Surface modification of polypropylene battery separator by direct fluorination with different gas components

Applied Surface Science 290 (2014) 137–141

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Surface modification of polypropylene battery separator by direct fluorination with different gas components Baoyin Li, Jie Gao, Xu Wang, Cong Fan, Huina Wang, Xiangyang Liu ∗ State Key Laboratory of Polymer Material and Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, PR China

a r t i c l e

i n f o

Article history: Received 22 June 2013 Received in revised form 13 August 2013 Accepted 4 November 2013 Available online 13 November 2013 Keywords: Polypropylene Separator Direct fluorination Alkali absorption ratio

a b s t r a c t Improvement in hydrophilicity of polypropylene (PP) separator and its stability is essential for enhancing the comprehensive performance of battery. In this study, the PP separators were surface modified by direct fluorination with F2 /N2 and F2 /O2 /N2 gas atmosphere. The alkali absorption ratios (AARs) of these two kinds of fluorinated separators are 302.7% and 418.4%, respectively, which is about nine and twelve times than that of the virgin PP separator. At the same time, the AARs of the fluorinated separators stored for 90 days at ambient temperature in air environment still remain. The surface energy of PP separators is increased from 37.8 mN/m to 47.7 mN/m and 48.9 mN/m determined by contact angle measurement after direct fluorination. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection infrared spectroscopy (ATR-FTIR) results indicate that polar groups, such as C O(OH) and C Fx , are introduced into the polymeric structures of the two fluorinated separator surfaces. Larger quantity of polar groups, especially C O(OH), are introduced on separator surface by the F2 /O2 /N2 modified route, which results in the difference of the AARs and behavior of alkali absorption. Scanning electron microscope (SEM) demonstrates that the size and shape of micropores of PP separators remain almost unchanged after direct fluorination. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Due to good mechanical properties and excellent chemical stability as well as a high safety to shut down when an unusually large amount of heat is generated, polypropylene (PP) microporous membrane has been widely used as battery separator material, which can ensure the security of battery in daily use [1–3]. However, because of its non-polar and hydrophobic surface resulting in poor compatibility with liquid electrolytes and low ionic conductivity, the battery capacity and cycle-life are seriously affected [4,5]. It is an urgent issue that how to improve wettability of PP separator and its stability to accommodate and retain liquid electrolytes. The capacity of absorption and retention of liquid electrolytes is generally estimated by alkali absorption ratio (AAR) [6,7]. Currently, a variety of modification methods have been focused to overcome the problem, such as corona treatment [8,9], sulfonation treatment [10,11], radiation-induced grafting treatment [12–14], plasma treatment [15–17], etc. Gineste et al. [12] investigated that radiation induced grafting of acrylic acid and diethyleneglycol dimethacrylate monomers onto the surface of the monolayer PP microporous membrane and influence of grafted

∗ Corresponding author. Tel.: +86 28 85403948; fax: +86 28 85405138. E-mail address: [email protected] (X. Liu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.015

monomer content on the hydrophilicity was investigated. Liu et al. [16] discussed the surface modification of PP non-woven fabric for battery separator carried out by low-temperature plasma treatment technology, which contributed to hydrophilic polar groups that resulted in an increase in surface energy and alkali absorption. However, radiation-induced grafting leads to increased expansion of battery separator and reduced permeability while plasma treatment deteriorates stability of the resultant hydrophilic separator. Compared with conventional chemical and physical methods, direct fluorination is a gas-phase chemical reaction of gaseous F2 and its mixtures with a polymer surface, which is an effective chemical method to modify and control physicochemical surface properties of polymers. Because F2 possesses strong electronegativity and high chemical reaction capability, when in contact with F2 , polar groups, such as C F covalent bond, etc., are introduced onto the polymer surface by free-radical substitution reaction [18–20]. In this study, two different gas atmosphere for direct fluorination were used. The fluorine–nitrogen or fluorine–oxygen–nitrogen mixture were employed to modify surface of PP separator. The effect of fluorination modification routes on AAR was observed. The correlation between AAR and chemical structure of PP separator has been studied by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance infrared spectroscopy (ATR-FTIR) to reveal the reasons for produced activation effect and influence of different gas component fluorination. In addition, contact angles, surface

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energy and scanning electron microscope (SEM) were utilized to characterize the surface polarity and morphology of PP separator before and after direct fluorination. 2. Experimental 2.1. Materials Polypropylene (PP) microporous membrane for battery separator with the thickness of 30 ␮m and porosity of 32% was obtained from Shenzhen Xingyuan Material Technology Co., Ltd. The PP separator possesses elliptical micropores, which are arranged in the same direction shown in the following figure. Major of the ellipse ranges from 0.06 to 0.45 ␮m, and minor axes of the ellipse ranges from 0 to 0.18 ␮m. The F2 /N2 (10 vol% for F2 ) mixed gases was supplied by China Nuclear Honghua Specialty Gases Co., Ltd. Potassium hydroxide, the analytical grade, was purchased from Chongqing East Sichuan Chemical Co., Ltd. (Chongqing, China). 2.2. Experimental methods 2.2.1. Material purifying Prior to direct fluorination, the samples were placed in acetone and ultrasonicated to remove oil stain and other impurities on the surface. Later samples were washed with ethanol and dried in a vacuum oven at 40 ◦ C for 12 h. 2.2.2. Direct fluorination treatment The treatment of samples by direct fluorination in a closed stainless steel reactor was carried out at room temperature. Samples were divided into two groups, and the direct fluorination process was described as follows. Route one: the air in the closed reactor was removed and replaced by nitrogen for three cycles, and then F2 /N2 (10 vol% for F2 ) mixture gas was passed into the chamber at room temperature for 10 min, and the mixture gases pressure was controlled at 10 kPa. After the completion of reaction, the gas in the reactor was pumped out. The treated sample was taken out from the reactor, which was denoted as F-PP. Route two: prior to passing F2 /N2 gases into chamber, O2 at 5 kPa was purged into the reactor. The other operations were the same as route one, the obtained sample was denoted as FO-PP. The virgin sample was denoted as U-PP. 2.3. AAR determination The AAR of the battery separator was determinated according to Chinese SJ/T10171.7-91 for AAR of alkaline battery separator. First, the mass fraction of 40% analytical grade potassium hydroxide solution was prepared. Samples of size 40 mm × 40 mm were left in 40% KOH solution for 4hr. The samples were taken out from the alkali solution, and weighed after 30 ± 2s on electronic balance, which could weigh up to 0.0001 g (each AAR value was the average of more than five successful measurements). The AAR (%) was calculated using the following equation: AAR (%) =

G2 − G1 × 100 G1

where G1 and G2 are the mass of the sample before and after immersion in the alkali solution, respectively. 2.4. Characterization Contact angles analysis were carried out on the Germany Krüss 100 type surface tension meter with deionized water and diiodomethane at room temperature, then surface energy was

Fig. 1. The AARs of PP separator before and after direct fluorination.

calculated according to a conventional method [21]. XPS measurement was performed on a Kratos ASAM 800 spectrometer (Kratos Analytical Ltd., U.K.) with nonmonochromatic Al K␣ (1486.6 eV) X-ray source (a voltage of 15 kV, a wattage of 250 W) radiation, and the vacuum chamber pressure was controlled at a range of 10−6 –10−7 Pa. Attenuated total reflection infrared spectroscopy (ATR-FTIR) was operated on the Nicolet 560 infrared spectrometer employing attenuated total reflection mode for observing the chemical changes on the samples. Scanning electron microscope (SEM) was operated with FEI Inspect F (FEI company, USA) at 20 kV and the magnification was set at 40,000×. 3. Results and discussion 3.1. AARs The AARs of PP separators before and after direct fluorination are shown in Fig. 1. The AAR of the U-PP is only 35.2% for its non-polar hydrophobic structure, while that of F-PP and FO-PP are increased to 302.7% and 418.4%, respectively, which are greatly enhanced through the two kinds of gas atomsphere direct fluorination routes. Besides, the AAR of FO-PP is about 38.2% more than that of F-PP, which indicates that the direct fluorination with oxygen employed is more efficient in improving the AAR. After storage for 90 days at ambient temperature in air environment, the AARs of both modified separators are 312.4% and 425.2%, respectively, which provides evidence that the fluorinated separators have excellent storage stability. The relevant literatures show that the liquid electrolyte absorption capacity of PP microporous membrane and its stability are closely related to the surface morphology and chemical structure [12,16,22]. 3.2. The contact angles and surface energy The photographs of static contact angle of U-PP, F-PP and FOPP with deionized water are presented in Fig. 2. As shown in Fig. 2, contact angles of F-PP and FO-PP with deionized water are significantly reduced, decreasing from 110◦ to 79.4◦ and 49.7◦ , respectively, compared with that of U-PP. It can be found that the hydrophilicity of the fluorinated PP separators is increased. Besides, the PP separator modified by direct fluorination with oxygen has a smaller contact angle. The surface energy of U-PP, F-PP and FOPP is tabulated in Table 1. The surface energy of PP separator is significantly improved by two different direct fluorination routes, which is enhanced from 37.8 mN/m to 47.7 mN/m and 48.9 mN/m, p respectively. Polar component (s ) of F-PP and FO-PP is increased

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Fig. 2. The photographs of static contact angle of PP separator with deionized water before and after direct fluorination: (a) U-PP; (b) F-PP(c); FO-PP.

Table 1 Surface energy of PP battery separator. p

Sample

s (mN/m)

sd (mN/m)

Surface energy (mN/m)

U-PP F-PP FO-PP

5.1 8.9 12.5

32.7 38.8 36.4

37.8 47.7 48.9

to 8.9 mN/m and 12.5 mN/m, about 74.5% and 145.1% improvement compared with that of the U-PP. The dispersion component (sd ) of F-PP and FO-PP is only about 18.7% and 11.3% improvement compared with U-PP shown in Table 1. p The s of the surface energy of a solid reflects the intermolecular dipole interaction and hydrogen bond on the surface, while sd mainly characterizes the change in surface morphology of a solid p [4,21,23,24]. s of the surface energy for PP separator is significantly enhanced through the two direct fluorination modification routes, indicating that the direct fluorination modification can indeed increases the polarity of PP separator surface. The increase p in the s of FO-PP is greater than that of F-PP, which reveals that the surface of PP separator modified by direct fluorination with fluorine–oxygen atmosphere has a higher polarity. In addition, both direct fluorination modification routes can slightly increase sd of PP separator, indicating that the surface morphology of PP separator changes slightly after direct fluorination, which is consistent with SEM result that surface morphology of fluorinated PP separators is almost the same to the virgin PP separator. The contact angle and surface energy analyses mentioned above demonstrate that the improvement in AAR of the fluorinated PP separator is mainly attributed to polar groups on the surface caused by direct fluorination instead of the change in surface morphology.

3.3. ATR-FTIR The ATR-FTIR spectra of U-PP, F-PP and FO-PP are shown in Fig. 3. Compared with U-PP, some new absorption peaks in both F-PP or FO-PP are observed at near 3400 cm−1 , 1720 cm−1 , and 1180 cm−1 , corresponding to OH, C O and C Fx stretching vibration absorption peaks, respectively [18]. The absorption peak at 1720 cm−1 is further attributed to COOH group. Besides, there is a band at 1839 cm−1 in spectra of FO-PP in the Fig. 3, attributed to C( O)F group stretching vibration absorption peaks, which are transformed into COOH groups by hydrolysis of C( O)F group: COF + H2 O → COOH + HF [25,26]. Due to the same thickness of all samples, C O and C Fx absorption peaks area of F-PP is calculated as 76.6 and 725.4, and that of FO-PP is 474.4 and 646.5, which roughly shows that more polar groups are bonded on the surface of PP separator through direct fluorination with fluorine–oxygen–nitrogen gas atmosphere. Besides, it is found that the C Fx absorption peaks in FO-PP is wider with respect to the F-PP, which is caused by more kinds of chemical environments around carbon–fluorine bond.

Fig. 3. FTIR spectra of PP separators before and after direct fluorination.

3.4. XPS analysis XPS characterization was performed to further study the surface chemical structure of PP separator treated by the only F2 /N2 gases or by F2 /O2 /N2 mixtures. The surface elemental analysis of PP separator before and after direct fluorination is given in Table 2. As shown in Table 2, the content of C decreases from 99.2% to 68.7% and 62.5%, respectively, after fluorination. Meanwhile, the content of F is 28.6% and 19.7%, and the content of O is increased to 2.6% and 17.8% after fluorination, corresponding to F-PP and FO-PP, respectively. Oxygen atoms introduced on the surface of F-PP is mainly derived from F2 /N2 mixed gases containing a trace amount of oxygen. The F/C and O/C ratios of the F-PP are 0.416 and 0.038, respectively, while that of FO-PP are 0.315 and 0.285, respectively, which indicates a large number of fluorine-containing and oxygen-containing polar groups are bonded on the surface of F-PP and FO-PP by direct fluorination compared with the U-PP. From the Table 2, the F/C ratio of F-PP is about 45% more than the FO-PP, but the O/C ratio of FO-PP is about seven times than that of F-PP. It can be found that FOPP possesses more oxygen atoms and higher content of total polar atoms. To further investigate the relationship between the bonded form of fluorine and oxygen atoms on the surface of PP separator and AAR, Fig. 4a–c presents the U-PP, F-PP and FO-PP C1s spectra, respectively. Only strong C C single absorption peak Table 2 The surface element content of PP battery separator. PP battery separator

U-PP F-PP FO-PP

Element content (%)

Element ratio

C

F

O

F/C

O/C

99.2 68.7 62.5

0 28.6 19.7

0.8 2.6 17.8

0 0.416 0.315

0.008 0.038 0.285

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Fig. 4. C1s spectra of polypropylene separators before and after direct fluorination.

at 284.80 eV in the U-PP C1s spectrum is observed in Fig. 4a. The F-PP and FO-PP C1s spectra are fitted to six peaks with binding energy near about 284.8 eV, 285.7 eV, 286.6, 287.9 eV, 289.1 eV and 291.2 eV, which are attributed to C C , C C O, C CFx , C O, C F and C F2 absorption peaks shown in Fig. 4b and c [19,27–31]. Simultaneously, we can find a different peak with binding energy at 290.2 eV in FO-PP (Fig. 4c), which is corresponded to C( O)F groups, which are easily transformed into COOH groups under atmospheric moisture action. These chemical groups increase the polarity of PP separator surface and consequently enhance AAR. The polar groups produced by direct fluorination on the surface of PP separator can be divided into two categories: the one is C O that is in form of the C( O)OH group, which induce covalent adsorption with alkali solution; the other is C F and C F2 ,

where hydrogen bond adsorption mainly occurs. The former polar group content of F-PP is 2.7%, and the latter polar group content of F-PP is 18.5% (12.8% + 5.7%). The total polar group content of F-PP is 21.2% in Fig. 4b. Meanwhile, the former content of FO-PP is 16.1% (14.6% + 1.5%), and the latter content of FO-PP is 12.2% (11.0% + 1.2%). The total polar group content of FO-PP is 28.3% in Fig. 4c. The above results support that FO-PP possesses a higher AAR. The content of polar groups ( C O) inducing covalent adsorption with alkali solution in FO-PP is about five times than that of F-PP. However, the content of polar groups ( C Fx ) inducing hydrogen bond adsorption with alkali solution in F-PP is about 51.6% more than that of FO-PP. The analysis of the above results shows that F-PP may primarily be based on hydrogen bond adsorption, while FO-PP may be primarily based on both covalent

Fig. 5. Scanning electron micrographs of polypropylene separator before and after treatment, from left to right, U-PP (a), F-PP (b) and FO-PP (c) samples.

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adsorption and hydrogen bond adsorption. The difference in alkali absorption mechanism of both fluorinated PP is obvious. The AAR of FO-PP is 38.2% more than that of F-PP, while the content of polar groups is 33.5% more than that of F-PP. It can be found that effect of alkali absorption based on the covalent adsorption is better. Therefore, the direct fluorination with oxygen employed is more efficient in improving AAR. In addition, the above XPS analysis also indicates that oxygen and fluorine atoms produced by direct fluorination are in the form of covalent bonds on the surface of PP separator, so the AARs of both the treated PP separators have excellent storage stability. 3.5. Surface morphology The surface morphology of U-PP, F-PP and FO-PP was investigated by SEM. Fig. 5 presents the SEM morphology photographs with magnification of 40,000×. There is no obvious change of the polymer texture before and after direct fluorination shown in Fig. 5. The shape and size of the both modified PP microporous separators remain almost unchanged by whether only fluorine–nitrogen gas used or fluorine–oxygen–nitrogen mixture gas atmosphere direct fluorination, so permeability of the separator should be still maintained. In addition, the influence of improvement in hydrophilicity of PP separator induced by the direct fluorination surface modification on the performance of battery deserves further studying. 4. Conclusion The AAR of PP separator is substantially enhanced through fluorine–nitrogen gas and fluorine–oxygen–nitrogen mixture two different gas atmosphere direct fluorination surface modification. A large number of fluorine-containing and oxygen-containing polar groups are bonded onto the surface macromolecular chains, resulting in high hydrophilicity and providing strong covalent adsorption and hydrogen bond adsorption with alkali solution. Larger number of polar groups are introduced by the fluorine–oxygen–nitrogen mixture atmosphere, and the activation effect of direct fluorination with oxygen employed is more obvious. Simultaneously, the surface physical structure has not been changed, and the porous structure is still maintained after direct fluorination. The direct fluorination, especially for direct fluorination with oxygen employed, is an effective method to improve the hydrophilicity of PP separator. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 50973073) and by the Combination Project of Guangdong Province and Ministry of Education (No. 2011A090200014). References [1] P. Arora, Z. Zhang, Battery separators, Chemical Reviews-Columbus 104 (2004) 4419–4462. [2] Q. Yang, Z. Xu, Z. Dai, J. Wang, M. Ulbricht, Surface modification of polypropylene microporous membranes with a novel glycopolymer, Chemistry of Materials 17 (2005) 3050–3058. [3] M. Pantoja, N. Encinas, J. Abenojar, M.A. Martínez, Effect of tetraethoxysilane coating on the improvement of plasma treated polypropylene adhesion, Applied Surface Science 280 (2013) 850–857. [4] M. Hu, Q. Yang, Z. Xu, Enhancing the hydrophilicity of polypropylene microporous membranes by the grafting of 2-hydroxyethyl methacrylate via a synergistic effect of photoinitiators, Journal of Membrane Science 285 (2006) 196–205.

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