Article
Limiting Internal Short-Circuit Damage by Electrode Partition for Impact-Tolerant Li-Ion Batteries Michael Naguib, Srikanth Allu, Srdjan Simunovic, Jianlin Li, Hsin Wang, Nancy J. Dudney
[email protected] (S.S.)
[email protected] (N.J.D.)
HIGHLIGHTS New design for Li-ion battery electrodes to mitigate mechanical impact damage Electrodes break along preset patterns to limit current across a short circuit After mechanical damage, the battery with breakable electrodes remained functional
In this study we report on a new design concept for Li-ion battery electrodes to mitigate mechanical impact without catastrophic failure for the battery. The concept is based on introducing breakable electrodes that, upon impact, isolate the damaged part from the rest of the electrode to limit the current going through any short circuit. We used patterns of slits to realize the breakable electrodes, which in principle add little cost and can be produced by the roll-to-roll process.
Naguib et al., Joule 2, 155–167 January 17, 2018 ª 2017 Elsevier Inc. https://doi.org/10.1016/j.joule.2017.11.003
Article
Limiting Internal Short-Circuit Damage by Electrode Partition for Impact-Tolerant Li-Ion Batteries Michael Naguib,1,4 Srikanth Allu,2 Srdjan Simunovic,2,* Jianlin Li,3 Hsin Wang,1 and Nancy J. Dudney1,5,*
SUMMARY
Context & Scale
We report on a unique safety mechanism introduced to the Li-ion battery design to mitigate the effects of a mechanical impact event by limiting the current moving through resulting internal shorts, thereby preventing thermal runaway. ‘‘Slitted’’ electrodes and current collectors would electrically isolate the impacted parts of the electrodes before puncturing the separator. Batteries with such ‘‘slitted’’ electrodes were shown to perform normally prior to the mechanical impact. A proof-of-concept experiment showed that the battery with modified electrodes survived significant mechanical deformation without any change in the open-circuit voltage of the battery. It is interesting to note that, after the impact event, the modified battery was still viable with a reversible capacity of about 93% of that before the indentation test, while the standard battery was no longer functional.
Mechanical abuse of Li-ion batteries (LIBs), through events such as automobile accidents, can lead to complete failure. Our research represents a promising new manufacturing method that will enable portions of LIBs to remain functional once damaged sections have ceased to function. Our approach for mitigating the severity of internal electrical shorts in LIBs involves using electrodes designed to break upon impact into electrically isolated parts before the separator is punctured and an internal electric short occurs. The electric current passing through the internal short will be reduced, preventing the onset of exothermic reactions and thermal runaway. The new design introduces slits to the electrode that adds minimal cost and does not require significant changes in roll-to-roll production, making this approach scalable. While more research is needed to optimize the slit patterns for various scenarios, this work opens a new avenue for incorporating inherent safety features into battery designs.
INTRODUCTION Compared with other electrochemical energy storage systems, Li-ion batteries (LIBs) have very high energy and power densities.1 They have been used in portable electronic devices for more than two decades and more recently in electric vehicles. However, safety is one of the main challenges facing the widespread deployment of LIB technology.2 If damaged, the flammable organic electrolyte in LIBs may lead to fires with toxic fumes.3 LIB safety upon impact has been the subject of extensive research into possible failure mechanisms and the design of necessary safety features.4,5 For batteries in electric or hybrid vehicles, safety is typically ensured by reinforcing the enclosure of the battery pack, which adds to the weight of a vehicle5 and reduces the overall Wh/mile performance.6 Designing batteries to protect them from a variety of impact scenarios is a difficult task.7 During impacts, there is an increasing risk for the cathodes and anodes to puncture the separator and connect, causing a short circuit that can trigger a series of highly exothermic events (i.e., thermal runaway) and result in the battery’s catastrophic failure.8 Additional safety components can be used to prevent thermal runaway, such as a positive thermal coefficient barrier layer,9 current interruption device, shutdown separator,10 thermal fuses, and vents.11 These safety mechanisms respond to a critical temperature or current and shut down an entire cell under failure conditions. Our design for a mechanical abuse-tolerant LIB disrupts the electrical connectivity and/or conductivity of a battery electrode at the site of an internal short. The electrically isolated regions reduce or eliminate the electrical current through the
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Figure 1. Schematic for Mechanical Abuse-Tolerant Battery Design (A) Schematic showing predetermined breakable pattern in the battery electrodes. (B) The mechanically impacted part of the battery is separated upon prescribed deformation from the rest of the battery. This design limits the battery capacity that can be discharged through the isolation subregions to less than the capacity that causes sufficient I 3 R heating to initiate thermal runaway.
internal short, thereby avoiding the onset of thermal runaway and allowing the cell to continue operating. Such electrical isolation may be achieved by introducing slits along which the electrodes will break before the anode and cathode fragments can come into contact. Computational modeling was used to develop a rectangular ‘‘slitting’’ pattern for an effective fragmentation of the electrodes. Similar perforation patterns have been used for manufacturing morphable structures out of perforated thin sheets,12 and for auxetic planar materials.13,14 Under mechanical abuse, the electrodes spontaneously break up into smaller, electrically insulated fragments (Figure 1), confining the discharge domain of a short circuit event to a small area (i.e., small capacity) and thereby limiting the current going through the short circuit. The electrically connected region outside the fragmented region remains functional following the internal short circuit event between the fragments of the opposite polarity. A similar concept by Wang et al.15—to design electrodes to break the electrical contact upon impact—was implemented using a multistep processing approach and tested at length scale different from that reported by us here (in our study, the fragments are on the mm-to-cm scale).
RESULTS AND DISCUSSION Experimental Testing of the Main Hypothesis To test our main hypothesis that electrically isolating the shorted part from the rest of the battery restricts the short-circuit event to the isolated parts, we punctured two batteries using a stainless-steel dart, resulting in short circuits. The semi-automated fabrication of these balanced LiNi0.5Mn0.3Co0.2O2 (NMC)/graphite electrodes is described in detail in Experimental Procedures. The first battery is a standard single-layer pouch cell with an active area of 50 cm2 (Figures 2A–2C). The second battery was modified by cutting a 1 3 1 cm2 corner, isolating it electrically from the remaining portion of the cell but sharing the separator and electrolyte (Figures 2D–2F). Both cells were punctured by a stainless steel game dart (26.4 g) dropped from a height of 75 cm through a tube onto the corner of each battery. The fins were removed from the game darts for this test. Before the short-circuit test, both batteries were charged (C/25) to 4.2 V. For the modified battery, the tabs for anodes of the corner and the remaining portion were connected, and similarly for the cathodes, during charging. During the shortcircuit test, the tabs for the corner and remaining portions were disconnected to monitor the open-circuit voltage (OCV) of each portion separately. An infrared (IR) camera was used to monitor the change in temperature during the short-circuit test.
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1Materials
Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
2Computer
Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
3Energy
and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
4Present
address: Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118, USA
5Lead
Contact
*Correspondence:
[email protected] (S.S.),
[email protected] (N.J.D.) https://doi.org/10.1016/j.joule.2017.11.003
Figure 2. Experimental Proof of Concept with Pouch Batteries (A) Schematic for a pouch cell shorted at the corner. (B) Thermographic image for a standard single-layer pouch cell at 1 s after shorting at the corner. (C) OCV of the battery in (B) during the short-circuit test. (D) Schematic for a modified battery design in which the corner is completely isolated electrically from the rest of the battery but shares the separator and electrolyte with the rest of the battery. (E) Thermographic image for a cell described in (D) at 1 s after shorting. (F) OCV for both corner and large portions of the battery shown in (D) and (E).
The thermographic image recorded 1 s after dropping the dart on the standard battery (Figure 2B) shows that the temperature of the entire battery increased, while Figure 2E shows that the temperature increase was limited to the corner of the modified battery. The maximum increase in the temperature close to the dart was found to be 19 C after 3 s for the standard battery; it was only 2 C after 4 s for the modified one. The temperature inside the pouch is no doubt higher, but the trend can be expected to continue. Movie S1 shows the IR images for both standard and modified batteries. As shown in Figure 2C, the OCV of the standard battery dropped to zero when the dart impacted the battery. For the modified battery, the OCV of the corner also dropped to zero, but the OCV of the remaining portion was unchanged (Figure 2F). This agrees with the IR images that showed no contribution from the large part of the modified battery, and proves that electrically isolating the shorted part from the rest of the battery limits the discharge capacity through the short, therefore preventing thermal runaway. Computational Model for Isolation of Cell Electrodes Selecting an effective electrode safety feature will allow a damaged LIB to avoid thermal runaway by isolating damaged electrodes. We conducted electrochemical simulations for a series of configurations to understand the effect of various levels
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Figure 3. Contour Plots of Li-Ion Concentration (mol/cm3) in the Solid Phase across the Segmented Electrodes during the End of the Discharge Process (A) Current collectors are isolated, but electrode materials are intact. (B) Current collectors and cathode electrodes are isolated with an anode electrode in place. (C) Current collectors and electrodes are completely isolated. Initial Li + concentration in graphite was 66% of maximum concentration in all cases.
of separation on electrode sections that may be in partial contact with the rest of the cell components (either measured for the exact electrodes or reported for the same system,16 as listed in Table S2). The simulated configurations included (1) isolated current collectors with the electrodes’ films intact, (2) current collectors and cathode film isolated with the anode electrode intact, and (3) complete isolation of the current collectors and electrodes’ films. Several current and voltage boundary conditions were evaluated to simulate a short circuit, including a sudden decrease in the cell potential to 1 mV. Figure 3 shows results obtained when a constant 2C (normalized to the quadrant) current was applied to the isolated cathode current collector quadrant to evaluate whether the neighboring regions participate in the electrochemical transport. This numerical setup incorporates the necessary processes during the onset of an internal short that elucidate the dominant transport mechanisms where the currents going through the shorted portion can rise exponentially. We used two initial Li concentrations in graphite: 66% (Figure 3) and 95% (Figure S1) of the maximum Li concentration. Comparable results were obtained for both initial concentrations. Figure 3 (66% initial Li concentration) shows contour plots of the solid-phase Li concentration (mol/cm3) computed across the regions toward the end of discharge. For ease of comparison, all the solutions are plotted with the same color scale; red indicates the maximum solid-phase Li concentration attained among all simulations, and blue indicates an Li content of zero. The cross-section views at the top of Figure 3 highlight the isolated current collectors, the separator region filled with electrolyte, and the electrodes’ films with colors referring to the initial concentration state. The first case shows that, even though the metal current collectors are completely isolated, the island shorted with constant current boundary condition drives the transport of Li to equilibrate the entire electrodes. This indicates that breaking the metal foils alone may not be sufficient to prevent high current and heating because the remaining electrodes contribute to the extracted battery capacity. In the second case, we demonstrate that if the cathode electrodes and aluminum foils are completely isolated, the entire anode with an isolated copper foil can still contribute to the discharge current, indicating higher heat generation that may trigger thermal runaway. In the final case, we investigate
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the discharge of the completely isolated anode and cathode electrodes along with current collectors under constant current. Electrodes separated directly across from each other participate in the transport as we can see Li concentration develop across the separator and cathode interface. However, there are no concentration gradients in the adjacent electrode regions; three quadrants that are electrically isolated retain the same initial concentration states and do not contribute to the discharge capacity extracted because the ion diffusion through the electrolyte system is extremely slow. All three designs could improve safety compared with standard battery construction, as they could delay heat propagation; however, to eliminate the possibility of thermal runaway, complete electrical isolation of both the current collectors and electrodes is best. Design of Breakable Electrodes Our cell safety concept provides its main advantage by electrically isolating the electrodes before the onset of an internal short. After the breakup, the electrode fragments should be electrically isolated from each other; in this way, if electrodes of opposite polarities do come into contact, the related electrical discharge does not involve the entire battery capacity. For this concept to be effective, the electrode fragmentation should occur before any global fault formation. At this time, models for the formation of global faults in the cell are insufficient; therefore, we are considering only electrode fragmentation. The basic electrode fragmentation mechanism is the introduction of areal slit patterns into the electrodes. Under external loading, these slits guide the failure by breaking the remaining ligaments, which separates the electrode into electrically isolated fragments. The slits can be introduced at the last step, when the electrodes are cut using a clicker die and automatic punch and before cell assembly. This maintains the roll-to-roll electrode coating and calendaring processes currently used in battery manufacturing, and adds only minimum cost by modifying the standard design of the clicker die used to cut the electrodes for a pouch configuration. Guiding tearing along a prescribed path using slits is common in the paper and cardboard manufacturing industry.17 Slit patterns have been used for linear cutting by creating a ‘‘running crack’’ that connects the adjacent slits, but we have not found any instances in the literature of using areal slit patterns to create surface fragmentation. Two- or three-dimensional slits have been used as experimental or computational analogs for material failure by void coalescence,18–20 but not for guiding coalesced cracks into domain fragmentation. Research into cracking of thin sheets and foils by tensile in-plane forces shows that this phenomenon occurs under low constraints in the crack tip.21 The mechanical and fracture properties involved in the event are strongly influenced by the thickness and the microstructure of the material.22–24 Macroscopically, metal foils are very brittle. They fracture by forming localized necks with negligible thinning outside the neck zone. Under tension tests, foils usually break as soon as yield strength is reached for the smallest cross section.25 This is strongly influenced by the surface roughness of the foil, which may be a substantial fraction of the foil thickness. Growth of large cracks in thin metallic sheets is usually described using a crack-tip opening angle (CTOA) method.26 In thin sheets, considerable energy is spent in the neck shrinking ahead of the crack tip to the extent that the fracture energy varies with the sheet thickness.27 However, for stable cracks much longer than the sheet thickness, the CTOA acquires a constant value after about one thickness of crack extension.28 Therefore, driving cracks in tension opening (K1) mode is an efficient method for controlled tearing of metal foils. A threshold for separation can be related to imparted deformation by equating
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the final crack length to the initial ligament length. Analytical expressions for stress fields29 and computational simulations can be combined with experimental data for specific loading and crack configuration. We wanted to create relatively large fragments, on the order of 10 mm in length, for which the slitting of the electrode sheet would not require a complicated technological process. Our first, unsuccessful attempt involved cutting a regular square slit pattern, as described in Supplemental Information and shown in Figures S1 and S2. The perforation was cut using a laser tool leaving a wide slot of exposed edges. For this pattern, complete separation of areal segments outlined by the regular square perforations depends on several cracks converging to the same point, which is difficult to achieve.30 We then designed an alternative slit pattern that creates locally intense stress fields at the tips of each slit and promotes K1 tensile cracking. The slit pattern and the schematic tearing associated with in-plane tensile loading are shown in Figures 4A and 4B, respectively. Each crack path is attracted to a single free boundary so that it becomes increasingly stable by approaching the boundary. Through thickness, impact forces effectively constrain the out-of-plane wrinkling and further suppress the diffusion of local deformation. Similar crack growth patterns can be found in skin fragmentation and mud cracking before the setting in of stationary crack networks.31 We used the finite element method (FEM)32 commercial code LS-DYNA33 to guide the selection of the slit pattern. Our simulation model is intended to illustrate how the indentation deformation will result in stress concentrations that encourage the cracks to grow in the direction of electrode patch separation. Due to the complexity of interactions between various cell layers, we have not modeled the crack propagation in the electrodes nor the overall cell response. Simulations of deformation of in-plane uniaxial and biaxial tension of a plate with slits in Figures 4C and 4D illustrate the global kinematic mechanisms as described in Figure 4B. A simulation of an out-of-plane indentation of thin copper foil with slits is shown in Figure 4E. The colors denote the effective plastic strain. More details about the modeling can be found in Experimental Procedures. We conducted a series of indentation simulations on monolithic sheets of copper for increasingly larger slits, to simulate crack growth in cells, then used the resulting stress and strain distributions as indicators of the stability of crack growth. The actual values for indentation simulation of a copper thin sheet are shown in Figure 4E. Other slit patterns can also be used; for example, a triangular pattern with similar characteristics is shown in Figure S4. The solid lines denote the perforation pattern, and the dashed lines denote the schematics of a crack opening for kinematically admissible deformation. Experimental Testing of the LIB Cells with Breakable Electrodes Our modeling suggested a slit pattern, which was introduced by modifying the design of the clicker die as shown in Figure 4F (dimensions given in Figure S5). Using an automatic punch, the slit electrodes were punched from the large rolls of electrodes in a single operation. The knife edges were limited to the center of the die to simplify use of the automated stacking equipment available at Oak Ridge National Laboratory (ORNL) for cell assembly. Complete details of the fabrication of the single-sided LiNi0.8Co0.15Al0.05O2 (NCA) on Al electrodes and graphite on Cu electrodes are given in Experimental Procedures. The optical
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Figure 4. Design and Function of the ‘‘Slitted’’ Electrodes (A) The slits pattern used in this study. (B) Schematic tearing associated with in-plane tensile loading. (C and D) Simulations of (C) uniaxial and (D) biaxial tension of thin copper foil with slits based on the pattern shown in (A) using FEM. (E) Snapshot at an intermediate time for a sphere shape pressing into an electrode with slits. In (C), (D), and (E) the colors denote the effective plastic strain. (F) Drawing for the clicker die used to introduce slits to the electrodes.
micrograph of an electrode with slits (Figure S6) shows no sign of delamination, exposed metal edges, metal burrs, or electrode debris for these commercial-quality electrode coatings. While all our experiments used single-sided coated current collectors, it should not be difficult to extend this work to double-sided coated collectors. Figure S7 shows both front and back sides of single- and double-sided coated cathodes. Fully functioning batteries were assembled, as well as ‘‘dry’’ batteries that did not contain liquid electrolyte. To validate our tests, we assembled two electrolyte-free dry cells—one of them with ‘‘slitted’’ electrodes and the other with standard electrodes—and deformed them using a brass ball. It is important to mimic loading conditions on pouch cells that can occur from external impact on battery packs.4,34 Inside battery packs and modules, pouch cells
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Figure 5. Testing the Efficacy of the Slit Design for Electrolyte-free Cells (A) Photograph of the indentation test configuration used in this study; inset shows a schematic for the experimental setup. (B) OCV of AA battery connected to the electrolyte-free cells (as shown in the schematic) as function of time and stroke for both standard and modified cells. (C and D) Postmortem photographs after the indentation test for ‘‘slitted’’ (C) and standard (D) cells. The broken fragments covered by the magnifying glass symbol in (C) and (D) are shown at higher magnification on the left.
are typically packed in stacks under compression, which allows the cells to provide flexible support to one another. During external impact on a battery pack, the deformation of cells is constrained by the pack enclosures and neighboring cells. Thus, the cells experience long-range strain and stress fields in all directions. The pouch cell safety test using localized indentations against a rigid surface35 does not apply to this deformation scenario because it does not account for the flexibility and constraints provided by the adjacent cells and enclosures. In our experiment, we used ballistic clay36 to provide a deformable backing to cells to include a large-scale deformation. A 3-mm-thick polycarbonate plastic sheet was placed on top of the cell to reduce free surface effects such as wrinkling. The clay, the cell, and the polycarbonate sheet were contained in an aluminum box, as shown in Figure 5A, to simulate multicell battery stack packaging. During the indentation test, the electrolyte-free cells were connected to commercial 3-V AA batteries. The OCV of the AA batteries was monitored as shown in Figure 5B. Based on this configuration, if a low-resistance short takes place in the test cell, a drop in the AA battery’s voltage will occur. As shown in Figure 5B, a drop in the
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voltage was observed for the standard cell as soon as an indentation achieved a depth of about 0.4 inch. For the modified slitted cell, no change was observed in the commercial battery’s OCV. Once the indentation depth reached 1 inch the load was released, allowing for partial recovery in the AA battery’s OCV. After the test, the two cells were opened carefully to observe the electrodes’ physical condition and study the failure patterns. Figure 5C shows the modified cell in which the center of the slitted area has fallen apart; 3 3 3 squares become completely disconnected from the 5 3 5 slit portion. It is worth noting that the separator was also torn. This suggests that the broken squares of the slitted area were isolated from the rest of the electrodes before the separator was torn. Figure 5D shows that a few pieces of the standard battery’s electrodes were randomly torn from the electrodes, although other damaged pieces were still connected to the rest of the electrodes. This explains the electrical shorting that caused the drop in the AA battery’s OCV during the indentation test. Live cells, containing 1.2 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate electrolyte, were assembled with standard and modified electrodes and cycled between 2.5 V and 4.2 V at C/10 for the first five cycles and then cycled at C/5. Both graphite and NCA cathodes were again used in this experiment. As shown in Figure 6A, the cells’ voltage profiles are almost identical. Likewise, the capacities of both were found to be similar (inset of Figure 6A). Even after more than 30 cycles at C/5 (Figure 6E), there was no measurable capacity fading. This suggests that introducing the slits does not affect the electrochemical performance of batteries. More extensive cycling and quality tests must be conducted with a statistically significant number of samples to further validate the performance. We predict that the slits will not adversely affect the electrodes’ typical power performance. After this precycling, both cells were charged back to 4.2 V and mechanically abused using the indentation test in the same configuration discussed above and shown in Figure 5A. Unlike the electrolyte-free cell tests discussed above, where an external AA battery was used to monitor voltage, the OCV of the full cell was monitored during the indentation test. No change in the OCV was observed during the indentation test for the modified cell, but the OCV of the standard cell dropped to zero during the test. Because of the setup, it was not possible to also record the temperature distribution during indentation. After the tests, both cells were removed from the testing box and connected to the battery cycler. As shown in Figures 6C and 6D, both cells were deformed significantly during this leg of testing. Unsurprisingly, the standard cell was not functional in the test and could not be cycled electrochemically. However, the modified cell still measured voltage close to 4.2 V, and we were able to discharge it to 2.5 V. Moreover, the cell was cyclable at C/5, as shown in Figure 6E, with a reversible capacity of 93% of that before the indentation test. It is worth noting that isolating 9 squares of 0.64 3 0.64 cm from the 50 cm2 electrode would result in about 7% loss of the capacity. After postindentation electrochemical cycling, the cell was charged to 4.2 V and allowed to rest for more than 30 days; no significant change was observed in the OCV, indicating no detectable leakage paths or self-discharge processes. These findings are very important because the modified battery avoided a catastrophic thermal runaway and was functional after mechanical abuse. If one imagines an electric vehicle crashing, a battery that retains the ability to function could allow the driver to safely drive the car to a repair shop. This is advantageous over other commercially available safety mechanisms, such as thermal fuses, which do not isolate the damage, causing the battery to shut down permanently.
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Figure 6. Standard Battery Compared with Modified Battery (A) Precycling voltage profile for both standard (blue) and modified (red) cells cycled at C/10 for the first five cycles and then cycled at C/5 (inset shows capacity versus cycle number for the same results). (B) OCV during the indentation test for both standard and modified cells. (C and D) Photographs of modified (C) and standard (D) cells after the indentation test. (E) Capacity versus cycle number for the modified cell before and after the indention test.
This research represents a first step in developing a new approach to battery safety. Additional experiments and model simulations will help determine whether slitted electrodes provide a viable approach that can be reliably manufactured. Applications that may benefit from the avoidance of total battery shutdown have yet to be explored. Perhaps the research community will identify alternatives to cutting slits into electrodes to isolate impact damage, faster than what is possible with fuses that respond to current or heat. Conclusions We have introduced the use of breakable electrodes for mitigating mechanical abuse to LIBs, such as what may occur during a car crash. This technology involves electrically isolating the internally shorted part of the battery from the rest of the cell before the separator is punctured. By limiting the current passing through the shorted area, heat generation can be minimized to prevent thermal runaway. Using an IR camera, we showed that this approach leads to a significantly lower increase in the temperature compared with a standard battery experiencing mechanical abuse.
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We performed computer simulations on electrochemical systems to identify the level of separation needed for mitigating thermal runaway. FEM analysis was conducted to validate the proposed slit pattern for fragmenting the electrodes and current collectors under cell indentation. The breakable electrodes were realized by introducing a certain slit pattern using a modified clicker die without affecting the conventional roll-to-roll production for battery electrodes. Batteries with slitted electrodes exhibited capacities and voltage profiles similar to those of standard batteries. When mechanically abused by heavy deformation, the modified battery did not short-circuit, while the standard one shorted and became nonfunctional. The modified battery retained 93% of its capacity after the mechanical abuse test and was electrochemically viable.
EXPERIMENTAL PROCEDURES Fabrication In all the full cells reported here, graphite was used as the anode with a composition of 92 wt.% active material (graphite A12, ConocoPhillips) + 6 wt.% polyvinylidene fluoride (Kureha 9300 PVDF) binder + 2 wt.% carbon black (P-Li, Timca), with an areal loading of 2.13 mAh/cm2. For cathodes, both NMC and NCA were used as active materials, with an areal loading of 1.8 mAh/cm2 and a cathode formula of 90 wt.% active material (Toda America) + 5 wt.% PVDF (Solvay Solef 5130) binder + 5 wt.% carbon black (Denka). The electrode balance (negative/positive) is 1.15 to minimize lithium plating and dendrite formation.37 The electrodes were fabricated using a pilot-scale automated slot-die coater. Detailed electrode coating, drying, and calendaring procedures have been reported in previous work.38 One layer of Celgard 2325 (25 mm thick) was used as a separator. Metal foils of copper (9 mm thick) and aluminum (15 mm thick) were used as current collectors for anodes and cathodes, respectively. The electrode dimensions were 8.64 3 5.8 cm for anode and 8.44 3 5.6 cm for cathode. An electrolyte of 1.2 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate solvents (3:7 by weight) was used, except when electrolyte-free cells were tested to study the failure pattern. Laminated pouch material of polypropylene/aluminum/polyethylene, with a total thickness of 60 mm, was used to vacuum-seal and package the electrode stack. Cell assembly and electrode fabrication were carried out using semi-automated equipment at ORNL’s Battery Manufacturing R&D facility. The slit patterns were introduced in the electrodes by modifying the design of clicker dies used to cut the electrodes. The die is made of steel rules with razor-blade shapes embedded into a wooden base (Apple Steel Rule Die). Figure S5 shows a drawing for the clicker die used in this study. A semi-automated punch was used to slit the battery electrodes. The modified design created a 5 3 5 pattern of squares in the middle of each electrode. Each square was approximately 0.64 cm on a side, each slit being 0.96 cm long. The square pattern was designed so that each slit was separated from the next with a spacing of one-sixth of the slit length, or 0.16 cm in this example. The slits extended through the thickness of the battery sheet including active material and current collector. Testing Electrochemical charging and cycling was carried out at room temperature using a Series 4000 MACCOR battery cycler (Maccor). To monitor the change in the surface temperature during the dart short-circuit test, we used an FLIR A325 camera. Its micro-bolometer focal-plane-array detector operates at 8–12 mm wavelength with a 320 3 240 pixel format. The indentation test was conducted using a 1-inch-diameter brass sphere connected with a rigid rod to the grips and load cell of a servo-hydraulic mechanical testing machine (MTS Systems with Model
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407 controller). The indentation test was set with a limit of 1 inch stroke, and loading rate of 0.015 inch/s for electrolyte-free and 0.01 inch/s for full cell. Simulation The 3D volume-averaged formulation for electrochemical transport developed in AMPERES was used to conduct numerical simulations.39 The conservation equations defining the species and charge transport in the battery are described in the Supplemental Information. For the FEM, the modeled square domain contains 100 10 3 10-mm patches in the x, y plane, with slits 16 mm long that result in starting ligaments of 2 mm. The thickness of the copper sheets was 10 mm. The indentation simulation utilized a 50-mm-radius steel punch that incrementally indented for 20 mm into the foil at a rate of 2 mm/s. A fully integrated shell element with three integration points through the thickness was used to study the copper foil material using a bilinear elastic-plastic model. The elastic modulus used in this study was 117 GPa, Poisson ratio was 0.34, yield stress was 100 MPa, and tangent hardening modulus was 1.12 GPa.
SUPPLEMENTAL INFORMATION Supplemental Information includes seven figures, two tables, and one movie and can be found with this article online at https://doi.org/10.1016/j.joule.2017.11.003.
AUTHOR CONTRIBUTIONS N.J.D. proposed the concept and led the team. All of the authors contributed to planning, analysis of experiments, and composing the manuscript. Specifically: M.N. conducted the battery assembly, electrochemical cycling, and proof-ofconcept experiments; J.L. prepared the slot die cast electrodes; H.W. captured the IR images; S.S., S.A., H.W., and M.N. conducted the mechanical indentation test. For modeling: S.S. carried out the finite element analysis to design slit patterns and test rig; S.A. conducted the electrochemical modeling.
ACKNOWLEDGMENTS This research was sponsored by the RANGE program of Advanced Research Projects Agency-Energy (ARPA-E), award DE-AR0000869-1707, US Department of Energy (DOE). It was conducted at ORNL, managed by UT Battelle for DOE under contract DE-AC05-00OR22725. The authors thank Drs. Ping Liu, Sue Babinec, and Julian Sculley at ARPA-E for their support and advice. The authors thank Seong Jin An for the drawing of clicker die illustration and Donald Erdman for assistance with the indentation test. This manuscript has been authored by UT-Battelle LLC under Contract No. DE-AC05-00OR22725 with the US Department of Energy. The United States Government retains—and the publisher, by accepting the article for publication, acknowledges that the United States Government retains—a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Received: May 31, 2017 Revised: October 11, 2017 Accepted: November 2, 2017 Published: December 13, 2017
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