Lithium-ion cell response to mechanical abuse: Three-point bend

Journal of Energy Storage 28 (2020) 101244

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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est

Lithium-ion cell response to mechanical abuse: Three-point bend a,⁎

a

a

a

T b

Johanna K. Stark Goodman , Jay T. Miller , Steven Kreuzer , Joel Forman , Sungun Wi , Jae-man Choib, Bookeun Ohb, Kevin Whitea a b

Exponent, Inc., 1075 Worcester St., Natick, MA 01760, USA Samsung Electronics Co., Ltd., 129, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16677, Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Lithium-ion Cycling Mechanical abuse Defect Short-circuit

Previous studies have investigated the conditions required to immediately initiate thermal run-away, however there is little information available about the development of internal cell defects in abused cells that lead to thermal run-away or failure as a result of continued cycling. In this study cells are subjected a three-point bending abuse at levels that physically damage the cells but do not cause immediate thermal run-away. The abused cells are then cycled at multiple temperatures for up to 1000 cycles. Samples were examined at 100, 200, 500 and 1000 cycles using techniques including electrical analysis, computed tomography X-ray imaging, destructive physical analysis, and cross-section analysis. The electrical performance (capacity, impedance) of the abused cells was not significantly different than that of the control group. The damage imposed during the abuse protocol did not lead to thermal runaway of the cells used in this study; instead, failures presented themselves as high resistance short circuits that were sometimes not apparent during cell cycling. It is hypothesized that these short circuits formed during cycling as a result of the natural expansion and contraction of the electrodes.

1. Introduction Since the first commercial introduction by Sony in 1991, lithium-ion battery cells have experienced continuous improvement in design, manufacturing quality, performance, and safety. As a result, they have become ubiquitous in modern life. Widespread adoption as a power source for virtually every type of portable electronic device and countless hours of trouble-free service across a variety of use scenarios and conditions statistically supports the notion that, when used appropriately, lithium-ion cells are safe. However, improper use or abuse can result in performance loss and, in some instances, the uncontrolled exothermic breakdown of the cell known as thermal runaway. Thermal runaway is a series of undesirable exothermic chemical reactions within a lithium-ion cell that quickly heat the cell to the combustion temperature of the cell materials, producing combustible gases, smoke, and flames. The only requirement for the initiation of a thermal runaway event is heating of the cell, or a portion of the cell, to a temperature sufficient to start breakdown of the solid-electrolyte interface (SEI) layer [1,2]. In a properly designed and manufactured cell, operating within appropriate specifications, temperatures sufficient for SEI breakdown do not occur in the absence of external heating or mechanical intervention that causes a short circuit within the cell. Due to the catastrophic and potentially dangerous nature of lithium-

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ion thermal runaway, there is a large body of work focused on improving the thermal stability of the positive electrode [3–5], negative electrode [6,7], separator [33], and electrolyte [8–10]. Additionally, the literature and standards documents provide guidance for designing and building battery packs that are capable of maintaining an acceptable electrical, thermal, and mechanical environment for lithium-ion cells [11]. The effectiveness of a battery design is assessed by subjecting it to a set of controlled abuse tests outlined in industry accepted standards [12]. The general requirements for lithium-ion battery abuse tolerance are captured by multiple lithium-ion battery industry standards focusing on abuse scenarios that have the potential to cause heat generation within the cell that can lead to thermal runaway [12]. The testing required by most relevant standards can typically be broken down into two general categories: electrical abuse scenarios, whereby the cell is required to operate outside nominal voltage and current limitations and physical/environmental abuse scenarios, whereby the cell is subjected to temperature extremes or mechanical deformation. The goal of testing these scenarios is to show that when a cell is forced into a condition that results in self-heating, the ensuing failure occurs in a predictable manner and minimizes the chances for collateral damage. In some instances, passing criteria allows thermal runaway as long as the cell fails in an acceptable manner.

Corresponding author. E-mail address: [email protected] (J.K.S. Goodman).

https://doi.org/10.1016/j.est.2020.101244 Received 1 July 2019; Received in revised form 23 January 2020; Accepted 23 January 2020 2352-152X/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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The existing knowledge about the relationship between cell abuse and thermal runaway is largely focused on carefully understanding what abuse stressors result in thermal runaway and how to defend against them [34]. Post-mortem analysis of abused cells that have experienced thermal runaway has been used to infer conditions in the cell prior to the runaway event [13–16]. Several computational models exist that use chemical and electrochemical principles to predict cell changes as a function of time or use and the impact of those changes on cell behavior during thermal runaway [1,17–19]. Experimental studies on the mechanical abuse of cells have been performed under a variety of loading conditions and on a range of cell types but have been focused on determining the mechanical conditions that result in short circuits/thermal runaway [20–31]. Mechanical loading modes have included bending, crush, indentation, and nail penetration. These tests have been performed on cylindrical, prismatic, and pouch cells with a variety of orientations relative to the loading mechanism. The effect of the electrical state of the cell has been incorporated via testing at different SOCs [22,28,30]. These tests have provided a wealth of data on what causes thermal runaway or short circuits over a short time horizon as well as mechanical properties for computational simulations. However, these tests have generally not examined either (a) sub-critical loading (i.e., loads that do not induce a short-circuit or thermal runaway) or (b) electrical performance after mechanical testing. As a result, the damage patterns caused by subcritical abuse, the resulting electrical performance loss, the propensity for latent damage to progress to thermal runaway, or the propensity of mechanical damage to induce known cell ageing mechanisms such as lithium plating has not been robustly examined. As most commercial lithium-ion batteries for electronic devices produced by mature manufacturers are robustly designed and assembled with advanced manufacturing techniques, modern lithium-ion cells rarely have problems associated with obvious design flaws and internal defects. Therefore, it is important to understand not only the effects of stressors on cell failure, but also the effects of complex latent problems accumulated during long-term use. However, to our knowledge, there is no systematic published study that has examined the effects of the latent problems in the active materials associated with the application of mechanical stress to lithium-ion cells. In this work, we seek to add to this body of knowledge by exploring the effects of cell abuse in situations where damage is sustained by the cell but the cell does not immediately progress to a thermal runaway condition. Our goal is to form a deeper understanding of how cell damage that allows for continued cycling of the cell progresses toward failure by observing how the physical, chemical, and electrochemical properties of a damaged cell change as a function of cycle count.

Fig. 1. a) An aluminum load frame was constructed to lower the loading nose toward the lower supports along a linear slide; b) the loading nose was attached to the crosshead with a force sensor and contacted the cell at the mid-line of a 5 cm span between the lower supports.

across the different test conditions, a standard baseline cycle at room temperature was used to evaluate cells before mechanical damage, after mechanical damage, and after cycling. In addition, cells that could no longer continue the assigned cycling protocol were analyzed for their failure mode.

2. Experimental

2.2. Mechanical testing: three-point bend apparatus and sample preparation

2.1. Overview

Sample preparation consisted of three major steps: bending, flattening, and resting. Bending imparted the initial damage to the cell. Flattening returned the cell to its original shape, potentially causing further damage. Resting the cell and monitoring the voltage ensured that the cell was viable for cycling. A custom load frame was designed and built to perform the threepoint bend test, as shown in Fig. 1 (with major hardware components labeled in Fig. 1a). As can be seen in Fig. 1b, a cell was placed on two fixed lower supports separated by 5 cm. The supports were rounded to a 5 mm radius at the contact point with the cell. The cell was deflected from above by an assembly traveling along a linear slide. The deflecting assembly consisted of a loading nose (10 mm radius) that was attached to a 2224 N (500 lbf) load cell to measure the force on the cell. The linear slide was driven by a NEMA 23 stepper motor controlled by a microcontroller commanding a stepper motor driver. Sensor data monitored during testing included displacement (via a 0–31.75 cm string potentiometer), loading nose force, cell temperature, and cell voltage. Signals were acquired and recorded using a data logger at

The current work was conducted on 3.0 Ah wound pouch cells that were provided by Samsung Electronics Co., Ltd. The cells were constructed with a LiCoO2 positive electrode, graphite negative electrode, and a typical separator/electrolyte system. A three-point bend deformation was chosen as the abuse stressor because of the ability to precisely measure and control deflection and because such stress could occur as a result of user abuse in the field. The effects of mechanical damage with cycling were characterized for two stress levels and three temperatures: room temperature, high temperature (40 °C), and maximum temperature (50 °C). Three-point bend samples were prepared in a custom apparatus that deflected the center of the cell by a specific distance corresponding to the two stress levels. Sets of cells were then cycled at three different temperatures up to 1000 cycles. Samples were removed from cycling at multiple points throughout the experiment to characterize the effects of the mechanical damage and temperature conditions on the cells. To compare cells 2

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presented current and voltage profiles typical of commercial cells. The room temperature (RT) condition represents a typical-use condition, simulating the current and voltage conditions that might occur in a device. The high temperature (HT) condition duplicates these current and voltage conditions at the highest temperature allowed by the specification sheet, with the purpose of accelerating any chemical or electrochemical degradation that might occur during cycling. The maximum temperature condition (MT) examines a further temperature increase, but at lesser current and voltage operating conditions. Such de-rating of the current and voltage with increased temperature is typical of lithium-ion cells and the MT condition was the highest temperature condition allowed by the specification sheet. To compare the effects of mechanical damage, as well as the different cycling conditions, baseline electrical measurements, taken at room temperature, were acquired at three points during the testing: 1) before mechanical damage, 2) after mechanical damage, and 3) after cycling at one of the specific conditions outlined in Table 2. Additional details and sample voltage and current profiles can be found in the supplemental materials.

10 Hz during tests. As the microcontroller controlled the stepper motor via open-loop control, the results from each test were processed to ensure that the final displacement depth was within 0.5 mm of the target. To bend a cell, the cell was placed on the bottom supports and the loading nose assembly was lowered until the loading nose force indicated initial contact with the center of the supported span. A thermocouple was taped to the center of the cell opposite the loading nose contact point and electrical leads were attached to the cell terminals to monitor voltage during the test. The data logger was started, and the loading nose assembly was lowered at a rate of 2 mm s−1 to its target depth. This displacement rate was selected to result in an approximately 0.01 s−1 strain rate in the tested span of the cell over a 5 mm displacement. This was deemed to be slow enough to neglect dynamic loading effects, but fast enough to not be concerned with significant creep effects (particularly for a polymeric composite material). We believe this displacement rate to be representative of a wide range of quasi-static results, but future studies could explore rate effects, particularly at high displacement rates. After a 2 s delay at the bottom of travel, the loading nose was returned to its starting position and the cell was monitored for approximately 30 s for signs of thermal runaway. If there was no thermal runaway, the thermocouple was moved to the opposite side of the cell (the concave side after bending). The flattening process was conducted manually in a hydraulic press. The cell was placed between two parallel non-conductive plates, slowly pressed flat, and held for 15 s before being released. The goal of this process was not to compress the cell, but rather to return the cell to its original non-deflected state in a controlled manner. The temperature and cell voltage were recorded during flattening to monitor for thermal runaway and/or voltage drop. The cell was then allowed to rest for 16 h, after which the voltage was measured again to monitor for potential short circuits resulting from the bending and flattening process. In order to observe the effects of mechanical damage in the absence of catastrophic cell failure, it was desirable to find the maximum displacement level achievable while still being able to cycle the cells. The previously described mechanical test procedure was repeated for increasing displacement levels until cell failure was observed. A cell was deemed to have “passed” a displacement level if thermal runaway was not observed after bending or flattening and if the cell did not lose more than 0.1 V after a 16-h rest. To identify this threshold, cells were prepared at increasing displacement depths. If a cell passed, the displacement depth was increased by 2.5 mm and the test was repeated with a new cell. Once a cell did not pass a bending level (in which case the cell was said to have “failed”), tests were repeated at the failed level and the level immediately prior to the failure to confirm the repeatability of the threshold. After the threshold was determined, the two bending levels immediately below the threshold were chosen to be the “High” and “Low” stress levels.

2.4. Characterization at selected intervals The effect of cycling the mechanically damaged cells was monitored via their cycle performance during testing, as well as with non-destructive and destructive examination at regular intervals. Cells that experienced a failure during the cycling protocol, such as a short circuit or swelling, were also analyzed via the same procedure. Cells were sampled from the cycling populations for analysis after 100, 200, and 500 cycles at each of the three temperature conditions (RT, HT, and MT). A control group that had not undergone any mechanical damage was also examined at the same intervals. Computed tomography (CT) analysis was used to non-destructively evaluate each cell. This technique distinguishes materials based on their ability to absorb X-rays. Heavier components, such as the lithium cobalt dioxide of the positive electrode, will appear brighter than lighter components, such as the graphite of the negative electrode. The resolution of the scans performed was 40–50 µm. The mechanical state of the separator was of particular interest, but the separator could not be adequately studied in CT due to its low X-ray absorption compared to other cell components and its small thickness compared to the typical CT scan resolution. Physical cross-sections where prepared so that the separator could be examined via optical and scanning electron microscopy (SEM). These cross-sections were prepared by vacuum infiltrating cells with epoxy, sectioning at the plane of interest, and then polishing to a finish suitable for microscopy. After cycling at various conditions, cells were opened and unrolled to examine the individual electrodes. This allowed for visual observation of discoloration and deformation at the damage region, as well as additional microscope and elemental analysis via SEM and energy dispersive X-ray spectroscopy (EDS).

2.3. Cycling conditions

3. Results

Cycling protocols, summarized in Table 1, were chosen to include a range of conditions that did not exceed the specifications of the cell. Cycling was conducted with a typical constant current/constant voltage (CC/CV) charge step and a constant current discharge step. The cells were of a well characterized chemistry (graphite/ LiCoO2) and

3.1. Control samples A control group that did not undergo the three-point bend served as

Table 1 Cell cycling conditions at the three temperatures examined RT (~23 °C), HT (40 °C)

MT (50 °C)

Capacity (Ah)

Discharge Current (A)

Voltage Min (V)

Charge Current (A)

Charge Cutoff (A)

Voltage Max (V)

Discharge Current (A)

Voltage Min (V)

Charge Current (A)

Charge Cutoff (A)

Voltage Max (V)

3.0

3.0

3.4

2.160

0.060

4.35

3.0

3.4

1.080

0.060

4.15

3

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Table 2 Cell baseline conditions for comparison across different temperature conditions Capacity (Ah)

Discharge Current (A)

Voltage Min (V)

Charge Current (A)

Charge Cut-off (A)

Voltage Max (V)

3.0

3.0

2.75

2.40

0.150

4.35

progressively with original bend depth, as shown in Fig. 2c. Fig. 2b and d illustrate the localized damage from the hinging behavior of samples bent to 5.0 mm and 7.5 mm on the face opposite contact with the loading nose. The sample bent to 2.5 mm shows relatively little visible pouch deformation. After flattening, the appearance of the deformed region appears to be wrinkle-dominated in the 5.0 mm bent cell, but crease-dominated in the 7.5 mm bent cell. In total, 24 cells were bent and flattened at each stress level. The reaction force and displacement of the loading nose assembly was recorded at a rate of 10 Hz for each test. No tested cells went into thermal runaway at either the high or low stress levels. The reaction force is plotted vs. displacement in Fig. 3, with the Low stress tests on the left represented by blue data points and the High stress tests on the right represented by red data points. Consistent reaction force profiles within each stress level were observed, as well as consistent behavior between the two stress levels. In both experiments, initial force-displacement measurements are approximately linear, with a slight softening before a peak force of ~375 N. This peak force is reached at approximately 2 mm displacement (Fig. 3, point a), and is followed by a drop in reaction force until approximately 2.5 mm (Fig. 3, point b). In the Low stress tests, the loading nose assembly reverses once the 2.5 mm displacement is reached. In the High stress tests, the loading nose assembly continues to 5.0 mm displacement and a reduced stiffness is observed throughout the remainder of the test. In both the High and Low stress tests, the loading nose assembly stops and holds at the desired displacement depth for 2 s before returning. This can be seen by the locus of points at the maximum displacement, indicating that the reaction force decreases slightly over the rest period. As the loading nose assembly starts to return (decreasing displacement), the force-displacement behavior is approximately linear, with a similar slope to that of the initial loading. As the reaction force goes to zero, the loading nose loses contact with the bent cell (Fig. 3, point c). Note that the displacement measured is that of the crosshead, not the cell, so the force measurement does not capture elastic return of the cell that occurs more slowly than the crosshead velocity. As is also shown in Fig. 3, there was an acceptable level of variation from sample to sample in force-displacement magnitude between tests, especially after peak forces were achieved and the samples yielded. Data from all tests is plotted as a locus of points, with data from a single run shown in a contrasting color. Due to the open-loop nature of the load frame control, there was also variation in the final displacement of tests, which contributed to intra-test variability during the unloading phase of experiments.

a reference for evaluating capacity fade and changes to the physical characteristics of the test samples. Specifically, the regions that would be affected by the mechanical damage were examined for defects or construction features that might obscure the effects of the imparted mechanical damage. The three-point bend affects the electrode layers in the center of the individual electrode panels, with the most stress placed on the exterior windings opposite the deflection. CT and physical examination of this region in the control cells did not show features that would obscure the effects of the imparted mechanical damage. Electrodes of the control group did not exhibit significant deformation or discoloration in this region at any temperatures tested. No swelling or cell failure was observed in the control group. Initial capacity values were consistent with the specified capacity of the cells and capacity fade was typical of lithium-ion cells of this form-factor.

3.2. Mechanical characterization High and Low stress displacements were chosen by determining a bending threshold for cell failure and then stepping back from this maximum displacement. The thresholding procedure, previously outlined in Section 2.2, resulted in a cell failure at a displacement of 7.5 mm. Repeatability of the threshold was tested by bending five cells at both 5.0 mm and 7.5 mm. As can be seen in Table 3, no evidence of temperature rise or voltage loss was observed in the five cells bent to 5 mm at any stage of testing. However, all five cells bent to 7.5 mm exhibited temperature increases of over 3 °C during flattening. One cell bent to 7.5 mm underwent partial thermal run away during flattening resulting in pouch breach, sparking, and elevated temperatures. Note that no cells included in the thresholding experiments exhibited signs of thermal runaway during bending. All temperature rises and voltage drops were recorded during or after the flattening procedure. If flattening was not a part of the procedure, the cells would likely be able to withstand a much larger displacement. As a result of the threshold procedure, a 5.0 mm bend was selected as the “High” stress level and a 2.5 mm bend for the “Low” stress level. Fig. 2 shows representative cells after bending (a, b) and after bending and flattening (c, d) for bend depths of 2.5 mm, 5.0 mm, and 7.5 mm. In Fig. 2a, samples bent at 5.0 mm and 7.5 mm show the majority of deformation occurring in a local area of the span with a morphology often referred to as hinging. After pressing cells flat, one can observe that some residual bend remains in the cells, increasing Table 3 Summary of measurements from the threshold procedure examining 5.0 and 7.5 mm bend Bend Indentation (mm)

Temperature Rise During Bending (°C)

Temperature Rise During Flattening (°C)

Wait Period Voltage Loss (%)

5

0.1 0.6 0.2 0.5 0.4 0.6 0.0 0.0 0.3 0.9

0.1 0.1 0.1 0.1 0.1 58.2* 3.1 5.8 8.7 9.4

0.00% −0.21% −0.21% −0.03% −0.03% −99.92% −66.78% −91.21% −77.45% −90.29%

7.5

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3.3. Macroscopic characterization A lithium-ion cell consists of multiple layers of positive electrode, separator, and negative electrode. In these bending experiments, a cell failure occurs when a breach of the separator allows for contact between the positive and negative electrodes. The three-point bend results in compression of the electrode layers closest to the loading nose and tension in the layers opposite the loading nose, hereafter referred to as “upward-facing” and “downward-facing,” respectively. As the cell is bent, the compression of the upward-facing layers results in surface wrinkling and shear band formation at the indentation site. In this case, the separator deforms, but does not develop breaches in this region. The downward-facing layers are placed in tension. The displacement applied in these tests exceeds the fracture stress of both the negative copper current collector and the positive aluminum current collector,

Pouch breach + spark during flattening. 4

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Fig. 2. Bent cell (a) profiles and (b) planar views (opposite loading nose), as well as flattened cell (c) profiles and (d) planar views

layers. At 2.5 mm deflection, this separation was negligible and did not appear to affect the innermost windings of the electrode assembly. At 5.0 mm and 7.5 mm, the separation between the fractured edges of the outermost downward-facing electrode layers was 0.35 mm and 1.1 mm respectively, as measured from the CT images in Fig. 4. The distance from fractured edge to fractured edge decreases toward the center of the cells but the damage pattern does extend to the innermost windings. The physical cross-section images in Fig. 4 provide an optical view of the bent regions at the three different displacement levels. In physical cross-section, the separator is better resolved. The separator is observed to be continuous in both the Low and High stress cases, despite the tearing and separation of the electrode foils. Cross-section allows for visualization of a single plane, thus it is possible for separator breaches to be present along the bend in regions that were not imaged. In the Threshold case, the separator was observed to be torn in three locations on the downward-facing side of the cell. Despite the significant damage to the electrodes, no cells failed during the bending process. The upward-facing layers are compressed, resulting in local delamination of the active materials and abrasion and deformation of the separator. While this process has the potential to bring the positive and negative electrode in contact, no separator breach was observed in this region supporting that the fracture stress of the separator was not exceeded. The downward-facing layers are placed in tension, resulting in elongation of the separator. The rupture stress of both electrode foils is exceeded, as observed by the torn foils and gapping. Flattening the cells results in closing the gap established in the downward-facing electrode layers. The integrity of the separator is paramount at this stage, as it maintains separation between electrodes of opposite polarity. Failures, such as loss of voltage or temperature rise, occurred primarily during flattening of the cells. Fig. 5 shows a series of CT and physical cross-sections showing cells bent and flattened with differing bend displacement. A physical cross-section could not be prepared for the Threshold case, as the cell experienced gas swelling during the preparation process (cell failure). Cell swelling is a common side-effect of short-circuit and over-discharge. This failure mode defines the Threshold case. The surface wrinkles and gaps formed during bending on the upward-facing electrodes were partially removed during the flattening process of the cells bent to 2.5 mm and 5 mm, with electrodes returning to a more planar state and aligning as they would in an undamaged cell. The upward-facing electrodes exhibited increasing shear band zones

Fig. 3. Force-displacement curves of the low stress (2.5 mm bend depth) and high stress (5.0 mm bend depth) levels. A single run for both tests is shown in contrasting colors. Peak force is reached at approximately 2 mm displacement (point a) and is followed by a drop in reaction force until approximately 2.5 mm (point b). As the loading nose retreats, it loses contact with the bent cell (point c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

resulting in tearing of both layers. The separator, being more compliant, stretches further than the foil layers before reaching its fracture stress. Depending on the degree of bending, the separator may or may not rupture. To characterize these phenomena, all cells in this study were CT scanned. One representative sample at each stage was selected for physical cross-section. Images from these analysis techniques are shown in Figs. 4 and 5. In the bent state, separation of the downward-facing electrodes can be observed in CT cross-sections and physical cross-sections of the damaged regions. Fig. 4 shows a series of CT and physical cross-sections with cells bent to 2.5 mm, 5.0 mm, and 7.5 mm. In Fig. 4, the loading nose impinges from the left side of each image, with the downwardfacing layers on the right. Successively higher degrees of bending result in greater tearing and separation of the downward-facing electrode 5

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Fig. 4. CT (top) and physical cross-sections (bottom) of cells bent to specific displacements. The loading nose impinges from the left side of each image, with the downward-facing layers on the right. All scale bars (bottom right of each image) are 500 µm.

from the current collector, thus becoming electrochemically inactive. The expected side effects of this process are immediate capacity loss due to the isolation of active material, and increased local degradation, due to local electrode imbalance that results from this isolation. The copper and aluminum current collector foils exhibit very limited elastic behavior, tearing rather than stretching during bending. The 2.5 mm sample exhibited regions where, after flattening, copper current collector foils that had torn were now overlapping with their mating edge. This was observed only for the copper current collector, not the aluminum current collector. The inner layers of the 5.0 mm sample also exhibited this behavior. This overlap was on the order of 50 µm. Two possible explanations exist for this observation: 1) the copper current collector plastically deformed at the bending site, resulting in a slightly longer foil that then overlapped with its mating side when the cell was flattened, or 2) the electrode layers slipped relative to each other during bending, resulting in the local overlap during flattening. Additional images of the physical cross sections can be found in the supplemental materials. Displacements of 2.5 mm and 5.0 mm resulted in electrode damage largely limited to the bending site, without apparent delamination further along the electrode windings. In the 7.5 mm case, the bending and flattening resulted in long-range deformation, such as de-lamination of electrode active material and gapping between electrode layers. From CT observations, no gapping extended to the top or bottom of the cell, but movement of the electrodes may not result in gapping observable in CT. Bending occurs on the plane of individual electrode layers, thus it is possible for the electrodes to slip relative each other to

with increased deformation. At 2.5 mm deflection the cells typically exhibited a single shear band that extended through a majority, but not necessarily all, upward facing windings. This band can be observed in the bent cells (Fig. 4) as well as in the flattened cells (Fig. 5). At 5.0 mm deflection, a second shear band was observed in some cells. The 7.5 mm samples all exhibited two shear bands in the upward-facing electrodes. The shear bands disturbed the composite electrode layers, resulting in small gaps between the active material particles. These remained in the electrode after the flattening process, even as the overall electrodes returned to a more aligned state. The downward-facing electrodes also returned to a more aligned state; however a bending deflection of the torn electrodes towards the inner windings was observed after the flattening process. The CT and physical cross-section images show that if a breach in the separator had formed during the bending process due to locally exceeding the separator tensile strength, the flatting procedure would have resulted in bringing the damaged electrodes back into contact through this breach. This observation is consistent with failures occurring during the flattening process, rather than the initial bending, and with the high failure rate at the 7.5 mm threshold. The imparted level of elongation of the separator results in inelastic behavior, where the separator does not tear, but also does not recover to its original length after flattening. This leaves excess separator in the downward-facing region when the cell is flattened. This excess separator forms wrinkles and inclusions of active material. Examples of this behavior were observed primarily in the 5.0 mm sample, and to a much lesser extent in the 2.5 mm sample. The active material is isolated 6

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Fig. 5. CT (top) and physical cross-sections (bottom) of cells bent to a specific displacement, and then flattened. The loading nose originally impinged from the left side of each image, with the downward-facing layers on the right. All scale bars (bottom right of each image) are 500 µm.

observed. Slipping of electrode layers due to the sheer stresses generated was not clearly observed as a change in the negative electrode overlap. Maintaining sufficient electrode overlap at the top and bottom of the cell protects the cell from shorting at those locations as a result of the bending stress, making it more robust to this kind of abuse.

relieve some of the stress imparted by the bending protocol. To characterize this possible slipping, the electrode alignment of control cells was compared to that of cells bent to 2.5 mm and 5.0 mm and then flattened. Lithium-ion cells are constructed such that the negative electrode extends past the positive electrode at the top and bottom of the cell. This overlap was measured at various positions on cell winding from the CT data collected during the study. Three cells at each condition were evaluated along with the CT cross-section for a control cell. No correlation was observed between the overlap measurements and the stress level at the resolution available in CT (40–50 µm). The positive and negative electrodes were aligned appropriately and consistently, however the cell-to-cell variation for a given position was typically ~125 µm, thus trends smaller than this would not be

3.4. Electrical/cycling performance The baseline protocol described in Section 2.3 was conducted before and after the mechanical damage, as well as on cells selected for analysis at 100, 200, and 500 cycles. This baseline protocol allows for comparison of cells across different cycling conditions. Mechanical damage resulted in electrode gapping, compromised 7

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Fig. 6. Baseline capacity of cells before (blue bars) and after (green bars) mechanical damage on the leftmost plot. Capacity plotted for Control, Low Stress, and High Stress cells cycled at the Room, High, and Max Temperature conditions on the right plots. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.5. Sample analysis and discussion

electrical and ionic contact of active material, and interruption of the electron conduction pathway through the current collector. These factors were expected to immediately affect the capacity of the cell. The leftmost plot in Fig. 6 shows the capacity measured before and after cycling as well as individual values to inform the spread of the values. Each bar represents the average of 25 cells. A slight decrease in the mean capacity is observed after both the High and Low Stress procedures compared to the control and initial mean. The average capacity for the control group was 2.983 Ah, while the average capacity for the Low and High stress groups after mechanical damage was 2.974 Ah and 2.980 Ah respectively. This relatively small decrease in the mean capacity is overshadowed by the much larger capacity variability inherent in the population and is in agreement with the local damage resulting from the three-point bend method. The three-point bend only affects a region ~1 mm tall, whereas the active region in a cell is 74 mm in height. Further, only a small amount of active material within this region loses ionic/electronic contact and the loss of that contact is preferentially on the downward-facing layers of the cell, with the upwardfacing side suffering little isolation. The current collector foils are torn at the downward-facing side of the cell but the electron conduction path remains intact on the upward-facing side, and is thus not a limiting factor at the currents used for these tests. Initial decreases in capacity due to the mechanical damage were minimal. The compromised electrical and ionic contact of active material at the damage location was expected to lead to undesirable local electrochemical activity. Loose active material or gapping could lead to localized electrolyte degradation, which consumes lithium ions resulting in capacity loss and local impedance increases. Local capacity imbalance could also lead to lithium plating in regions where the negative active material delaminates, but the positive does not. As cells were sampled for analysis at the various cycle intervals, the baseline cycle was repeated to compare the effects of the different cycling conditions. The baseline capacity data is plotted for the cells cycled at the three temperature conditions in the three plots on the right side of Fig. 6. Each data point represents the average of two cells.

In addition to the electrical analysis described above, cells were removed from cycling at selected intervals and were opened to physically examine the electrodes for local degradation at the damage region. Tearing of the current collector foils was confirmed by physical examination for both the negative and positive electrode. As observed in cross-section, physical examination confirmed that tearing was limited to the downward-facing layers. Disturbance of the electrode active material layers in the bend region was characterized by cracking and delamination. The disturbed region spanned only a few millimeters of the long axis of the cell but extended along the entire width of the electrode, perpendicular to the long axis of the cell, with more severe cracking on the downward facing layers. Disturbed active material would be expected to cause electrochemical degradation; however, the negative electrode showed only minor discoloration in and around the bend region at 500 cycles. The discoloration observed was typical of cells having undergone cycling and not an indication of excessive electrolyte degradation. Lithium plating might be expected in this region due to the local disturbance of the active materials; however, even after 500 cycles, no lithium plating was observed in any cells examined. The laminated nature of the electrode assembly likely contributed to maintaining electrical and ionic contact through the mechanical damage protocol. Images of the opened cells can be found in the supplemental materials. Of the forty-eight (48) bent cells cycled at various temperature and damage levels, four cells developed failures during the cycling protocol. All four cells were part of the High Stress group (5.0 mm displacement) cycled at maximum temperature (50 °C). The cycle conditions and nature of the failures are summarized in Table 4. All failures were characterized by voltage loss followed by swelling, with no failures resulting in thermal runaway. The root cause of all failures could be traced to breaches in the separator on the downward-facing side of the cells. In each case, the breach was between two coated electrodes. The location of each breach is shown in the winding diagram in Fig. 8, with the failed cells highlighted. The cells did not exhibit capacity loss prior 8

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Table 4 List of cell failures observed during and as a result of cycling Cell ID

Stress Level

Temp.

Completed Cycles

Description of Failure

BHM4 BHM6 BHM5 BHM7

High High High High

Max Max Max Max

200 500 500 78

No issues during cycling, post-test voltage loss/swelling No issues during cycling, post-test voltage loss/swelling Recovered from short during cycling, post-test voltage loss/swelling Charge time-out during cycling, voltage loss/swelling

T T T T

Fig. 7. a) Capacity plotted vs cycle for all High Stress cells. The failed samples, BHM4, BHM5, BHM6, and BHM7 are highlighted. b) Charge capacity for all High Stress Max temperature cells. c) Current vs time plot of the constant voltage portion of Cell BHM7’s last charging step. d) Constant voltage portion of the charge for cell BHM5 before, during, and after the short. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

recorded on the previous cycle. Interestingly, the cell recovered from this short, exhibiting a decrease in charge capacity with cycles after the initial short. At cycle 60, the cell exhibited a charge step consistent with the pre-short cycle and similar to other cells in the same group. Cell BHM7 was removed from its cycling protocol due to a charge time-out protection programmed into the protocol. During the constant voltage step, the current did not decay sufficiently to terminate the charge step. Instead, the current suddenly increased and continued the constant voltage step, dissipating energy through the short, as observed in Fig. 7c. The cell did not go into thermal runaway, indicating that the heat generated from the internal short was not sufficient to initiate a thermal event. The current increase was 0.2 A while the voltage was held at 4.15 V. This can be used to calculate an approximate resistance for the short circuit of 20 Ω. The behavior of the four failures is consistent with a high resistance short in each cell. The failed cells were opened to determine the locations of the short circuits. In all cases, the cause for short circuit was found to be a breach of the separator. The location of these breaches in the context of the full cell is shown in Fig. 8. The BHM6 breach did not exhibit any morphology consistent with heat damage. The breach was linear in nature and oriented along the winding direction of the cell. The separators commonly used in lithiumion cells are anisotropic in nature, with higher tensile strength in the winding/machine direction, and lower tensile strength in the transverse direction. This results in easy tear propagation in the machine direction, which is evident in the breach. The breaches in BHM4 and BHM5

to failure compared to other cells in the same category, or the control cells. In the case of BHM4 and BHM6, the cells completed their assigned number of cycles with no significant charge or discharge capacity changes noted during the cycling or post-cycle baseline procedures. Their discharge capacity is plotted versus cycle number in Fig. 7a along with other cells cycled at the same condition (gray) and the control group (blue). No abnormal capacity fade, impedance changes, or selfdischarge were observed during cycling. Rather the failures were observed as voltage loss and swelling during post-test processing. Post-test processing involved a longer rest period, typically a few days, compared to the 10 min rest between charge and discharge during the cycling protocol. The observation of voltage loss during this longer rest period, but not during the 10 min rest at top of charge, indicates that both failures were due to high resistance short circuits in the electrode windings. Cell BHM5 also completed its assigned 500 cycles and did not exhibit abnormal discharge capacity fade or trigger the charge time-out safety. The cell did develop and recover from a high resistance short circuit at cycle 42. While the discharge capacity remains normal (Fig. 7a), the charge capacity suddenly increases at Cycle 42 (Fig. 7b), indicating the cell is dissipating current through an internal short. The short is of high enough resistance that the cell is still able to hit the current cut-off and terminate charge within the safety time-out limit. The dissipation of current can be observed in the constant voltage portion of cycle 42 (Fig. 7d), where the current decay lags behind that 9

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eventually lead to the high resistance short circuits observed. Lithium-ion cells exhibit volume change due to the lithiation and de-lithiation of the negative and positive active materials used in the cell. The overall volume of a typical lithium-ion cell increases during charge and is highest at the top of charge. This volume change is reversed during discharge, and is typically 5–10% [32]. It is hypothesized that on a particle level, such a volume change could result in slight movement of active particles, including expansion of particles into an existing separator breach. A short resulting from this process would be between the positive and negative active materials, and involve only a small number of particles. This type of short would have a high resistance and is unlikely to result in a thermal runaway, consistent with the observations of this study. A future study employing higher resolution CT scanning to better understand changes on the particle level could be used to evaluate this hypothesis. Three of four failures did not result in immediate termination of the cycling protocol. Rather, the cells were able to continue cycling and exhibited normal capacity values. The resistance of the internal shorts was likely high enough not to significantly affect cell operation. One of these cases exhibited increased charge capacity, which could have been used to diagnose the issue, but this cell recovered to normal operation with continued cycles. In this case, the continued expansion and contraction of particles likely resulted in movement of particles to break the short in later cycles. One internal short did result in immediate failure. This short occurred towards the end of the constant current charging step, when particle expansion would be highest, and is thus also consistent with the hypothesis. Two of four failures did not exhibit obvious electrical signals, such as increased charge capacity or impedance changes, that could have been used to inform their state-of-health or terminate their use. Electrical signals of the high resistance shorts would have manifested by monitoring extended rest periods, or comparing a detailed cell model and high accuracy current measurements. As a result, this systematic study of lithium-ion cell response to mechanical abuse not only helps with understanding the behavior of the lithium-ion cells with latent problems as a function of cycle count and damage level but also provides a guidance to achieve further improvement in the robustness of lithium-ion cells. We believe that the results of this study can provide insight for future development of lithium-ion battery systems with improved tolerance to abuse.

Fig. 8. Cell winding diagram showing the locations of separator failure (top) and optical microscope images of separator breached in failed cells. Scale bars are 500 µm.

exhibited edges with a rounded and thickened morphology, consistent with melting and re-solidification of the polymer; however, heat generation was low enough that thermal damage was not observed on the mating electrodes. The separator breach in BHM7 exhibited clear signs of heat generation, with the separator edges around the breach appearing discolored, and scorch marks observed on the mating positive and negative active material. BHM7 was the only cell to fail before completing its assigned number of cycles, but it did still complete 78 cycles without incident. Two of four failures, BHM 4 and BHM5, occurred in the same location, directly adjacent to the insulting tape that is applied to prevent local capacity imbalance at the coating edges. By its design, this region contains several thickness changes: the addition of two layers of insulating tape, the start of the positive electrode coatings (~55 µm each), and further removed, the start of the negative electrode coatings (~65 µm each), and negative electrode foil (15 µm). In addition to resulting in local thickness differences, each layer exhibits different mechanical properties. During the bending and flattening process, the juxtaposition of different mechanical properties can result in increased stresses on the components. All failures in this study occurred in the high stress, maximum temperature group and resulted from breaching of the separator via mechanical, rather than electrochemical, means. It is hypothesized that the bending and flattening protocol resulted in weakening of the separator at the damage region or may have resulted in an immediate breach. Cells did not immediately fail because contact between the positive and negative electrode was not immediately established. As the cells cycled, the volume change associated with cycling resulted in loose particle movement and additional stress on the separator that

4. Conclusions In this work, a systematic investigation into the latent effects of mechanical abuse on the performance of lithium-ion cells as a function of cycle life and mechanical damage level was conducted. The effects of mechanical damage imposed by a three-point bend deformation on cell performance were characterized via electrochemical, non-destructive, and destructive analyses. The bending and flattening of cells resulted in severe mechanical deformation of the electrodes and separator. Despite this damage, cells did not exhibit significantly diminished performance characteristics. Long range deformation of the electrode assembly was not observed due to good adhesion between the separator, separator coating, and electrodes, keeping the electrode assembly intact in the regions away from the damaged region. This good adhesion limited mechanical damage to a highly localized region compared to the overall electrochemical area available in the cell and prevented the slipping of the electrode stack as well as gap propagation in the electrode layers beyond the damage region. These physical observations were consistent with the electrical performance of the damaged cells. As the lack of significant performance degradation is largely attributed to the construction and assembly of the cells, the implication of this study is that the construction of these cells, specifically the laminated nature of the various layers and the mechanical properties of the separator, can be applied to any chemistry to also make it robust to mechanical abuse. The results of this study are specific to wound cells 10

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that use a single continuous current collector for the positive and negative electrode. The single conductor used in this construction format ensures that all parts of the cell remain electrically connected to the cell tabs. Both the negative and positive current collectors tore as a result of the imparted mechanical damage, thus a stacked format that employs many individual current collector sheets would result in isolation of portions of the electrodes and exhibit a lower cell capacity after mechanical damage. No cells, deformed below the systematically established threshold level, experienced thermal runaway during this testing. All short circuit failures observed occurred in the high stress and maximum temperature group and were found to result from mechanical damage to the separator rather than chemical or electrochemical degradation of the cell. With regard to the four short circuit failures, it was shown through stable voltage measurements that contact between the positive and negative electrodes was not immediately established during the bending and flattening process. However, the bending and flattening protocol was shown to have caused weakening and potential tearing of the separator at the damage region. The comprehensive analysis presented in this work supports the hypothesis that as the cells are cycled, the volume change associated with the cycling results in movement of material in the damaged region, eventually leading to the establishment of high resistance, and sometimes transient, short circuits.

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Funding This work was supported by Samsung Electronics Co., Ltd. CRediT authorship contribution statement Johanna K. Stark Goodman: Conceptualization, Methodology, Investigation, Supervision, Writing - original draft. Jay T. Miller: Methodology, Investigation, Writing - original draft. Steven Kreuzer: Conceptualization, Writing - review & editing. Joel Forman: Formal analysis, Writing - review & editing. Sungun Wi: Resources, Writing review & editing. Jae-man Choi: Resources, Writing - review & editing. Bookeun Oh: Funding acquisition, Resources, Supervision. Kevin White: Funding acquisition, Supervision, Writing - review & editing. Declaration of Competing Interest Sungun Wi, Jae-man Choi, and Bookeun Oh are employees of Samsung Electronics Co., Ltd., which provided the materials and partial funding for this study. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2020.101244. References [1] S. Abada, M. Petit, A. Lecocq, G. Marlair, V. Sauvant-Moynot, F. Huet, Combined experimental and modeling approaches of the thermal runaway of fresh and aged lithium-ion batteries, J. Power Sources 399 (2018) 264–273. [2] D.P. Finegan, M. Scheel, J.B. Robinson, B. Tjaden, I. Hunt, T.J. Mason, J. Millichamp, M.D. Michiel, G.J. Offer, G. Hinds, D.J.L. Brett, P.R. Shearing, Inoperando high-speed tomography of lithium-ion batteries during thermal runaway, Nat. Commun. 6 (2015) Article number: 6924. [3] J.R. Dahn, E.W. Fuller, M. Obrovac, U. von Sacken, Thermal stability of LixCoO2, LixNiO2 and λ-MnO2 and consequences for the safety of Li-ion cells, Solid State Ion. 69 (Issues 3–4) (1994) 265–270. [4] B. Satishkumar, D. Chikkannanavara, M. Bernardi, L. Liu, A review of blended cathode materials for use in Li-ion batteries, J. Power Sources 248 (2014) 91–100.

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