cell is of structural response rather than a single material response. To develop the detailed FE model, each of the individual component materials has to be carefully tested and characterized.
public domain. Kisters et al. conducted quasi-static and dynamic tests of pouch and elliptic cells, which shows that the impact velocity and the existence of electrolyte significantly influence critical force and stiffness of batteries [13]. Xu et al. studied the dynamic mechanical integrity of cylindrical cells under transverse crushing by taking into consideration the strainrate and inertia effect in the simulations [14,15]. Chen et al. and Xia et al. performed the dynamic impact tests of stacked pouch cells and the battery modules, analyzed the integrated failure behaviors under various loading conditions, and in the meantime, built a FE model of the battery module based on homogenization of pouch cells [16,17]. From the perspective of simulation, Battery is a multi-layered structure of cathode, anode and polymer separators, which make the modeling a challenging work. Sahraei [7], Pan [12] and Zhang [18] separately developed the micro-scale representative volume element (RVE) to study the deformation mechanism. Another modeling method treats the battery jellyroll as a homogenized material. The homogenized FE models were built by employing and calibrating the crushable foam model [4–6], or the honeycomb model [3]. However, the accuracy of homogeneous simplifications strongly relies on the selection of material model. It is difficult for them to properly reflect the underlying physics. A systematic comprehension from the constituent level behavior to the full-cell response is essential to the safety focused battery study [1,3,5,19]. A detailed model of the 18650 cell developed by Zhu captured the typical deformation stages during the axial loading, exhibiting the benefits from the detailed modeling [20]. Wang et al. also performs their attempts on the cylindrical cells [21]. So far there is only few reports of a research focusing on the detailed modeling of pouch cells [22,23]. Although the battery safety studies have not been settled, quite a few standards and regulations for batteries have already been composed out of safety concerns. Drafted by the organizations like ISO, SAE, NHTSA, the standard tests require examining the safety performance of lithium-ion batteries subjected to typical mechanical abuse loading, such as drop tests, shock tests, nail penetration tests, crush tests [24]. For instance, UN 38.3:2015 defines an impact test as follows. A 15.8 mm diameter steel bar is placed across the center of the sample while a 9.1 kg mass is released from a height of 610 mm at the intersection of the bar and to impact the sample cell, as shown in Fig. 1(a). It requires that the battery after the impact should not be smoky and fired. This impact test is often run by the regulation agencies and any battery undergoing air transportation should pass this standard test. Therefore, the mechanical response and the failure mechanism of pouch cells in this test type are worthy of in-depth analysis. The objective of the present paper is to explore the deformation and failure progress of pouch cell under the impact loading of UN 38.3 type by the aid of experiments and FE simulations. A detailed finite element model of a pouch cell is developed. The metal current collectors, electrode active layers and separators are modeled as individual components and the interfaces between various layers are carefully calibrated and treated. By comparing the simulations and test results, mechanisms of internal short circuit are investigated through the deformation and damage analysis of the electrodes and separators. This method provides a picture of the dynamic deformation progress inside the battery cells and helps to figure out the possible access to improve pouch cell crash safety.
2.2. Material test preparation
2. Experimental preparation
To develop the detailed FE model, all the component materials are tested under various strain-rate and stress conditions, and the material properties are identified. Asymmetric tension-compression behavior is one of the typical mechanical features of the electrodes, which is closely related to the sandwich structure of the electrodes. When the electrode is compressed from the direction normal to its surface, i.e. the out-of-plane direction, the active layers contribute the most to the deformation. Nevertheless, in the tensile tests it is the metal foil current collector that affords the largest proportion of the force. Regarding this, in the FE Model developed in the present study both the metal foil current collector and the active layer are carefully characterized. The metal foil current collectors are extracted from the batteries for testing by treating the cathode and anode with N-methyl pyrrolidone (NMP) to remove the active layers. Thereafter, we measure the mechanical properties of the electrodes directly and the current collectors, while the tensile and compression properties of the active layer can be indirectly identified by comparing the test results between the electrode and the current collector. The other components of the battery cell, i.e. the separator and the pocket that play essential roles in ISC prevention and protection, are also tested. All the component materials are extracted from the batteries at 0 SOC to mitigate possible influence of SOC on material properties. Fig. 2(a) shows the specimen die-cutting molds for tension and compression tests. The specimen shape is chosen to meet the battery geometry. Cathode, anode and separator samples are extracted from a disassembled battery cell and cut into the specified shapes by the molds. All the tension tests are performed in 3 directions, MD(0°), DD(45°), TD(90°), which is the angle relative to the battery rolling direction, in order to examine the anisotropy of the component materials. For compression tests, round shape discs of 16 mm diameter are cut from the thin sheets of component and stacked in the thickness direction: 35 layers for electrodes and 70 layers for separators. By this way, the total thickness is a measurable quantity and it makes the relative error of deformation measurement small enough in the compression test. The quasi-static tests of component materials are performed on a Zwick Z020TE test machine with a loading speed of 0.6 mm/min, and the dynamic intermediate strain-rate tests are performed with a hydraulic high-speed test machine, which is able to reach a speed of 2 m/ s. A high speed camera is adopted to record the deformation information and then Digital Image Correlation (DIC) method is used to acquire the deformation field. Every test type is performed more than 6 times to exclude individual differences.
2.1. The structure of the tested pouch cell
2.3. Pouch cell test set-up
The tested object in the present study is a commercial pouch battery cell that serves in consumer electronics, as shown in Fig. 1(b), The capacity of battery is 16.98 Wh, and the open circuit voltage is 3.85 V. The major part of the battery cell is the jellyroll, a tightly wound structure of the cathode-separator-anode-separator laminate soaked
As mentioned above, the UN 38.3:2015 T.6 impact/crush Test scenario is focus in this study. Drop tower is employed to release a dropped weight of 9.1 kg from the height of 610 mm. Setup of the test is shown in Fig. 2(b). The fixture is placed on a force sensor, which acquires the force signal during the impact. The time of the whole loading process is 2
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Fig. 1. Impact test of pouch cell.
3. Constitutive mechanical behavior of component materials
Table 1 Geometry of battery cell. Parameters
Value (mm)
Length of the cell Width of the cell Height of the cell Parameters Pouch pocket Cathode current collector Cathode active layer Anode current collector Anode active layer Separator
71.0 54.0 5.46 Thickness (µm) 105 10 58 8 65 9
3.1. Tension behavior The quasi-static and dynamic tensile test results of cathode, anode, aluminum foil, copper foil, separator and the pouch pocket are exhibited in Fig. 3, respectively. The comparison of experimental results shows that the active layer only affords about 5% of total force, and electrodes exhibit a similar elastoplastic behavior just like the metal foils. Moreover, the failure strain of electrodes is consistent with that of current collectors, showing that the failure of current collector happens to trigger the electrode failure at both quasi-static and dynamic condition. Since the thickness of aluminum and copper foils are less than 10 μm, both of the current collectors are too fragile to bear large deformation. Attributed to the difference between aluminum and copper foils, the cathode only has failure strain of 0.01 and yield strength of 140 MPa while anode has a failure strain at 0.02 and a yield strength of 220 MPa. As the cathode and anode are closely placed and exposed to the similar mechanical loading, this incompatibility of cathode and anode in strength and stiffness could be the reason of battery internal failure. Tensile tests of different directions are performed for electrode materials to clarify the in-plane anisotropic behaviors. It turns out that both cathode and anode have limited anisotropy; the yield strength and the failure strain have a little difference in 3 directions. The corresponding current collectors have the similar behaviors. In addition, for the strain-rate effect, electrodes and current collectors share the same trends: as the strain rate increases, the yield stress increases but the failure strain of metal foil decreases. The
less than 2 ms. Displacement of the steel bar and battery deformation is recorded by a Phantom high-speed camera with a frame rate of 100000 fps. The temperature of battery is recorded by a thermocouple. Meanwhile, the voltage of battery is monitored using a data acquisition card and synchronized with force and displacement signals. Moreover, corresponding quasi-static tests with a continuous loading of 2 mm/min are also performed on a universal testing machine to investigate the speed effect of the battery. The influence of state of charge (SOC) on the battery mechanical behavior is also concerned, so battery cells under 4 states of charge (SOC=0%, 30%, 60%, 100%) are tested for dynamic loading conditions.
3
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Fig. 2. Pouch cell dimensions and the preparation for the material tests and impact tests.
exhibited in Fig. 3(k). Limited by the load capacity of the testing machine sensor, the maximum stress only reaches 250 MPa, and cathode and separator have not been compressed to failure. Cathode has a three-stage stress-strain plot under the compression. At the early stage, the stiffness increases, representing the densifying stage. When the stress increases to 135 MPa, the curve encounters a turning point and enters the second stage. The active particles of cathode are being crushed to fill the space between them and the force reaches a plateau at the strain range from 0.18 to 0.32. Then it comes to the third stage, where the stiffness increases again and the material undergoes another densification process. For anode, the compression tests have an opposite trend with tensile tests: the strength and failure strain of anode are much smaller than those of cathode. Anode specimen collapses at 56 MPa under compression and cracks go through the thickness direction, making the specimen split into pieces. The failure of anode is much easier than the cathode under compression. Since the failure strain of aluminum foil and copper foil do not have such large difference, this phenomenon implies the different transverse deformation of cathode and anode active layers under compression. The compression stress-strain behavior of separator has an exponential form and the stiffness keeps increasing. For further validation, the integrated battery sample containing 18 repetitions of cathode-separator-anode-separator laminate is directly cut from the battery cell and tested, as shown in Fig. 3(k). It can be
aluminum foil under dynamic loading is so brittle that it nearly fails at the elastic stage. In this study, both cathode and anode are treated and calibrated as transverse isotropic elastic-plastic materials with strainrate effects. As a porous polymer material, separator has the following characteristics: superelasticity, visco-plasticity, and anisotropy. hyperelasticity, viscoplasticity and anisotropy. The failure strain of separator reaches 100%, as shown in Fig. 3(g). It should also be noted that the anisotropy of presented separator is quite lower than separators from literature [25], which can be attributed to the wet-processed manufacture. The difference between plastic behaviors in 0° and 90° is less than 20% and the failure strains in different directions are almost the same. The strain rate effect of this separator is remarkable: the stress level increases by 100% from quasi-static condition to dynamic condition and the failure strain decreases to 0.4 at a strain-rate of 2.5 s−1. Pouch pocket is a type of composite made of aluminum foil and polymer material, serving as the major protection for the pouch cell. It has a failure strain of 0.18 and isotropic yielding behavior, as shown in Fig. 3(h). With the increase of strain-rate, both the strength and failure strain of the aluminum-plastic film both increase, which could lead to better performance under impact condition. 3.2. Compression behavior The compression results of cathode, anode and separator are 4
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Fig. 3. Experimental results of all component materials: (a) Cathode (b) Aluminum foil (c) Comparison of cathode and cathode current collector (d) Anode (f) Comparison of anode and anode current collector (e) Copper foil (g) Separator (h) Pouch Pocket (k) Material compression results.
the same:
observed that the integrated battery sample has the stress-strain curve shape and the failure strain quite similar to the anode. This implies that the compression failure of the whole battery cell structure is triggered by the fracture onset of anode. In cathode and anode, the thickness of active material coating is about 10 times larger than that of the metal foil current collector and the stiffness of active coating is much weaker. When the electrode is compressed during the impact, the active layer will take the most deformation and dominate the total behavior. The active layer is composed of small-size granules of active material stuck together by the binder, which has the natural porosity and compressibility. The active layer can be assumed isotropic since the microscale structure has no apparent orientation. The compression through the thickness direction can represent its mechanical behavior.
The average compression strain consists of both components of electrode based on the ratio of thickness t: (3)
εe te = εcc tcc + εa ta
Since the current collectors are made of metal foils, the properties of which are obtained through the tensile tests, then Eq. (3) is used to get the exact compression behavior of the active layers. As the properties of current collectors have been obtained from tensile tests and the mechanical behavior of current collectors is considered as tension-compression symmetric, the compression stressstrain properties of active layers could be exacted using Eqs. (2) and (3). The crushable foam material model (Mat_63) in LS-DYNA is applied to characterize the active layers of electrodes, in which the curve of nominal yield stress versus volumetric strain needs to be defined. In the present study the models of the cathode and anode active layers are calibrated with the aforementioned test results, respectively. Due to the limited anisotropy, the cathode and anode current collectors are characterized with an isotropic elastoplastic material model with the Cowper-Symonds formulation of strain-rate effect (Mat_24 in LS-DYNA),
3.3. Material calibration Although the active layers have the thickness of about 0.12 mm in total, the tensile behaviors of electrodes mainly depend on the metal current collector of a thickness about 0.01 mm.Assuming that the metal foil and active layer share the same strain during the tensile tests, the force equilibrium of the sandwich structure is considered as:
σe Se = σcc Scc + σa Sa
(2)
σe = σcc = σa
(1)
σ = E : (ε − εp) ⎧ σ¯ ⎪ ε˙p = ε¯˙p σ ⎪ ⎨ ⎪ σy (εp, ε¯˙p) = σy (εp) ⎡1 + ⎢ ⎪ ⎣ ⎩
Here, the σe, σcc and σa represent the stress of electrode, current collector and active layer, while Se, Scc, Sa refer to the section area. The contribution of metal foil and active layer to the total force can be portioned by the product of the stress and the section area of each component. The compression testing results of electrodes are the overall response of two active material layers and the current collector, which means some assumptions are needed to obtain the properties of active material. The σ for each layer through the thickness direction is
1 p
( ) ⎤⎥⎦ ε¯˙p c
(4)
where ɛ and ɛp denote the total and plastic strain tensors, and ε˙p is the equivalent plastic strain rate. The Young's Modulus E and plastic hardening curve σy(ɛp) is calibrated through the quasi-static tests while 5
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SOCs have been smoky and temperature of which increase above 100 °C, revealing the ISC occurred inside. The batteries of 0%, 100% SOC only have a temperature shifting to 50 °C and pass the impact tests. In Fig. 4(c), the top and section views of the battery cells after tests of both quasi-static and dynamic conditions are exhibited. Focusing on the quasi-static loaded battery shown in the left pictures, 2 pouch pocket cracks appears on the side below the indenter and through the width direction of the battery. When we observe from the cross-section direction, these two cracks correspond to the shear slipping line on the either side of the battery. The indenter squeezes the battery directly below and creates the smooth shearing fracture surface even if the battery is made of different component materials. This observation explains why the quasi-static force curve has a sharp peak, as the peak illustrates that all the battery components fail at the same time due to the concentrated shearing under quasi-static loading. The battery deformation is localized in the area under the indenter, and the bottom face of pouch pocket and seals of both sides are intact. When it comes to the dynamic impact tests, the situation is different. During the exact moment of about 2 ms, the different cell components will have the combining deformation of tension and compression, leading to the failures. The initial kinetic energy of indenter is much larger than the necessary energy to break the battery, which means the cell will directly be squeezed into 2 pieces by indenter and flying in 2 opposite directions. The broken jellyroll parts are pushed horizontally to move and break the head seal and the pouch pocket on the tail. This movement of jellyrolls could benefit the battery safety since the detachment of the fracture areas would lead to few internal short circuits. Unlike the compressed area of quasi-static tests can still hold the shape, the materials under the indenter have been crushed into powdery residue, which may bring larger ISC probability. If we look closely at the battery area near the fracture, we can find the local kink forming at the several top layers and the shearing fracture surface is rougher than the quasistatic conditions. Attributed to the battery rolling-up structure, battery top layers and bottom layers are actually separated and afford different loads and deformation.
parameters C and p is calibrated from all the strain rate tests. According to the limited anisotropic behaviors of separator and pouch pocket, these two materials are simplified and characterized using the same isotropic elasto-plastic constitutive model. 3.4. Interface strength Due to the multi-layered structure, the battery cell has plenty of interface contacts, which would have large influence on the battery mechanical response. There are two major categories of interfaces: the interfaces between the active layer and the current collector in cathode/anode (referred to as cathode/anode interface in this paper) and the interfaces between the active layer of electrode and the separator (referred to as separator interface in this paper). Luo et al. have introduced a testing method for the interface strength of electrodes under combined tension-shear loading [26], which is also employed in the present study. The test results show that the normal failure strength of the cathode interface is 2 MPa, while that of anode interface is 0.3 MPa. The separator interface is much weaker, just about 0.01 MPa, but existence of this interface strength is essential for keeping the integrity of the battery cell. 4. Standard impact test The force and voltage measurements over the battery intrusion of the quasi-static and the UN38.3 standard impact tests are shown in Fig. 4(a). The dash lines are the force-intrusion plots and the thinner solid lines represent the voltage trend relative to the battery intrusion. There are several obvious differences between the mechanical responses of the battery cell under quasi-static loading and the dynamic loading. An important feature in quasi-static tests is that force plot has only one peak but that of dynamic tests have 3 peaks. Under the quasi-static condition, the load is continuously applied on the battery with no interrupt. However, the dropped weight is not connected to the indenter at beginning that results in three-body collision. At first, the dropped weight will hit the steel bar indenter and give it a higher speed, making it detached and load the battery. The steel bar will accelerate and decelerate between the dropped weight and battery, squeezing the battery, which leads to the multiple peaks of force plot under the dynamic impact. At the initial stage of impact, the battery stiffness under dynamic loading is far larger than the quasi-static loading conditions while the first peak of the dynamic curve appears at the smaller intrusion than the quasi-static one. This phenomenon is widely observed in dynamic tests different battery types [13], which indicates the battery becoming stronger and more vulnerable under dynamic conditions. The second stage of impact can be defined as the intrusion range from 0.5 mm to 2.5 mm, from the first peak to the second peak. The decrease of the force suggests the failure inside the battery. Under quasi-static condition, all battery components seem to fail at the same intrusion level because of its sharp force peak, but under the impact condition, the collapse of battery last a larger range. It infers that here the failure of different layers is a gradual process. The voltage of battery is supposed to reveal the battery internal short-circuit condition in tests. As we can observe, the voltage drops around the force peak under the quasi-static condition, however, voltage under dynamic impact drops before the first peak force, implying that the internal short circuit may happen earlier under the impact. The final stage starts at the intrusion around 2.5 mm. After this point, the force response of the battery decreases to a low level, and the battery is being separated to 2 parts by the continuously downward indenter. Impact test results with various SOCs are shown in Fig. 4(b). It demonstrates a slight increase of the force with the higher SOC. It could be contributed to that the higher SOC can inflate the battery cell and increase the internal stress. Additionally, the batteries of 30%, 60%
5. Finite element simulation 5.1. FE model To reflect the battery response as real as possible, all the battery components should be taken into the simulation. Then, the detailed finite element model of battery cell is established in LS-DYNA R7, as shown in Fig. 5. The pouch pocket, cathode current collector, anode current collector and separator are built as shell elements, while the cathode and anode active layers are built as solid elements. To capture the meticulous mechanical features, the element size of concentrated deformation area is 0.1 mm and totally 2217242 elements are employed here. All the components material definitions are summarized in Table 2. All the shell elements are using isotropic material card MAT24. The plastic yielding behaviors of all the component materials are calibrated directly from the testing results and defined with curve input. The strain rate effects of metal foils, separators and pouch pockets are also considered and defined in the material cards. Active layers are defined with material card MAT63, which is a crushable foam card. The failure criteria obey these rules: shell elements are based on maximum tension strain because of the stretched scenario and the solid elements are defined by maximum principle strain and effective strain combining both tension/compression conditions. The contact interfaces between layered component structures are carefully defined. The contact between the electrodes and current collectors are CONTACT_ TIED_ SURFACE_ TO_ SURFACE; and the contact between the active layers and separators are CONTACT _TIEBREAK _SURFACE _ TO_ SURFACE; and the other possible contact are controlled by global-defined CONTACT_ AUTOMATIC_ SINGLE_ SURFACE. 6
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Fig. 4. battery test result.
stiffness of dynamic modeling has a very good agreement with that of experiments, demonstrating the detailed model with the dynamic loading boundary condition is able to reveal the stiffness strengthen observed in the dynamic tests. The detailed simulation is also capable of capturing the damaging process during the impact. The intrusion range for the force peak is about 1 mm, and the quasistatic condition make the deformation more concentrated. Since the failure achieved in FE simulation is by element deletion, the damaged elements under the impactor cannot afford the compression any more. Then simulation force of the final stage directly come to 0, while the experimental data still have about 5 kN left due to the remains. As all the material tests are based on 0 SOC battery cells, the dynamic impact simulation results comparing to the 0 SOC standard impact tests are exhibited in Fig. 6(b). At the initial stage, the initial stiffness of dynamic modeling has a very good agreement with that of experiments, demonstrating the detailed model with the dynamic loading boundary condition is able to reveal the stiffness strengthen observed in the dynamic tests. The detailed simulation is also capable of capturing the damaging process during the impact. The green circle A has highlighted an inflection point of the experiment curve, which is clearly exhibited on the simulation curve. It indicates the crushed anode material starting to fail. The reason of the force drop is that the collapse of anode is achieved by element deletion
Boundary condition is the same as the standard impact tests, the drop mass of 9.1 kg and the indenter are also integrated in the simulation, and the initial speed of the drop mass is 3.46 m s−1 corresponding to the drop height of 0.61 m. The simulation was carried on the parallel computing system and the computational time for the impact simulation was about 34 h with 16 Xeon E5-2690v4 CPUs, while the quasi-static condition took more than 200 h.
5.2. Force-intrusion simulation The force-intrusion results of the simulation are shown in Fig. 6. Under Quasi-static condition, the force-intrusion curve matches well with the experiments results. Before the force peak, the hardening trend of the simulation at initial stage is consistent with that acquired from the tests, and failure happens at around 1.6 mm intrusion. Since the failure achieved in FE simulation is by element deletion, the damaged elements under the indenter cannot afford the compression anymore. So simulation force of the final stage directly come to 0, while the experimental data still have about 5 kN left due to the remains. As all the material tests are based on 0 SOC battery cells, the dynamic impact simulation results comparing to the 0 SOC standard impact tests are exhibited in Fig. 6(b). At the initial stage, the initial 7
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Fig. 5. Finite element model of the pouch cell battery.
being compressed into debris by the indenter, while the other 2 parts of battery cell absorb the kinetic energy of the indenter and start the lateral motion. These movements break the pouch pocket and the head seal. Finally, the battery is cut into 2 parts and flying in the opposite directions. The simulation meets the battery dynamics during the impact experiment, and make the detail observation for the battery deforming process accessible. From the cross-section view of the simulation, the von-Mises stress contour is plotted in Fig. 7(b). To be specific, it could be found that there is no fracture inside the battery at 0.5 ms. The stress is concentrated on the central area of battery and has a bell-type distribution. Afterwards at 0.6 ms, cracks first appear at the top of the battery and propagate immediately cross the battery section in the middle of compressed zone. The crack propagation forms a tilt line, which has a certain angle (about 70°) to the horizontal direction, consistent with experimental observation. Since the impact is a dynamic process, the high residual stress on both sides of battery leads to the more fractures after 1.2 ms. Thus, there are more fractures in the dynamic impact condition than the quasi-static condition, creating more debris in the central area.
in FE software. Meanwhile, the maximum force also matches the testing results, the only problem is that the intrusion for maximum force have a difference of 0.1 mm. The force peak of battery under the dynamic loading is lower and wider than the quasi-static condition. Another important feature for battery under the dynamic conditions is the multiple peaks of the force, which come from the reciprocating motion of the indenter and the battery vibration. The proper boundary condition makes the simulation able to produce this feature as the experiment does. However, the vibration period is smaller.
5.3. Battery deformation process This section is supposed to reveal the dynamic impact simulation results, since we mainly focus on the standard impact condition. Fig. 7(a) is the section view of battery deformation process for the impact simulation from 0 to 1.2 ms. The initial stage for the battery impact is from 0 to 0.6 ms, the intrusion of the indenter makes the central part of battery compressed and nearby parts slightly tilted. Winding center plane of the not-directly-impacted battery part starts to separate and the pouch pocket bends under the indenter. At this stage, the material fracture has not occurred. As the anode is compressed to its compression failure criterion, the crack appears at 0.6 ms and immediately crosses through the layers. The failure of crushed anode will trigger the neighbor layers successively. This point is also corresponded to the force peak on the force-intrusion plot. At the final stage, from 0.9 ms to 1.2 ms, battery failure keeps developing with the further intrusion of the indenter. Two more cracks extend cross the whole battery at the edge of the impacted area, separating the battery apart. The crush zone in the middle is still
5.4. The post-crash features of battery The detailed battery model can not only reproduce the battery deforming process but also capture the important post-crash characteristics of battery. The detailed model is able to reflect the rational morphological features of the fracture. The fracture surface of the battery grows along the certain angle and two fracture lines are likely to form the fracture surface as the blue circles shown in Fig. 8(a).
Table 2 Component modeling information. Components
Material card
Young's Modulus (GPa)
Yielding stress (MPa)
Failure Strain
Pocket Cathode current collector Cathode active layer Anode current collector Anode active layer Separator
MAT24 MAT24 MAT63 MAT24 MAT63 MAT24
11.9 46.0 1.2 52.7 0.8 1.5
32.7 137.5 Curve defined 184.2 Curve defined 13.5
0.028 0.01 0.5 0.015 0.3 0.4
8
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Fig. 6. Force intrusion plots from simulations.
Fig. 7. Simulation results for dynamic impacts. 9
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Fig. 8. Comparison of simulation and experiment: (a) Formation of local kinks and fracture surfaces (b) Failure of the pouch seals at the edge of battery.
not inevitable. However, if the remaining parts of the battery could separate from each other and keep a clear fracture surface, ISC will not progress afterwards and the accumulated heat may be not enough for the material ignition. Then the battery could still be safe even if it is broken during the impact. Otherwise, if the residual debris caused by the impact keeps different electrode layers connected, there would be the continuous ISC after impact and the severe thermal runaway at final. From the detailed model simulation, the generation of debris can be recognized and the central crushed area could have contacts with the remaining parts of battery. Meanwhile, the local kinks appeared on the battery top layers and irregular battery fracture section observed during experiments could also make the contact of cathode and anode happen, denoting the higher ISC risks. From the dynamic tests of batteries, the battery safety could benefit from the break of the seal and pouch pocket, because the broken battery part could move laterally and detach from the residue debris in the central crushed zone. Inside the electronic devices and the electrical vehicles, the batteries are embedded in a certain slot. This strong restraint, limiting the lateral movement of broken battery, will maintain the ISC condition and increase the thermal run-away probability. Some suggestion could be drawn to improve the battery safety under the impact condition. Firstly, inhibiting the generation debris will be a possible solution. It can be achieved by employing electrode current collectors of higher strength and elongation. Nowadays the battery manufacturer tends to use the thinner metal foil to serve as the current collector. The thinner foils of the lower strength and elongation, are more likely to broken into pieces and create more fractures surface inside the battery. The strengthening of the current collector will create fewer debris and keep a uniform fracture surface. Secondly, we need to avoid the rough fracture surface and the local kinks. If we are able to increase the interface strength inside the battery, the integrity of battery will improve and the dislocation of different layers will not emerge. By this way, the broken electrode will still be protected by the broken separator, the ISC will not appear at the fracture surface. At last, to preset space for the battery deformation and movement is necessary. If the battery slot is crushable and allow the broken parts of battery move laterally and detached, the risk of thermal runaway will decrease.
Comparing the simulation results with the battery after tests, the inconsistent failure of different component layers results in the rough and rugged fracture surface. Moreover, it is clear that the detailed model is capable of identifying the local kinks generated at the top layers of the battery, in Fig. 8(a). From the simulation we could know that the intrusion of indenter keeps pushing the upper layers of batteries move aside, so that the uncoordinated motions result in the local kinks. It is worth noting that material failure is a necessary and insufficient condition for ISC. Both kinks and rough fracture surface are potential risks for ISC, because the dislocation of electrode layers creates risk of interaction between cathode and anode. If the cathode and anode contacts directly at the fracture surfaces or kinks, the ISC could happen and even lead to severe thermal runaway and fire. However, if the fracture surface is smooth and debris does not connect different layers, the cathode and anode can still be separated by the remained separator. In this case, even if severe damage has happened to the battery, the ISC would stop without thermal runaway. This can be verified through the comparison of quasi-static and dynamic tests that the thermal runaway is only observed under the dynamic condition. Additionally, the detailed model is also able to capture the failure of the sealing and the rush-out of the battery parts. Fig. 8(b) shows the seals breaking on the both side of battery. The seal of the battery is manufactured using the thermoplastic sealing process. During the impact, the battery divides into two parts laterally and push the separated core breaking the seals on the side of the pouch pocket. When the separated battery parts could break the seals and move out, the possibility of ISC decreases. It indicates that the weakness of seals can benefit the battery safety to some extent. 6. Discussions In the presented tests and simulations, the mechanical characteristics of the battery under the impact loading is well demonstrated. When a pouch cell is applied a mechanical abuse loading, the voltage of battery drops will drop at the force peak, which corresponds to the material fracture. At the same time, the inner material failure could lead to the internal short circuit (ISC), and even the dangerous thermal runaway. The standard impact tests are supposed to verify whether the thermal runaway would happen during the impact. From our experiment results of this pouch cell, the occurrence of the material failure is 10
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7. Conclusion
Sources 357 (2017) 126–137, https://doi.org/10.1016/j.jpowsour.2017.04.103. [4] E. Sahraei, J. Meier, T. Wierzbicki, Characterizing and modeling mechanical properties and onset of short circuit for three types of lithium-ion pouch cells, J. Power Sources 247 (2014) 503–516, https://doi.org/10.1016/j.jpowsour.2013.08. 056. [5] E. Sahraei, R. Hill, T. Wierzbicki, Calibration and finite element simulation of pouch lithium-ion batteries for mechanical integrity, J. Power Sources 201 (2012) 307–321, https://doi.org/10.1016/j.jpowsour.2011.10.094. [6] T. Wierzbicki, E. Sahraei, Homogenized mechanical properties for the jellyroll of cylindrical lithium-ion cells, J. Power Sources 241 (2013) 467–476, https://doi. org/10.1016/j.jpowsour.2013.04.135. [7] E. Sahraei, E. Bosco, B. Dixon, B. Lai, Microscale failure mechanisms leading to internal short circuit in li-ion batteries under complex loading scenarios, J. Power Sources 319 (2016) 56–65, https://doi.org/10.1016/j. jpowsour.2016.04.005. [8] S.H. Chung, T. Tancogne-Dejean, J. Zhu, H. Luo, T. Wierzbicki, Failure in lithiumion batteries under transverse indentation loading, J. Power Sources 389 (2018) 148–159, https://doi.org/10.1016/j.jpowsour.2018.04.003. [9] H. Luo, Y. Xia, Q. Zhou, Mechanical damage in a lithium-ion pouch cell under indentation loads, J. Power Sources 357 (2017) 61–70, https://doi.org/10.1016/j. jpowsour.2017.04.101. [10] W. Li, Y. Xia, G. Chen, E. Sahraei, Comparative study of mechanical electricalthermal responses of pouch, cylindrical, and prismatic lithium ion cells under mechanical abuse, Sci. Chin. Tech. Sci. 61 (2018) 1472–1482, https://doi.org/10. 1007/s11431-017-9296-0. [11] W.-J. Lai, M.Y. Ali, J. Pan, Mechanical behavior of representative volume elements of lithium-ion battery cells under compressive loading conditions, J. Power Sources 245 (2014) 609–623, https://doi.org/10.1016/j.jpowsour.2013.06.134. [12] W.-J. Lai, M.Y. Ali, J. Pan, Mechanical behavior of representative volume elements of lithium-ion battery modules under various loading conditions, J. Power Sources 248 (2014) 789–808, https://doi.org/10.1016/j.jpowsour.2013.09.128. [13] T. Kisters, E. Sahraei, T. Wierzbicki, Dynamic impact tests on lithium- ion cells, Int. J. Impact Eng. 108 (2017) 205–216, https://doi.org/10.1016/j.ijimpeng.2017.04. 025. [14] J. Xu, B. Liu, L. Wang, S. Shang, Dynamic mechanical integrity of cylindrical lithium-ion battery cell upon crushing, Eng. Fail. Anal. 53 (2015) 97–110, https:// doi.org/10.1016/j.engfailanal.2015.03.025. [15] Y. Jia, S. Yin, B. Liu, H. Zhao, J. Xu, Unlocking the coupling mechanical-electrochemical behavior of lithium-ion battery upon dynamic mechanical loading, Energy (2019) 166. [16] G. Chen, W. Li, H. Luo, Y. Xia, Influence of mechanical interaction between lithiumion pouch cells in a simplified battery module un- der impact loading, ASME 2017 International Mechanical Engineering Congress and Exposition, 2017. [17] Y. Xia, G. Chen, Q. Zhou, X. Shi, F. Shi, Failure behaviors of 100% modules under different impact loading conditions, Eng. Fail. Anal. 82 (2017) 149–160, https:// doi.org/10.1016/j.engfailanal.2017.09.003. [18] C. Zhang, S. Santhanagopalan, M.A. Sprague, A.A. Pesaran, Coupled mechanicalelectrical-thermal modeling for short-circuit prediction in a lithium-ion cell under mechanical abuse, J. Power Sources 290 (2015) 102–113, https://doi.org/10.1016/ j.jpowsour.2015.04.162. [19] B. Liu, Y. Jia, C. Yuan, L. Wang, X. Gao, S. Yin, J. Xu, Safety issues and mechanisms of lithium-ion battery cell upon mechanical abusive loading: a review, Energy Storage Mater. (2019), https://doi.org/10.1016/j.ensm.2019.06.036. [20] J. Zhu, X. Zhang, E. Sahraei, T. Wierzbicki, Deformation and failure mechanisms of 18650 battery cells under axial compression, J. Power Sources 336 (2016) 332–340, https://doi.org/10.1016/j.jpowsour.2016.10.064. [21] L. Wang, S. Yin, J. Xu, A detailed computational model for cylindrical lithium-ion batteries under mechanical loading: from cell deformation to short-circuit onset, J. Power Sources 413 (2019) 284–292, https://doi.org/10.1016/j.jpowsour.2018.12. 059. [22] C. Breitfuss, W. Sinz, F. Feist, G. Gstrein, B. Lichtenegger, C. Knauder, C. Ellersdorfer, J. Moser, H. Steffan, M. Stadler, P. Gollob, V. Hennige, A microscopic structural mechanics FE model of a lithium-ion pouch cell for quasi-static load cases (2013). doi: 10.4271/2013-01-1519. [23] J. Zhu, W. Li, T. Wierzbicki, Y. Xia, J. Harding, Deformation and failure of lithiumion batteries treated as a discrete layered structure, Int. J. Plastic. 121 (2019) 293–311, https://doi.org/10.1016/j.ijplas.2019.06.011. [24] V. Ruiz, A. Pfrang, A. Kriston, N. Omar, P. Van den Bossche, L. Boon- Brett, A review of international abuse testing standards and regulations for lithium ion batteries in electric and hybrid electric vehicles, Renew. Sustain. Energy Rev. 81 (2018) 1427–1452, https://doi.org/10.1016/j.rser.2017.05.195. [25] H. Luo, X. Jiang, Y. Xia, Q. Zhou, Fracture mode analysis of lithium ion battery under mechanical loading, Proceedings of the ASME 2015 International Mechanical Engineering Congress Exposition, 2015. [26] H. Luo, J. Zhu, E. Sahraei, Y. Xia, Adhesion strength of the cathode in lithium-ion batteries under combined tension/shear loadings, RSC Adv. 8 (2018) 3996–4005, https://doi.org/10.1039/c7ra12382e.
This paper conducts research on the mechanical response of the pouch cell battery under the impact loading condition defined in the UN 38.3 regulation and its influence on the ISC condition. It could help to improve the battery safety design for the pouch battery cells. Not only the cylindrical compression tests under quasi-static loading and dynamic conditions are performed, but also the elaborate material experiments for all kinds of component materials are tested under various loading conditions and loading speeds. A detailed finite element model was established. The FE model has a good predicting capability on the battery behavior under the impact loading and the internal failure process of the battery. A clear crack propagation is demonstrated, which is unable to achieve directly from the tests. This crack is the major reason for the internal short circuit. This detailed FE model can reveal the battery deformation information for all component parts. The convenience gives us a reasonable method to optimize the battery safety design by varying the material parameters. This method will be much easier for the battery designer to find the best combination by choosing the proper battery materials. The major findings based on the battery tests and detailed modeling can be summarized as the following aspects: (1) The differences of battery mechanical response under the quasistatic loading and impact loading are identified. Under the quasistatic condition, the deformation is more concentrated and the fracture surface is smoother, which could benefit the battery from further internal short circuit. (2) The failure mechanism of battery is identified. The vulnerability of anode under compression is the trigger for the whole battery collapse, and the tilted fracture surface will go through the battery. (3) The internal short circuit accompanied with the battery material failure will not directly lead to the battery thermal runaway. However, the battery layer kinks, the rough fracture surface and residue debris of crushed battery may be the real reason for the continuous internal short circuit and further thermal danger. To cut down the further contact of electrodes could protect battery from thermal runaway. (4) We find that the break of the battery seal could let the broken battery parts departed from each other and decrease the safety risk of further internal short circuit. This could be the possible way for battery safety strategy. Acknowledgements We acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 51675294 and U1564205) and the International Science & Technology Cooperation Program of China (Grant No. 2016YFE0102200). Thanks also to Amperex Technology Limited (ATL) and Ford URP (University Research Program). References [1] 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, https:// doi.org/10.1016/j.jpowsour.2017.12.034. [2] H. Maleki, J.N. Howard, Internal short circuit in li-ion cells, J. Power Sources 191 (2009) 568–574, https://doi.org/10.1016/j.jpowsour.2009.02.070. [3] C. Zhang, J. Xu, L. Cao, Z. Wu, S. Santhanagopalan, Constitutive behavior and progressive mechanical failure of electrodes in lithium-ion batteries, J. Power
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Experiments and 3D detailed modeling for a pouch battery cell under impact loading Pan Zhexin, Li Wei, Xia Yong
T
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State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion pouch cell Impact loading Detailed modeling Fracture Internal short circuit
Mechanical behavior of Lithium-ion batteries under dynamic impact loading is crucial in assessing and improving the crash safety of batteries. To understand the possible causes of internal short circuit (ISC) in the impacted batteries, both experimental and numerical methods are necessary. Quasi-static and dynamic tests for a type of pouch-type battery cell are performed according to a UN standard. The mechanical characterization of all the component materials in the battery is carefully carried out under different strain rate and loading conditions. Then a detailed 3D Finite Element (FE) Model of the pouch cell with all the components is established. Combining the experimental and numerical approaches, the dynamic deformation process under the UN 38.3 Impact tests and the corresponding failure mechanism of pouch cells are firstly captured. Meanwhile, the conditions for the occurrence of thermal runaway are investigated with the comparison of quasi-static and dynamic tests. It is believed that the rough fracture surface and the crushed powdery residue are responsible for the continuous internal short circuits and thermal runaway of the broken batteries. To improve the crash safety performance of the pouch cell, some safety suggestions are proposed.
1. Introduction With high demands in markets of consumer electronics and electric vehicles, the production and applications of lithium-ion pouch cell batteries come to an explosive growth. As a daily-use energy storage unit, lithium-ion batteries have received primary safety concerns. The batteries under external mechanical abuse conditions may lead to the internal short-circuit (ISC) and even fire or explosion subsequently. To provide technical basis for lithium-ion battery safety in crash events, the mechanical failure behaviors and ISC mechanism should be systematically studied. In recent years, mechanical responses of cylindrical cells, pouch cells and prismatic cells have attracted interest of quite a few research teams worldwide, and valuable experimental and simulation investigations have been carried out, as summarized in [1]. We are going to elaborate the results from battery mechanical experiments under different loading conditions to various approaches of simulations. From the perspective of battery experiment, Maleki and Howard suggested that the pinching tests of pouch cells is quite suitable to mimic the high-risk ISC situations [2], which inspires the similar indentation tests with indenters in different shape such as a hemispherical punch [3,4]. Wierzbicki et al. conducted various quasi-static tests of both cylindrical and pouch cells, and revealed the correlation between ⁎
mechanical failure and occurrence of ISC [4–8]. In the meantime, Sahraei et al. developed an effective method to equivalently characterize the mechanical properties of pouch batteries under local indentation as a homogeneous material [4,5]. Luo et al. studied the damage evolution of a pouch cell subjected to indentation by performing indentation tests at different force levels and inspection of change of constituents inside the post-mortem samples [9]. Li et al. performed comparative tests of several typical battery types and observed different failure patterns due to different indenter shapes [10]. Most researchers tested the pouch cells through the thickness direction by considering the fact that the large flat area has higher probability to be severely loaded, while Pan et al. for example investigated the mechanical behavior of pouch cells subjected to in-plane compression, and observed the local buckling, kink and shear-band deformations of cell components [11,12]. Above-mentioned researches usually focused on the quasi-static loading conditions, which is indispensable for understanding the basic mechanical response of the battery structures. However, in reality the safety issue of lithium-ion batteries sometimes has to be addressed in impact scenarios like electric vehicle crash or electronic device drop. Accordingly, the dynamic tests could be more representative. The dynamic tests of batteries are relatively difficult in terms of test setting-up and technical operation. Thereby fewer reports can be tracked in the
Corresponding author. E-mail address: [email protected] (Y. Xia).
https://doi.org/10.1016/j.est.2019.101016 Received 14 August 2019; Received in revised form 30 September 2019; Accepted 8 October 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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with electrolyte. Jellyroll is enveloped inside a pocket of aluminumplastic film. The cathode is a sandwich of an aluminum-foil current collector and two coating layers of Lithium Cobalt Oxide, while the anode is a sandwich of a copper-foil current collector and two coating layers of graphite, also shown in Fig. 1(b). The separator is made of polyethylene. All the geometrical parameters of the pouch cell are summarized in Table 1. Considering the structural complexity, any test directly upon the full battery cell is of structural response rather than a single material response. To develop the detailed FE model, each of the individual component materials has to be carefully tested and characterized.
public domain. Kisters et al. conducted quasi-static and dynamic tests of pouch and elliptic cells, which shows that the impact velocity and the existence of electrolyte significantly influence critical force and stiffness of batteries [13]. Xu et al. studied the dynamic mechanical integrity of cylindrical cells under transverse crushing by taking into consideration the strainrate and inertia effect in the simulations [14,15]. Chen et al. and Xia et al. performed the dynamic impact tests of stacked pouch cells and the battery modules, analyzed the integrated failure behaviors under various loading conditions, and in the meantime, built a FE model of the battery module based on homogenization of pouch cells [16,17]. From the perspective of simulation, Battery is a multi-layered structure of cathode, anode and polymer separators, which make the modeling a challenging work. Sahraei [7], Pan [12] and Zhang [18] separately developed the micro-scale representative volume element (RVE) to study the deformation mechanism. Another modeling method treats the battery jellyroll as a homogenized material. The homogenized FE models were built by employing and calibrating the crushable foam model [4–6], or the honeycomb model [3]. However, the accuracy of homogeneous simplifications strongly relies on the selection of material model. It is difficult for them to properly reflect the underlying physics. A systematic comprehension from the constituent level behavior to the full-cell response is essential to the safety focused battery study [1,3,5,19]. A detailed model of the 18650 cell developed by Zhu captured the typical deformation stages during the axial loading, exhibiting the benefits from the detailed modeling [20]. Wang et al. also performs their attempts on the cylindrical cells [21]. So far there is only few reports of a research focusing on the detailed modeling of pouch cells [22,23]. Although the battery safety studies have not been settled, quite a few standards and regulations for batteries have already been composed out of safety concerns. Drafted by the organizations like ISO, SAE, NHTSA, the standard tests require examining the safety performance of lithium-ion batteries subjected to typical mechanical abuse loading, such as drop tests, shock tests, nail penetration tests, crush tests [24]. For instance, UN 38.3:2015 defines an impact test as follows. A 15.8 mm diameter steel bar is placed across the center of the sample while a 9.1 kg mass is released from a height of 610 mm at the intersection of the bar and to impact the sample cell, as shown in Fig. 1(a). It requires that the battery after the impact should not be smoky and fired. This impact test is often run by the regulation agencies and any battery undergoing air transportation should pass this standard test. Therefore, the mechanical response and the failure mechanism of pouch cells in this test type are worthy of in-depth analysis. The objective of the present paper is to explore the deformation and failure progress of pouch cell under the impact loading of UN 38.3 type by the aid of experiments and FE simulations. A detailed finite element model of a pouch cell is developed. The metal current collectors, electrode active layers and separators are modeled as individual components and the interfaces between various layers are carefully calibrated and treated. By comparing the simulations and test results, mechanisms of internal short circuit are investigated through the deformation and damage analysis of the electrodes and separators. This method provides a picture of the dynamic deformation progress inside the battery cells and helps to figure out the possible access to improve pouch cell crash safety.
2.2. Material test preparation
2. Experimental preparation
To develop the detailed FE model, all the component materials are tested under various strain-rate and stress conditions, and the material properties are identified. Asymmetric tension-compression behavior is one of the typical mechanical features of the electrodes, which is closely related to the sandwich structure of the electrodes. When the electrode is compressed from the direction normal to its surface, i.e. the out-of-plane direction, the active layers contribute the most to the deformation. Nevertheless, in the tensile tests it is the metal foil current collector that affords the largest proportion of the force. Regarding this, in the FE Model developed in the present study both the metal foil current collector and the active layer are carefully characterized. The metal foil current collectors are extracted from the batteries for testing by treating the cathode and anode with N-methyl pyrrolidone (NMP) to remove the active layers. Thereafter, we measure the mechanical properties of the electrodes directly and the current collectors, while the tensile and compression properties of the active layer can be indirectly identified by comparing the test results between the electrode and the current collector. The other components of the battery cell, i.e. the separator and the pocket that play essential roles in ISC prevention and protection, are also tested. All the component materials are extracted from the batteries at 0 SOC to mitigate possible influence of SOC on material properties. Fig. 2(a) shows the specimen die-cutting molds for tension and compression tests. The specimen shape is chosen to meet the battery geometry. Cathode, anode and separator samples are extracted from a disassembled battery cell and cut into the specified shapes by the molds. All the tension tests are performed in 3 directions, MD(0°), DD(45°), TD(90°), which is the angle relative to the battery rolling direction, in order to examine the anisotropy of the component materials. For compression tests, round shape discs of 16 mm diameter are cut from the thin sheets of component and stacked in the thickness direction: 35 layers for electrodes and 70 layers for separators. By this way, the total thickness is a measurable quantity and it makes the relative error of deformation measurement small enough in the compression test. The quasi-static tests of component materials are performed on a Zwick Z020TE test machine with a loading speed of 0.6 mm/min, and the dynamic intermediate strain-rate tests are performed with a hydraulic high-speed test machine, which is able to reach a speed of 2 m/ s. A high speed camera is adopted to record the deformation information and then Digital Image Correlation (DIC) method is used to acquire the deformation field. Every test type is performed more than 6 times to exclude individual differences.
2.1. The structure of the tested pouch cell
2.3. Pouch cell test set-up
The tested object in the present study is a commercial pouch battery cell that serves in consumer electronics, as shown in Fig. 1(b), The capacity of battery is 16.98 Wh, and the open circuit voltage is 3.85 V. The major part of the battery cell is the jellyroll, a tightly wound structure of the cathode-separator-anode-separator laminate soaked
As mentioned above, the UN 38.3:2015 T.6 impact/crush Test scenario is focus in this study. Drop tower is employed to release a dropped weight of 9.1 kg from the height of 610 mm. Setup of the test is shown in Fig. 2(b). The fixture is placed on a force sensor, which acquires the force signal during the impact. The time of the whole loading process is 2
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Fig. 1. Impact test of pouch cell.
3. Constitutive mechanical behavior of component materials
Table 1 Geometry of battery cell. Parameters
Value (mm)
Length of the cell Width of the cell Height of the cell Parameters Pouch pocket Cathode current collector Cathode active layer Anode current collector Anode active layer Separator
71.0 54.0 5.46 Thickness (µm) 105 10 58 8 65 9
3.1. Tension behavior The quasi-static and dynamic tensile test results of cathode, anode, aluminum foil, copper foil, separator and the pouch pocket are exhibited in Fig. 3, respectively. The comparison of experimental results shows that the active layer only affords about 5% of total force, and electrodes exhibit a similar elastoplastic behavior just like the metal foils. Moreover, the failure strain of electrodes is consistent with that of current collectors, showing that the failure of current collector happens to trigger the electrode failure at both quasi-static and dynamic condition. Since the thickness of aluminum and copper foils are less than 10 μm, both of the current collectors are too fragile to bear large deformation. Attributed to the difference between aluminum and copper foils, the cathode only has failure strain of 0.01 and yield strength of 140 MPa while anode has a failure strain at 0.02 and a yield strength of 220 MPa. As the cathode and anode are closely placed and exposed to the similar mechanical loading, this incompatibility of cathode and anode in strength and stiffness could be the reason of battery internal failure. Tensile tests of different directions are performed for electrode materials to clarify the in-plane anisotropic behaviors. It turns out that both cathode and anode have limited anisotropy; the yield strength and the failure strain have a little difference in 3 directions. The corresponding current collectors have the similar behaviors. In addition, for the strain-rate effect, electrodes and current collectors share the same trends: as the strain rate increases, the yield stress increases but the failure strain of metal foil decreases. The
less than 2 ms. Displacement of the steel bar and battery deformation is recorded by a Phantom high-speed camera with a frame rate of 100000 fps. The temperature of battery is recorded by a thermocouple. Meanwhile, the voltage of battery is monitored using a data acquisition card and synchronized with force and displacement signals. Moreover, corresponding quasi-static tests with a continuous loading of 2 mm/min are also performed on a universal testing machine to investigate the speed effect of the battery. The influence of state of charge (SOC) on the battery mechanical behavior is also concerned, so battery cells under 4 states of charge (SOC=0%, 30%, 60%, 100%) are tested for dynamic loading conditions.
3
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Fig. 2. Pouch cell dimensions and the preparation for the material tests and impact tests.
exhibited in Fig. 3(k). Limited by the load capacity of the testing machine sensor, the maximum stress only reaches 250 MPa, and cathode and separator have not been compressed to failure. Cathode has a three-stage stress-strain plot under the compression. At the early stage, the stiffness increases, representing the densifying stage. When the stress increases to 135 MPa, the curve encounters a turning point and enters the second stage. The active particles of cathode are being crushed to fill the space between them and the force reaches a plateau at the strain range from 0.18 to 0.32. Then it comes to the third stage, where the stiffness increases again and the material undergoes another densification process. For anode, the compression tests have an opposite trend with tensile tests: the strength and failure strain of anode are much smaller than those of cathode. Anode specimen collapses at 56 MPa under compression and cracks go through the thickness direction, making the specimen split into pieces. The failure of anode is much easier than the cathode under compression. Since the failure strain of aluminum foil and copper foil do not have such large difference, this phenomenon implies the different transverse deformation of cathode and anode active layers under compression. The compression stress-strain behavior of separator has an exponential form and the stiffness keeps increasing. For further validation, the integrated battery sample containing 18 repetitions of cathode-separator-anode-separator laminate is directly cut from the battery cell and tested, as shown in Fig. 3(k). It can be
aluminum foil under dynamic loading is so brittle that it nearly fails at the elastic stage. In this study, both cathode and anode are treated and calibrated as transverse isotropic elastic-plastic materials with strainrate effects. As a porous polymer material, separator has the following characteristics: superelasticity, visco-plasticity, and anisotropy. hyperelasticity, viscoplasticity and anisotropy. The failure strain of separator reaches 100%, as shown in Fig. 3(g). It should also be noted that the anisotropy of presented separator is quite lower than separators from literature [25], which can be attributed to the wet-processed manufacture. The difference between plastic behaviors in 0° and 90° is less than 20% and the failure strains in different directions are almost the same. The strain rate effect of this separator is remarkable: the stress level increases by 100% from quasi-static condition to dynamic condition and the failure strain decreases to 0.4 at a strain-rate of 2.5 s−1. Pouch pocket is a type of composite made of aluminum foil and polymer material, serving as the major protection for the pouch cell. It has a failure strain of 0.18 and isotropic yielding behavior, as shown in Fig. 3(h). With the increase of strain-rate, both the strength and failure strain of the aluminum-plastic film both increase, which could lead to better performance under impact condition. 3.2. Compression behavior The compression results of cathode, anode and separator are 4
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Fig. 3. Experimental results of all component materials: (a) Cathode (b) Aluminum foil (c) Comparison of cathode and cathode current collector (d) Anode (f) Comparison of anode and anode current collector (e) Copper foil (g) Separator (h) Pouch Pocket (k) Material compression results.
the same:
observed that the integrated battery sample has the stress-strain curve shape and the failure strain quite similar to the anode. This implies that the compression failure of the whole battery cell structure is triggered by the fracture onset of anode. In cathode and anode, the thickness of active material coating is about 10 times larger than that of the metal foil current collector and the stiffness of active coating is much weaker. When the electrode is compressed during the impact, the active layer will take the most deformation and dominate the total behavior. The active layer is composed of small-size granules of active material stuck together by the binder, which has the natural porosity and compressibility. The active layer can be assumed isotropic since the microscale structure has no apparent orientation. The compression through the thickness direction can represent its mechanical behavior.
The average compression strain consists of both components of electrode based on the ratio of thickness t: (3)
εe te = εcc tcc + εa ta
Since the current collectors are made of metal foils, the properties of which are obtained through the tensile tests, then Eq. (3) is used to get the exact compression behavior of the active layers. As the properties of current collectors have been obtained from tensile tests and the mechanical behavior of current collectors is considered as tension-compression symmetric, the compression stressstrain properties of active layers could be exacted using Eqs. (2) and (3). The crushable foam material model (Mat_63) in LS-DYNA is applied to characterize the active layers of electrodes, in which the curve of nominal yield stress versus volumetric strain needs to be defined. In the present study the models of the cathode and anode active layers are calibrated with the aforementioned test results, respectively. Due to the limited anisotropy, the cathode and anode current collectors are characterized with an isotropic elastoplastic material model with the Cowper-Symonds formulation of strain-rate effect (Mat_24 in LS-DYNA),
3.3. Material calibration Although the active layers have the thickness of about 0.12 mm in total, the tensile behaviors of electrodes mainly depend on the metal current collector of a thickness about 0.01 mm.Assuming that the metal foil and active layer share the same strain during the tensile tests, the force equilibrium of the sandwich structure is considered as:
σe Se = σcc Scc + σa Sa
(2)
σe = σcc = σa
(1)
σ = E : (ε − εp) ⎧ σ¯ ⎪ ε˙p = ε¯˙p σ ⎪ ⎨ ⎪ σy (εp, ε¯˙p) = σy (εp) ⎡1 + ⎢ ⎪ ⎣ ⎩
Here, the σe, σcc and σa represent the stress of electrode, current collector and active layer, while Se, Scc, Sa refer to the section area. The contribution of metal foil and active layer to the total force can be portioned by the product of the stress and the section area of each component. The compression testing results of electrodes are the overall response of two active material layers and the current collector, which means some assumptions are needed to obtain the properties of active material. The σ for each layer through the thickness direction is
1 p
( ) ⎤⎥⎦ ε¯˙p c
(4)
where ɛ and ɛp denote the total and plastic strain tensors, and ε˙p is the equivalent plastic strain rate. The Young's Modulus E and plastic hardening curve σy(ɛp) is calibrated through the quasi-static tests while 5
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SOCs have been smoky and temperature of which increase above 100 °C, revealing the ISC occurred inside. The batteries of 0%, 100% SOC only have a temperature shifting to 50 °C and pass the impact tests. In Fig. 4(c), the top and section views of the battery cells after tests of both quasi-static and dynamic conditions are exhibited. Focusing on the quasi-static loaded battery shown in the left pictures, 2 pouch pocket cracks appears on the side below the indenter and through the width direction of the battery. When we observe from the cross-section direction, these two cracks correspond to the shear slipping line on the either side of the battery. The indenter squeezes the battery directly below and creates the smooth shearing fracture surface even if the battery is made of different component materials. This observation explains why the quasi-static force curve has a sharp peak, as the peak illustrates that all the battery components fail at the same time due to the concentrated shearing under quasi-static loading. The battery deformation is localized in the area under the indenter, and the bottom face of pouch pocket and seals of both sides are intact. When it comes to the dynamic impact tests, the situation is different. During the exact moment of about 2 ms, the different cell components will have the combining deformation of tension and compression, leading to the failures. The initial kinetic energy of indenter is much larger than the necessary energy to break the battery, which means the cell will directly be squeezed into 2 pieces by indenter and flying in 2 opposite directions. The broken jellyroll parts are pushed horizontally to move and break the head seal and the pouch pocket on the tail. This movement of jellyrolls could benefit the battery safety since the detachment of the fracture areas would lead to few internal short circuits. Unlike the compressed area of quasi-static tests can still hold the shape, the materials under the indenter have been crushed into powdery residue, which may bring larger ISC probability. If we look closely at the battery area near the fracture, we can find the local kink forming at the several top layers and the shearing fracture surface is rougher than the quasistatic conditions. Attributed to the battery rolling-up structure, battery top layers and bottom layers are actually separated and afford different loads and deformation.
parameters C and p is calibrated from all the strain rate tests. According to the limited anisotropic behaviors of separator and pouch pocket, these two materials are simplified and characterized using the same isotropic elasto-plastic constitutive model. 3.4. Interface strength Due to the multi-layered structure, the battery cell has plenty of interface contacts, which would have large influence on the battery mechanical response. There are two major categories of interfaces: the interfaces between the active layer and the current collector in cathode/anode (referred to as cathode/anode interface in this paper) and the interfaces between the active layer of electrode and the separator (referred to as separator interface in this paper). Luo et al. have introduced a testing method for the interface strength of electrodes under combined tension-shear loading [26], which is also employed in the present study. The test results show that the normal failure strength of the cathode interface is 2 MPa, while that of anode interface is 0.3 MPa. The separator interface is much weaker, just about 0.01 MPa, but existence of this interface strength is essential for keeping the integrity of the battery cell. 4. Standard impact test The force and voltage measurements over the battery intrusion of the quasi-static and the UN38.3 standard impact tests are shown in Fig. 4(a). The dash lines are the force-intrusion plots and the thinner solid lines represent the voltage trend relative to the battery intrusion. There are several obvious differences between the mechanical responses of the battery cell under quasi-static loading and the dynamic loading. An important feature in quasi-static tests is that force plot has only one peak but that of dynamic tests have 3 peaks. Under the quasi-static condition, the load is continuously applied on the battery with no interrupt. However, the dropped weight is not connected to the indenter at beginning that results in three-body collision. At first, the dropped weight will hit the steel bar indenter and give it a higher speed, making it detached and load the battery. The steel bar will accelerate and decelerate between the dropped weight and battery, squeezing the battery, which leads to the multiple peaks of force plot under the dynamic impact. At the initial stage of impact, the battery stiffness under dynamic loading is far larger than the quasi-static loading conditions while the first peak of the dynamic curve appears at the smaller intrusion than the quasi-static one. This phenomenon is widely observed in dynamic tests different battery types [13], which indicates the battery becoming stronger and more vulnerable under dynamic conditions. The second stage of impact can be defined as the intrusion range from 0.5 mm to 2.5 mm, from the first peak to the second peak. The decrease of the force suggests the failure inside the battery. Under quasi-static condition, all battery components seem to fail at the same intrusion level because of its sharp force peak, but under the impact condition, the collapse of battery last a larger range. It infers that here the failure of different layers is a gradual process. The voltage of battery is supposed to reveal the battery internal short-circuit condition in tests. As we can observe, the voltage drops around the force peak under the quasi-static condition, however, voltage under dynamic impact drops before the first peak force, implying that the internal short circuit may happen earlier under the impact. The final stage starts at the intrusion around 2.5 mm. After this point, the force response of the battery decreases to a low level, and the battery is being separated to 2 parts by the continuously downward indenter. Impact test results with various SOCs are shown in Fig. 4(b). It demonstrates a slight increase of the force with the higher SOC. It could be contributed to that the higher SOC can inflate the battery cell and increase the internal stress. Additionally, the batteries of 30%, 60%
5. Finite element simulation 5.1. FE model To reflect the battery response as real as possible, all the battery components should be taken into the simulation. Then, the detailed finite element model of battery cell is established in LS-DYNA R7, as shown in Fig. 5. The pouch pocket, cathode current collector, anode current collector and separator are built as shell elements, while the cathode and anode active layers are built as solid elements. To capture the meticulous mechanical features, the element size of concentrated deformation area is 0.1 mm and totally 2217242 elements are employed here. All the components material definitions are summarized in Table 2. All the shell elements are using isotropic material card MAT24. The plastic yielding behaviors of all the component materials are calibrated directly from the testing results and defined with curve input. The strain rate effects of metal foils, separators and pouch pockets are also considered and defined in the material cards. Active layers are defined with material card MAT63, which is a crushable foam card. The failure criteria obey these rules: shell elements are based on maximum tension strain because of the stretched scenario and the solid elements are defined by maximum principle strain and effective strain combining both tension/compression conditions. The contact interfaces between layered component structures are carefully defined. The contact between the electrodes and current collectors are CONTACT_ TIED_ SURFACE_ TO_ SURFACE; and the contact between the active layers and separators are CONTACT _TIEBREAK _SURFACE _ TO_ SURFACE; and the other possible contact are controlled by global-defined CONTACT_ AUTOMATIC_ SINGLE_ SURFACE. 6
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Fig. 4. battery test result.
stiffness of dynamic modeling has a very good agreement with that of experiments, demonstrating the detailed model with the dynamic loading boundary condition is able to reveal the stiffness strengthen observed in the dynamic tests. The detailed simulation is also capable of capturing the damaging process during the impact. The intrusion range for the force peak is about 1 mm, and the quasistatic condition make the deformation more concentrated. Since the failure achieved in FE simulation is by element deletion, the damaged elements under the impactor cannot afford the compression any more. Then simulation force of the final stage directly come to 0, while the experimental data still have about 5 kN left due to the remains. As all the material tests are based on 0 SOC battery cells, the dynamic impact simulation results comparing to the 0 SOC standard impact tests are exhibited in Fig. 6(b). At the initial stage, the initial stiffness of dynamic modeling has a very good agreement with that of experiments, demonstrating the detailed model with the dynamic loading boundary condition is able to reveal the stiffness strengthen observed in the dynamic tests. The detailed simulation is also capable of capturing the damaging process during the impact. The green circle A has highlighted an inflection point of the experiment curve, which is clearly exhibited on the simulation curve. It indicates the crushed anode material starting to fail. The reason of the force drop is that the collapse of anode is achieved by element deletion
Boundary condition is the same as the standard impact tests, the drop mass of 9.1 kg and the indenter are also integrated in the simulation, and the initial speed of the drop mass is 3.46 m s−1 corresponding to the drop height of 0.61 m. The simulation was carried on the parallel computing system and the computational time for the impact simulation was about 34 h with 16 Xeon E5-2690v4 CPUs, while the quasi-static condition took more than 200 h.
5.2. Force-intrusion simulation The force-intrusion results of the simulation are shown in Fig. 6. Under Quasi-static condition, the force-intrusion curve matches well with the experiments results. Before the force peak, the hardening trend of the simulation at initial stage is consistent with that acquired from the tests, and failure happens at around 1.6 mm intrusion. Since the failure achieved in FE simulation is by element deletion, the damaged elements under the indenter cannot afford the compression anymore. So simulation force of the final stage directly come to 0, while the experimental data still have about 5 kN left due to the remains. As all the material tests are based on 0 SOC battery cells, the dynamic impact simulation results comparing to the 0 SOC standard impact tests are exhibited in Fig. 6(b). At the initial stage, the initial 7
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Fig. 5. Finite element model of the pouch cell battery.
being compressed into debris by the indenter, while the other 2 parts of battery cell absorb the kinetic energy of the indenter and start the lateral motion. These movements break the pouch pocket and the head seal. Finally, the battery is cut into 2 parts and flying in the opposite directions. The simulation meets the battery dynamics during the impact experiment, and make the detail observation for the battery deforming process accessible. From the cross-section view of the simulation, the von-Mises stress contour is plotted in Fig. 7(b). To be specific, it could be found that there is no fracture inside the battery at 0.5 ms. The stress is concentrated on the central area of battery and has a bell-type distribution. Afterwards at 0.6 ms, cracks first appear at the top of the battery and propagate immediately cross the battery section in the middle of compressed zone. The crack propagation forms a tilt line, which has a certain angle (about 70°) to the horizontal direction, consistent with experimental observation. Since the impact is a dynamic process, the high residual stress on both sides of battery leads to the more fractures after 1.2 ms. Thus, there are more fractures in the dynamic impact condition than the quasi-static condition, creating more debris in the central area.
in FE software. Meanwhile, the maximum force also matches the testing results, the only problem is that the intrusion for maximum force have a difference of 0.1 mm. The force peak of battery under the dynamic loading is lower and wider than the quasi-static condition. Another important feature for battery under the dynamic conditions is the multiple peaks of the force, which come from the reciprocating motion of the indenter and the battery vibration. The proper boundary condition makes the simulation able to produce this feature as the experiment does. However, the vibration period is smaller.
5.3. Battery deformation process This section is supposed to reveal the dynamic impact simulation results, since we mainly focus on the standard impact condition. Fig. 7(a) is the section view of battery deformation process for the impact simulation from 0 to 1.2 ms. The initial stage for the battery impact is from 0 to 0.6 ms, the intrusion of the indenter makes the central part of battery compressed and nearby parts slightly tilted. Winding center plane of the not-directly-impacted battery part starts to separate and the pouch pocket bends under the indenter. At this stage, the material fracture has not occurred. As the anode is compressed to its compression failure criterion, the crack appears at 0.6 ms and immediately crosses through the layers. The failure of crushed anode will trigger the neighbor layers successively. This point is also corresponded to the force peak on the force-intrusion plot. At the final stage, from 0.9 ms to 1.2 ms, battery failure keeps developing with the further intrusion of the indenter. Two more cracks extend cross the whole battery at the edge of the impacted area, separating the battery apart. The crush zone in the middle is still
5.4. The post-crash features of battery The detailed battery model can not only reproduce the battery deforming process but also capture the important post-crash characteristics of battery. The detailed model is able to reflect the rational morphological features of the fracture. The fracture surface of the battery grows along the certain angle and two fracture lines are likely to form the fracture surface as the blue circles shown in Fig. 8(a).
Table 2 Component modeling information. Components
Material card
Young's Modulus (GPa)
Yielding stress (MPa)
Failure Strain
Pocket Cathode current collector Cathode active layer Anode current collector Anode active layer Separator
MAT24 MAT24 MAT63 MAT24 MAT63 MAT24
11.9 46.0 1.2 52.7 0.8 1.5
32.7 137.5 Curve defined 184.2 Curve defined 13.5
0.028 0.01 0.5 0.015 0.3 0.4
8
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Fig. 6. Force intrusion plots from simulations.
Fig. 7. Simulation results for dynamic impacts. 9
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Fig. 8. Comparison of simulation and experiment: (a) Formation of local kinks and fracture surfaces (b) Failure of the pouch seals at the edge of battery.
not inevitable. However, if the remaining parts of the battery could separate from each other and keep a clear fracture surface, ISC will not progress afterwards and the accumulated heat may be not enough for the material ignition. Then the battery could still be safe even if it is broken during the impact. Otherwise, if the residual debris caused by the impact keeps different electrode layers connected, there would be the continuous ISC after impact and the severe thermal runaway at final. From the detailed model simulation, the generation of debris can be recognized and the central crushed area could have contacts with the remaining parts of battery. Meanwhile, the local kinks appeared on the battery top layers and irregular battery fracture section observed during experiments could also make the contact of cathode and anode happen, denoting the higher ISC risks. From the dynamic tests of batteries, the battery safety could benefit from the break of the seal and pouch pocket, because the broken battery part could move laterally and detach from the residue debris in the central crushed zone. Inside the electronic devices and the electrical vehicles, the batteries are embedded in a certain slot. This strong restraint, limiting the lateral movement of broken battery, will maintain the ISC condition and increase the thermal run-away probability. Some suggestion could be drawn to improve the battery safety under the impact condition. Firstly, inhibiting the generation debris will be a possible solution. It can be achieved by employing electrode current collectors of higher strength and elongation. Nowadays the battery manufacturer tends to use the thinner metal foil to serve as the current collector. The thinner foils of the lower strength and elongation, are more likely to broken into pieces and create more fractures surface inside the battery. The strengthening of the current collector will create fewer debris and keep a uniform fracture surface. Secondly, we need to avoid the rough fracture surface and the local kinks. If we are able to increase the interface strength inside the battery, the integrity of battery will improve and the dislocation of different layers will not emerge. By this way, the broken electrode will still be protected by the broken separator, the ISC will not appear at the fracture surface. At last, to preset space for the battery deformation and movement is necessary. If the battery slot is crushable and allow the broken parts of battery move laterally and detached, the risk of thermal runaway will decrease.
Comparing the simulation results with the battery after tests, the inconsistent failure of different component layers results in the rough and rugged fracture surface. Moreover, it is clear that the detailed model is capable of identifying the local kinks generated at the top layers of the battery, in Fig. 8(a). From the simulation we could know that the intrusion of indenter keeps pushing the upper layers of batteries move aside, so that the uncoordinated motions result in the local kinks. It is worth noting that material failure is a necessary and insufficient condition for ISC. Both kinks and rough fracture surface are potential risks for ISC, because the dislocation of electrode layers creates risk of interaction between cathode and anode. If the cathode and anode contacts directly at the fracture surfaces or kinks, the ISC could happen and even lead to severe thermal runaway and fire. However, if the fracture surface is smooth and debris does not connect different layers, the cathode and anode can still be separated by the remained separator. In this case, even if severe damage has happened to the battery, the ISC would stop without thermal runaway. This can be verified through the comparison of quasi-static and dynamic tests that the thermal runaway is only observed under the dynamic condition. Additionally, the detailed model is also able to capture the failure of the sealing and the rush-out of the battery parts. Fig. 8(b) shows the seals breaking on the both side of battery. The seal of the battery is manufactured using the thermoplastic sealing process. During the impact, the battery divides into two parts laterally and push the separated core breaking the seals on the side of the pouch pocket. When the separated battery parts could break the seals and move out, the possibility of ISC decreases. It indicates that the weakness of seals can benefit the battery safety to some extent. 6. Discussions In the presented tests and simulations, the mechanical characteristics of the battery under the impact loading is well demonstrated. When a pouch cell is applied a mechanical abuse loading, the voltage of battery drops will drop at the force peak, which corresponds to the material fracture. At the same time, the inner material failure could lead to the internal short circuit (ISC), and even the dangerous thermal runaway. The standard impact tests are supposed to verify whether the thermal runaway would happen during the impact. From our experiment results of this pouch cell, the occurrence of the material failure is 10
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7. Conclusion
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This paper conducts research on the mechanical response of the pouch cell battery under the impact loading condition defined in the UN 38.3 regulation and its influence on the ISC condition. It could help to improve the battery safety design for the pouch battery cells. Not only the cylindrical compression tests under quasi-static loading and dynamic conditions are performed, but also the elaborate material experiments for all kinds of component materials are tested under various loading conditions and loading speeds. A detailed finite element model was established. The FE model has a good predicting capability on the battery behavior under the impact loading and the internal failure process of the battery. A clear crack propagation is demonstrated, which is unable to achieve directly from the tests. This crack is the major reason for the internal short circuit. This detailed FE model can reveal the battery deformation information for all component parts. The convenience gives us a reasonable method to optimize the battery safety design by varying the material parameters. This method will be much easier for the battery designer to find the best combination by choosing the proper battery materials. The major findings based on the battery tests and detailed modeling can be summarized as the following aspects: (1) The differences of battery mechanical response under the quasistatic loading and impact loading are identified. Under the quasistatic condition, the deformation is more concentrated and the fracture surface is smoother, which could benefit the battery from further internal short circuit. (2) The failure mechanism of battery is identified. The vulnerability of anode under compression is the trigger for the whole battery collapse, and the tilted fracture surface will go through the battery. (3) The internal short circuit accompanied with the battery material failure will not directly lead to the battery thermal runaway. However, the battery layer kinks, the rough fracture surface and residue debris of crushed battery may be the real reason for the continuous internal short circuit and further thermal danger. To cut down the further contact of electrodes could protect battery from thermal runaway. (4) We find that the break of the battery seal could let the broken battery parts departed from each other and decrease the safety risk of further internal short circuit. This could be the possible way for battery safety strategy. Acknowledgements We acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 51675294 and U1564205) and the International Science & Technology Cooperation Program of China (Grant No. 2016YFE0102200). Thanks also to Amperex Technology Limited (ATL) and Ford URP (University Research Program). References [1] 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, https:// doi.org/10.1016/j.jpowsour.2017.12.034. [2] H. Maleki, J.N. Howard, Internal short circuit in li-ion cells, J. Power Sources 191 (2009) 568–574, https://doi.org/10.1016/j.jpowsour.2009.02.070. [3] C. Zhang, J. Xu, L. Cao, Z. Wu, S. Santhanagopalan, Constitutive behavior and progressive mechanical failure of electrodes in lithium-ion batteries, J. Power
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