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Self-Extinguishing Lithium Ion Batteries Based on Internally Embedded Fire-Extinguishing Microcapsules with TemperatureResponsiveness Taeeun Yim,† Min-Sik Park,† Sang-Gil Woo,† Hyuk-Kwon Kwon,† Jung-Keun Yoo,‡ Yeon Sik Jung,*,‡ Ki Jae Kim,*,† Ji-Sang Yu,† and Young-Jun Kim† †

Advanced Batteries Research Center, Korea Electronics Technology Institute, 68 Yatap-dong, Bundang-gu, Seongnam, Gyeonggi-do 463-816, Republic of Korea ‡ Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 302-701, Republic of Korea S Supporting Information *

ABSTRACT: User safety is one of the most critical issues for the successful implementation of lithium ion batteries (LIBs) in electric vehicles and their further expansion in large-scale energy storage systems. Herein, we propose a novel approach to realize self-extinguishing capability of LIBs for effective safety improvement by integrating temperature-responsive microcapsules containing a fire-extinguishing agent. The microcapsules are designed to release an extinguisher agent upon increased internal temperature of an LIB, resulting in rapid heat absorption through an in situ endothermic reaction and suppression of further temperature rise and undesirable thermal runaway. In a standard nail penetration test, the temperature rise is reduced by 74% without compromising electrochemical performances. It is anticipated that on the strengths of excellent scalability, simplicity, and cost-effectiveness, this novel strategy can be extensively applied to various high energydensity devices to ensure human safety. KEYWORDS: Lithium ion battery, safety, microcapsules, fire-extinguishing

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researchers have made intensive efforts to exclude these components by (1) employing a nonflammable or extinguishing agent to the electrolytes or by (2) utilizing a nonflammable separator. It has been reported that a nonflammable or extinguishing additive effectively suppresses additional fire spread in LIBs because the nonflammable agent reduces flash points of conventional electrolytes12−14 and extinguishing materials scavenge active radical species that can accelerate thermal runaway via exothermic radical chain reactions.15−17 In most cases, however, huge amounts of additives are required to ensure a high level of cell safety, which consequently degrades the overall ionic conductivity of the electrolyte, resulting in poor electrochemical performance. Another noteworthy approach is adopting a nonflammable separator instead of a conventional poly(olefin)-based separator because the remarkable thermal/mechanical properties of the former prevent them from shrinking even at high temperature, which greatly inhibits the occurrence of internal shorts without significant electrochemical fading.18−21 However, this cannot

ithium ion batteries (LIBs) are considered the most promising power sources for electric vehicles (EVs) because of their high specific energy density, lack of memory effect, and excellent cycling performances.1−4 With increasing demand for EV applications, assuring a sufficient level of safety has become one of the most important issues given the drastic increment of the energy density of LIBs and potentially high risk of battery explosion by car collisions.5,6 It is generally accepted that the safety of LIBs is closely associated with the internal presence of combustive components such as electrolytes and electrodes.7−9 Once ignition is initiated by internal/ external electric shorts, the combustible ingredients (especially delithiated oxide-based cathodes and flammable electrolytes) facilitate thermal runaway, which is responsible for a rapid rise of internal temperature due to undesirable exothermic reactions, eventually leading to battery explosion. These types of accidents have frequently been reported since the mid-2000s and as a result considerable attention has been devoted to improving the safety of LIBs.10,11 It should be noted that propagation of a fire in a LIB cell requires the existence of heat, fuel, and an oxidizing agent (fire-triangle) and thus at least one of the fire-triangle components has to be removed in order to stop a continuous combustion reaction. In this respect, © XXXX American Chemical Society

Received: March 25, 2015 Revised: June 18, 2015

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DOI: 10.1021/acs.nanolett.5b01167 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Synthesis of the fire-suppression microcapsules. (a) Schematic illustrations for the synthesis route of the microcapsules containing firesuppression agent (DMTP). The DMTP droplets were encapsulated with a rigid PMMA shell via an oil-in-water emulsion-based polymerization reaction using MMA monomer, EGDMA as a cross-linking agent, and ADVN as a polymerization initiator. (b) SEM, (c,d) TEM, and (e) cryo-TEM image of the synthesized microcapsules. (f) 19F-NMR spectra of pure DMTP (black) and extract solution from DMTP-containing microcapsules (blue).

and inexpensive and can be utilized in various energy storage applications. In order to enable controlled and timely release of extinguishing agent molecules, the functional structure of microcapsules with a temperature-responsive surface capping layer was designed as shown in Figure 1a. Stimuli-triggered release of microcapsule content has been widely studied for drug-delivery, fragrance release, self-healing materials/devices, and other applications.29−31 However, the use of thermoresponsive microcapsules for the control of surrounding temperature thus far has not been demonstrated to the best of our knowledge. The requirements for the polymeric capping layer include sufficient robustness to withhold the highly volatile agent inside, good chemical/electrochemical stabilities, and swift responsiveness to temperature change. In this study, selfextinguishing microcapsules with a temperature-responsive and mechanically rigid polymeric shell were prepared through an oil-in-water emulsion-based polymerization reaction.32−37 For secure polymer-based encapsulation of DMTP, methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) were used as a monomer and a cross-linking agent, respectively. The cross-linking reaction provides electrochemical and mechanical stability of the microcapsules, as will be discussed in detail. One of the challenges for the development of this polymerization process was finding a suitable initiator with an appropriate reaction temperature during the microcapsule synthesis. This is because at high temperature (>50 °C), the oil-in-water emulsion can easily be destroyed due to the high volatility of the contained DMTP, resulting in loss of DMTP in

ultimately suppress serious thermal runaway because the highly combustible components, which still are present in the cell, can be ignited by an external short, and thus a fire can spread easily. These limitations demand the development of a systematic approach that can circumvent the aforementioned critical issues. Here we suggest the use of temperature-responsive “selfextinguishing” microcapsules to achieve highly reliable safety of the cell and also maintain the desired electrochemical performance. Accelerating rate calorimetry (ARC) studies on LIBs have demonstrated that significant exothermic heat, which is produced by decomposition of the electrolyte and active material, accelerates thermal runaway through violent combustion reactions.22−26 Upon this background, we successfully incorporated a commonly used extinguishing agent (1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane) (DMTP)) with remarkable endothermic properties (ΔH = +102.1 J g−1)27,28 in an LIB to effectively suppress a drastic temperature rise. The extinguishing agent can be vaporized in a timely manner by the absorption of external heat and finally extinguishes the fire before the cell reaches a serious thermal runaway state. However, in general the direct mixing of DMTP in an electrolyte is seriously restricted due to poor miscibility with conventional electrolytes. In response, we encapsulated the DMTP with a temperature-responsive polymeric layer to avoid direct contact between DMTP and the electrolyte. This provides multiple advantages of excellent miscibility in the electrolyte, suppression of temperature rise, and reasonable electrochemical performance. Moreover, the proposed encapsulation approach is highly scalable, convenient, B

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Figure 2. Temperature responsiveness test on the microcapsules. (a) DSC curves of the DMTP-containing microcapsules. The inset SEM images present the morphology change of the microcapsules upon temperature increase. While the as-synthesized microcapsules have spherical shapes, the release of DMTP resulted in a considerable change of shape (similar to flat balloons). (b) Fire retardancy test results for the microcapsules. The combustible electrolyte was instantly ignited using a conventional lighter, and the DMTP-containing microcapsules successfully quenched the fire. (c) SET of electrolyte with no additive (black), PMMA (red), and microcapsules (blue).

from the supernatant sample and were in precise agreement with the spectrum of the DMTP reference. We therefore concluded that the extinguishing agent, DMTP, was successfully encapsulated through the proposed synthesis process. We now demonstrate the thermal responsiveness of the microcapsules. To analyze their fundamental thermal properties, a differential scanning calorimetry (DSC) test was performed in a temperature range of 40 to 160 °C. As shown in Supporting Information Figure S2, the boiling point of pure DMTP (TDMTP ) was found at 88 °C and the glass transition of b pure poly(methyl methacrylate) (PMMA) (TgPMMA) was observed at 122 °C. After the encapsulation of DMTP, the thermal properties were almost identical to those of each constituent, although the boiling point and glass transition temperature of the microcapsules (TPMMA = 127 °C and TDMTP g b = 95 °C) were slightly higher. This is attributed to the additional cross-linking reaction of EGDMA (Figure 2a). From the observation that TDMTP (95 °C) is lower than TPMMA (127 b g °C) for the microcapsules, it is reasonable to hypothesize that the temperature-triggered opening of the PMMA shell and release of DMTP is caused by internal pressure build-up due to the rapid endothermic vaporization of DMTP. A comparison of SEM images (Figure 2a, insets) of the microcapsules at two different temperatures supports our assumption that there are considerable morphological changes in the spherical microcapsules depending on the external exposure temperature. While the initial spherical microcapsules were well-preserved

the microcapsules. On the other hand, at low temperature ( 70 °C due to the rapid evaporation of DMTP near its boiling point. Once again, these results substantiate the temperature-responsiveness of the microcapsules. In addition, we could not detect any significant abnormal behaviors in the linear sweep voltammetry (LSV) profiles (Figure S4a,b in the

Supporting Information); there was no additional electrochemical degradation of the microcapsules coated on the PE separator within a normal cell-operating potential range of 0.0− 4.3 V versus Li/Li+. Moreover, the microcapsule-coated PE showed only slightly decreased ionic conductivity as well as permeability compared to bare PE (Table S1 and Figure S5 in the Supporting Information) without significant degradation, and these data suggest that the microcapsule-coated separator are suitably applicable for conventional LIB systems.43−45 It can therefore be concluded that the microcapsules are compatible with a conventional separator and electrolyte without any significant deterioration. Although the overall self-extinguishing capability can be improved by loading more microcapsules on the separator, excessive coating of additional components on the separator will limit the kinetics of electrochemical processes by disturbing Li-ion migration, as reported in previous studies.18,46,47 An E

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Figure 5. Characterization of lithium-ion batteries containing the fire-suppression microcapsules. (a) Initial voltage profiles of graphite/ LiNi0.5Co0.2Mn0.3O2 (NCM523). The cells were tested in a range of 2.5−4.2 V. Black, bare PE; red, PMMA-coated PE; green, microcapsule-coated PE; and blue, microcapsule-coated PE + microcapsule-added electrolyte. (b) Capacity retention test up to 50 cycles. (c) A schematic illustration of the nail penetration test. The photographs show the pouch cell before and after the nail test, resulting in a hole at the center of the cell. (d) Temperature profiles during the nail penetration test of the full cell (graphite/LiNi0.5Co0.2Mn0.3O2(NCM523)). The cells were fully charged to 4.35 V before testing.

alternative approach to further enhance the fire-extinguishing capability is to add microcapsules to the electrolyte. It should be noted that the good compatibility of the microcapsules with the electrolyte (Figure 3 and Supporting Information Figure S6) and the separator (Figure 5 and Supporting Information Figure S4) is confirmed with negligible trade-off effects. With these considerations, the self-extinguishing microcapsules were adopted in full-cells composed of a LiNi0.5Co0.2Mn0.3O2 cathode and a graphite anode with a microcapsule-coated PE separator (mass loading of microcapsules = 0.636 g g−1 of PE) and electrolyte mixed with microcapsules (10 wt %), and their cycling performance was investigated (Figure 5a,b). For comparison, cells with a pristine PE separator, a PMMA-coated PE separator, and a microcapsule-coated PE separator, respectively, were also prepared with the standard electrolyte and used for the test. In the galvanostatic charge and discharge profiles of the cells (Figure 5a), no significant difference in the capacity and polarization was observed at the first cycle with self-extinguishing microcapsules. In addition, there was no notable difference in cycle performance between the cells, which exhibited a high capacity retention of more than 95% after 50 cycles, as shown in Figure 5b. It should be noted that the cell cycled with an electrolyte mixed with uncapped DMTP showed poor electrochemical performance, low initial capacity together with much larger polarization, because poor miscibility between the extinguishing agent and the electrolyte severely disturbs the internal kinetics of Li-ion migration (Supporting Information

Figure S6). These results strongly suggest that the incorporation of microcapsules does not compromise the electrochemical performance of LIBs. In order to investigate the safety characteristics of the LIBs with microcapsules embedded separator and electrolyte, fully charged stacked-cells with a capacity of 500 mA h were penetrated by nails (1.5 mm × 1.5 mm), and the changes in internal temperature of the cells were subsequently monitored. This nail penetration test (schematically illustrated in Figure 5c) can artificially induce and simulate the occurrence of an internal short circuit that can be caused by external collision or the incorporation of undesirable conductive materials in LIBs. The temperature measured by the nails can be monitored and analyzed as a useful and standard measure of the internal temperature of LIBs.48−50 In the nail penetration test, a significant enhancement of safety was observed for the cell with self-extinguishing microcapsules (Figure 5d). The cell assembled with a pristine PE separator showed a drastic temperature increase (72.3 °C) immediately upon the occurrence of an internal short. In contrast, the maximum cell temperature was estimated to be only 37.2 °C for the cell with self-extinguishing microcapsules. Considering that the initial temperature is 25.0 °C, the temperature rise during the test can be suppressed by 74% with incorporation of the microcapsules. It can be explained that the significant heat generated by an intentional short circuit and resultant thermal runaway can be effectively absorbed by the endothermic evaporation of DMTP released from the collapsed microF

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and then completely washed out by water and ethanol. Finally, the product was dried under room temperature for 1 day. Material Characterizations. Structural analyses of microcapsules were conducted by using a field-emission-scanning electron microscope (JEOL, JSM-7000F) and transmission electron microscope (JEOL, JEM100) at 80 kV. For the cryoTEM analysis, specimens were prepared as follows. A Lacey carbon grid (Agar Scientific, U.K.) was treated by a plasma cleaner (PDC-32G-2, Harrick Plasma, U.S.A.) followed by washing with 5 μL of distilled water. The sample grid was vitrified by Vitrobot Mark IV (FEI, U.S.A.) under 100% relative humidity at 4 °C. During sample transfer into the measurement stage, the vitrified sample was maintained at the liquid nitrogen temperature and images were acquired using a cryo transfer holder (cryo holder-626, Gatan, U.S.). The sample temperature was maintained at approximately −177 °C under the stage of the electron microscope, which was carefully monitored by a SmartSet Controller (Gatan, U.S.A.). A Tecnai G2 Sprit Twin equipped with lanthanum hexaboride (LaB6) gun at 120 kV was adopted and each image was recorded by Ultracan 4000 CCD detector (Gatan, U.S.A.) under low dose imaging mode with a typical electron dose of 10−20 e/Å2. The existence of a liquid core was confirmed by 19F-NMR (19F-NMR, Bruker, ASCEND 400) in acetone-d6. The thermal behaviors were analyzed by differential scanning calorimetry (Mettler Toledo, DSC-1) with a rate of 5 °C min−1 in a range of 25 to 160 °C. The flammability test to calculate the SET value was performed as follows: 3.0 g of electrolytes (EC/EMC = 3:7 + 1.15 M LiPF6) with or without PMMA and microcapsules was ignited by a burner. Each extinguishing time was recorded and was normalized by the mass of the electrolyte. Coating of Microcapsules on Separator. Poly(ethylene) (Ashahi Kasei) separators coated with the functional microcapsules were prepared by a dip-coating method in an acetone solution incorporating microcapsules and poly(vinylidene fluoride-co-hexafluoropropylene) copolymer (Kynar 2801, MW: 470 000) with a 1:1 ratio. The total solid content in the solution was 3.75 wt %. The PVDF-HFP copolymer (1 g) was dissolved in 80 mL of acetone and then 2 g of microcapsules was finely dispersed by an automated mixer. The PE separator was then dipped in a coating solution and dried at room temperature for 1 h. To further probe the temperature responsiveness of DMTP-containing microcapsules, we used an in situ pressure measurement device, which can simultaneously record pressure and system temperature. More specifically, the samples were placed at the center of an analysis cell (bottom part) and then the cell was sealed tightly with an upper part equipped with pressure and temperature sensors. The internal pressure of the closed cell was then recorded with increasing temperature of the measurement system until 100 °C. Electrochemical Analyses. Linear sweep voltammograms were obtained using a three-electrode configuration composed of glassy carbon as a working electrode and lithium metal as counter and reference electrodes with a scan rate of 10 mV s−1. Galvanostatic discharge−charge was performed with a beakertype cell in which a piece of lithium foil (HONJO Metal), a poly(ethylene) separator (Asahi Kasei), electrolyte (EC/EMC = 3:7 + 1.15 M LiPF6 + 2% VC), and electrodes were placed. To prepare the anodes a mixture of graphite (size of about 7 μm, provided by Posco Chemtech, Korea), Super P carbon black, carboxymethyl cellulose (Cellogen, DKS), and styrene− butadiene rubber (BM 400B, Zeon) (mixed in a weight ratio of

capsules. More specifically, the internal temperature increase can be minimized by the heat absorption reaction of DMTP and thus ignition of combustible electrolytes can be effectively prevented. The microcapsules are thus beneficial to enhance the safety characteristics of LIBs without any adverse effects on the electrochemical performance, leading to the realization of self-protective, self-extinguishing LIBs that can safely operate under high-power conditions. In summary, novel design and synthesis of thermally triggered microcapsules filled with a fire-extinguishing agent were introduced and the microcapsules were applied as a promising inhibitor for undesirable thermal runaway in LIBs. On the basis of our synthesis strategy, a fire-extinguishing agent was securely encapsulated with a rigid polymeric shell via an oilin-water emulsion-based polymerization reaction with the assistance of a cross-linking agent. The capping of DMTP with a cross-linked PMMA shell prevented direct contact between the extinguisher agent and the electrolyte and also afforded excellent structural stability in the electrolyte. From a standard nail penetration test, LIBs equipped with the DMTPcontaining microcapsules markedly showed a suppressed increase of cell temperature. This highly effective role of the self-extinguishing microcapsules was achieved by the temperature-induced collapse of the capsules and the consequent release of internal DMTP, which absorbs the heat produced by an internal short-circuit. It also should be emphasized that the safety of LIBs can be considerably improved by employing the proposed microcapsules without any performance fading. Furthermore, the usage of these microcapsules is not limited to separators; it can be utilized with other cell components such as electrolytes or electrodes with optimized forms. Moreover, a variety of other extinguisher agents with different reaction temperatures and reaction kinetics also can be applied for tailor-made application of the microcapsules for various devices. The excellent scalability and cost-effectiveness of this approach may pave a new pathway for assuring personal safety during the use of high-power, high-density energy sources. Experimental Procedure. Materials. Methyl methacrylate (Aldrich, 99%) was used as a monomer and ethylene glycol dimethacrylate (Aldrich, 98%) was used as a cross-linking agent for polymerization to control the solubility of the microcapsules. As a functional additive, 1,1,1,2,2,3,4,5,5,5-decafluoro3-methoxy-4-(trifluoromethyl)-pentane (3M, 99%) was selected based on its high nonflammability. Triton X-100 (Aldrich, 99%) and 2,2-azobis(2,4-dimethylvaleronitrile) (Wako Chemicals, 98%) were used as a surfactant and initiator for polymerization, respectively. Deionized water was used as a reaction medium for microcapsule synthesis. Preparation of Microcapsules. MMA (monomer, 30 mL) and EGDMA (cross-linker, 0.3 mL) were added into a solution of deionized water (500 mL) and Triton X-100 (surfactant, 2.5 g), while purging with N2 gas to remove remaining active oxygen, which can disturb polymerization by terminating the reactivity of the initiator. This mixture was stirred for 30 min until a homogeneous solution formed. DMTP (core liquid, 30 mL) and ADVN (initiator, 0.3 g) were then added into the solution under rigorous stirring at 2000 rpm for 10 min. The stirring rate was then fixed at 200 rpm and the mixture was heated to 45 °C for 5 h for polymerization. After the reaction was completed, the synthesized capsules were precipitated by adding excess sodium chloride and the precipitate was filtered G

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96:1:1:2) was dispersed in distilled water. The formed slurry was coated on Cu foil, and the resulting electrode was dried in a vacuum oven at 120 °C for 12 h. The loading and electrode density were fixed at 6.93 mg cm−2 and 1.50 g cm−3, respectively. To prepare the cathode, a mixture of LiNi0.5Co0.2Mn0.3O2 (provided by Ecopro, Korea), PVDFHFP (KF1100, Kureha), and Super P carbon black (mixed in a weight ratio of 92:4:4) was dispersed in N-methyl-2pyrrolidone. The formed slurry was coated on Al foil, and then dried in a vacuum oven at 120 °C for 12 h. The loading and electrode density were fixed at 15.19 mg cm−2 and 2.95 g cm−3, respectively. Galvanostatic discharge−charge cycling of the full cell was performed using a pouch-type cell (34 mm × 50 mm in size), which was assembled using an anode, cathode, separator, and electrolyte (EC/EMC = 3:7 + 1.15 M LiPF6 + 2% VC). The microcapsules were also added to the electrolyte (10 wt %) and they were galvanostatically charged to 4.2 V and discharged to 2.5 V repeatedly at a constant current of 0.5 C at 60 °C using a charge/discharge unit (TOSCAT-3100, TOYO). Nail Penetration Tests of Full-Cell (Graphite/NCM523). To characterize safety of the full cell, a 500 mAh pouch-type cell (34 mm × 50 mm in size) was fabricated using an anode, cathode, separator, and electrolyte with additional PMMA or microcapsules (10 wt % of the total cell weight) and then galvanostatically charged to 4.35 V. After charging of the full cell was completed, a nail (1.5 mm × 1.5 mm) was gradually penetrated into the center of the charged pouch cell, and the internal cell temperature with time was then recorded.

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ASSOCIATED CONTENT

* Supporting Information S

Additional table and figures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b01167.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.J.K.) *E-mail: [email protected] (Y.S.J.). Author Contributions

T.Y. and M.-S.P. equally contributed to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Energy Efficiency & Resources Core Technology Program (20132010101890), Global Excellent Technology Innovation (20135020900030) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea), and the R&D Convergence Program (National Research Council of Science & Technology, Project No. CAP-14-2-KITECH) of Republic of Korea. We also thank Profesor Jin-Woong Kim (Hanyang University) for grateful support to design a synthetic scheme for preparation of the microcapsule and RouteJD for the measurement of nail penetration test of pouch-type full-cell.

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REFERENCES

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DOI: 10.1021/acs.nanolett.5b01167 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.5b01167 Nano Lett. XXXX, XXX, XXX−XXX