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Poly (ether ether ketone) (PEEK) porous membranes with super high thermal stability and high rate capability for lithium-ion batteries
Journal of Membrane Science 530 (2017) 125–131
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Poly (ether ether ketone) (PEEK) porous membranes with super high thermal stability and high rate capability for lithium-ion batteries
MARK
⁎
Dan Lia,b, Dingqin Shia, Kai Fenga, Xianfeng Lia,c, , Huamin Zhanga,c,⁎⁎ a b c
Division of energy storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100039, China Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Lithium-ion battery Poly (ether ether ketone) Porous membrane Thermal stability
Lithium-ion batteries are receiving intensive interest due to their promising prospect for electric vehicles. However, the safety issues and low rate capability of lithium-ion batteries limited their further development due to the poor dimensional thermo-stability and low electrolyte wettability of commercial polyolefin separators. Herein, we report a sponge-like porous poly (ether-ether-ketone) (PEEK) membrane with super high thermal stability and good rate capability for lithium ion batteries. The porous PEEK membrane showed no fusion deformation even at temperature of 350 °C, showing excellent thermal stability. An electrolyte uptake of 251% was obtained for a porous PEEK membrane, which was more than twice than that of a commercial PE separator, showing very good electrolyte wettability. As a result, the lithium-ion battery with a porous PEEK membrane showed a discharge capacity of 124.1 mA h g−1 at 5 C, showing good rate performance. The battery performance of a PEEK membrane after treating at 350 °C changed rarely. The PEEK membranes provide more options for cost-effective high power battery separators.
1. Introduction Lithium-ion batteries have attracted great interest in electric vehicles (EV) and hybrid electric vehicles (HEV) due to their higher energy density, no memory effect and environmental friendliness comparing with conventional secondary batteries [1–6]. However, the further promotion of EV and HEV is highly hindered by safety issues of lithium ion batteries. The safety issue of a lithium battery is mainly induced by the poor dimensional thermo-stability of currently used separators [7,8]. In a lithium battery, a separator acts as an interlayer to prevent the direct contact of positive electrode and negative electrode, while providing channels for transferring lithium ions. Consequently, an ideal separator should be resistant to chemical and electrochemical interaction from electrodes and electrolyte, and possess high dimensional thermo-stability to prevent the safety issues [9,10], which caused mostly by overcharge, short circuit, thermal runaway and even explosion. Currently, the commercial separators for lithium-ion batteries are porous polyolefin membranes, such as polyethylene (PE), polypropylene (PP) and their composite for their low cost, excellent chemical and electrochemical stability, high mechanical strength, and fair microstructure [11,12]. However, the poor wettability with electrolyte and ⁎
poor dimensional thermo-stability of polyolefin separators further hinder their application in high power battery. The poor thermal stability of polyolefin separators is derived from the intrinsic low melting point of polymers (PE/130 °C, PP/160 °C) [13,14], causing some security issues for lithium-ion batteries at elevated temperature. The poor wettability with electrolyte of polyolefin separators is due to the hydrophobicity and low porosity of polyolefin membranes, leading to low ionic conductivity and poor rate performance [11,15]. Therefore, it is in urgent need of developing a separator with high dimensional thermo-stability and good wettability for high power lithium-ion batteries. Up to now, many efforts have been introduced to improve the thermal stability of the current separators. Surface modification, such as introducing inorganic (SiO2 [16], Al2O3 [17], TiO2 [18]) or organic (polyimide [19], polyarylate [20]) materials to substrate membranes was widely employed to enhance the thermal stability of separators. In addition, constructing a heat-resistant skeleton can also solve this issue, such as porous polyether imide separators [8] and polyoxyzole nanofiber membranes [21]. To improve the electrolyte wettability, extensive efforts have been taken to fabricate a membrane with polar surface by introducing inorganic ceramic [18,22] or polymer layer [15] on commercial polyolefin separator, thus the wettability of the mem-
Corresponding author at: Division of energy storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China. Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (H. Zhang).
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http://dx.doi.org/10.1016/j.memsci.2017.02.027 Received 22 October 2016; Received in revised form 14 February 2017; Accepted 19 February 2017 Available online 21 February 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
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Nomenclature PEEK EV HEV PE PP TG LSV NMP SEM PVDF EIS SS mo m1 ρ0
ρ1 U Mo
Poly (ether ether ketone) Electric vehicles Hybrid electric vehicles Polyethylene Polypropylene Thermogravimetric Linear sweep voltammetry 1-methyl-2-pyrrolidinone Scanning electronic microscopy poly (vinylidene fluoride) Electrochemical impedance spectroscopy Stainless steels Weight of dry membrane (g) Weight of n-butanol (g) Density of polymer (g/cm3)
Me Rb σ d S So S A R T Ea SEI
Density of n-butanol (g/cm3) Electrolyte uptake (%) Weight of the membrane before immersing in the electrolyte (g) Weight of the membrane after immersing in the electrolyte (g) Bulk impedance (Ω) Ionic conductivity (S cm−1) Thickness of the membranes (μm) Effective area of the membranes (cm2) Area of the membrane before heat-treatment (cm2) Area of the membrane after heat-treatment (cm2) Pre-exponential factor Gas constant Absolute temperature Activation energy Solid electrolyte interface
2. Experimental
brane can be highly enhanced due to the strong interaction between polar surfaces of prepared membrane with the carbonate electrolyte. Nevertheless, these strategies also brought many adverse effects on their applicability for lithium-ion battery due to the relative expensive materials and multi-step manufacture process. Poly (ether-ether-ketone) (PEEK) is one kind of semi-crystalline polymer, it owns characteristics of high chemical stability and thermal stability. Thus it could be used as high temperature structural materials and electrical insulating materials. In addition, due to the fact that the polar oxygen atoms and the carbon-oxygen double bond in PEEK polymer may have strong interaction with the carbonate electrolyte, the PEEK based membranes are expected to have high thermal stability and good wettability with electrolyte, which can further resolve the safety issues and improve the power density of lithium-ion batteries. What's more, it is worth noting that the cost of PEEK polymer is relatively low. Even though, the poor solubility of PEEK polymers limited their application in membranes field. Quite recently, Andrew et al. [23] first reported the application of PEEK porous membranes in organic solvent filtration, where, this membranes were proved to be prepared by a typical phase inversion method and the method was proved to be easily scaled up. Therefore, in this paper, a porous PEEK membrane with symmetric sponge-like structure was designed and prepared by phase inversion method (Scheme 1) for the first time, we investigated their application in lithium ion battery application. The obtained sponge-like porous PEEK membranes have superior wettability with electrolyte, owning 251% electrolyte uptake and high thermal stability. Our work will provide more options for high power battery separators.
2.1. Fabrication of membranes The sponge-like porous membranes were fabricated through phase inversion method. Firstly, a proper amount of PEEK (Changchun Jida plastic engineering research co., Ltd, China) was dissolved in a mixture solution of sulfuric acid and methyl sulfonic acid (mass ratio of sulfuric acid/methyl sulfonic acid was 1/10) to prepare a 12 wt% casting solution at room temperature. Then the solution was cast onto a clean glass plate using a doctor blade (Elcometer 3545 adjustable Bird Coater, Scraper, Elcometer 3545/8), then transferred to water for about 10 min. Afterword, the prepared membranes were completely immersed in water for 48 h. Finally, the prepared membranes were dried in a vacuum oven at 60 °C for 24 h. The average thickness of the membrane was 30 ± 2 µm. 2.2. Physical characterization The surface and cross-section morphologies of the membranes including the prepared PEEK membrane and the commercial PE separator (Dongguan Saidiou Electronic Technology Co. Ltd.) were observed with scanning electron microscope (JSM-7800F). Cross section samples were obtained by breaking the membranes in liquid nitrogen. The thermal stability of a membrane was investigated by thermogravimetric analysis (TG) using a thermal analyzer thermogravimetry analyses (Pyris-Elmer), which was operated at a heating rate of 5 °C/min from 50 °C to 800 °C under nitrogen gas flow. The contact angle measurement of a membrane was conducted on a
Scheme 1. Schematic principle of membranes with sponge-like structure.
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2.4. Thermal shrinkage measurement
drop shape analysis system (Powereach JC2000C1, Shanghai Zhongchen Digtal Technology Apparatus Co., Ltd.) in air atmosphere and at room temperature to analyze the wettability of the electrolyte. The porosity of the membranes was examined by measuring the weight of the dry membrane and n-butanol in a saturated membrane, which was immersed in the n-butanol for 12 h. Then, the porosity of the membrane was calculated according to the Eq. (1).
The dimension stability of a membrane at evaluated temperature was investigated by treating it at different temperatures for 1 h. The thermal shrinkage rate was determined by measuring the dimensional change before and after exposing a membrane at different temperatures for 1 h and the shrinkage was calculated according to the Eq. (4):
Porosity = (m1/ρ1)/(m1/ρ1 + m 0 /ρ0 ) × 100%
Shrinkage(%) =
(1)
where m0 and m1 are the weight of a dry membrane and n-butanol respectively. The ρ0 and ρ1 represent the density of polymer and nbutanol. The electrolyte uptake (U) of a membrane was analyzed by measuring the weight of the dry membrane and the saturated membrane after immersing the membrane in the electrolyte (1 M LiPF6 in ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (EC/DMC/EMC, 1:1:1 vol)) for 12 h. The electrolyte uptake (U) was calculated according to the Eq. (2):
U=
Me − Mo ×100% Mo
2.5. Electrochemical performance of batteries Electrochemical stability of a membrane was measured by the linear sweep voltammetry (LSV) using CR2016 coin cell, with the metallic lithium and SS as the reference and counter electrode respectively. The LSV was conducted on an electrochemical system (PARSTAT 2273, Princeton Applied Research, USA) over voltage range from 2.5 to 5.5 V with sweep rate of 0.5 mV s−1. Battery performance of a membrane was tested using CR2016 coin cell, which was assembled with a membrane as a separator, 1 M LiPF6 in EC/DMC/EMC as electrolyte, metallic lithium as the anode and LiFePO4 as the cathode in an argon-filled glove box. The cathode was made from 80 wt% LiFePO4, 10 wt% Super-p and 10 wt% poly (vinylidene fluoride) (PVDF). The composite was added to 1-methyl2-pyrrolidinone (NMP) to form a slurry, then the slurry was coated on an aluminum foil and the prepared electrode was dried overnight at 70 °C. The galvanostatic charge−discharge was performed in the voltage window of 2.0−4.2 V on a Land automatic battery tester (Wuhan, China).
(2)
2.3. Ionic conductivity measurement The bulk impedance (Rb) of a membrane was conducted on an electrochemical impedance spectroscopy (EIS) (Solartron 1287 electrochemical work station) by sandwiching an electrolyte-soaked membrane between two stainless steels (SS). Impedance spectra were recorded over a frequency range from 0.1 Hz to 106 Hz with the AC amplitude of 10 mV under open circuit potential condition. The ionic conductivity (σ ) was calculated according to the Eq. (3):
d Rb × S
(4)
where the So and S are the area of the membrane before and after heattreatment.
where the Mo and Me are the weight of the membrane before and after immersing in the electrolyte, respectively.
σ=
So − S ×100% So
3. Results and discussion A porous PEEK membrane was fabricated by a typical phase inversion process. Fig. 1 showed the SEM images of a prepared PEEK porous membranes and a PE separator. Figs. 1a and b were the low and high magnified cross-section images of the PEEK mem-
(3)
where the d and S are the thickness of the membranes and effective area of the membranes, respectively.
Fig. 1. The SEM images of PEEK membranes and PE separators. (a) Cross-section and (b) magnified cross-section images of PEEK membranes. Surface images of (c) PEEK membranes and (d) PE separators.
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PEEK membrane was lower than that of a PE separator. Consequently, the PEEK membrane possessed a higher ionic conductivity of 10.64×10−4 S cm−1 (Table 1). In addition, the dependence of the ionic conductivity of liquid electrolyte-soaked PE separator and PEEK membrane on temperature (30–70 °C) was shown in Fig. S2. And it was well fitted by Arrhenius formula:
brane. A 30 µm thick with high uniformity sponge-structured membrane was obtained. The formation of sponge-like membranes in phase inversion process was determined by thermodynamics and mass transfer properties [24,25]. Firstly, a homogeneous fluid gel-like precipitate, composed of polymer, solvent and nonsolvent throughout the whole membranes, was obtained. Then the homogeneous fluid gellike precipitate would be solidified at similar time over the entire film cross-section and formed a sponge-like structure. During the process of transition from fluid gel-like precipitate to solidification, the casting solution can be separated into two domains forming the matrix and pores in the final membranes, once the process of solidification occurred, the polymer-rich and polymer-poor phases were formed, and droplets were formed due to the initiation of the polymer-poor phase by the nucleation and growth mechanism, resulting in a solid polymer with a separated micron-sized cellular, which was filled with non-solvent. In addition, a liquid layer appeared on the surface of the casting solution once it was placed in the nonsolvent, which would lead to a solvent concentration gradient through the casting solution, and result in a low concentration near the bottom. And the viscosity of the cast solution increased with increasing polymer concentration. As a result, a relatively dense surface was formed at the bottom, where the viscosity was relatively high. The PEEK membranes showed 50– 250 nm pores in size, which was similar to commercial PE separators. The physical properties were crucial features for lithium ion batteries. The porosity and other physical properties of PEEK membranes and PE separators were shown in Table 1. The PEEK membrane exhibited 78% porosity, which was almost twice of a PE separator (40%). Such high porosity of a PEEK membrane was closely related to the unique porous microstructure in Fig. 1 formed by phase inversion process. Fig. 2 showed the photographs of electrolyte wetting behavior of the membranes by spreading test [7]. A certain amount of electrolyte was dipped onto the membranes, then the pictures were fixed immediately. The electrolyte on a PEEK membrane was spreading immediately, while the electrolyte kept as a droplet for a long time on a PE separator. Consequently, the PEEK exhibited high affinity between the PEEK chains and carbonate electrolyte, however, there was a relativity low affinity between the PE chains and carbonate electrolyte. The same phenomenon can also be observed in electrolyte contact angle experiment in Fig. 2c. The liquid electrolyte contact angle of a prepared PEEK membrane was 29°, which was much lower than that of a PE separator (53°). The affinity between the polymer chains and electrolyte was closely related to the membrane surface characteristic. The polar oxygen atoms and the carbon-oxygen double bond in PEEK polymer exhibited a relatively strong interaction with carbonate electrolyte (shown in Scheme 1). However, PE separators showed relativity lower interaction with the electrolyte due to their hydrophobic chains. The electrolyte uptake of a membrane was of high significance for a battery. Fast and uniform wetting of electrolyte through the whole membrane will facilitate the transport of ions to obtain high ionic conductivity, which was beneficial to improve rate performance of a lithium battery. The electrolyte uptake of a PEEK membrane was 251% by soaking them into electrolyte for enough time (Table 1), which was more than twice than that of a PE membrane. For comparison, a PE separator exhibited only 111% electrolyte uptake. The porosity and the affinity between membranes and electrolyte were two crucial factors that affecting the electrolyte uptake. Namely, the higher the membrane porosity, and the larger the mutual affinity of membrane and electrolyte, the higher the electrolyte uptake will be obtained. The mechanical properties of prepared PEEK membranes were shown in Fig. S1. The tensile strength of PEEK membranes was 11.99 MPa. The relativity low result were due to the much higher porosity of PEEK membranes. The ionic conductivity of the membranes was determined by electrochemical impedance spectroscopy (EIS) accurately. The x-intercept on the EIS curves, which represented the bulk resistance of the membranes, was exhibited in Fig. 3. It was obviously that the bulk resistance of a
σ = Aexp(−Ea /RT) where A is the pre-exponential factor, R is the gas constant, T is the absolute temperature and Ea is the activation energy. Ea can be calculated according to the Arrhenius formula by the slope of lines in Fig. S2. The calculated Ea values were 5.9 kJ/mol and 6.4 kJ/mol for PEEK membrane and PE separator, respectively. The result indicated that the transport of ions in electrolyte soaked PEEK membranes was easier than that in PE separators. Fig. 4 showed the thermogravimetric (TG) curves of a commercial PE separator and a prepared PEEK membrane under an atmosphere of nitrogen with temperature range from 50 to 800 °C at a rising rate of 5 °C/min. The PEEK membrane demonstrated an excellent thermal stability according to the TG curve. It indicated that no obvious weight loss below 530 °C was observed, which was well agreement with the reported results in previous literature [26]. However, for commercial PE separator, it decomposed sharply at about 460 °C. These results indicated a better thermal stability for PEEK membranes than commercial PE separators. The basic function of a separator within a battery was preventing the direct contact between a positive and a negative electrode. Otherwise, the short circuit would generate lots of heat and cause unwanted side reactions, and then further induce even a combustion or explosion. Therefore, the dimensional thermo-stability of a membrane was one of most important factors for a high power battery. We studied the dimensional thermo-stability of a commercial PE separator and a prepared PEEK membrane by treating them at certain temperature for 1 h and then measured the dimensional change (area-based) before and after heat-treatment. The results showed in Figs. 5 and 6. The PE separators exhibited a remarkable dimensional shrinkage 41.8% after storing at 130 °C for 1 h, and 100% after treating them at 150 °C (higher than the melting point of PE), namely, the PE separators melted completely at this temperature. However, for PEEK membranes, there was no obvious dimensional shrinkage until 150 °C, and they showed higher dimensional shrinkage with the increasing of temperature. More precisely, the PEEK membranes exhibited a thermal shrinkage of 37.7% at 150 °C, and 53.2% at 350 °C, without melting process occurring. These results demonstrated a better thermal stability from the view of dimension change, and it could be predicted that the PEEK membrane would show the excellent safety performance for high power batteries even under extreme conditions. Electrochemical stability of membranes, as one of the most important features for their application in battery system, was evaluated by the linear sweep voltammetry (LSV) method (Fig. 6). As can be seen, from the LSV curve of PE separators, the current onset at about 4.5 V versus Li+/Li indicated an electrochemical stability up to 4.5 V, at which oxidative decomposition of an electrolyte-soaked PE separator happened. The result was in well agreement of the reported results [27]. And the PEEK membrane was also stable until around 4.5 V. Table 1 Physical properties of PEEK membranes and PE separators.
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Sample
Thickness [um]
Porosity [%]
Electrolyte contact angle [deg]
Electrolyte uptake [%]
Ion conductivity [S cm−1, 25 °C]
PE PEEK
20 30
40 78
53 29
111 251
6.65×10−4 10.64×10−4
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Fig. 4. The thermogravimetric (TG) curves of PE separators and PEEK membranes.
Fig. 2. The wettability of PEEK membranes and PE separators with liquid electrolyte. (a) Without electrolyte, (b) with electrolyte, (c), electrolyte contact angle.
Fig. 5. Thermal shrinkage of PE separators and PEEK membranes.
Fig. 3. The electrochemical impedance spectroscopy (EIS) curves for PEEK membranes and PE separators. Fig. 6. The linear sweep voltammetry curves of PE separators and PEEK membranes in the electrolyte of 1 M LiPF6 in EC/DMC/EMC.
The cycle performance of batteries with a PE separator and a PEEK membrane at a rate of 0.5 C was shown in Fig. 7a. As can be seen, the initial discharge capacities of batteries with the PE separator and the PEEK membrane were 148.5 mA h g−1 and 147.4 mA h g−1 respectively. A notable finding was that, after 100 cycles, the discharge capacity retention of a battery with a PEEK membranes was 97.8%, which was similar with that of a PE separator (96.5%). Every cycle during the test was very stable and showed a coulombic efficiency of nearly 100% (Fig S3). Comparing to a battery with a PE membrane, a little higher discharge capacity of a battery with a PEEK membrane during the 100 charge-discharge cycles was achieved. The difference was probably due to the higher ionic conductivity of the PEEK membranes, which would favor intercalation and deintercalation of lithium ions on a cathode, and thus resulting in a higher discharge
capacity. The impedance spectra of batteries with a PE separator and a PEEK separator after the 10th and 50th cycle at 0.5 C were conducted and shown in Fig. S4. The interfacial stability was characterized by the change of impedance. The impedance change of batteries with a PEEK membrane (~75 Ω) was smaller than that of a PE separator (~85 Ω), which indicated that the enhanced interfacial stability was observed in a battery with a PEEK membrane. The result may be due to the better wettability of a PEEK membrane with the liquid electrolyte, then resulting in more benign contact of the PEEK membranes with the electrode, and thus suppressed the growth of the solid electrolyte interface (SEI) film. 129
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Fig. 7. The electrochemical performance of Li/separator/LiFePO4 batteries. (a) The discharge capacity and (b) the rate performance of batteries assembled with PE separators and PEEK membranes. The discharge curves of batteries with (c) PEEK membranes and (d) PE separators at the rate of 0.2 C, 0.5 C, 1 C, 2 C, 5 C.
82.5% of 0.2 C compared with that of a battery with a PE separator (79.3%). The discharge capacity of a battery with a PEEK membrane was still slightly higher than the PE separators even at rate of 10 C (Fig. S5). A better capacity retention was resulted from the stronger affinity of PEEK membranes with carbonate electrolyte combining with lower surface porosity of PEEK membranes. To evaluate the electrochemical properties of the PEEK membranes at high temperature, the rate performance for Li /LiFePO4 battery with a PEEK membrane, which was treated at 350 °C for 1 h, was detected.
The rate performance was also of great significance for batteries. The discharge capacity at various rates was shown in Figs. 7b, c, and d. The discharge capacity of a battery with a PEEK membrane was 150.5, 148.3 143.5, 135.7, 124.1 mA h g−1 at rate of 0.2, 0.5, 1, 2, 5 C respectively. While, for a battery with a PE separator, the discharge capacity was 152.4, 145.2, 138.8, 131.2, 120.9 mA h g−1 respectively. As can be seen, the rate performance of battery with a PEEK membrane was better than that with a PE separator. At high current density of 5 C, a battery with a PEEK membrane showed higher capacity retention of
Fig. 8. The electrochemical performance of Li/LiFePO4 batteries with PEEK membranes after treatment at 350 °C for 1 h. (a) The rate performance of batteries and (b) the discharge curves of batteries at the rate of 0.2 C, 0.5 C, 1 C, 2 C, 5 C.
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From Fig. 8, the discharge capacity of the battery was 152.4, 151, 141.4, 130.9, 116.2 mA h g−1 at rate of 0.2 C, 0.5 C, 1 C, 2 C, 5 C respectively. At low rate, the battery showed capacity retention of 99.0% at 0.5 C, which was similar with that of batteries with a pristine PEEK membrane without heat-treatment. While, at a high rate discharge condition, it exhibited capacity retention of 92.8%, 85.9%, 76.2% at 1 C, 2 C, 5 C, respectively, which was a bit lower than that of a PEEK membrane without heat-treatment (95.3%, 90.2%, 82.5%). The electrochemical performance was affected rarely by the process of heat-treatment. Combining the low cost (around 1 Dollar/m2) and high electrochemical performance, PEEK porous membranes exhibited very promising prospect in lithium ion battery application.
[6] [7]
[8]
[9]
[10]
[11]
4. Conclusions [12]
In summary, a sponge-like PEEK porous membrane was fabricated by using phase inversion method. Comparing to a commercial polyolefin separator, the prepared PEEK membrane exhibited super high thermal stability, no fusion deformation occurred even at 350 °C, which improved the thermal stability of batteries greatly. In addition, the PEEK porous membranes showed high electrolyte wettability, with 251% electrolyte uptake, which facilitated the transfer of lithium ions, and resulted in a better rate performance. In addition, the PEEK membranes showed an excellent rate performance after heat-treatment at 350 °C. Moreover PEEK membranes could be easily extended to other electrochemical energy storage systems like sodium batteries, supercapacitors, and lithium-sulfur batteries etc.
[13]
[14]
[15]
[16]
[17]
Acknowledgments [18]
This research is supported by the financial support from the China Natural Science Foundation (Grant nos. 21476224), Key project of Frontier Science, Chinese Academy of Sciences CAS (QYZDB-SSWJSC032) and National Youth Top-notch Talent Program.
[19]
[20]
Appendix A. Supporting information [21]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2017.02.027.
[22]
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Poly (ether ether ketone) (PEEK) porous membranes with super high thermal stability and high rate capability for lithium-ion batteries
MARK
⁎
Dan Lia,b, Dingqin Shia, Kai Fenga, Xianfeng Lia,c, , Huamin Zhanga,c,⁎⁎ a b c
Division of energy storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100039, China Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Lithium-ion battery Poly (ether ether ketone) Porous membrane Thermal stability
Lithium-ion batteries are receiving intensive interest due to their promising prospect for electric vehicles. However, the safety issues and low rate capability of lithium-ion batteries limited their further development due to the poor dimensional thermo-stability and low electrolyte wettability of commercial polyolefin separators. Herein, we report a sponge-like porous poly (ether-ether-ketone) (PEEK) membrane with super high thermal stability and good rate capability for lithium ion batteries. The porous PEEK membrane showed no fusion deformation even at temperature of 350 °C, showing excellent thermal stability. An electrolyte uptake of 251% was obtained for a porous PEEK membrane, which was more than twice than that of a commercial PE separator, showing very good electrolyte wettability. As a result, the lithium-ion battery with a porous PEEK membrane showed a discharge capacity of 124.1 mA h g−1 at 5 C, showing good rate performance. The battery performance of a PEEK membrane after treating at 350 °C changed rarely. The PEEK membranes provide more options for cost-effective high power battery separators.
1. Introduction Lithium-ion batteries have attracted great interest in electric vehicles (EV) and hybrid electric vehicles (HEV) due to their higher energy density, no memory effect and environmental friendliness comparing with conventional secondary batteries [1–6]. However, the further promotion of EV and HEV is highly hindered by safety issues of lithium ion batteries. The safety issue of a lithium battery is mainly induced by the poor dimensional thermo-stability of currently used separators [7,8]. In a lithium battery, a separator acts as an interlayer to prevent the direct contact of positive electrode and negative electrode, while providing channels for transferring lithium ions. Consequently, an ideal separator should be resistant to chemical and electrochemical interaction from electrodes and electrolyte, and possess high dimensional thermo-stability to prevent the safety issues [9,10], which caused mostly by overcharge, short circuit, thermal runaway and even explosion. Currently, the commercial separators for lithium-ion batteries are porous polyolefin membranes, such as polyethylene (PE), polypropylene (PP) and their composite for their low cost, excellent chemical and electrochemical stability, high mechanical strength, and fair microstructure [11,12]. However, the poor wettability with electrolyte and ⁎
poor dimensional thermo-stability of polyolefin separators further hinder their application in high power battery. The poor thermal stability of polyolefin separators is derived from the intrinsic low melting point of polymers (PE/130 °C, PP/160 °C) [13,14], causing some security issues for lithium-ion batteries at elevated temperature. The poor wettability with electrolyte of polyolefin separators is due to the hydrophobicity and low porosity of polyolefin membranes, leading to low ionic conductivity and poor rate performance [11,15]. Therefore, it is in urgent need of developing a separator with high dimensional thermo-stability and good wettability for high power lithium-ion batteries. Up to now, many efforts have been introduced to improve the thermal stability of the current separators. Surface modification, such as introducing inorganic (SiO2 [16], Al2O3 [17], TiO2 [18]) or organic (polyimide [19], polyarylate [20]) materials to substrate membranes was widely employed to enhance the thermal stability of separators. In addition, constructing a heat-resistant skeleton can also solve this issue, such as porous polyether imide separators [8] and polyoxyzole nanofiber membranes [21]. To improve the electrolyte wettability, extensive efforts have been taken to fabricate a membrane with polar surface by introducing inorganic ceramic [18,22] or polymer layer [15] on commercial polyolefin separator, thus the wettability of the mem-
Corresponding author at: Division of energy storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China. Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (H. Zhang).
⁎⁎
http://dx.doi.org/10.1016/j.memsci.2017.02.027 Received 22 October 2016; Received in revised form 14 February 2017; Accepted 19 February 2017 Available online 21 February 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
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Nomenclature PEEK EV HEV PE PP TG LSV NMP SEM PVDF EIS SS mo m1 ρ0
ρ1 U Mo
Poly (ether ether ketone) Electric vehicles Hybrid electric vehicles Polyethylene Polypropylene Thermogravimetric Linear sweep voltammetry 1-methyl-2-pyrrolidinone Scanning electronic microscopy poly (vinylidene fluoride) Electrochemical impedance spectroscopy Stainless steels Weight of dry membrane (g) Weight of n-butanol (g) Density of polymer (g/cm3)
Me Rb σ d S So S A R T Ea SEI
Density of n-butanol (g/cm3) Electrolyte uptake (%) Weight of the membrane before immersing in the electrolyte (g) Weight of the membrane after immersing in the electrolyte (g) Bulk impedance (Ω) Ionic conductivity (S cm−1) Thickness of the membranes (μm) Effective area of the membranes (cm2) Area of the membrane before heat-treatment (cm2) Area of the membrane after heat-treatment (cm2) Pre-exponential factor Gas constant Absolute temperature Activation energy Solid electrolyte interface
2. Experimental
brane can be highly enhanced due to the strong interaction between polar surfaces of prepared membrane with the carbonate electrolyte. Nevertheless, these strategies also brought many adverse effects on their applicability for lithium-ion battery due to the relative expensive materials and multi-step manufacture process. Poly (ether-ether-ketone) (PEEK) is one kind of semi-crystalline polymer, it owns characteristics of high chemical stability and thermal stability. Thus it could be used as high temperature structural materials and electrical insulating materials. In addition, due to the fact that the polar oxygen atoms and the carbon-oxygen double bond in PEEK polymer may have strong interaction with the carbonate electrolyte, the PEEK based membranes are expected to have high thermal stability and good wettability with electrolyte, which can further resolve the safety issues and improve the power density of lithium-ion batteries. What's more, it is worth noting that the cost of PEEK polymer is relatively low. Even though, the poor solubility of PEEK polymers limited their application in membranes field. Quite recently, Andrew et al. [23] first reported the application of PEEK porous membranes in organic solvent filtration, where, this membranes were proved to be prepared by a typical phase inversion method and the method was proved to be easily scaled up. Therefore, in this paper, a porous PEEK membrane with symmetric sponge-like structure was designed and prepared by phase inversion method (Scheme 1) for the first time, we investigated their application in lithium ion battery application. The obtained sponge-like porous PEEK membranes have superior wettability with electrolyte, owning 251% electrolyte uptake and high thermal stability. Our work will provide more options for high power battery separators.
2.1. Fabrication of membranes The sponge-like porous membranes were fabricated through phase inversion method. Firstly, a proper amount of PEEK (Changchun Jida plastic engineering research co., Ltd, China) was dissolved in a mixture solution of sulfuric acid and methyl sulfonic acid (mass ratio of sulfuric acid/methyl sulfonic acid was 1/10) to prepare a 12 wt% casting solution at room temperature. Then the solution was cast onto a clean glass plate using a doctor blade (Elcometer 3545 adjustable Bird Coater, Scraper, Elcometer 3545/8), then transferred to water for about 10 min. Afterword, the prepared membranes were completely immersed in water for 48 h. Finally, the prepared membranes were dried in a vacuum oven at 60 °C for 24 h. The average thickness of the membrane was 30 ± 2 µm. 2.2. Physical characterization The surface and cross-section morphologies of the membranes including the prepared PEEK membrane and the commercial PE separator (Dongguan Saidiou Electronic Technology Co. Ltd.) were observed with scanning electron microscope (JSM-7800F). Cross section samples were obtained by breaking the membranes in liquid nitrogen. The thermal stability of a membrane was investigated by thermogravimetric analysis (TG) using a thermal analyzer thermogravimetry analyses (Pyris-Elmer), which was operated at a heating rate of 5 °C/min from 50 °C to 800 °C under nitrogen gas flow. The contact angle measurement of a membrane was conducted on a
Scheme 1. Schematic principle of membranes with sponge-like structure.
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2.4. Thermal shrinkage measurement
drop shape analysis system (Powereach JC2000C1, Shanghai Zhongchen Digtal Technology Apparatus Co., Ltd.) in air atmosphere and at room temperature to analyze the wettability of the electrolyte. The porosity of the membranes was examined by measuring the weight of the dry membrane and n-butanol in a saturated membrane, which was immersed in the n-butanol for 12 h. Then, the porosity of the membrane was calculated according to the Eq. (1).
The dimension stability of a membrane at evaluated temperature was investigated by treating it at different temperatures for 1 h. The thermal shrinkage rate was determined by measuring the dimensional change before and after exposing a membrane at different temperatures for 1 h and the shrinkage was calculated according to the Eq. (4):
Porosity = (m1/ρ1)/(m1/ρ1 + m 0 /ρ0 ) × 100%
Shrinkage(%) =
(1)
where m0 and m1 are the weight of a dry membrane and n-butanol respectively. The ρ0 and ρ1 represent the density of polymer and nbutanol. The electrolyte uptake (U) of a membrane was analyzed by measuring the weight of the dry membrane and the saturated membrane after immersing the membrane in the electrolyte (1 M LiPF6 in ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (EC/DMC/EMC, 1:1:1 vol)) for 12 h. The electrolyte uptake (U) was calculated according to the Eq. (2):
U=
Me − Mo ×100% Mo
2.5. Electrochemical performance of batteries Electrochemical stability of a membrane was measured by the linear sweep voltammetry (LSV) using CR2016 coin cell, with the metallic lithium and SS as the reference and counter electrode respectively. The LSV was conducted on an electrochemical system (PARSTAT 2273, Princeton Applied Research, USA) over voltage range from 2.5 to 5.5 V with sweep rate of 0.5 mV s−1. Battery performance of a membrane was tested using CR2016 coin cell, which was assembled with a membrane as a separator, 1 M LiPF6 in EC/DMC/EMC as electrolyte, metallic lithium as the anode and LiFePO4 as the cathode in an argon-filled glove box. The cathode was made from 80 wt% LiFePO4, 10 wt% Super-p and 10 wt% poly (vinylidene fluoride) (PVDF). The composite was added to 1-methyl2-pyrrolidinone (NMP) to form a slurry, then the slurry was coated on an aluminum foil and the prepared electrode was dried overnight at 70 °C. The galvanostatic charge−discharge was performed in the voltage window of 2.0−4.2 V on a Land automatic battery tester (Wuhan, China).
(2)
2.3. Ionic conductivity measurement The bulk impedance (Rb) of a membrane was conducted on an electrochemical impedance spectroscopy (EIS) (Solartron 1287 electrochemical work station) by sandwiching an electrolyte-soaked membrane between two stainless steels (SS). Impedance spectra were recorded over a frequency range from 0.1 Hz to 106 Hz with the AC amplitude of 10 mV under open circuit potential condition. The ionic conductivity (σ ) was calculated according to the Eq. (3):
d Rb × S
(4)
where the So and S are the area of the membrane before and after heattreatment.
where the Mo and Me are the weight of the membrane before and after immersing in the electrolyte, respectively.
σ=
So − S ×100% So
3. Results and discussion A porous PEEK membrane was fabricated by a typical phase inversion process. Fig. 1 showed the SEM images of a prepared PEEK porous membranes and a PE separator. Figs. 1a and b were the low and high magnified cross-section images of the PEEK mem-
(3)
where the d and S are the thickness of the membranes and effective area of the membranes, respectively.
Fig. 1. The SEM images of PEEK membranes and PE separators. (a) Cross-section and (b) magnified cross-section images of PEEK membranes. Surface images of (c) PEEK membranes and (d) PE separators.
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PEEK membrane was lower than that of a PE separator. Consequently, the PEEK membrane possessed a higher ionic conductivity of 10.64×10−4 S cm−1 (Table 1). In addition, the dependence of the ionic conductivity of liquid electrolyte-soaked PE separator and PEEK membrane on temperature (30–70 °C) was shown in Fig. S2. And it was well fitted by Arrhenius formula:
brane. A 30 µm thick with high uniformity sponge-structured membrane was obtained. The formation of sponge-like membranes in phase inversion process was determined by thermodynamics and mass transfer properties [24,25]. Firstly, a homogeneous fluid gel-like precipitate, composed of polymer, solvent and nonsolvent throughout the whole membranes, was obtained. Then the homogeneous fluid gellike precipitate would be solidified at similar time over the entire film cross-section and formed a sponge-like structure. During the process of transition from fluid gel-like precipitate to solidification, the casting solution can be separated into two domains forming the matrix and pores in the final membranes, once the process of solidification occurred, the polymer-rich and polymer-poor phases were formed, and droplets were formed due to the initiation of the polymer-poor phase by the nucleation and growth mechanism, resulting in a solid polymer with a separated micron-sized cellular, which was filled with non-solvent. In addition, a liquid layer appeared on the surface of the casting solution once it was placed in the nonsolvent, which would lead to a solvent concentration gradient through the casting solution, and result in a low concentration near the bottom. And the viscosity of the cast solution increased with increasing polymer concentration. As a result, a relatively dense surface was formed at the bottom, where the viscosity was relatively high. The PEEK membranes showed 50– 250 nm pores in size, which was similar to commercial PE separators. The physical properties were crucial features for lithium ion batteries. The porosity and other physical properties of PEEK membranes and PE separators were shown in Table 1. The PEEK membrane exhibited 78% porosity, which was almost twice of a PE separator (40%). Such high porosity of a PEEK membrane was closely related to the unique porous microstructure in Fig. 1 formed by phase inversion process. Fig. 2 showed the photographs of electrolyte wetting behavior of the membranes by spreading test [7]. A certain amount of electrolyte was dipped onto the membranes, then the pictures were fixed immediately. The electrolyte on a PEEK membrane was spreading immediately, while the electrolyte kept as a droplet for a long time on a PE separator. Consequently, the PEEK exhibited high affinity between the PEEK chains and carbonate electrolyte, however, there was a relativity low affinity between the PE chains and carbonate electrolyte. The same phenomenon can also be observed in electrolyte contact angle experiment in Fig. 2c. The liquid electrolyte contact angle of a prepared PEEK membrane was 29°, which was much lower than that of a PE separator (53°). The affinity between the polymer chains and electrolyte was closely related to the membrane surface characteristic. The polar oxygen atoms and the carbon-oxygen double bond in PEEK polymer exhibited a relatively strong interaction with carbonate electrolyte (shown in Scheme 1). However, PE separators showed relativity lower interaction with the electrolyte due to their hydrophobic chains. The electrolyte uptake of a membrane was of high significance for a battery. Fast and uniform wetting of electrolyte through the whole membrane will facilitate the transport of ions to obtain high ionic conductivity, which was beneficial to improve rate performance of a lithium battery. The electrolyte uptake of a PEEK membrane was 251% by soaking them into electrolyte for enough time (Table 1), which was more than twice than that of a PE membrane. For comparison, a PE separator exhibited only 111% electrolyte uptake. The porosity and the affinity between membranes and electrolyte were two crucial factors that affecting the electrolyte uptake. Namely, the higher the membrane porosity, and the larger the mutual affinity of membrane and electrolyte, the higher the electrolyte uptake will be obtained. The mechanical properties of prepared PEEK membranes were shown in Fig. S1. The tensile strength of PEEK membranes was 11.99 MPa. The relativity low result were due to the much higher porosity of PEEK membranes. The ionic conductivity of the membranes was determined by electrochemical impedance spectroscopy (EIS) accurately. The x-intercept on the EIS curves, which represented the bulk resistance of the membranes, was exhibited in Fig. 3. It was obviously that the bulk resistance of a
σ = Aexp(−Ea /RT) where A is the pre-exponential factor, R is the gas constant, T is the absolute temperature and Ea is the activation energy. Ea can be calculated according to the Arrhenius formula by the slope of lines in Fig. S2. The calculated Ea values were 5.9 kJ/mol and 6.4 kJ/mol for PEEK membrane and PE separator, respectively. The result indicated that the transport of ions in electrolyte soaked PEEK membranes was easier than that in PE separators. Fig. 4 showed the thermogravimetric (TG) curves of a commercial PE separator and a prepared PEEK membrane under an atmosphere of nitrogen with temperature range from 50 to 800 °C at a rising rate of 5 °C/min. The PEEK membrane demonstrated an excellent thermal stability according to the TG curve. It indicated that no obvious weight loss below 530 °C was observed, which was well agreement with the reported results in previous literature [26]. However, for commercial PE separator, it decomposed sharply at about 460 °C. These results indicated a better thermal stability for PEEK membranes than commercial PE separators. The basic function of a separator within a battery was preventing the direct contact between a positive and a negative electrode. Otherwise, the short circuit would generate lots of heat and cause unwanted side reactions, and then further induce even a combustion or explosion. Therefore, the dimensional thermo-stability of a membrane was one of most important factors for a high power battery. We studied the dimensional thermo-stability of a commercial PE separator and a prepared PEEK membrane by treating them at certain temperature for 1 h and then measured the dimensional change (area-based) before and after heat-treatment. The results showed in Figs. 5 and 6. The PE separators exhibited a remarkable dimensional shrinkage 41.8% after storing at 130 °C for 1 h, and 100% after treating them at 150 °C (higher than the melting point of PE), namely, the PE separators melted completely at this temperature. However, for PEEK membranes, there was no obvious dimensional shrinkage until 150 °C, and they showed higher dimensional shrinkage with the increasing of temperature. More precisely, the PEEK membranes exhibited a thermal shrinkage of 37.7% at 150 °C, and 53.2% at 350 °C, without melting process occurring. These results demonstrated a better thermal stability from the view of dimension change, and it could be predicted that the PEEK membrane would show the excellent safety performance for high power batteries even under extreme conditions. Electrochemical stability of membranes, as one of the most important features for their application in battery system, was evaluated by the linear sweep voltammetry (LSV) method (Fig. 6). As can be seen, from the LSV curve of PE separators, the current onset at about 4.5 V versus Li+/Li indicated an electrochemical stability up to 4.5 V, at which oxidative decomposition of an electrolyte-soaked PE separator happened. The result was in well agreement of the reported results [27]. And the PEEK membrane was also stable until around 4.5 V. Table 1 Physical properties of PEEK membranes and PE separators.
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Sample
Thickness [um]
Porosity [%]
Electrolyte contact angle [deg]
Electrolyte uptake [%]
Ion conductivity [S cm−1, 25 °C]
PE PEEK
20 30
40 78
53 29
111 251
6.65×10−4 10.64×10−4
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Fig. 4. The thermogravimetric (TG) curves of PE separators and PEEK membranes.
Fig. 2. The wettability of PEEK membranes and PE separators with liquid electrolyte. (a) Without electrolyte, (b) with electrolyte, (c), electrolyte contact angle.
Fig. 5. Thermal shrinkage of PE separators and PEEK membranes.
Fig. 3. The electrochemical impedance spectroscopy (EIS) curves for PEEK membranes and PE separators. Fig. 6. The linear sweep voltammetry curves of PE separators and PEEK membranes in the electrolyte of 1 M LiPF6 in EC/DMC/EMC.
The cycle performance of batteries with a PE separator and a PEEK membrane at a rate of 0.5 C was shown in Fig. 7a. As can be seen, the initial discharge capacities of batteries with the PE separator and the PEEK membrane were 148.5 mA h g−1 and 147.4 mA h g−1 respectively. A notable finding was that, after 100 cycles, the discharge capacity retention of a battery with a PEEK membranes was 97.8%, which was similar with that of a PE separator (96.5%). Every cycle during the test was very stable and showed a coulombic efficiency of nearly 100% (Fig S3). Comparing to a battery with a PE membrane, a little higher discharge capacity of a battery with a PEEK membrane during the 100 charge-discharge cycles was achieved. The difference was probably due to the higher ionic conductivity of the PEEK membranes, which would favor intercalation and deintercalation of lithium ions on a cathode, and thus resulting in a higher discharge
capacity. The impedance spectra of batteries with a PE separator and a PEEK separator after the 10th and 50th cycle at 0.5 C were conducted and shown in Fig. S4. The interfacial stability was characterized by the change of impedance. The impedance change of batteries with a PEEK membrane (~75 Ω) was smaller than that of a PE separator (~85 Ω), which indicated that the enhanced interfacial stability was observed in a battery with a PEEK membrane. The result may be due to the better wettability of a PEEK membrane with the liquid electrolyte, then resulting in more benign contact of the PEEK membranes with the electrode, and thus suppressed the growth of the solid electrolyte interface (SEI) film. 129
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Fig. 7. The electrochemical performance of Li/separator/LiFePO4 batteries. (a) The discharge capacity and (b) the rate performance of batteries assembled with PE separators and PEEK membranes. The discharge curves of batteries with (c) PEEK membranes and (d) PE separators at the rate of 0.2 C, 0.5 C, 1 C, 2 C, 5 C.
82.5% of 0.2 C compared with that of a battery with a PE separator (79.3%). The discharge capacity of a battery with a PEEK membrane was still slightly higher than the PE separators even at rate of 10 C (Fig. S5). A better capacity retention was resulted from the stronger affinity of PEEK membranes with carbonate electrolyte combining with lower surface porosity of PEEK membranes. To evaluate the electrochemical properties of the PEEK membranes at high temperature, the rate performance for Li /LiFePO4 battery with a PEEK membrane, which was treated at 350 °C for 1 h, was detected.
The rate performance was also of great significance for batteries. The discharge capacity at various rates was shown in Figs. 7b, c, and d. The discharge capacity of a battery with a PEEK membrane was 150.5, 148.3 143.5, 135.7, 124.1 mA h g−1 at rate of 0.2, 0.5, 1, 2, 5 C respectively. While, for a battery with a PE separator, the discharge capacity was 152.4, 145.2, 138.8, 131.2, 120.9 mA h g−1 respectively. As can be seen, the rate performance of battery with a PEEK membrane was better than that with a PE separator. At high current density of 5 C, a battery with a PEEK membrane showed higher capacity retention of
Fig. 8. The electrochemical performance of Li/LiFePO4 batteries with PEEK membranes after treatment at 350 °C for 1 h. (a) The rate performance of batteries and (b) the discharge curves of batteries at the rate of 0.2 C, 0.5 C, 1 C, 2 C, 5 C.
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From Fig. 8, the discharge capacity of the battery was 152.4, 151, 141.4, 130.9, 116.2 mA h g−1 at rate of 0.2 C, 0.5 C, 1 C, 2 C, 5 C respectively. At low rate, the battery showed capacity retention of 99.0% at 0.5 C, which was similar with that of batteries with a pristine PEEK membrane without heat-treatment. While, at a high rate discharge condition, it exhibited capacity retention of 92.8%, 85.9%, 76.2% at 1 C, 2 C, 5 C, respectively, which was a bit lower than that of a PEEK membrane without heat-treatment (95.3%, 90.2%, 82.5%). The electrochemical performance was affected rarely by the process of heat-treatment. Combining the low cost (around 1 Dollar/m2) and high electrochemical performance, PEEK porous membranes exhibited very promising prospect in lithium ion battery application.
[6] [7]
[8]
[9]
[10]
[11]
4. Conclusions [12]
In summary, a sponge-like PEEK porous membrane was fabricated by using phase inversion method. Comparing to a commercial polyolefin separator, the prepared PEEK membrane exhibited super high thermal stability, no fusion deformation occurred even at 350 °C, which improved the thermal stability of batteries greatly. In addition, the PEEK porous membranes showed high electrolyte wettability, with 251% electrolyte uptake, which facilitated the transfer of lithium ions, and resulted in a better rate performance. In addition, the PEEK membranes showed an excellent rate performance after heat-treatment at 350 °C. Moreover PEEK membranes could be easily extended to other electrochemical energy storage systems like sodium batteries, supercapacitors, and lithium-sulfur batteries etc.
[13]
[14]
[15]
[16]
[17]
Acknowledgments [18]
This research is supported by the financial support from the China Natural Science Foundation (Grant nos. 21476224), Key project of Frontier Science, Chinese Academy of Sciences CAS (QYZDB-SSWJSC032) and National Youth Top-notch Talent Program.
[19]
[20]
Appendix A. Supporting information [21]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2017.02.027.
[22]
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