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TiO2 ceramic-grafted polyethylene separators for enhanced thermostability and electrochemical performance of lithium-ion batteries
Journal of Membrane Science 504 (2016) 97–103
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
TiO2 ceramic-grafted polyethylene separators for enhanced thermostability and electrochemical performance of lithium-ion batteries Xiaoming Zhu a,b, Xiaoyu Jiang c, Xinping Ai c, Hanxi Yang c, Yuliang Cao c,n a
Hubei Collaboration Innovation Center of Non-power Nuclear Technology, Hubei University of Science and Technology, Xianning 437100, PR China School of Nuclear Technology & Chemistry and Biology, Hubei University of Science and Technology, Xianning 437100, PR China c College of Chemistry and Molecular Science, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 29 November 2015 Accepted 27 December 2015 Available online 8 January 2016
The separator is one of the most critically important components of lithium-ion battery to ensure battery safety. Herein, we introduce a novel TiO2 ceramic-grafted polyethylene (TiO2-grafted PE) separator prepared by electron beam radiation. The TiO2-grafted PE separator displays similar thickness and pore structure to the bare PE separator. Moreover, The TiO2-grafted PE separator exhibits not only stronger dimensional thermal stability (a shrinkage ratio of only 36% even at 150 °C), but also better electrochemical performance than the bare PE separator. Therefore, the TiO2-grafted PE separator is greatly beneficial for constructing safer lithium-ion batteries. & 2016 Elsevier B.V. All rights reserved.
Keywords: Ceramic-grafted separator Electron beam radiation Safety Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIBs) have demonstrated successful applications in a variety of portable electronic devices and are considered as one of the most promising candidates for electric vehicles and renewable power stations, due to their high energy and power density, minimal memory effects, and environmental friendliness [1,2]. However, the safety concern arising from the low thermal tolerance of the electrodes and electrolyte prevented the market acceptance of the LIBs in transportation applications. Once LIBs are subjected to extreme conditions, such as external or internal short-circuiting, overcharging, high-temperature thermal impacting, etc, these side-reactions may be triggered and subsequently accelerated with the temperature increasing through a dangerous positive feedback mechanism, producing excessive heat within a very short time, melting the separator and thus leading to thermal runaway, cell cracking, fire or even explosion [3]. In this regard, the separator plays an important role in determining not only the battery performance but also the safety [4,5]. Currently, most commercially employed separators for LIBs are based on polyolefin membranes, specifically microporous polyethylene (PE) or/and polypropylene (PP) separators. However, they may cause the safety issues because of their low thermal stability n
Corresponding author. E-mail address: [email protected] (Y. Cao).
http://dx.doi.org/10.1016/j.memsci.2015.12.059 0376-7388/& 2016 Elsevier B.V. All rights reserved.
[6,7]. Especially for applications of these separators in LIBs for electric vehicles, excessive heating should cause enormous dimensional shrinkage of the separator due to its melting, leading to an internal short circuit between the electrodes and consequently triggering thermal runaway reactions. To conquer these drawbacks of polymer-based separators, an effective strategy is to coat them with ceramic particles by using polymer as the binder [8,9]. Ceramic-coated separators display excellent thermal stability, negligible shrinkage at high temperatures and excellent electrolyte wettability due to the high thermostability and hydrophilicity of ceramic materials [4,10]. A small amount of ceramic particles such as SiO2 [11–13], Al2O3 [14,15] and TiO2 [16] have been introduced to coat onto the surface of the polymer to obtain the ceramiccoated separators. For instance, Yang et al. evaluated that a core– shell structured SiO2–PMMA sub-microspheres coated PE separator exhibited better thermal stability and improved cycle performance [13]. Lee et al. developed a PDA/Al2O3-coated PE separator which demonstrated superior thermal stability and cell performance [17]. Though the resultant separators exhibit good thermal stability and excellent wettability to liquid electrolytes, their increased thickness and partially blocked pores by modification of the ceramic nanoparticles impede their applications especially in high-energy and high-power systems. On the other hand, the insufficient binding between the nanoparticles and separator should cause particle shedding to lead to the alleviation of the thermal stability [8,18]. Induced graft polymerization of monomers by electron beam
98
X. Zhu et al. / Journal of Membrane Science 504 (2016) 97–103
radiation is a simple and effective method to modify the structure and properties of polymer materials, because the high intensity of electron beams can produce free radicals to uniformly initiate grafting in fast rate [19–22]. In this work, we report a novel TiO2 ceramic-grafted PE (TiO2-grafted PE) separator prepared by electron beam radiation technique. The TiO2-grafted PE separator exhibits not only similar thickness and pore structure to the bare separator, but also stronger dimensional thermal stability. In addition, the LiFePO4 cathode and graphite anode using TiO2-grafted PE separators show better electrochemical performance than those with bare PE separators.
2. Experimental 2.1. Preparation of the TiO2-grafted PE separator The PE separator (SKLiBS, SK Energy) with 8 mm thickness was washed with ethanol and dried at 60 °C prior to use. The PE separator was immersed in isopropyl trioleyl titanate (IPTT, supplied by Nanjing Upchemical Co. Ltd, Nanjing, China) and then radiated by an electron beam with doses of 80 kGy (1 Gy ¼1 J kg 1) at the dose rate of 20 kGy/pass at room temperature by a 1 MeV electron accelerator (Wasik Associates, USA). After grafting, the IPTT-grafted separator was immersed in the ethanol solution containing 0.12 mol/L tetrabutyl titanate, 0.05 mol/L hydrochloric acid, 0.5 mol/L water and 0.03 mol/L acetylacetone at 70 °C for 4 h to obtain the TiO2-grafted PE separator. Finally, the TiO2-grafted PE separator was cleaned with ethanol and dried at 60 °C before use. 2.2. Characterizations The surface functionalities of the separators were analyzed by FT-IR spectroscopy (Nicolet 6700) at room temperature in the wavenumber range of 4000–400 cm 1. 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 morphology and microstructure were characterized by a field emission scanning electron microscope (FE-SEM, ZEISS Merlin Compact VP, Germany). Surface element analysis was conducted using energy dispersive X-ray spectroscopy (EDS, Oxford Instruments Link ISIS).The mechanical intensity was measured at room temperature at a speed of 5 mm min 1 on a universal testing machine (CMT 6503, Shenzhen SANS Test Machine, Shenzhen, China) according to ISO 527-3, 1995 (E). Thermal analysis of the separators was carried out on a DSC Q200 system of TA Instrument in a temperature range of 60–180 °C at a heating rate of 10 °C/min under a 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. (1):
Shrinkage (%) =
Wi − Wf × 100 Wi
(1)
where Wi is the initial area and Wf is the final area of the separator after the storage test. 2.3. Electrochemical measurements The batteries for ionic conductivities tests of the separators were measured by electrochemical impedance spectroscopy (IM6, Zahner-elektrik, Kronach, Germany). A typical 2016 coin-type test cell was 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 an amplitude of 10 mV at room temperature. The ionic conductivity (s) was calculated using the following Eq. (2):
σ=
d RA
(2)
where d is the thickness of the separator, A is the area of the stainless steel electrode, and R is the bulk electrolyte resistance measured by one AC impedance test. The electrochemical stability window of the separators was measured by the linear sweep voltammetry (LSV) using a CHI600a electrochemical workstation (Shanghai Chenhua Inc., China). The separator was sandwiched between a steel working electrode and lithium metal counter electrode. The LSV test was carried out at a scan rate of 10 mV s 1 over a voltage range 3.0–5.0 V vs. Li þ /Li to check oxidative decomposition. The battery performance of the separators was examined using 2016 coin-type half-cells. The positive electrode consisted of 80 wt% LiFePO4, 10 wt% of PTFE and 10 wt% of acetylene black and the graphite electrode was composed of 90 wt% graphite, 2 wt% acetylene black and 8 wt/% PTFE. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 v/v/v). The cells were assembled in a high-purity argon-filled glove box with water/oxygen content lower than 1 ppm. The coin cells were cycled galvanostatically on a LAND cycler (Wuhan, China) at room temperature between 3.0 and 4.0 V at 40 mA g 1 for LiFePO4 cathode, and 0.01–3.0 V at 100 mA g 1 for graphite anode, respectively.
3. Results and discussion 3.1. Surface functionalization process of PE separator The preparation process of TiO2-grafted PE separator consists of two steps, including electron beam induced grafting and hydrolysis, as illustrated in Fig. 1. In the electron beam radiation process,
Fig. 1. The preparation process of the TiO2-grafted PE separator.
X. Zhu et al. / Journal of Membrane Science 504 (2016) 97–103
99
Fig. 2. (a) FTIR spectra of the bare, IPTT-grafted and TiO2-grafted PE separators and the contact angles of the (b) bare and (c) TiO2-grafted PE separators.
Fig. 3. FE-SEM photographs of (a) bare and (b,c) TiO2-grafted PE separators, (d) the element mapping images of Ti of TiO2-grafted PE separator in (c).
the high energy electron beam can produce large numbers of free radicals on the PE surface to initiate the grafting reaction of the isopropyl trioleyl titanate (IPTT) [22–24]. After being immersed into the freshly prepared titania sol, the grafted IPTT molecules were hydrolyzed and then reacts with titanium dioxide sol to obtain the TiO2-grafted PE separators. 3.2. Surface characterizations FT-IR spectra of bare PE, IPTT-grafted PE and TiO2-grafted PE separators are shown in Fig. 2a. For all of the spectra, the bands at 2850–3000 cm 1 and 1465 cm 1 respectively are related to C–H stretching and bending vibrations, indicating that the basic
structure of polyethylene still maintains [19]. Compared with bare PE separator, the bands around 500 cm 1 corresponding to Ti–O– Ti stretching vibrations are observed for IPTT-grafted PE and TiO2-grafted PE separators, and the bands gradually increase in intensity with increasing titania content [25]. In addition, the broad band near 3400 cm 1 is characteristic of the stretching vibration of Ti–OH [26]. The results give solid evidences to demonstrate that the titania has been introduced successfully onto the surface of the PE separator. The contact angle measurements are used to characterize the hydrophilic property of separators. As shown in Fig. 2b and c, the water contact angle on bare PE separator is 117°, whereas it decreases to 89° on the TiO2-grafted PE separator, confirming that the TiO2-grafted PE separator becomes
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Fig. 6. Tensile curves of the bare and TiO2-grafted PE separators. Fig. 4. DSC of the bare and TiO2-grafted PE separators.
3.3. Thermal and mechanical properties more hydrophilic than bare PE separator. Fig. 3 shows the FE-SEM images of the surfaces of the bare PE and TiO2-grafted PE separators. The bare PE separator presents a three-dimensional network structure with abundant open pores (Fig. 3a), which play important roles to provide fast ionic diffusion path. After the TiO2 ceramic grafted, the surface morphology remains almost unchanged but becomes rough (Fig. 3b). It is obvious to find that there are many nanoparticles distributed uniformly on the surface of the PE separator, indicating that the grafted TiO2 layer is thin enough so as to have insignificant effect on the porosity of PE separators [27]. Fig. 3d depicts an X-ray energy-dispersive spectroscopy (EDS) mapping of the TiO2-grafted PE separator, showing that TiO2 nanoparticles are distributed uniformly on the surface of PE separator.
Fig. 4 shows the DSC curves of bare PE and TiO2-grafted PE separators. The bare PE has endothermic peak associated with melting around 139 °C, which agrees with the reported data [28]. The melting temperature of TiO2-grafted PE separator slightly increases to 141 °C, indicating successful bonding of the TiO2 on the surface of PE separator, leading to improved thermostability of TiO2-grafted PE separator [29]. Fig. 5 shows the dimensional changes of the bare PE, IPTT-grafted PE and TiO2-grafted PE separators before and after storage at 150 °C for 0.5 h, respectively. The initial sizes of the separators are set at 5 cm 5 cm (width length). After being stored at 150 °C for 0.5 h, the bare PE separator suffers a drastically dimensional reduction (a dimensional shrinkage of about 95%) (Fig. 5a). However, the thermal shrinkage of IPTT-grafted and TiO2-grafted PE separators are about
Fig. 5. Photographs of the (a) bare, (b) IPTT-grafted, and (c) TiO2-grafted PE separator before/after storage at 150 °C for 0.5 h.
X. Zhu et al. / Journal of Membrane Science 504 (2016) 97–103
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Fig. 7. (a) The ionic conductivity and (b) linear sweep voltammetry curves of the bare and TiO2-grafted PE separators.
Fig. 8. Charge–discharge profiles for the (a) LiFePO4 and (c) graphite half-cells using the bare and TiO2-grafted PE separators. Cycling performances of the (b) LiFePO4 and (d) graphite half-cells using the bare and TiO2-grafted PE separators.
84% and 36%, respectively (Fig. 5b and c). It indicates that only organic IPTT grafting on the separator has very limited effect on improvement of the thermal stability. However, the introduction of heat-resistant TiO2 ceramic particles enhances significantly the thermal stability of the separator. Fig. 6 illustrates the tensile strength and elongation rate of bare PE and TiO2-grafted PE separators. The bare PE separator has a tensile strength of 129.8 MPa and elongation rate of 107.9%, whereas the TiO2-grafted PE separator shows comparative tensile strength and elongation rate (121.2 MPa and 107.1%). Generally, it is known that the polymer framework can be damaged by the
chain scission under electron beam radiation, thus leading to a decrease of the tensile strength. However, the slight decrease of tensile strength of the TiO2-grafted PE separator should have insignificant impact on its application in LIBs. 3.4. Electrochemical properties Electrochemical impedance spectra (EIS) are used to measure the ionic conductivity of the bare PE and TiO2-grafted PE separators, as shown in Fig. 7a. The ionic conductivities of the bare PE and TiO2-grafted PE separators infiltrated with electrolyte at room
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Fig. 9. (a) Discharge curves of the LiFePO4 half-cells and (b) Charge curves of the graphite half-cells using the bare and TiO2-grafted PE separators at different current rate.
temperature are calculated to be 0.32 and 0.50 mS cm 1 on the basis of Eq. (2), respectively. The higher ionic conductivity of TiO2-grafted PE separator can be attributed to its higher wettability (Fig. 2) and untouched pore structure (Fig. 3), which provide smooth ion diffusion path. Fig. 7b shows the electrochemical stability window of the bare PE and TiO2-grafted PE separators evaluated by a linear sweep voltammetry experiment. No anodic currents are observed below 4.5 V vs. Li þ /Li for both separators. The result indicates that the TiO2-grafted PE separator has sufficient electrochemical stability compatible with the carbonate electrolyte and can be applicable to a lithium ion battery. The electrode performance based on the bare PE and TiO2-grafted PE separators were examined using 2016 coin-type half-cells. Fig. 8a shows the initial charge–discharge curves of Li/LiFePO4 half-cells by using the bare PE and TiO2-grafted PE separators, respectively. The two electrodes exhibit similar characteristics. Compared to the electrode with bare PE separator, the LiFePO4 electrode with the TiO2-grafted PE separator has slightly higher initial reversible capacity, which can be attributed to the higher ionic conductivity. Both electrodes also show excellent cycling performance with almost no capacity decay over 100 cycles (Fig. 8b). For graphite anode, the reversible capacity and initial coulombic efficiency of the electrode with the TiO2-grafted PE separator are 303.9 mA h g 1 and 79% (Fig. 8c), respectively, which are slightly higher than the electrode with the bare separator (300.5 mA h g 1 and 76.8%). The cycling performance of the graphite with the bare PE and TiO2-grafted PE separators are shown in Fig. 8d. The graphite electrode with the TiO2-grafted PE separator shows slightly better cycling stability than that with the bare separator. The improvement of electrochemical performance of the electrodes results from the better wettability and higher ionic conductivity for the TiO2-grafted PE separator. Fig. 9a shows the discharge curves at different current densities for LiFePO4 half-cells using bare PE and TiO2-grafted PE separator. The discharge capacities drop with the current rate increasing for both cells. The discharge voltage plateaus of the cell using the TiO2-grafted PE separator are higher than that of the cell using bare PE separator especially at high current density, indicating lower electrochemical polarization. The cells show same capacity of about 133 mA h g 1 at 0.2 C (1 C ¼170 mA g 1). With the increase of the current rate, the electrode with TiO2-grafted PE separator exhibits improved capacity retention (60% at 5 C) compared with that with the bare PE separator (50% at 5 C). Fig. 9b shows charge curves at different current densities for graphite half-cells using bare PE and TiO2-grafted PE separator. The charge voltage plateaus increases with increasing current rate for both cells but the plateaus of the cell using TiO2-grafted PE separator is
lower. At the current density of 2 C (1 C ¼320 mA g 1), the cell as assembled with the bare PE separator shows only 18% capacity retention relative to the capacity at 0.2 C, whereas the cell using the TiO2-grafted PE separator has 34%. The superior rate capability of the cell with TiO2-grafted PE separator should correspond to the improved ionic conductivity (Fig. 7a) and better wetting ability (Fig. 2b and c) of the grafted separator.
4. Conclusion In summary, a novel TiO2-grafted PE separator is prepared by using electron beam radiation. The IR and SEM experimental results show that the TiO2 nanoparticles can be successfully grafted on the surface of the PE separator. The TiO2-grafted PE separator not only exhibits highly enhanced dimensional thermal stability, but also shows improved hydrophilicity and ionic conductivity. The electrochemical tests show that the TiO2-grafted PE separator can slightly improve the reversible capacity and rate performance of the LiFePO4 and graphite electrodes. Besides, the TiO2-grafted PE separator does not have any variation of the primitive thickness and pore structure compared with the PE separator, indicating that the TiO2-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 groupgrafted separators reported previously. In addition, the ceramicgrafted by electron beam radiation is a versatile avenue and applicable to other polymer materials in a variety of research fields such as energy, environment and biology.
Acknowledgments 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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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
TiO2 ceramic-grafted polyethylene separators for enhanced thermostability and electrochemical performance of lithium-ion batteries Xiaoming Zhu a,b, Xiaoyu Jiang c, Xinping Ai c, Hanxi Yang c, Yuliang Cao c,n a
Hubei Collaboration Innovation Center of Non-power Nuclear Technology, Hubei University of Science and Technology, Xianning 437100, PR China School of Nuclear Technology & Chemistry and Biology, Hubei University of Science and Technology, Xianning 437100, PR China c College of Chemistry and Molecular Science, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 29 November 2015 Accepted 27 December 2015 Available online 8 January 2016
The separator is one of the most critically important components of lithium-ion battery to ensure battery safety. Herein, we introduce a novel TiO2 ceramic-grafted polyethylene (TiO2-grafted PE) separator prepared by electron beam radiation. The TiO2-grafted PE separator displays similar thickness and pore structure to the bare PE separator. Moreover, The TiO2-grafted PE separator exhibits not only stronger dimensional thermal stability (a shrinkage ratio of only 36% even at 150 °C), but also better electrochemical performance than the bare PE separator. Therefore, the TiO2-grafted PE separator is greatly beneficial for constructing safer lithium-ion batteries. & 2016 Elsevier B.V. All rights reserved.
Keywords: Ceramic-grafted separator Electron beam radiation Safety Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIBs) have demonstrated successful applications in a variety of portable electronic devices and are considered as one of the most promising candidates for electric vehicles and renewable power stations, due to their high energy and power density, minimal memory effects, and environmental friendliness [1,2]. However, the safety concern arising from the low thermal tolerance of the electrodes and electrolyte prevented the market acceptance of the LIBs in transportation applications. Once LIBs are subjected to extreme conditions, such as external or internal short-circuiting, overcharging, high-temperature thermal impacting, etc, these side-reactions may be triggered and subsequently accelerated with the temperature increasing through a dangerous positive feedback mechanism, producing excessive heat within a very short time, melting the separator and thus leading to thermal runaway, cell cracking, fire or even explosion [3]. In this regard, the separator plays an important role in determining not only the battery performance but also the safety [4,5]. Currently, most commercially employed separators for LIBs are based on polyolefin membranes, specifically microporous polyethylene (PE) or/and polypropylene (PP) separators. However, they may cause the safety issues because of their low thermal stability n
Corresponding author. E-mail address: [email protected] (Y. Cao).
http://dx.doi.org/10.1016/j.memsci.2015.12.059 0376-7388/& 2016 Elsevier B.V. All rights reserved.
[6,7]. Especially for applications of these separators in LIBs for electric vehicles, excessive heating should cause enormous dimensional shrinkage of the separator due to its melting, leading to an internal short circuit between the electrodes and consequently triggering thermal runaway reactions. To conquer these drawbacks of polymer-based separators, an effective strategy is to coat them with ceramic particles by using polymer as the binder [8,9]. Ceramic-coated separators display excellent thermal stability, negligible shrinkage at high temperatures and excellent electrolyte wettability due to the high thermostability and hydrophilicity of ceramic materials [4,10]. A small amount of ceramic particles such as SiO2 [11–13], Al2O3 [14,15] and TiO2 [16] have been introduced to coat onto the surface of the polymer to obtain the ceramiccoated separators. For instance, Yang et al. evaluated that a core– shell structured SiO2–PMMA sub-microspheres coated PE separator exhibited better thermal stability and improved cycle performance [13]. Lee et al. developed a PDA/Al2O3-coated PE separator which demonstrated superior thermal stability and cell performance [17]. Though the resultant separators exhibit good thermal stability and excellent wettability to liquid electrolytes, their increased thickness and partially blocked pores by modification of the ceramic nanoparticles impede their applications especially in high-energy and high-power systems. On the other hand, the insufficient binding between the nanoparticles and separator should cause particle shedding to lead to the alleviation of the thermal stability [8,18]. Induced graft polymerization of monomers by electron beam
98
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radiation is a simple and effective method to modify the structure and properties of polymer materials, because the high intensity of electron beams can produce free radicals to uniformly initiate grafting in fast rate [19–22]. In this work, we report a novel TiO2 ceramic-grafted PE (TiO2-grafted PE) separator prepared by electron beam radiation technique. The TiO2-grafted PE separator exhibits not only similar thickness and pore structure to the bare separator, but also stronger dimensional thermal stability. In addition, the LiFePO4 cathode and graphite anode using TiO2-grafted PE separators show better electrochemical performance than those with bare PE separators.
2. Experimental 2.1. Preparation of the TiO2-grafted PE separator The PE separator (SKLiBS, SK Energy) with 8 mm thickness was washed with ethanol and dried at 60 °C prior to use. The PE separator was immersed in isopropyl trioleyl titanate (IPTT, supplied by Nanjing Upchemical Co. Ltd, Nanjing, China) and then radiated by an electron beam with doses of 80 kGy (1 Gy ¼1 J kg 1) at the dose rate of 20 kGy/pass at room temperature by a 1 MeV electron accelerator (Wasik Associates, USA). After grafting, the IPTT-grafted separator was immersed in the ethanol solution containing 0.12 mol/L tetrabutyl titanate, 0.05 mol/L hydrochloric acid, 0.5 mol/L water and 0.03 mol/L acetylacetone at 70 °C for 4 h to obtain the TiO2-grafted PE separator. Finally, the TiO2-grafted PE separator was cleaned with ethanol and dried at 60 °C before use. 2.2. Characterizations The surface functionalities of the separators were analyzed by FT-IR spectroscopy (Nicolet 6700) at room temperature in the wavenumber range of 4000–400 cm 1. 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 morphology and microstructure were characterized by a field emission scanning electron microscope (FE-SEM, ZEISS Merlin Compact VP, Germany). Surface element analysis was conducted using energy dispersive X-ray spectroscopy (EDS, Oxford Instruments Link ISIS).The mechanical intensity was measured at room temperature at a speed of 5 mm min 1 on a universal testing machine (CMT 6503, Shenzhen SANS Test Machine, Shenzhen, China) according to ISO 527-3, 1995 (E). Thermal analysis of the separators was carried out on a DSC Q200 system of TA Instrument in a temperature range of 60–180 °C at a heating rate of 10 °C/min under a 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. (1):
Shrinkage (%) =
Wi − Wf × 100 Wi
(1)
where Wi is the initial area and Wf is the final area of the separator after the storage test. 2.3. Electrochemical measurements The batteries for ionic conductivities tests of the separators were measured by electrochemical impedance spectroscopy (IM6, Zahner-elektrik, Kronach, Germany). A typical 2016 coin-type test cell was 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 an amplitude of 10 mV at room temperature. The ionic conductivity (s) was calculated using the following Eq. (2):
σ=
d RA
(2)
where d is the thickness of the separator, A is the area of the stainless steel electrode, and R is the bulk electrolyte resistance measured by one AC impedance test. The electrochemical stability window of the separators was measured by the linear sweep voltammetry (LSV) using a CHI600a electrochemical workstation (Shanghai Chenhua Inc., China). The separator was sandwiched between a steel working electrode and lithium metal counter electrode. The LSV test was carried out at a scan rate of 10 mV s 1 over a voltage range 3.0–5.0 V vs. Li þ /Li to check oxidative decomposition. The battery performance of the separators was examined using 2016 coin-type half-cells. The positive electrode consisted of 80 wt% LiFePO4, 10 wt% of PTFE and 10 wt% of acetylene black and the graphite electrode was composed of 90 wt% graphite, 2 wt% acetylene black and 8 wt/% PTFE. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 v/v/v). The cells were assembled in a high-purity argon-filled glove box with water/oxygen content lower than 1 ppm. The coin cells were cycled galvanostatically on a LAND cycler (Wuhan, China) at room temperature between 3.0 and 4.0 V at 40 mA g 1 for LiFePO4 cathode, and 0.01–3.0 V at 100 mA g 1 for graphite anode, respectively.
3. Results and discussion 3.1. Surface functionalization process of PE separator The preparation process of TiO2-grafted PE separator consists of two steps, including electron beam induced grafting and hydrolysis, as illustrated in Fig. 1. In the electron beam radiation process,
Fig. 1. The preparation process of the TiO2-grafted PE separator.
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Fig. 2. (a) FTIR spectra of the bare, IPTT-grafted and TiO2-grafted PE separators and the contact angles of the (b) bare and (c) TiO2-grafted PE separators.
Fig. 3. FE-SEM photographs of (a) bare and (b,c) TiO2-grafted PE separators, (d) the element mapping images of Ti of TiO2-grafted PE separator in (c).
the high energy electron beam can produce large numbers of free radicals on the PE surface to initiate the grafting reaction of the isopropyl trioleyl titanate (IPTT) [22–24]. After being immersed into the freshly prepared titania sol, the grafted IPTT molecules were hydrolyzed and then reacts with titanium dioxide sol to obtain the TiO2-grafted PE separators. 3.2. Surface characterizations FT-IR spectra of bare PE, IPTT-grafted PE and TiO2-grafted PE separators are shown in Fig. 2a. For all of the spectra, the bands at 2850–3000 cm 1 and 1465 cm 1 respectively are related to C–H stretching and bending vibrations, indicating that the basic
structure of polyethylene still maintains [19]. Compared with bare PE separator, the bands around 500 cm 1 corresponding to Ti–O– Ti stretching vibrations are observed for IPTT-grafted PE and TiO2-grafted PE separators, and the bands gradually increase in intensity with increasing titania content [25]. In addition, the broad band near 3400 cm 1 is characteristic of the stretching vibration of Ti–OH [26]. The results give solid evidences to demonstrate that the titania has been introduced successfully onto the surface of the PE separator. The contact angle measurements are used to characterize the hydrophilic property of separators. As shown in Fig. 2b and c, the water contact angle on bare PE separator is 117°, whereas it decreases to 89° on the TiO2-grafted PE separator, confirming that the TiO2-grafted PE separator becomes
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Fig. 6. Tensile curves of the bare and TiO2-grafted PE separators. Fig. 4. DSC of the bare and TiO2-grafted PE separators.
3.3. Thermal and mechanical properties more hydrophilic than bare PE separator. Fig. 3 shows the FE-SEM images of the surfaces of the bare PE and TiO2-grafted PE separators. The bare PE separator presents a three-dimensional network structure with abundant open pores (Fig. 3a), which play important roles to provide fast ionic diffusion path. After the TiO2 ceramic grafted, the surface morphology remains almost unchanged but becomes rough (Fig. 3b). It is obvious to find that there are many nanoparticles distributed uniformly on the surface of the PE separator, indicating that the grafted TiO2 layer is thin enough so as to have insignificant effect on the porosity of PE separators [27]. Fig. 3d depicts an X-ray energy-dispersive spectroscopy (EDS) mapping of the TiO2-grafted PE separator, showing that TiO2 nanoparticles are distributed uniformly on the surface of PE separator.
Fig. 4 shows the DSC curves of bare PE and TiO2-grafted PE separators. The bare PE has endothermic peak associated with melting around 139 °C, which agrees with the reported data [28]. The melting temperature of TiO2-grafted PE separator slightly increases to 141 °C, indicating successful bonding of the TiO2 on the surface of PE separator, leading to improved thermostability of TiO2-grafted PE separator [29]. Fig. 5 shows the dimensional changes of the bare PE, IPTT-grafted PE and TiO2-grafted PE separators before and after storage at 150 °C for 0.5 h, respectively. The initial sizes of the separators are set at 5 cm 5 cm (width length). After being stored at 150 °C for 0.5 h, the bare PE separator suffers a drastically dimensional reduction (a dimensional shrinkage of about 95%) (Fig. 5a). However, the thermal shrinkage of IPTT-grafted and TiO2-grafted PE separators are about
Fig. 5. Photographs of the (a) bare, (b) IPTT-grafted, and (c) TiO2-grafted PE separator before/after storage at 150 °C for 0.5 h.
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Fig. 7. (a) The ionic conductivity and (b) linear sweep voltammetry curves of the bare and TiO2-grafted PE separators.
Fig. 8. Charge–discharge profiles for the (a) LiFePO4 and (c) graphite half-cells using the bare and TiO2-grafted PE separators. Cycling performances of the (b) LiFePO4 and (d) graphite half-cells using the bare and TiO2-grafted PE separators.
84% and 36%, respectively (Fig. 5b and c). It indicates that only organic IPTT grafting on the separator has very limited effect on improvement of the thermal stability. However, the introduction of heat-resistant TiO2 ceramic particles enhances significantly the thermal stability of the separator. Fig. 6 illustrates the tensile strength and elongation rate of bare PE and TiO2-grafted PE separators. The bare PE separator has a tensile strength of 129.8 MPa and elongation rate of 107.9%, whereas the TiO2-grafted PE separator shows comparative tensile strength and elongation rate (121.2 MPa and 107.1%). Generally, it is known that the polymer framework can be damaged by the
chain scission under electron beam radiation, thus leading to a decrease of the tensile strength. However, the slight decrease of tensile strength of the TiO2-grafted PE separator should have insignificant impact on its application in LIBs. 3.4. Electrochemical properties Electrochemical impedance spectra (EIS) are used to measure the ionic conductivity of the bare PE and TiO2-grafted PE separators, as shown in Fig. 7a. The ionic conductivities of the bare PE and TiO2-grafted PE separators infiltrated with electrolyte at room
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Fig. 9. (a) Discharge curves of the LiFePO4 half-cells and (b) Charge curves of the graphite half-cells using the bare and TiO2-grafted PE separators at different current rate.
temperature are calculated to be 0.32 and 0.50 mS cm 1 on the basis of Eq. (2), respectively. The higher ionic conductivity of TiO2-grafted PE separator can be attributed to its higher wettability (Fig. 2) and untouched pore structure (Fig. 3), which provide smooth ion diffusion path. Fig. 7b shows the electrochemical stability window of the bare PE and TiO2-grafted PE separators evaluated by a linear sweep voltammetry experiment. No anodic currents are observed below 4.5 V vs. Li þ /Li for both separators. The result indicates that the TiO2-grafted PE separator has sufficient electrochemical stability compatible with the carbonate electrolyte and can be applicable to a lithium ion battery. The electrode performance based on the bare PE and TiO2-grafted PE separators were examined using 2016 coin-type half-cells. Fig. 8a shows the initial charge–discharge curves of Li/LiFePO4 half-cells by using the bare PE and TiO2-grafted PE separators, respectively. The two electrodes exhibit similar characteristics. Compared to the electrode with bare PE separator, the LiFePO4 electrode with the TiO2-grafted PE separator has slightly higher initial reversible capacity, which can be attributed to the higher ionic conductivity. Both electrodes also show excellent cycling performance with almost no capacity decay over 100 cycles (Fig. 8b). For graphite anode, the reversible capacity and initial coulombic efficiency of the electrode with the TiO2-grafted PE separator are 303.9 mA h g 1 and 79% (Fig. 8c), respectively, which are slightly higher than the electrode with the bare separator (300.5 mA h g 1 and 76.8%). The cycling performance of the graphite with the bare PE and TiO2-grafted PE separators are shown in Fig. 8d. The graphite electrode with the TiO2-grafted PE separator shows slightly better cycling stability than that with the bare separator. The improvement of electrochemical performance of the electrodes results from the better wettability and higher ionic conductivity for the TiO2-grafted PE separator. Fig. 9a shows the discharge curves at different current densities for LiFePO4 half-cells using bare PE and TiO2-grafted PE separator. The discharge capacities drop with the current rate increasing for both cells. The discharge voltage plateaus of the cell using the TiO2-grafted PE separator are higher than that of the cell using bare PE separator especially at high current density, indicating lower electrochemical polarization. The cells show same capacity of about 133 mA h g 1 at 0.2 C (1 C ¼170 mA g 1). With the increase of the current rate, the electrode with TiO2-grafted PE separator exhibits improved capacity retention (60% at 5 C) compared with that with the bare PE separator (50% at 5 C). Fig. 9b shows charge curves at different current densities for graphite half-cells using bare PE and TiO2-grafted PE separator. The charge voltage plateaus increases with increasing current rate for both cells but the plateaus of the cell using TiO2-grafted PE separator is
lower. At the current density of 2 C (1 C ¼320 mA g 1), the cell as assembled with the bare PE separator shows only 18% capacity retention relative to the capacity at 0.2 C, whereas the cell using the TiO2-grafted PE separator has 34%. The superior rate capability of the cell with TiO2-grafted PE separator should correspond to the improved ionic conductivity (Fig. 7a) and better wetting ability (Fig. 2b and c) of the grafted separator.
4. Conclusion In summary, a novel TiO2-grafted PE separator is prepared by using electron beam radiation. The IR and SEM experimental results show that the TiO2 nanoparticles can be successfully grafted on the surface of the PE separator. The TiO2-grafted PE separator not only exhibits highly enhanced dimensional thermal stability, but also shows improved hydrophilicity and ionic conductivity. The electrochemical tests show that the TiO2-grafted PE separator can slightly improve the reversible capacity and rate performance of the LiFePO4 and graphite electrodes. Besides, the TiO2-grafted PE separator does not have any variation of the primitive thickness and pore structure compared with the PE separator, indicating that the TiO2-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 groupgrafted separators reported previously. In addition, the ceramicgrafted by electron beam radiation is a versatile avenue and applicable to other polymer materials in a variety of research fields such as energy, environment and biology.
Acknowledgments 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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