Temperature-responsive microspheres-coated separator for thermal shutdown protection of lithium ion batteries

Abstract

Safety issues have severely retarded the commercial applications of high-capacity and high-rate lithium ion batteries (LIBs) in electric vehicles and renewable power stations. Thermal runaway is a major cause for the hazardous behaviors of LIBs under extreme conditions. In this paper, a new thermal shutdown separator with a more reasonable shutdown temperature of ∼90 °C is developed by coating thermoplastic ethylene-vinyl acetate copolymer (EVA) microspheres onto a conventional polyolefin membrane film and tested for thermal protection of lithium-ion batteries (LIBs). The experimental results demonstrate that owing to the melting of the EVA coating layer at a critical temperature, this separator can promptly cut off the Li⁺ conduction between the electrodes and thus shut down the battery reactions, so as to protect the cell from thermal runaway. In addition, this type of the separator has no negative impact on the normal battery performance, therefore providing an internal and self-protecting mechanism for safety control of commercial LIBs.

---

Introduction

With the advantages of high energy density and excellent cycling performance, lithium ion batteries (LIBs) have dominated consumer electronic markets and are also increasingly being developed as preferred power sources for hybrid electric vehicles (HEV) and electric vehicles (EV).^1,2 However, the safety concern arising from the low thermal-abuse tolerance of the electrodes and electrolyte prevented the market acceptance of LIBs in transportation applications.^3,4 It is known that except for the normal charge–discharge reactions, there still exist a number of potential exothermic side-reactions in LIBs, including thermal decomposition of the solid electrolyte interphase (SEI), reduction of the electrolyte on the graphite anode, decomposition of the cathode material, etc.⁵ Once LIBs are subjected to extreme conditions such as overcharging, external or internal short-circuiting, high-temperature thermal impacting, and so on, these side-reactions may be triggered and subsequently accelerated with the temperature increasing through a dangerous positive feedback mechanism, producing excessive heat and flammable gas within a very short time and thus leading to thermal runaway, cell cracking, fire or even explosion.^6,7

To improve the safety of LIBs, shutdown separators typically having PP (polypropylene)/PE (polyethylene) bilayer or PP/PE/PP trilayer structure are commonly used as a fail-safe device in commercial cells.^8,9 Once the internal temperature in batteries rises up to the melting point of PE under abusive conditions, the PE layer softens and melts to close off the pores of the separator and thereby to shut down the battery reactions, thus preventing thermal runaway from happening. However, this type of separator often loses control to thermal runaway in practical applications, because the difference between the melting point of PE (135 °C) and PP (165 °C) is only 30 °C, thermal inertia after shutdown can easily cause the cell temperature to keep going onto the melting point of PP, resulting in shrinking of separator and then internal short-circuiting of the electrodes.^10,11 Although ceramic coating layer can effectively enhance the mechanical strength and the dimensional stability of separator, the shrinking and melting of the polyolefin substrate appearing after thermal shutdown is still a problem for the ceramic-coated separator.^12–15

To get a better safety control for LIBs, a number of strategies such as temperature-sensitive electrode materials,¹⁶ positive-temperature-coefficient (PTC) electrodes,^17–19 and thermal shutdown electrode^20,21 and electrolyte^22–25 have been proposed as a self-activating protection mechanism to prevent the overheated cells from thermal runaway. Once the internal temperature of the batteries reaches to a risky value regardless of any reason, these protection mechanisms would be triggered to cut off the electrons or ions transport within or between electrodes, so as to interrupt the battery reactions, thus ensuring the battery safety under abusive conditions. But unfortunately, these methods often involve in either difficult material synthesis or complicated electrode processing, making them inconvenient for battery applications. In addition, the thick coating layers of temperature-responsive materials both in the PTC electrodes and shutdown electrodes would cause a substantial decrease in energy density of the batteries, hindering their practical use in commercial batteries.

From the viewpoint of industrial application, thermal shutdown separator is still the most attractive means for safety protection of LIBs, because of its reliability, cost effectiveness and easy-to-use. However, as aforementioned, the thermal shutdown effect in this conventional separator can only last for a short duration and then fail to act because the melting points of PE and PP are so close that the heat accumulated in the cell can cause the internal temperature to keep going up, leading to the melting of the polyolefin substrate and the exposing of electrodes. If the thermal shutdown could occur earlier at a relatively lower temperature, dimensional stability of the separators could maintain for a long period after thermal shutdown. As a result, the safety of the overheated cells would be substantially improved. Based on this consideration, we synthesized a new type of thermoresponsive microspheres of ethylene-vinyl acetate copolymer (EVA) and coated these microspheres on conventional polyolefin membrane to prepare a thermal-shutdown separator. Also, the thermal and electrochemical responses of this shutdown separator are described in this paper.

Experimental

Materials preparation and characterization

The LiCoO₂ positive material used in this study was obtained from Dongguan Amperex Technology Co., Ltd. (Dongguan, China). Ethylene-vinyl acetate copolymer (vinyl acetate 12 wt%, Aladdin chemistry Co., Ltd.) was used as received without further purification. The electrolyte solution used was 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylene methyl carbonate (EMC) (1 : 1 : 1 by vol.), purchased from Guotai-Huarong New Chemical Material Co., Ltd (Zhangjiagang, China). The surfactant used in this study was sodium dodecyl sulfate (Chemically pure. Sinopharm chemical reagent Co., Ltd).

The thermoplastic EVA microspheres were synthesized by a solvent evaporation method. 2 g EVA copolymer was dissolved in 75 ml chloroform at 70 °C to form a homogeneous polymer solution. Then, the as-prepared solution was slowly dropped into a 2 wt% sodium dodecyl sulfate aqueous solution under continuous mechanical stirring. This mixture was stirred at 2000 rpm at ambient temperature until chloroform was completely evaporated off. The microsphere precipitate was then filtrated, rinsed with deionized water three times to remove excess surfactant and finally vacuum dried at 60 °C for 24 h.

The EVA microspheres-coated separator was prepared firstly by dispersing 1 g EVA microspheres into 9 g n-methyl pyrrolidone (NMP) solution containing 5 wt% polyvinylidene fluoride (PVDF) as binder to get a homogeneous coating-slurry and then coating the EVA slurry onto a commercial porous PP/PE/PP membrane (UBE UP3074, 20 μm thick, 50% porosity), and finally, drying the separator at room temperature. The finished separator has a thickness of ∼26 μm.

The structural characterization of the EVA microspheres and the EVA-modified separator were separately characterized by differential scanning calorimetry (DSCQ 200) and scanning electron microscopy (SEM Quanta-200).

Electrochemical measurements

The thermal shutdown behavior of the separators was investigated by calibrating the electrochemical response of LiCoO₂/Li coin cells at various temperatures. The LiCoO₂ cathode was consisted of 80 wt% active material, 10 wt% acetylene black and 10 wt% PVDF binder and prepared by casting the electrode slurry onto a 20 μm thick aluminum foil. Coin-type (2016) cells with a lithium disk as counter electrode were assembled in an argon-filled glove box. The galvanostatic charge and discharge tests were carried out on a computer-controlled battery charger (CT2001A Land Battery Testing System, Wuhan, China) at a voltage interval of 4.3–3.0 V. For thermal testing, the cells were fixed in the thermal testing clamps mounted in a calorstat oven for program-controlled heating. The electrochemical impedances of the coin cells were measured at room temperature on an Impedance Measuring Unit (IM6e, Zahner) with oscillation amplitude of 5 mV in a frequency range of 100 mHz to 100 kHz. The test cells for this measurement were firstly cycled at predetermined temperatures for three cycles and then taken out from the calorstat oven for cooling down.

Results and discussion

The structure and working mechanism of the temperature-responsive microspheres-coated separator are illustrated in Fig. 1. At normal working temperature, the microspheres-coated layer on the separator substrate is porous and ion-permeable, thus ensuring the battery reactions to take place normally. At a predetermined trigger temperature, this coating layer melts and then collapses to form a blocked barrier, thus cutting off the ion conduction between the two electrodes and shutting down the battery reactions eventually. Such a thermal safety shutdown can prevent a runaway chemical reaction in LIBs at a risky temperature. To realize this concept, the key criteria is to find a suitable thermoresponsive polymer, which has a melting point slightly higher than the highest allowable working temperature of LIBs (∼80 °C) and considerably lower than the melting point of the conventional polyolefin separator membrane so that the dimensional stability of the separator substrate can maintain after thermal shutdown.

Fig. 1 Schematic illustration of the thermoresponsive microspheres-coated separator.

In this work, we chose an EVA copolymer as a thermoresponsive material mainly because the melting point of EVA can be varied between 60 °C to 100 °C depending on the weight fraction of vinyl acetate in the copolymer, which enables us to adjust the shutdown temperature of the separator conveniently. In this work, an EVA copolymer with 12 wt% vinyl acetate was optimized as the start material to synthesize the thermoresponsive microspheres by a solvent evaporation method.

Fig. 2 displays the DSC curves of the as-prepared EVA microspheres and EVA microspheres-coated separator. As displayed in Fig. 2a, only an endothermic band appears at 90 °C in the DSC curve, indicating the melting point of the plastic EVA microspheres used in this study, which is a right temperature for thermal shutdown protection of LIBs. From the DSC curve of the EVA-coated separator in Fig. 2b, it can be seen that except the endothermic peak attributed to EVA melting, another two endothermic peaks appear at about 130 °C and 165 °C, corresponding to the melting of PE and PP in the separator substrate, respectively. Nevertheless, the endothermic peak attributed to the PVDF binder in the coating layer is not observed in Fig. 2b. A possible reason is that the phase-transition enthalpy of PVDF binder is much smaller than that of EVA, PP and PE, resulting in that the endothermic peak representing the melting of PVDF is too small to be clearly observed. These results indicate that EVA microspheres, as a thermoplastic coating layer, can effectively lower the shutdown temperature of the separator and thereby improve the dimensional stability of the separator in the post-shutdown duration.

Fig. 2 The DSC curves of the as-prepared EVA microspheres (a) and (b) EVA microspheres-coated separator at a heating rate of 5 °C min⁻¹.

Fig. 3 shows the morphology of the as-prepared EVA microspheres. As observed in the SEM image, the EVA microspheres have a smooth surface and a polydispersed size with a diameter ranging from hundreds of nanometers to several micrometers. Such small sized particles easily produce a sufficiently thin EVA-coating layer on the separator substrate.

Fig. 3 SEM image of the as-prepared EVA microspheres.

The thermal shutdown behavior of the EVA microspheres can be visualized by comparing the morphological changes of the EVA-coating layer on the polyolefin substrate at different temperatures. As shown in Fig. 4a, the random packing of microspheres on the surface of separator substrate forms a porous coating layer, which provides sufficient channels for electrolyte to pass through. Once the coated separator was heated up to 90 °C (as shown in Fig. 4b), the EVA microspheres started to melt and then fuse together to block off the most of pores within the coating layer and to cut off the Li⁺ conduction between two electrodes, thus interrupting the battery reactions and preventing the battery from thermal runaway.

Fig. 4 The cross-sectional-view SEM images of the EVA microspheres-coated separator before (a) and (b) after heated at 90 °C.

To evidence the thermal shutdown behavior of the EVA microspheres in LIBs, laboratory LiCoO₂/Li coin cells were assembled using an EVA-coated separator and tested at various temperatures. Fig. 5 shows the charge–discharge curves of the cells cycled at a constant current of 40 mA g⁻¹ in a controlled voltage range of 4.3–3.0 V. The cells were firstly cycled twice at room temperature for stabilizing their electrochemical performance prior to testing at higher temperatures. As displayed in Fig. 5, the cells show a reversible capacity of about 150 mA h g⁻¹ LiCoO₂ at room temperature and similar charge/discharge plateaus as observed from the cells using conventional separator, suggesting that the thin EVA-coating layer on the separator does not affect the electrochemical behaviors of the electrode. As the temperature was elevated from 25 °C to 60 °C and then to 80 °C, no significant change was observed in the charge–discharge profile except for a slightly increase in capacity, most likely due to the accelerated kinetics of lithium insertion reaction at elevated temperatures. Once the temperature was further increased to 90 °C, the cells were almost incapable to charge or discharge with the charging voltage steeply up to the upper limit of 4.3 V and the discharging voltage suddenly down to the lower limit of 3.0 V, giving no any discernible capacity. This result indicates that the EVA microspheres-coated separator can serve as a safety device to provide thermal-shutdown protection for LIBs at risky temperatures.

Fig. 5 Charge/discharge curves of LiCoO₂/Li coin cells using EVA-coated separator at various temperatures.

The thermal shutdown function can also be confirmed by electrochemical impedance data (EIS) of LiCoO₂/Li coin cells with the EVA-coated separator at various temperatures. As reflected in Fig. 6, all the EIS spectra are similar with a semicircle at high frequencies and a sloping line at low frequencies, representing the interfacial charge transfer resistance (R_CT), and Warburg impedance (W) of Li⁺ diffusion in the bulk of LiCoO₂ phase, respectively. The intercept of the semicircle at high frequency region on the real axis relates to the electrolyte solution resistance (R_s). It can be found from Fig. 6 that the R_s and R_CT remain almost unchanged during the temperature increase up to 80 °C. However, as the temperature continuously increases to 90 °C, the R_s value and also the diameter of the semicircle are increased enormously. Such a rapid increase in the impedance is exclusively resulted from the melting and collapsing of the EVA-coating layer, which blocks off the pores of the separator and therefore substantially slows down the ionic conduction between the electrodes and eventually cuts off the electrode reactions at elevated temperature.

Fig. 6 Nyquist plots of experimental impedance data for LiCoO₂/Li coin cells using EVA-coated separator at various temperatures. The inset is an expanded view in the high frequency region.

To reveal the effects of the modified separator on the normal performance of the cells, we compared the charge–discharge behaviors of LiCoO₂/Li coin cells using both conventional and EVA-coated separators at ambient temperatures. Fig. 7a shows the cycling performance of LiCoO₂/Li coin cells at various C-rates from 0.5 to 10 C. As displayed in Fig. 7a, the LiCoO₂ electrode in the cells with EVA-coated separator can deliver a capacity of 150 mA h g⁻¹ at 0.5 C, 148 mA h g⁻¹ at 1.0 C and 140 mA h g⁻¹ at 5 C. Even at a very high rate of 10 C, this electrode can still deliver a stable capacity of ∼120 mA h g⁻¹, which is only slightly lower than that of the cells using conventional bare separator. These results indicate that the EVA-coating layer only produces a marginally negative effect on the ionic transport in the cells and the high rate capability of the LiCoO₂ electrode.

Fig. 7 Comparison for rate capability (a) and (b) cycling performance of Li/LiCoO₂ coin cells using both conventional bare separator and EVA-coated separator at ambient temperatures.

Fig. 7b compares the cycling performance of LiCoO₂/Li coin cells using both conventional and EVA-coated separators at a current density of 40 mA g⁻¹ at ambient temperature. As shown in Fig. 7b, these two types of cells exhibit no any discernable difference in capacity during 100 cycles of charge and discharge, demonstrating that the EVA-coated separator has no adverse impact on the normal cycling performance of the cells.

Conclusions

In summary, we prepared an EVA microsopheres-coated separator and investigated it as a thermal shutdown separator for safety protection of LIBs. The experimental results have demonstrated that the thermoresponsive EVA microsopheres could melt to form a polymer barrier on the surface of the separator substrate at temperature ∼90 °C, cutting off the Li⁺ conduction between the electrodes to interrupt the battery reaction, so as to prevent the cell from thermal runaway. In addition, this separator has no significant impacts on the normal charge–discharge performance of cells, showing a great prospect for practical application in LIBs.

Acknowledgements

This work was financially supported by the National High-tech R&D Program of China (2012AA110102, 2012AA052201), the National Science Foundation of China (21373154) and the National Basic Research Program of China (no. 2015CB251105).

Notes and references

1. J. B. Goodenough and Y. Kim, Chem. Mater., 2009, 22, 587–603.
2. J. M. Tarascon and M. Armand, Nature, 2001, 414, 359–367.
3. J. M. Tarascon, Philos. Trans. R. Soc., A, 2010, 368, 3227–3241.
4. B. Scrosati and J. Garche, J. Power Sources, 2010, 195, 2419–2430.
5. D. D. MacNeil, Z. Lu, Z. Chen and J. R. Dahn, J. Power Sources, 2002, 108, 8–14.
6. E. P. Roth and D. H. Doughty, J. Power Sources, 2004, 128, 308–318.
7. Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun and C. Chen, J. Power Sources, 2012, 208, 210–224.
8. P. Arora and Z. Zhang, Chem. Rev., 2004, 104, 4419–4462.
9. X. Huang, J. Solid State Electrochem., 2011, 15, 649–662.
10. P. G. Balakrishnan, R. Ramesh and T. Prem Kumar, J. Power Sources, 2006, 155, 401–414.
11. E. P. Roth, D. H. Doughty and D. L. Pile, J. Power Sources, 2007, 174, 579–583.
12. J.-A. Choi, S. H. Kim and D.-W. Kim, J. Power Sources, 2010, 195, 6192–6196.
13. M. Kim and J. H. Park, J. Power Sources, 2012, 212, 22–27.
14. S. M. Kang, M.-H. Ryou, J. W. Choi and H. Lee, Chem. Mater., 2012, 24, 3481–3485.
15. S. S. Zhang, J. Power Sources, 2007, 164, 351–364.
16. L. Xia, S.-L. Li, X.-P. Ai, H.-X. Yang and Y.-L. Cao, Energy Environ. Sci., 2011, 4, 2845–2848.
17. H. Zhong, C. Kong, H. Zhan, C. Zhan and Y. Zhou, J. Power Sources, 2012, 216, 273–280.
18. X. M. Feng, X. P. Ai and H. X. Yang, Electrochem. Commun., 2004, 6, 1021–1024.
19. J. Li, J. Chen and H. Lu, Int. J. Electrochem. Sci., 2013, 8, 5223–5231.
20. M. Baginska, B. J. Blaiszik, R. J. Merriman, N. R. Sottos, J. S. Moore and S. R. White, Adv. Energy Mater., 2012, 2, 583–590.
21. M. Baginska, B. J. Blaiszik, T. Rajh, N. R. Sottos and S. R. White, J. Power Sources, 2014, 269, 735–739.
22. C. L. Cheng, C. C. Wan, Y. Y. Wang and M. S. Wu, J. Power Sources, 2005, 144, 238–243.
23. Y. Wang, W.-H. Zhong, T. Schiff, A. Eyler and B. Li, Adv. Energy Mater., 2014 DOI:10.1002/aenm.201400463.
24. L. Xia, D. Wang, H. Yang, Y. Cao and X. Ai, Electrochem. Commun., 2012, 25, 98–100.
25. C.-C. Lin, H.-C. Wu, J.-P. Pan, C.-Y. Su, T.-H. Wang, H.-S. Sheu and N.-L. Wu, Electrochim. Acta, 2013, 101, 11–17.

---