A Highly Thermostable Ceramic-Grafted Microporous Polyethylene

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A Highly Thermostable Ceramic-grafted Microporous Polyethylene Separator for Safer Lithium-ion Batteries Xiaoming Zhu, Xiaoyu Jiang, Xinping Ai, Hanxi Yang, and Yuliang Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07230 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015

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ACS Applied Materials & Interfaces

A Highly Thermostable Ceramic-grafted Microporous Polyethylene Separator for Safer Lithium-ion Batteries Xiaoming Zhu a,b, Xiaoyu Jiang c, Xinping Ai c, Hanxi Yang c and Yuliang Cao c,* a

Hubei Collaboration Innovation Center of Non-power Nuclear Technology, Hubei University of

Science and Technology, Xianning 437100, (P. R. China) b

School of Nuclear Technology & Chemistry and Bioloy, Hubei University of Science and

Technology, Xianning 437100, (P. R. China) c

College of Chemistry and Molecular Science, Hubei Key Laboratory of Electrochemical Power

Sources, Wuhan University, Wuhan 430072, China. *

Corresponding author E-mail: [email protected]

Abstract: The safety concern is a critical obstacle to large-scale energy storage applications of lithium-ion batteries. Thermostable separator is one of the most effective means to construct the safe lithium ion batteries. Herein, we demonstrate a novel ceramic (SiO2)-grafted PE separator prepared by electron beam irradiation. The separator shows similar thickness and pore structure to the bare separator, while displaying strong dimensional thermostability, as the shrinkage ratio is only 20 % even at elevated temperature of 180 °C. Besides, the separator is highly electrochemical inert, showing no adverse effect on the energy and power output of the batteries.

ACS Paragon Plus Environment

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Considering the excellent electrochemical and thermostability, the SiO2-grafted PE separator developed in this work is greatly beneficial for constructing safer lithium-ion batteries. Keywords: Electron beam irradiation; Ceramic-grafted separator; Thermal property; Safety; Lithium-ion battery

1. Introduction Lithium-ion batteries have demonstrated successful applications in a variety of portable electronic devices and received worldwide interest for electric storage applications in electric vehicles and renewable power stations, due to their high energy and power density, long-term cycle life and environmental friendliness.1 A major concern for these large-scale applications is the safety issue due to the use of flammable carbonate electrolyte and the existence of a series of potential thermal runaway reactions during the abuse of lithium ion batteries.2-3 Separator is a critical component in Li-ion batteries, significantly determining the battery performance and the safety.4-7 Particularly, the separator in lithium ion batteries must have a strong dimensional stability at elevated temperatures to keep the anode and cathode apart to prevent electrical short circuit, which is a major cause for the thermal runaway of the batteries. Therefore, development of high strength and thermostable separators is an effective way to enhance the safety of Li-ion batteries for large-scale electric energy storage applications.7 Currently, microporous polyethylene (PE) film is widely used as separator for lithium-ion batteries because of their good electric resistance, high electrochemical stability and good mechanical strength8. However, the melting point of PE membrane is only 135 °C, which easily causes the swelling and fusing of the membrane at elevated temperatures and possibly the shutdown of the pores to cutoff the ionic transport. Although the thermal shutdown of the PE


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separator also can effectively suppress the occurrence of a series of exothermal reactions, the melting of the PE membrane often cause enormous dimensional shrinkage, leading to internal short circuit between the electrodes and consequently trigging thermal runaway reactions. To conquer this drawback, a number of efforts have been devoted to use non-woven separators, ceramic-coated

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and irradiation-treated separators instead of conventional PE membranes.

Although non-woven separators have a strong dimensional thermostability, their practical use in lithium-ion battery is limited due to their poor mechanical strength, inhomogeneous pores and thickness and high cost compared to the PE separator.15 Ceramic-coated PE separators display greatly improved dimensional thermostability and excellent electrolyte wettability due to the high thermal stability and hydrophilicity of ceramic materials;16-17 Different designs have been reported for ceramic-coated separators. For instants, Wang et al.18 showed that a PEI/SiO2/PE separator prepared by self-assembly process can improve the electrolyte wetting, the thermal stability, the ionic conductivity of PE separator. Yanilmaz et al.19 demonstrated a SiO2/nylon 6,6 nanofiber membranes by fabricated by electrospun technique. The SiO2/nylon 6,6 nanofiber membranes showed no apparent dimensional change after thermal exposure at 150 °C for 0.5 h. Lee et al.20 evaluated that a PDA/Al2O3-coated PE separator revealed improved thermal stability and cycle performance for lithium secondary batteries. However, their increased thickness and blocked pores by modification of the ceramic nanoparticles and polymeric binder impede their applications especially in high-energy and -power systems.4 Irradiation treated PE separators by high energy irradiations such as plasma,21 gamma ray

22-23

and electron beam technologies

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have very different pore and surface structures. The high energy irradiation can produce free radicals to initiate the cross-linking reaction of the polymer chains, so as to greatly improve the mechanical strength of the separator. It was reported that the irradiated separator exhibited


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enhanced thermostability without variation of thickness.22, 24 However, excess radiation dose can cause serious polymer chain damage. Therefore, it is very tricky to control the chain crosslink while avoiding the breakdown of the polymer skeleton. Surface grafting under appropriate low irradiation is a potential modification method to fabricate thermostable separator without causing significant impairment of the polymer matrix.2526

In this work, we report a novel ceramic-grafted PE separator that free of polymer binders

prepared by grafting vinyltrimethoxysilane on the PE separator under electron beam irradiation and then silicating the separator with a freshly prepared silica sol to obtain the SiO2-grafted PE separator. Compared to conventional γ-ray irradiation, this new technique has remarkable advantages of easy controllability, safety, on-line process and high efficiency. Such prepared separator shows similar thickness and pore structure to the bare separator, while displaying strong dimensional thermostability with a shrinkage ratio of only 20 % even at elevated temperature of 180 °C. In addition, this separator shows no adverse effect on the electrochemical performances of Li-ion batteries.

2. Experimental Section 2.1．．Preparation of the ceramic-grafted PE separator. PE separator (SKLiBS, 8 µm thickness, SK Energy) was washed with ethanol and dried at 60 °C prior to use. The PE separator was immersed in VTMS solution (vinyltrimethoxysilane ) and then irradiated by an electron beam to doses of 80 kGy (1 Gy=1 J/Kg) at the dose rate of 20 kGy/pass at room temperature by 1 MeV electron accelerator (Wasik Associates, USA). After the irradiation, the VTMS-grafted separator was immersed in a solution including 0.15 mol/L sodium measilicate and 0.36 mol/L hydrochloric acid for 4 h at 60 °C to obtain SiO2-grafted


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separator. Finally, the SiO2-grafted separator was washed with ethanol and dried at 60 °C. The degree of grafting was calculated by using Eq. (1): DG % =

Wg − W 0 × 100 W0

(1)

2.2 Characterization. The surface functionalities of the separators were analyzed by a FT-IR spectroscopy (Nicolet 6700) in the wave number range of 4000–400 cm−1 at ambient conditions. The morphologies were observed on a field emission scanning electron microscopy (FE-SEM, ZEISS Merlin Compact VP, Germany). Contact angles were determined by the sessile drop method with distilled water (2 µL) as a probe liquid on a Dataphysics OCA20 CA system at room temperature. The mechanical intensity was measured at room temperature on an universal testing machine (CMT 6503, Shenzhen SANS Test Machine, Shenzhen, China) according to ISO 527-3, 1995 (E) at a speed of 5 mm min-1. Thermal analysis of the separators were carried out on a DSC Q200 system of TA instrument in a temperature range of 60-160 °C at a heating rate of 10 °C/min under N2 atmosphere. The thermal shrinkage of the separators was determined by measuring the dimensional changes after storage at 150 °C for 0.5 h. The degree of thermal shrinkage was calculated by using Eq. (2): Shrinkage (%) =

wi − w f wi

× 100 (2)

where Wi is the initial area and Wf is the final area of the separator after the storage test. 2.3 Electrochemical Measurements The ionic conductivities of the separators were measured by electrochemical impedance spectroscopy (IM6, Zahner-elektrik, Kronach, Germany). 2016 coin-type test cells were


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assembled by sandwiching the separator between two stainless steel (SS) electrodes and soaking it into the liquid electrolyte (1 M LiPF6 in EC/DEC/DMC 1:1:1 by volume) for AC impedance measurements. Impedance data were obtained in the frequency range of 10 Hz–100 kHz with amplitude of 10 mV at room temperature. The ionic conductivity (σ) was calculated using the following Eq. (3):

σ=

d RA

(3)

Where d is the thickness of the separators, A is the area of the stainless steel electrode and R is the electrolyte resistance measured by AC impedance. The electrochemical performance of the separators was examined using 2016 coin-type half cells, where the separator were sandwiched between a lithium metal anode and a LiFePO4 cathode and soaked with the electrolyte of 1M LiPF6 in EC/DEC/DMC (1:1:1, by volume). The cathode was composed of 80 wt. % of LiFePO4, 10 wt. % of PTFE and 10 wt. % of acetylene black. The coin cells were cycled between 3.0 and 4.0 V at room temperature at 40 mA g-1 on a LAND cycler (Wuhan, China). The AC impedance spectra of the 2016 coin-type half cells were obtained using electrochemical impedance spectroscopy (IM6,Zahner-elektrik, Kronach, Germany) in the frequency range of 0.01 and 106 Hz with an amplitude of 10 mV at room temperature.

3. Results and Discussion 3.1. Formation mechanism of the ceramic-grafted PE separator Irradiation-induced grafting is a facile technique to tailor the structure as well as to modify the surface of polymers. Figure 1 depicts the preparation process of the ceramic-grafted PE separator. As it shows, VTMS molecules are firstly grafted onto the active surface sites of the PE separator through its carbon double bonds under electron beam irradiation (EBI).27-28 After


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immersed into sodium measilicate solution, the grafted VTMS molecules was hydrolyzed to form active -OH groups, which react with measilicate by the silication reaction to form the resulted ceramic (SiO2)-grafted PE separator.29 The degree of grafting SiO2 of ceramic-grafted PE separator calculated by Eq. 1 is 16.1%.

Figure 1. Mechanism for the preparation of ceramic-grafted PE separators by irradiation grafting of VTMS and silication.

3.2. Surface characterizations To confirm the successful grafting of SiO2 onto the PE separator, the bare, VTMS- and SiO2grafted separators were analyzed using FT-IR. As shown in Fig. 2a, the characteristic bands associated with PE separator appear at 2850–3000 cm-1 and 1465 cm-1 in all of the three spectra, corresponding to C–H stretching and bending vibration, respectively.30 Compared to the bare PE separator, the VTMS-grafted separator gives a new peak at 1100 cm-1, which is characteristic of the stretching vibration of Si–OH,29 indicating successful VTMS grafting onto the separator by


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EB induced irradiation. For the SiO2-grafted PE separator, a signal at 1080 cm-1 is clearly observed, which corresponds to the asymmetric stretching vibration of Si–O–Si.31 Besides, the asymmetric stretching vibration near 800 cm-1 and the symmetric stretching and bending vibration near 460 cm-1 of Si–O–Si are also clearly detected.29 These results give a further solid proof to verify that the silica can be introduced successfully onto the surface of the PE separator. The wettability of the separators was characterized by the contact angle measurement to confirm the effect of the SiO2 grafting. As shown in Fig. 2b and c, the water contact angle on the bare PE separator is 117o, whereas it decreases to 79o on the ceramic-grafted PE separator. The great reduction of the contact angle is obviously resulted from the introduction of the SiO2 ceramic layer on the grafted PE separator, which can obviously improve the wettability and ionic conductivity of the separator.

Figure 2. (a) FTIR spectra of the bare, VTMS-grafted and ceramic-grafted PE separators and the contact angles of the bare (b) and ceramic-grafted (c) PE separators. Figure 3 shows the FE-SEM images of the surfaces for the bare and the ceramic-grafted PE separators. The bare PE separator presents three-dimensional network structure with abundant


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open pores (Fig. 3a, inset), which can facilitate the soakage of electrolyte to form fast ion diffusion path. After the ceramic grafting, a large number of SiO2 nanoparticles are found to distribute uniformly on the surface of the PE (Fig. 3b, inset) while the pore structure shows no change compared to the bare PE separator. Fig. 3d depicts an X-ray energy dispersive spectroscopy (EDS) mapping of SiO2-PE separator of Fig. 3c, suggesting that SiO2 nanoparticles are distributed uniformly on the surface of PE separator. Thus, the as prepared separator exhibits obvious advantages over those ceramic-coated separators by traditional coating synthesis.9,

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The separator shows no significant increase in thickness so as to avoid the occupation of the restricted space of the active materials in a full cell. Moreover, the original pore structure is remained in good condition so that the ionic transportation inside the separator is unaffected

Figure 3. FE-SEM photographs of bare (a) and ceramic-grafted (b,c) PE separators and the


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element mapping images (d) of Si of ceramic-grafted PE separator in (c). The insets show the magnification SEM images. 3.3 Thermal and mechanical properties The thermal stabilities of the bare and SiO2-grafted PE separators were investigated by DSC and thermal shrinkage tests. Figure 4 displays the DSC curves of the bare and SiO2-grafted PE separators. The bare PE separator melts at 139 °C, agreeing with the reported data in the previous reference.24 The melting temperature of the SiO2-grafted PE separator slightly increases to 143 °C, owing to the grafting of the SiO2 nanoparticles. The thermal shrinkage behavior is a direct measure of the thermal stability of separators.

Figure 4. DSC of the bare and ceramic-grafted PE separators. Figure 5 shows the dimensional changes of the separators before and after storage at 150 °C and 180 °C for 0.5 h respectively. The initial sizes of the separators were set at 5 cm × 5 cm (Width × Length) (Fig. 5a and 5d for bare separator and for SiO2-grafted separator). After stored at 150 °C for 0.5 h, the bare PE separator shrank drastically and showed a dimensional reduce of more than 90 % (Fig. 5b). The VTMS-grafted PE separator showed slightly less dimensional


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reduction of about 80% (Fig. 5c), indicating that organic VTMS grafting has very limited effect on the thermal stability improvement of the PE separator. In contrast, the SiO2-grafted PE separator showed almost no shrinkage at 150 °C for 0.5 h at all as its dimensional size maintained as the original one (Fig. 5e). When the separators were stored at 180 °C for 0.5 h, the dimensional reduction of the SiO2-grafted PE separator was only 20%, even though the PE was actually melted intrinsically as the separator became transparent (Fig. 5f). The excellent thermal shrinkage resistance of the SiO2-grafted PE separator apparently favors from the uniformly grafted SiO2 nanoparticles on the surface of the PE separator, which act as a thermostable framework to resist the dimensional variation of PE during melting. Considering thermal safety of the separator, the shutdown function of the separator at high temperature is also very important. In order to evaluate the shutdown function of the separator, the SEM images of the SiO2-grafted PE separator after high temperature storage were carried out (Fig. 6). As can be seen from Figure 6, the SiO2-grafted PE separator has closed most pores at 150 °C and closed all pores at 180 °C, indicating being partially melt at 150 °C and completely melt at 180 °C. It implies that the SiO2-grafted separator can not only realize the shutdown function, but also maintain high dimensional thermostability at high temperature. Taking into account these two aspects, the grafted separator can further improve the battery safety according to its thermal shutdown function and dimensional thermostability.


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Figure 5. Photographs of the bare (a) and ceramic-grafted (d) PE separator. Photographs of bare (b), VTMS-grafted (c) and ceramic-grafted (e) PE separators after thermal storage at 150 °C for 0.5 h. (f) Photographs of the ceramic-grafted PE separator after thermal storage at 180 °C for 0.5 h.


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Figure 6. SEM of the ceramic-grafted PE separator after 150 °C (a) and 180 °C (b) storage for 0.5 h.

The mechanical intensity of the separator is also an important parameter for battery applications. The tensile strength and elongation rate of the bare and the SiO2-grafted PE separators are compared in Fig. 7. It can be seen that the bare PE separator has a tensile strength of 16.78 MPa and elongation rate of 116 %, while the SiO2-grafted PE separator shows a lower tensile strength of 14.46 MPa and elongation rate of 110 %, respectively. The slightly lower tensile strength and elongation rate of the SiO2-grafted PE separator might be caused by the chain scission of PE framework under electron beam irradiation,25 which should have insignificant impact on its application in lithium-ion batteries.


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Figure 7. The tensile curves of the bare and ceramic-grafted PE separators. 3.4 Electrochemical properties The ionic conductivity of the separator is a major parameter determining the electrochemical performance of batteries. Electrochemical impedance spectra (EIS) were used to measure the ionic conductivity of the bare and SiO2-grafted PE separators (Figure 8a). Based on the Eq. 3 described in the Experimental Section, the ionic conductivities of the bare and the SiO2-grafted PE separators at room temperature were calculated to be 0.32 and 0.45 mS cm-1, respectively. The higher ionic conductivity of the SiO2-grafted PE separator should be originated from the higher wettability (Fig. 2) and untouched pore structure (Fig. 3), which provide smooth ion diffusion path.9 The compatibility of the electrode with separator was also evaluated by EIS. Figure 8b shows the Nyquist plots of Li/separator/LiFePO4 cells at open circuit potential. The cell using the SiO2grafted PE separator showed a lower electrochemical resistance than the bare separator,


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indicating more fluent ion transportation between the ceramic-grafted PE separator and electrodes.

Figure 8. (a) The ionic conductivity of the bare and ceramic-grafted PE separators. (b) Nyquist plots of the Li/bare PE/LiFePO4 cell and Li/ceramic-grafted PE/ LiFePO4 cell at the open circuit potential. The charge-discharge measurement can directly assess the effect of the SiO2-grafted PE separator on the electrochemical performance of batteries. Figure 9a displays the initial chargedischarge profiles of Li/separator/LiFePO4 cells cycled at 40 mA g-1. It can be seen that the LiFePO4 electrode using the SiO2-grafted PE separator exhibits slightly higher initial reversible capacity and lower charge-discharge potential gap compared to that using the bare PE separator, because of the higher ionic conductivity (Fig. 8a) and lower the electrochemical impedance of the LiFePO4/ SiO2-grafted PE separator interface (Fig. 8b). The half cells using the two separators exhibit similar cycle performance (Fig. 9b), implying that the SiO2-grafted PE separator has reliable electrochemical stability during the repeating charge and discharge, which is one of the most important factors determining the feasibility of the SiO2-grafted PE separator in lithium-ion batteries. Fig. 9 c depicts the rate capability of LiFePO4/Li cells with bare PE and


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SiO2-grafted PE separator. The discharge current is varied from 0.2 C to 5 C (1 C= 170 mA g-1). With the increase of the current rate, the SiO2-grafted PE separator exhibits improved capacity retention compared with the bare PE separator. For instant, at the current density of 5 C, the cell as assembled with bare PE separator maintain 49.8% capacity relative to the capacity at 0.2 C, whereas SiO2-grafted PE separator maintain 58.4%. The superior rate performance of SiO2grafted PE separator can be attributed to better wetting ability (Fig. 1b,c) and higher ion conductivity32 (Fig. 8a). In addition, the binder-free, thin-layer SiO2 facilitates efficient ion transport without affecting the pore structure (Fig. 3b).


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Figure 9. (a) Charge-discharge profiles, (b) cycling performances and (c) rate performances of the LiFePO4 half cells using the bare and ceramic-grafted PE separators.

4. Conclusions In this work, we present a novel ceramic (SiO2)-grafted PE separator by using electron beam irradiation. The as-prepared separator exhibits highly enhanced dimensional thermal-stability without any variation of its primitive thickness and pore structure. The separator also displays enhanced ionic conductivity owing to the improved wettability and excellent electrochemical stability. Thus, the SiO2-grafted PE separator would not affect the energy or power density when used in practical batteries, which is obviously superior to those ceramic-coated and organic functional group-grafted separators reported previously. Besides, the experimental results also demonstrate that the electron beam irradiation technology is controllable, safe, in-line and efficient, more suitable for large-scale manufacture of thermostable and high-performance separator than gamma ray irradiation. Nevertheless, for practical applications, some problems need to be pre-solved, such as irradiation uniformity, the choice of more stable grafting agents,


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the control of the silication reaction, etc. It is expected that these issues will be addressed with the development of the electron beam irradiation technology and material science. Moreover, the ceramic-grafting by electron beam irradiation can also be extended to modify and functionalize other polymer materials in a variety of research fields such as energy, environment and biology.

Author Information Corresponding Authors *E-mail: [email protected]

Acknowledgements The authors gratefully acknowledge the financial support by the 2011 Program of Hubei Province, Natural Science Foundation of Hubei Province, China (Grant No. 2015CFC774), Program for New Century Excellent Talents in University (NCET-12-0419) and Hubei National Funds for Distinguished Young Scholars (2014CFA038).

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Table of Contents Graphic

Title: A Highly Thermostable Ceramic-grafted Microporous Polyethylene Separator for Safer Lithium-ion Batteries Xiaoming Zhu, Xiaoyu Jiang, Xinping Ai, Hanxi Yang and Yuliang Cao,*

ToC Figure:

A SiO2-grafted microporous Polyethylene separator is prepared by using electron beam irradiation method. The as-prepared SiO2-grafted PE separator exhibits highly enhanced dimensional thermostability without any variation of its primitive thickness and pore structure, as well as enhanced ionic conductivity and excellent electrochemical stability, which is a promising separator for safer lithium-ion battery.


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224x175mm (96 x 96 DPI)


A Highly Thermostable Ceramic-Grafted Microporous Polyethylene

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