REGULAR PAPER
Japanese Journal of Applied Physics 52 (2013) 11NM07 http://dx.doi.org/10.7567/JJAP.52.11NM07
Tailoring Surface Properties of Polymeric Separators for Lithium-Ion Batteries by 13.56 MHz Radio-Frequency Plasma Glow Discharge Chia-Han Liang1 , Ruey-Shin Juang1;2 , Ching-Yuan Tsai1 , and Chun Huang1 1
Department of Chemical Engineering and Materials Science, Yuan Ze University, Chungli 32003, Taiwan Department of Chemical and Materials Engineering, Chang Gung University, Kweishan, Taoyuan 33302, Taiwan E-mail:
[email protected] 2
Received February 27, 2013; accepted July 21, 2013; published online November 20, 2013 The hydrophilic surface modification of the polymeric separator is achieved by low-pressure 13.56 MHz radio-frequency Ar and He gas plasma treatments. The changes in surface hydrophilicity and surface free energy were examined by static contact angle analysis. The static water contact angle of the plasma-modified polymeric separator particularly decreased with the increase in treatment time. An obvious increase in the surface energy of polymeric separators owing to the crosslinking by activated species of inert gases effect of monatomic-gas-plasma treatments was also observed. Optical emission spectroscopy was carried out to analyze the chemical species generated after Ar and He gas plasma treatments. The variations in the surface morphology and chemical structure of the polymeric separators were confirmed by scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements. XPS analysis showed significantly higher surface concentrations of oxygen functional groups for monatomic-gas-plasma-modified polymeric separator surfaces than for the unmodified polymeric separator surface. The experimental results show the important role of chemical species in the interaction between Ar and He gas plasmas and the polymeric separator surface, which can be controlled by surface modification to tailor the hydrophilicity of the polymeric separator. # 2013 The Japan Society of Applied Physics
1. Introduction
More and more attention has been paid to scientific works on lithium ion batteries because of their admirable features, such as design flexibility and low rates of safety failures.1,2) The separator is important to lithium ion batteries because its surface property significantly affects the battery performance, including the power densities, cycle life, and safety.3,4) The separator for lithium ion batteries commonly consists of a polymeric membrane forming a microporous layer.5) Polypropylene (PP) or polyethylene (PE) and their laminates have been extensively utilized as separator materials for lithium ion batteries. However, their poor wetting in cyclic electrolyte solutions with high dielectric constant and the increased internal resistance of the cells restricts their use.6–11) For this reason, the surface of the polymeric separator must be rendered hydrophilic through chemical modification of the separator surface property prior to its use in electrolyte uptake and/or charge transfer. Plasma surface modification is an environmentally friendly method of modifying the surface by introducing functional groups onto the surface of a polymeric material.12–20) The surface modification of a polymeric separator by a plasma process is relevant to the tailoring of the surface properties of polymeric materials.21–25) However, despite the extensive use of the plasma modification process, the surface activation of polymeric separators by a monatomic-gasplasma process has not been extensively examined or reported. Depending on the input gas used for plasma formation and the operational parameters, the polymeric surface can be activated by inserting active species and/or by a cross-linking process. The diverse reactive species in the plasma state cause the generation of different free radicals in the polymeric chain and insert different functional groups onto the surface of the polymer, thereby tailoring the surface properties of the polymeric separators.26) It is desirable to investigate and compare the monatomic-gas-plasma characteristics under the same experimental conditions.
In our present study, microporous polymeric separators were modified by the 13.56 MHz radio-frequency (RF) argon (Ar) and helium (He) gas plasmas to improve the intrinsic low surface properties. Changes in the hydrophilicities of the monatomic-gas-plasma-modified polymeric separators were characterized by measuring the water contact angle. Optical emission spectroscopy (OES) was adopted to examine the various chemical species that cause plasma surface modification. The surface morphology of the monatomic-gasplasma-modified polymeric separator was analyzed by scanning electron microscopy (SEM). Moreover, changes in the chemical compositions of monatomic-gas-plasmamodified separator surfaces were characterized by X-ray photoelectron spectroscopy (XPS). This work is the first step in exploring the potential of monatomic-gas-plasma modification as a means of controlling the surfaces of microporous polymeric separators. 2. Experimental Procedure
Both Ar and He gases used to induce monatomic-gas-plasma surface modification were of industrial grade with 99.999% purity. Celgard 2320 with a thickness of 20 m and a porosity of 42% was used as the polymeric separator. The polymeric separator was based on the PP/PE/PP trilayer architecture. The plasma reactor system used in this investigation was a Pyrex glass tubular reactor (6.0 cm OD, 5.3 cm ID, and 120 cm length), as shown in Fig. 1. A copper coil was used for the generation of a capacitively coupled-mode electrical plasma power. The copper coil was placed on the Pyrex tube at a distance of 20 cm from one end, where the gas inlet was located. Ar and He gases were used as the input gas for plasma surface modification. The plasma power was applied at 13.56 MHz RF with a required matching network unit (Huttinger Elektronik PFG-300RF generator). The static contact angle of the plasma-modified polymeric separator was measured by projecting an image of an automatic sessile droplet resting on the membrane surface with a Magic Droplet Model 100SB video contact angle system (Sindatek Instruments). To understand the nature of
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Fig. 1. (Color online) Schematic diagram of low-pressure tubular plasma system.
Fig. 2. Optical emission spectra of Ar and He gas plasmas. Plasma conditions: RF power of 5 W, Ar and He gas flow rate of 10 sccm at 100 mT for 60 s.
the surface change of the polymeric separator, the dispersion and polar interaction contributions to the surface energy of the materials were calculated using the Owens–Wendt model.27) The liquids used for calculating the surface energies of the unmodified and plasma-modified polymeric separators were water and di-iodomethane of known p (polar component) and d (disperse component). The surface energy of a solid ( s ) has two components, namely, a polar component and a disperse component. Both components contribute to the total surface energy. The major plasma diagnostic apparatus of the monatomic gas plasma is an OES system. This equipment consists of both the instrumentation and spectrum analysis software programs, which were supplied by Hong-Ming Technology, Inc. The observable spectral range was 250–950 nm with a resolution of 2 nm. The chemical structure of the polymeric separator was characterized using an FTIR spectrometer (Perkin-Elmer LX 20000G). Each spectrum was obtained from an average of 256 scans in the range of 400–4000 cm1 at a resolution of 4 cm1 . The images were processed with VK Viewer control software. The surface morphology and roughness of the plasma-modified polymeric separators were
examined by SEM analysis performed with a JEOL JSM6701f scanning electron spectroscopy apparatus. Au coatings 20 nm thick were evaporated onto the polymeric separator surface (1 1 cm2 ) to obtain a conducting surface. A tungsten filament was used as the electron source. A 20 keV accelerator voltage was used for scanning the polymeric separator surfaces. XPS measurements were carried out on a VG Scientific Microlab 310F system, using nonmonochromatic Mg K-radiation (h ¼ 1253:6 eV) and Al K-radiation (h ¼ 1486:6 eV) operated at 25 kV. Spectra are acquired with the angle between the direction of the emitted photoelectrons and the surface of microporous polymeric separators equal to a take-off analysis angle of 70 . 3. Results and Discussion
OES was carried out to detect the excited plasma reactive species generated by monatomic gas plasma. OES is expected to clarify the reactions of plasma reactive species that may contribute to surface modification. To compare the emission spectra between He and Ar plasmas, only the gas flow system was changed, while other experimental conditions were fixed. Figure 2 shows the OES emission spectra
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Fig. 3. (Color online) Average contact angles and surface free energy changes of Ar- and He-gas-plasma-modified polymeric separators with treatment time. Plasma conditions: RF power of 5 W, Ar and He gas flow rate of 10 sccm at 100 mT.
from 300 to 900 nm in He and Ar plasmas. From optical emission spectra of monatomic gas plasmas, the distinctive He and Ar emission lines are dominant at 300–800 nm. Regarding the strong He and Ar plasma species from OES analysis, the crosslinking via activated species of inert gases (CASING) effect can be recognized as the possible major reason behind plasma surface modification on polymeric separators. A possible mechanism of active site generation is the interaction of radicals released from plasma with the polymeric separator surface to generate dangling bonds, which then leads to the formation of surface active sites.28,29) The results of optical emission analysis of monatomic gas plasma indicate that this mechanism corresponds to the possible surface modification effect of He and Ar plasma species, which provide an ion bombardment effect onto the separator surface. Figure 3 shows a summary of the static contact angles of two suitable liquids (deionized water and di-iodomethane) and surface free energies of unmodified and monatomicgas-plasma-modified polymeric separators. The effects of plasma treatment time on surface energy and the contact angles of both liquids are shown in Fig. 3. It can be seen that the unmodified polymeric separator has a static water contact angle of 112 , which makes it very hydrophobic and difficult for electrolyte solution to penetrate. In contrast, the monatomic-gas-plasma-modified polymeric separator showed improved hydrophilicity with a static water contact angle lower than 70 , which is due to the plasma reaction on the surface. For improving the hydrophilicity of the polymeric separator, a longer treatment time was more effective than a shorter treatment time. Figure 3 also shows a plot of surface energy from the measured static contact angles on the monatomic-gas-plasma-modified polymeric separator surfaces as a function of treatment time. The surface energy of the unmodified polymeric separator is 25 mJ/m2 . The changes in the surface energy of monatomicgas-plasma-modified polymeric separator surfaces particularly improved after a very short treatment time (60 s). For changes in the surface energy, He plasma (54.9 mJ/m2 ) was more effective than Ar plasma (53.5 mJ/m2 ). These results show that the monatomic gas plasma efficiently modifies the hydrophilicity of the polymeric separator surface.
(a)
(b)
(c)
(d)
Fig. 4. (Color online) Luminous gas phases of monatomic gas plasmas: (a) Ar plasma without separator, (b) He plasma without separator, (c) Ar plasma with separator, and (d) He plasma with separator. Plasma conditions: RF power of 5 W, Ar and He gas flow rate of 10 sccm at 100 mT for 60 s.
Upon the addition of a polymeric separator into the plasma reactor system, the glow color and emission of argon and helium plasmas become highly dissimilar, as shown in Figs. 4 and 5, respectively. The luminous glow of plasma discharge represents the electron energy level/chemical reaction therein.30) From this aspect, the glow characteristics in plasma discharge have important implications in understanding the creation mechanisms of chemically reactive species in monatomic-gas-plasma surface modification. When the polymeric separator was added into the Ar plasma system, an obvious color change (purple–white) was observed, as shown in Fig. 4. In addition, when the polymeric separator was introduced into the He plasma system, the glow color change also occurred and a very short lightpink flame was formed. This indicates that chemical reactions rapidly occurred in the low-pressure monatomicgas-plasma surface modification process. The interaction of plasma species in the plasma with the polymeric separator surface can be recognized as the enhancement of surface modification from the results of glow characteristic analysis. OES analysis is expected to clarify the reactions of plasma reactive species that may contribute to low-pressure monatomic gas plasma modification. The typical emission spectrum of the low-pressure monatomic gas plasmas in Fig. 5 is from 250 to 900 nm. The dominant features in the emission spectra with the polymeric separator are due to excited species corresponding to the relevant gases, with the strongest argon or helium lines remaining visible. The low intensity of emission derived from hydrocarbon emission lines observed at 386 nm is also observed in the spectra, as well as the emission line of hydrogen at 656 and 486 nm. This result supports the assumption that monatomic-gasplasma surface modification is a result of the CASING effect. Figure 6 shows the FTIR spectra of the monatomic-gasplasma-modified polymeric separators along with those of the unmodified control polymeric separator. The spectra show the evolution of the peaks associated with the functional groups located in the FTIR spectra at 3,200– 3,600 and 1,720 cm1 , which correspond to hydroxyl [O–H]
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Fig. 5. (Color online) Optical emission spectra of monatomic gas plasmas with/without separators. Plasma conditions: RF power of 5 W, Ar and He gas flow rate of 10 sccm at 100 mT for 60 s.
Fig. 6. (Color online) FTIR spectra of Ar- and He-gas-plasma-modified polymeric separators. Plasma conditions: RF power of 5 W, Ar and He gas flow rate of 10 sccm at 100 mT for 60 s.
and carbonyl [C¼O],31) respectively. In comparison with the case of the unmodified control polymeric separator, the strong IR band at 1720 cm1 generated by the vibration of C¼O bonds, which is observed in the spectra of monatomicgas-plasma-modified polymeric separators, indicated the implementation of functional groups onto the polymeric separator surfaces. This results in efficient plasma treatment using the He and Ar monatomic gases and thus, polar functionalities on the polymeric separator surfaces. The increase in peak intensity indicates an increase in the number of oxygen-containing functional groups corresponding to the activation of the polymeric separator surface by He and Ar plasma treatments.
Figure 7 shows the static contact angle changes of Heand Ar-monatomic-gas-plasma-modified polymeric separators. The lines with squares in Fig. 7 represent the static water contact angle measurements directly after the modification treatment, whereas the lines with stars in Fig. 7 represent the contact angles of the samples washed in a deionized water ultrasonic bath for 3 min, then dried by blowing with compressed air and left in air for 10 min. In monatomic plasmas, the plasma species could lose its reactivity in a short time because of the much higher collision frequency among the plasma particles. As a result, the plasmas could significantly lose their reactivity in a remote position (away from the glow). To study the surface modification effects in a remote region, the polymeric separators were exposed to plasma at a position 5 cm away from the glow. Figure 7 shows that the He and Ar monatomic gas plasmas can efficiently modify the polymeric separator surface in the glow region compared with those in a remote region. The post-washing results of the He and Ar monatomic gas plasma treatments shown in Fig. 7 for He-plasma-modified polymeric separators showed a slight recovery of hydrophobicity, as seen by the static water contact angle of
