Preventing Li-ion cell explosion during thermal runaway with reduced pressure

Preventing Li-ion cell explosion during thermal runaway with reduced pressure

Applied Thermal Engineering 124 (2017) 539–544 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier...

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Applied Thermal Engineering 124 (2017) 539–544

Contents lists available at ScienceDirect

Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Preventing Li-ion cell explosion during thermal runaway with reduced pressure Andreas Hofmann a,⇑, Nils Uhlmann b, Carlos Ziebert b, Olivia Wiegand c, Alexander Schmidt c, Thomas Hanemann a,d a Karlsruher Institut für Technologie (KIT), Institut für Angewandte Materialien – Werkstoffkunde (IAM-WK), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b Karlsruher Institut für Technologie (KIT), Institut für Angewandte Materialien – Angewandte Werkstoffphysik (IAM-AWP), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany c Karlsruher Institut für Technologie (KIT), Institut für Nanotechnologie – Projekt Competence-E (INT-PCE), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany d Universität Freiburg, Institut für Mikrosystemtechnik, Georges-Köhler-Allee 102, 79110 Freiburg, Germany

h i g h l i g h t s  Increase of Li-ion cell safety by reduced pressure.  Li-ion cell thermal runaway without cell explosion.  Reduced pressure via vacuum pump or suction unit.

a r t i c l e

i n f o

Article history: Received 16 November 2016 Revised 8 May 2017 Accepted 11 June 2017 Available online 13 June 2017 Keywords: Lithium ion battery Thermal runaway Reduced pressure Vacuum Safety Accelerating rate calorimeter

a b s t r a c t Concerning Li-ion cells it is demonstrated by overcharging tests both on the shelf in a fume-hood and in an accelerating rate calorimeter that the application of reduced pressure in the moment of a thermal runaway accident can prevent a fire and in particular a cell explosion, caused by the electrolyte. Within the experiment, pouch-bag Li-ion cells (88 mAh and 264 mAh) composed of graphite and NMC (LiNi1/3Mn1/3Co1/3O2) were overcharged by 10 C in order to induce a thermal runaway. In spite of a strong temperature increase, the cell remains tightly close during the thermal runaway without any fire or explosion in case of vacuum application. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Although considerable efforts were made to increase the cell safety of Li-ion based cells and battery packs due to fire accidents and high-energy materials, one of the main critical issues remains their abuse behaviour in terms of thermal runaway which usually results in cell explosions and fire accidents [1–3]. Additionally, toxic gases like HF and decomposition products are released to the environment during a cell opening [4]. Such an issue is very important when using large-scale battery packs or single cells with high energy content. ⇑ Corresponding author. E-mail address: [email protected] (A. Hofmann). http://dx.doi.org/10.1016/j.applthermaleng.2017.06.056 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.

Large efforts were made on the material side so far by developing safer anode (e.g. lithium titanate [5,6]) or cathode materials (e.g. lithium iron phosphate [7–10]), electrolyte additives [11,12], electrolytes with reduced flammability [13–17], or the concept of solid electrolytes [18–23], gel-polymer electrolytes [24–28] and ceramic coated separators [29,30]. In multilayer separators, the resistance rises rapidly when the inner layer melts and blocks the lithium transport. Additional concepts on cell level are cell casing, overpressure valves which can release gases in a controlled manner and aerogel material [31] inside the cell which is able to absorb gaseous products. On battery level, heat absorbing material between single cells, battery pack casing and the cell relay including safety features are used to increase the safety of the Li-ion based cell packs. Nevertheless, state-of-the-art cell chemistry

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technology is still the use of highly flammable electrolytes, the use of polyolefine separators and electrode materials with a high content of combustible material (binder, carbon black and graphite). Thus, concepts are strongly needed which can increase the safety level of state-of-the-art Li-ion cells. During a thermal runaway, the temperature inside the cell increases rapidly so that the electrolyte solvents start to boil and come into gaseous state [3,32]. This behaviour leads to a strong increase in pressure [33] and volume expand. As a consequence, a flexible cell-casing starts to blow up till the cell opens and highly flammable gaseous products are released to the environment. Usually, the temperature in this state already reached the auto-ignition temperature and the gaseous products start to burn or explode. An interrupt during this process can be obtained by a strong endothermic process. So it can be observed that during the endothermic boiling process of the solvent heat is absorbed. Nevertheless, there is a short delay which cannot set the cell back into a safe condition based on the solvent amount compared to the heat release during a thermal runaway. Additionally, the boiling temperature shifts to higher boiling temperatures at elevated pressure so much that the endothermic heat absorbance is attenuated compared to the boiling point temperature under standard conditions. In order to remove some of the heat from the cell, it is possible to assemble the cell into a heat-absorbing material [31]. However, such prevention can work only outside of the cell and not intervene in the pressure increase directly. Another option to extract heat from the cell could be the use of reduced pressure. In this case, the boiling of the solvent is enabled at lower temperature. During this process, much more heat could be absorbed in a short period of time in dependence of the reduced pressure strength and gas flow characteristics. Additionally, the pressure inside the cell decreases rapidly and all gaseous and highly flammable products are removed from the cell. In this paper the increase of cell safety level by using reduced pressure is introduced for the first time with the best of our knowledge and experimental evidence. The proof-of-concept is demonstrated in case of pouch-bag cells. Additionally, the method is discussed for use in larger pouch-bag cells and battery design. 2. Material and methods The abuse tests were done with 44 mAh, 88 mAh and 264 mAh pouch bag cells with commercial available NMC cathode material (LiNi1/3Mn1/3Co1/3O2; 5.0  5.0 cm; 90 wt.% active material), graphite anodes (5.2  5.2 cm; 90 wt.% active material) and ceramic coated PET-based separator (5.5  5.5 cm). The cell characteristics are summarized in Table 1. The weight of the cell casing (coated aluminium bag and contact strips) is 5.50 ± 0.05 g for all cells. An ethylene carbonate/dimethyl carbonate (EC: DMC = 50:50 wt.%) based electrolyte with 1 M LiPF6 and 3 wt.% vinylene carbonate is used for all cells. The cell assembling was

performed in a dry room with dew point of < 70 °C. Before the abuse tests, all cells were cycled (100 cycles) at 2 C/3 C between 3 and 4.2 V to cause cell aging. The reference cells are used with no additional features. For adapting the reduced pressure unit, a small tube (Festo, PAN-6x1) was fixed inside the pouch cell with hot glue or super glue with temperature stability up to 160 °C. The cell opening, tube adaption and closing of the pouch-bag cell were put inside of an argon-filled glovebox (oxygen and water level <0.5 ppm). A manual valve enables the switch on of the reduced pressure in terms of a small vacuum pump (Pfeiffer, Duo 1.5 A, 1.5 m3/h) or a suction device (Filtertrolley 2.1, ULT Jumbo, ACD). A laboratory DC power supply (Knürr-Heinzinger, LNG 504) is used for charging during the overcharge test (charging at current rate of 10 C up to 50 V; 88 mAh cells at I = 0.90 A; 264 mAh cells at I = 2.70 A; constant current). As the cell voltage rises to 15–25 V under strong cell swelling, the reduced pressure is engaged with the manual valve. The overcharging tests were performed both on the shelf in a fume hood and inside of an accelerating rate calorimeter (ARC, es-ARC, Thermal Hazard Technology); the reduced pressure was switched on at 15–20 V. The calorimeter chamber, with an inner diameter of 0.1 m and a height of 0.10 m, has one heater and one thermocouple located in the lid and bottom as well as two heaters and two thermocouples (all type N) in the side wall. The heaters adjust the required temperatures depending on the measurement conditions. The cell temperature is recorded by a main or so-called bomb thermocouple attached onto the surface of the cell by temperature-resistant (up to 250 °C) adhesive tape. This bomb thermocouple of type N is placed in the center of the region where the electrode sheets are located. In addition to the automated temperature recording from the calorimeter, the temperature was logged with the device Siemens Thermizet B4001. Standardly the temperature is measured at the front side; a control is done at the backside for individual cells with a second temperature sensor. The experimental flow chart is depicted in Fig. 1. Within this experimental set-up, the ARC was not used as a heating source for the cell. All heat was generated by the overcharging procedure. In order to estimate the temperature of no return, a hot-boxtest (5 °Cmin 1 heating) was performed on ARC (es-ARC, Thermal Hazard Technology) calorimeter. The starting temperature was set to 50 °C. At the same time, the temperatures of the cell and the calorimeter (top/side/bottom) were recorded. The sensitivity rate of the calorimeter was set to 6 °Cmin 1. The cell voltage at the beginning of the test was 4.2 V. Within the test, no overcharging was applied. 3. Results and discussion During preliminary tests it is proved that it is very difficult via overcharging at moderate current rates (5–10 C) to bring small pouch bag cells of 44 mAh nominal capacity to any kind of

Table 1 Cell characteristics of the pouch bag cells. The density d of the electrolyte is d = 1.27 gcm 3. A Teflon-based tube (0.51 ± 0.05 g) is installed in each pouch-bag cell for gathering gas which is built during cell formation. The contact strips are composed of Al (135 mg/sheet) and Cu (242 mg/sheet). A ceramic coated separator (135 mg/sheet) is used for all tests between both electrodes. Based on the composition of the cell, a Cp-value of 1.2 J(Kg) 1 was calculated for the 88 mAh cells according to literature methods at 60 °C assuming an additive contribution of the individual components to Cp [34,35]. Nominal cell capacity/mAh

Electrolyte amount/ ml

Nominal voltage before over-charging/V

44 88 264

450 ± 3 900 ± 5 2700 ± 20

4.2 ± 0.1 4.2 ± 0.1 4.2 ± 0.1

Electrode sheets (o = one side coated, t = two side coated)

Electrode material weight/mg (including inactive material, without aluminum or copper)

NMC

C

NMC

C

1 (o) 1 (t) 3 (t)

1 (o) 2 (t) 4 (t)

326 ± 1 652 ± 2 1956 ± 6

256 ± 1 1024 ± 4 2048 ± 8

Mass of pouch-bag cell (total)/g

7.1 ± 0.3 9.6 ± 0.3 15.6 ± 0.3

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Fig. 1. Flow chart of the experimental setup.

cell-explosion or to a thermal runaway reaction. Usually, these small cells only swell until 50 V but do not burst. It is assumed that this behaviour is caused by very small amounts of electrolyte/electrode material combined with large gas space inside the pouch-bag cell (Table 1). Therefore, 44 mAh cells were not used for calorimetry measurements. Principally, pouch-bag cells were used for the tests due to their straightforward possibility to adapt a tube into the interior of the cell in order to apply reduced pressure manually. Other cell types which are equipped with an overpressure valve, such as cylindrical 18650 cells, could be used as well. However, such a valve usually opens at relatively high pressure in order that the reduced pressure unit can operate very late during the thermal runaway. In a recent study it was shown that the internal pressure of an 18650 cell reaches up to 12 bar until the safety valve opens [33]. Therefore, such commercial cells were not used for in this study. The pouch bag cells were equipped with a tube and a control valve to ensure that vacuum from a vacuum pump or a suction unit can be switched on immediately. These so-called ‘‘modified cells” were tightly closed with hot glue and evacuated again for a short period of time to ensure the normal cell condition. The cell opening and installation of the tube was done under inert conditions inside of an argon filled glove box to avoid water contamination of the electrolyte. In the first step, 88 mAh pouch bag cells were overcharged (10 C) under visual control inside of a fume cupboard (Fig. 2). The reference cells without cell modification exhibit a typical behaviour during the thermal runaway including strong potential increase above 6–7 V, temperature increase, cell inflating, cell bursting and sometimes ignition of the fume (Fig. 2a). Afterwards, these tests were performed with modified cells including a tube connection. During the experiment, a first thermal runaway could be prevented applying vacuum at U  15 V and T (surface) > 60 °C (measured manually via temperature probe). In this case, the cell was inflated strongly but immediately after applying the reduced pressure, the cell shrank, the cell voltage dropped to 6.5 V again and cell charging was continued. It should be noted that the removal of electrolyte via vacuum will alter the salt concentration as well as the electrolyte solvent composition. After reaching U  20 V and T (cell surface) > 90 °C, a strong swelling occurred

again. Under applying the reduced pressure, the outer plastic layer of the aluminum foil melted on both sides (Fig. 3a/b) due to the strong heat release. It is assumed that the heat formation is based on exothermic electrode, Li-electrode and electrode-electrolyte (conducting salt and ethylene carbonate) reactions [36]. The exothermic reaction with external oxygen is prohibited successfully and internal oxygen (e.g. formed by decomposition of cathode material) and gaseous products (H2, CO, CH4, C2H4, C2H6) [37] are removed from the cell as well as gaseous decomposition products and electrolyte vapor. As a result, the cell did not open or broke in any way but was tightly close all the time (finally U = 0 V). Under practical conditions, the use of a vacuum pump is quite difficult, expensive and unrealizable. Therefore, another method was investigated in the manner that reduced pressure is applied with a suction unit including a charcoal unit to clean the gas before emitting to the environment. A pouch bag cell (88 mAh) was used for the test again (Fig. 2c). In this case, the same conditions were used compared to the vacuum pump test. The effect of the suction unit to the thermal runaway accident is almost identical compared to the vacuum pump test and a similar effect and cell appearance is observed (Fig. 3c/d vs. a/b). After opening the cell, all parts were completely dry and the electrode material exfoliated from the current collector. In order to study the thermal runaway process more precisely and systematically, the abuse tests were repeated inside the accelerating calorimeter with 88 mAh and 264 mAh pouch bag cells. During overcharging, the calorimeter was not heated or cooled but the temperature of the calorimeter as well as the cell temperature (measured outside in the middle of the electrode sheet) was detected and monitored. Due to missing visible control, the reduced pressure was applied when the cell potential exceeded 15 V. Exemplarily, the measurement profiles for 264 mAh cells are depicted in Fig. 4 in comparison with each other. After the measurement, the amount of material loss during cell opening or reduced pressure is in the same order of magnitude with and without vacuum control (264 mAh cells: 3.5 ± 0.2 g; 88 mAh cells: 0.92 ± 0.11 g). The main weight loss is caused by the removal of electrolyte. Additionally, chemical reactions of the active material (release of oxygen) and a partial removal of active material during thermal runaway became apparent as well. In

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Fig. 2. Cell tests during thermal runaway caused by overcharging at 10 C. (a) Overcharging experiment without any additional prevention. The pouch bag cell explodes in an exothermic reaction. (b) During thermal runaway vacuum is applied by a vacuum pump. (c) During thermal runaway reduced pressure is applied by a suction unit. The first picture from each series shows the swallowed cell immediately before the explosion or applying reduced pressure.

Fig. 3. Pouch bag casing after thermal runaway during reduced pressure with vacuum pump (a/b) and with a suction unit (c/d).

Fig. 4 it can be observed that the temperature on both sites of the cell distinguish to some extent which can be explained by a detaching the temperature sensor during cell inflating. For both 264 mAh cells (100% SOC at 264 mAh), the thermal runaway reaction started at 240% SOC (extra capacity of 375 mAh). Applying reduced pressure it was possible to bring the cell back into ‘‘stable” conditions for additional 30 min although the cell temperature increased up to 130 °C. The cell voltage after vacuum utilization was measured to be 5.3–6.2 V which is in same order of

magnitude than the voltage plateau before the thermal runaway in spite of the alteration of the electrolyte composition. Additionally, it can be observed that the temperature increase inside of the calorimeter during the thermal runaway is less pronounced for the vacuum-type cell (without considering the sharp peak in the moment of the thermal runaway; without/with vacuum; top: 2.1 °C/1.3 °C; side: 2.1 °C/1.3 °C). This is due to hot fume which is released to the calorimeter without reduced pressure under cell opening and is an additional sign that the vacuum controlled cell

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Fig. 4. Overcharging measurement inside an accelerating rate calorimeter with 264 mAh pouch bag cells without (a) and with vacuum control (b). Depicted from bottom to top are cell temperatures for both sites, calorimeter temperature (top and side temperature), the cell voltage and the vacuum control (for (b) only).

remains tightly closed during the thermal runaway reaction. However, it was observed that the maximum cell temperature during thermal runaway is higher when reduced pressure is used (without/with vacuum: 240 °C/265 °C for 264 mAh cells; 160 ± 20 °C/225 ± 20 °C for 88 mAh cells). Such behaviour can be explained because of the close distance between temperature sensor and active material for vacuum-controlled cells. Finally, the principle can be summarized as following: Since reduced pressure conditions (vacuum) are applied to a Li-ion cell in the moment of overpressure (e.g. caused by gas formation, decomposition, temperature increase) the thermal runaway accident can be delayed and the cell can be hindered to burst/explode. Additionally, the temperature of exothermic reaction was determined for 88 mAh cells without vacuum control by a hotbox-test. The temperature, at which the cell exceeds the sensitivity of 6 °Cmin 1 of the calorimeter, was 210 °C. The electrolyte amount inside of a 88 mAh cell is 900 ml, thus a volume in the order of roughly 0.16 l gaseous DMC can be formed (n = 5 mmol DMC at p = 1 bar and T = 100 °C; ideal gas equation: pV = nRT). Ethylene carbonate exhibits a boiling point of 248 °C at p = 1 bar, thus during thermal runaway this temperature is exceeded and also EC will evaporate. The amount inside of the cell can form approximately 0.17 l of gaseous EC. However, much more volume is formed at larger cell sizes, of course. Thus, a 20 A h pouch bag cell is composed of 80 mL electrolyte solvent, so approximately 0.130 m3 gaseous products could be formed (assuming m = 100 g electrolyte solvent, T = 150 °C, p = 0.3 bar; ideal gas equation). Empirical values suggest an amount of gaseous products in the order of 200 l from a 40 A h cell [38]. To remove such an amount of gas within a timescale of a few seconds, it is necessary to scale the reduced pressure unit in an accurate order. Nevertheless, suction unit devices are commercially available which can handle an exhaust air capacity of >200 m3/h.

Another difficulty is the fact that the reduced pressure adaption is done inside of the cell in the experiment. Usually, a cell is hermetically closed and any device or adapter on the cell will increase the cell manufacturing cost significantly. To avoid this deficiency, it is possible to use the reduced pressure unit within a battery which is composed of individual cells. Briefly, each pouch cell could be equipped with a predetermined breaking point in the aluminium closing which opens the cell at a defined overpressure. The reduced pressure unit which can be controlled by temperature or pressure sensors could be assembled on the battery casing using a rupture disk. Thus, a mechanical straightforward response is ensured. Since the fume comes in contact with the battery atmosphere, it could be helpful to equip the battery interior with protective nonflammable gas. Ideally, between each cells heat absorbing material is used which significantly reduces the danger of continued thermal runaway processes in neighbouring cells [39]. However, there might be still the possibility of ignition from a hot-spot caused by hot-gas-flow inside of the battery. Nevertheless, the lithium cells do not necessarily have to contain additional features/connections and the number of reduced pressure connections can be reduced significantly.

4. Conclusion It is demonstrated in a first proof-of-concept that the use of reduced pressure via vacuum pump or suction unit can be appropriate to prevent an explosion/fire caused by Li-ion cells during a thermal runaway. In the study, the reduced pressure is applied when the cell potential exceeds 15 V, which has been enforced by overcharging (10 C) the Li-ion pouch-bag cell. At this time, the cell temperature reached already >60 °C. Another possibility to detect a thermal runaway reaction in a closed system could be a

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pressure sensor. After the thermal runaway reaction, the pouchbag cell was still tightly closed, despite a significant temperature increase, and the cell components were almost dry with an overall cell potential of 0 V. It is supposed that this technique can be used to attenuate and defuse the thermal runaway accident in commercial battery packs by using predetermined breaking points combined with reduced pressure units. It is expected that the practical testing of larger cells or whole batteries can give more insights into the usability of this approach and the adaptability to commercial Li-ion based products. Acknowledgements Andreas Hofmann acknowledges the support by Deutsche Forschungsgemeinschaft (Sachbeihilfe, HO 5266/1-1). Carlos Ziebert acknowledges the funding of the ARC by the German Research Foundation Priority Programme SPP1473WeNDeLIB. References [1] H. Yang, S. Amiruddin, H.J. Bang, Y.K. Sun, J. Prakash, J. Ind. Eng. Chem. 12 (2006) 12–38. [2] Q. Wang, B. Jiang, B. Li, Y. Yan, Renew. Sustain. Energy Rev. 64 (2016) 106–128. [3] T.M. Bandhauer, S. Garimella, T.F. Fuller, J. Electrochem. Soc. 158 (2011) R1– R25. [4] P. Ribière, S. Grugeon, M. Morcrette, S. Boyanov, S. Laruelle, G. Marlair, Energy Environ. Sci. 5 (2012) 5271–5280. [5] B. Zhao, R. Ran, M. Liu, Z. Shao, Mat. Sci. Eng. R: Rep. 98 (2015) 1–71. [6] T.F. Yi, S.Y. Yang, Y. Xie, J. Mater. Chem. A 3 (2015) 5750–5777. [7] K. Zaghib, M. Dontigny, A. Guerfi, P. Charest, I. Rodrigues, A. Mauger, C.M. Julien, J. Power Sources 196 (2011) 3949–3954. [8] X. Liu, Z. Wu, S.I. Stoliarov, M. Denlinger, A. Masias, K. Snyder, Fire Safety J. 85 (2016) 10–22. [9] F. Larsson, B.E. Mellander, J. Electrochem. Soc. 161 (2014) A1611–A1617. [10] M. Brand, S. Gläser, J. Geder, S. Menacher, S. Obpacher, A. Jossen, D. Quinger, World Electr. Veh. J. 6 (2013) 572–580. [11] X. Zhu, X. Jiang, X. Ai, H. Yang, Y. Cao, Electrochim. Acta 165 (2015) 67–71. [12] L. Xia, Y. Xia, Z. Liu, J. Power Sources 278 (2015) 190–196.

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