Surface-modified polyethylene separator via oxygen plasma treatment for lithium ion battery

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Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3

Surface-modified polyethylene separator via oxygen plasma treatment for lithium ion battery

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Q1 So

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a

Yeon Jin a, James Manuel b, Xiaohui Zhao b, Won Ho Park a,*, Jou-Hyeon Ahn b,c,**

Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Chemical Engineering and Research Institute for Green Energy Convergence Technology, Jinju 52828, Republic of Korea c Department of Materials Engineering and Convergence Technology and RIGET, Gyeongsang National University, Jinju 52828, Republic of Korea b

A R T I C L E I N F O

Article history: Received 2 August 2016 Received in revised form 22 August 2016 Accepted 24 August 2016 Available online xxx Keywords: Lithium ion batteries Separator Plasma treatment Electrolyte wettability Surface modification

A B S T R A C T

The separator is an important component in lithium ion batteries (LIBs). However, commercial separators such as polyethylene (PE) and polypropylene (PP) are in urgent need to be modified to improve the surface properties to meet requirements for high performance LIBs. In this study, oxygen plasma has been applied to modify the surface of PE separating membrane with functional groups, which has greatly improved the electrolyte wettability and retention of PE separators. The cells with plasmatreated PE separators showed improved charge–discharge capability with lower interfacial resistance and stable cycling performance. This result demonstrates a high potential of the plasma-treated PE separator in LIBs. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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Introduction

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A secondary battery can be discharged and charged a large number of times unlike the primary battery. Secondary batteries include many kinds of batteries, such as a nickel–cadmium battery, nickel–metal hybrid battery, lithium ion batteries (LIBs), redox flow battery, lithium/sodium sulfur batteries (Li–S/Na–S), and so on, among which LIBs are the most popular rechargeable batteries for portable electronics such as mobile products, communication devices, and electronic equipment. Recently, the importance and demands of LIBs has been remarkably increased, because LIBs have high energy density, excellent cycle life, and relatively low selfdischarge property [1,2]. The separator is placed between the negative and positive electrodes in the battery. It plays a key role in LIB, not only does it control the lithium ions that pass through its inner channel, but also it prevents the physical contact of the cathode and anode while serving as the electrolyte reservoir to allow the transport of ionic charge carriers [3–5]. Polyolefin separators, such as polypropylene (PP) and polyethylene (PE), have been used widely for LIBs. Polyolefin separators have superior mechanical and

* Corresponding author. Fax: +82 42 823 3736. ** Corresponding author. Fax: +82 55 772 1789. E-mail addresses: [email protected] (W.H. Park), [email protected] (J.-H. Ahn).

chemical stabilities, and also effectively prevent electrical shortcircuits or overcharging by keeping the cathode and anode apart. However, they have hydrophobic surface with low surface energy, and thus have poor wetting/retaining ability to the electrolyte solutions [6]. Therefore, the development of polyolefin separators with good wettability is one of the most urgent challenges for high performance LIBs. To overcome these drawbacks, various methods including inorganic particle coating [7–12], polymer coating [13], physical/chemical vapor deposition [14], and grafting hydrophilic polymer [15–18] have been employed to improve the wettability of the polyolefin separators. Even though the modification of hydrophobic polyolefin separators with suitable hydrophilic monomers to impart enough hydrophilicity to readily absorb the electrolyte solution is one of the best methods, they still have the drawbacks such as complex multi-steps and relatively high expense. Plasma treatment is an energy-saving, pollution-free, and dry process for surface modification. Plasma treatment has several advantages over other competitive techniques, such as environmental safety, and uniformity, reproducibility, and selective modification without a serious loss of bulk properties. In particular, the oxygen plasma treatment could generate a significant amount of hydrophilic oxygen-containing functional groups (C5 5O, C–OH, COOH) on the polymer surfaces [15]. The hydrophilicity and wettability of the separators are increased by the oxidation reaction. In this study, the oxygen plasma treatment was employed

http://dx.doi.org/10.1016/j.jiec.2016.08.021 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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to selectively modify the surface of commercial PE separator and its effect on mechanical and electrochemical properties was investigated in detail.

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Experimental

59

Materials

CA measurement was carried out at room temperature and the droplet amount was limited to about 10 ml. The etching rate of the treated PE samples during exposure to oxygen plasma treatment was determined by weighing equal sized separators (5 cm  5 cm) before and after plasma treatment, by the following equation: Weight loss ðmg=cm2 Þ ¼

W 0 W S

(1)

60 61 62 63 64 65 66 67 68

Commercialized PE separator (thickness, 9 mm) made by wet process was kindly supplied from Celgard Co. (Korea). The electrolyte solution of 1 M lithium hexafluorophosphate (LiPF6) dissolved in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC:DMC = 1:1, by volume), was purchased from Leechem Co. (Korea). Anhydrous 1-butanol (99.8% purity) was purchased from Sigma–Aldrich Co. (USA). Lithium metal (thickness, 300 mm) was purchased from Cyprus Foote Mineral Co. (USA).

where W0 and W indicate the weight of the separators before and after plasma treatment, respectively, and S indicates the surface area of separator. The electrolyte uptake of separators was determined by soaking the separators in the electrolyte solution. The weight of the separators was taken after removing the electrolyte remaining on the surface of separators with filter paper and the electrolyte uptake was calculated using the equation:

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Plasma treatment process

Electrolyte uptake ¼

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PE separator was surface-modified by oxygen plasma treatment. Surface treatment equipment with RF plasma source (PLASMART MINIPLASMA-station) was used to modify the surface of the PE separator. Oxygen plasma treatment of the PE separator was carried out for various time durations in a chamber (26 cm  26 cm  10 cm) connected to a two-stage rotary pump via a liquid nitrogen cold trap (base pressure of 4  108 mbar). A PE separator was placed in the center of the chamber, followed by evacuation to base pressure. The experimental power input from RF generator was 50 W and the treatment time varied from 0 to 10 min. The gas flow rate was 100 sccm (standard cm3/min). Schematic oxygen plasma treatment for the PE separator is presented in Fig. 1. The original PE separator was denoted as ‘‘untreated’’. The separator samples treated with oxygen plasma for 1, 3, 5, and 10 min were referred to 1 min, 3 min, 5 min, and 10 min, respectively.

where W0 and W are the weight of the dry and wet separators. The porosity was determined from the uptake value after immersing separators in 1-butanol for 30 min, and calculated with the following Eq. (3):

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Physical and structural characterization of modified separator

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In order to examine the chemical structure of oxygen plasmatreated PE separators, X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific MultiLab 2000 system using Al Ka source (1486.6 eV). All binding energies were referenced to the C1s neutral carbon peak at 284.5 eV. The untreated and oxygen plasma-treated PE separators were characterized by attenuated total reflectance infrared spectroscopy (ATR-IR, ALPHA-P, Diamond crystal) in wavenumber range of 3500–500 cm1 and a maximum resolution of 0.9 cm1 at room temperature. The contact angle (CA) measurement of the untreated and modified PE separators (1, 3, 5, and 10 min) was carried out by using a contact angle measurement (DSA100, Kruss) to verify the effect of the oxygen plasma treatment on the wettability of the electrolyte. The CA was determined by means of sessile drop method, using two different testing liquids: distilled water and the electrolyte liquid (1 M lithium hexafluorophosphate (LiPF6) dissolved in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC:DMC = 1:1, by volume)). The

Porosity ¼

WW 0 100 W0

mb =Pb 100 ðmb =P b Þ þ ðms =P s Þ

105 106 107 108 109 110

112 111 113 114 115 116 117 118 119

(2) 121 120 122 123 124

(3)

where ms and mb are the mass of the separators and 1-butanol, respectively, and Ps and Pb are the density of the separator and 1butanol, respectively. The mechanical strength of separators was measured on a tensile tester (Instron-4467) at room temperature with 50 N capacity load cell and stretching rate of 500 mm/min. The dimension of test specimens was 10  100 mm.

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Cell assembly and electrochemical characterizations

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The electrochemical stability of the separators was determined by linear sweep voltammetry (LSV) of Li/separator with liquid electrolyte (LE)/stainless steel (SS) cells at a scan rate of 1 mV/s over the range of 2.5–5.5 V at 25 8C. The interfacial resistance between the separator and lithium metal electrode was measured by electrochemical impedance spectroscopy (EIS) over the frequency range 100 mHz to 2 MHz at an amplitude of 20 mV at room temperature with the Li/separator with LE/Li cells, the schematic of which is given in Fig. 8. For further electrochemical testing, coin cells were fabricated by placing a separator with LE between lithium metal anode and in-house prepared carbon coated lithium iron phosphate (LiFePO4) cathode. LiFePO4 cathode was prepared by casting the slurry of the synthesized LiFePO4 powder, conductive Super-P (SP) carbon, and poly(vinylidene fluoride) (PVDF) binder at a weight ratio of 80:10:10 in N-methyl2-pyrrolidone (NMP) onto an Al foil, dried at 60 8C for 12 h, and thereafter at 80 8C for 24 h under vacuum to get the electrode of 22 mm thickness and the active material loading of 3.18 mg/ cm2. The electrochemical performances of the Li/separator with LE/ LiFePO4 cells were performed in an automatic galvanostatic charge–discharge unit, WBCS3000 battery cycler (WonA Tech.

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Fig. 1. Schematic representation of surface modification of the PE separator via oxygen plasma treatment.

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Co.), between 2.5 and 4 V at room temperature with current densities of 0.1, 0.2, 0.5, 1 and 2C, respectively.

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Results and discussion

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Table 1 Elemental compositions of PE separators with respect to oxygen plasma treatment time obtained from XPS analysis. Sample

C1S

O1S

O/C

Untreated 1 min 3 min 5 min 10 min

100 86.92 84.78 82.92 80.59

0 13.08 15.22 17.08 19.47

0 0.15 0.18 0.21 0.24

XPS analysis is a useful tool to clarify the surface elemental composition of the separators. The change in chemical compositions of the untreated and plasma-treated separators (1, 3, 5, and 10 min) was investigated by XPS spectra as shown in Fig. 2. The original PE separator showed an intense and narrow peak at 285 eV, corresponding to the signal of C1s core level. On the other hand, the PE separators treated with oxygen plasma exhibited a new O1s peak at 532.5 eV, indicating newly formed oxygencontaining functional groups such as carbonyl groups. The oxygen/ carbon (O/C) ratio was gradually increased from an initial value of 0–0.24 with respect to oxygen plasma treatment time (Table 1). This can be attributed to the increase in the formation of oxygencontaining functional groups (carbonyl or peroxyl groups) on the surface of the PE separator with oxygen plasma treatment. Notably, the surface of the PE separator contained abundance of oxygen atoms with a 15% increase even for the separator treated for 1 min and up to 24% increase for 10 min treatment (Table 1). Hence, the hydrophobic surface of PE separator was effectively modified to hydrophilic one by oxygen plasma. Fig. 3 shows the ATR-IR spectra of the untreated and treated PE separators. All the spectra showed the characteristic bands of PE. The strong bands at 2918 and 2948 cm1 are assigned to asymmetric and symmetric C–H stretching vibration in –CH2–, respectively, while the peaks at 1463 cm1 and 717 cm1 are assigned to C–H bending deformation and C–C rocking deformation, respectively. The new peaks at 1029, 1195, and 1700–1730 cm1 are assigned to C–O, C–O–C, and C5 5O groups, respectively [19]. From the XPS and ATR-IR spectra, the plasma-treated samples with short exposure time up to 10 min clearly showed the rapid appearance of functional groups on their surface. The surface functionalization occurs by complex reactions of polymeric radicals with oxygen-containing active species, and thus depends on the surface area of polymeric membrane. In this study, the fast surface reaction by oxygen plasma maybe attributed to high surface area due to porous structure of PE separator, unlike dense polyolefin films. These results indicate that the oxygencontaining functional groups have been rapidly introduced to the surface of PE separator by oxygen plasma treatment.

To evaluate quantitatively the wetting properties of the untreated and treated PE separators, the CAs between the separators and the LE were measured (Fig. 4). The quick and complete wetting property of a separator in typical battery electrolytes is important for cell assembling process and excellent electrochemical performance. The untreated PE separator was hardly wetted by the distilled water or LE due to the hydrophobic surface of the untreated separator. In contrast, the plasma-treated separators showed the improved wettability with distilled water and LE. Even the PE separator with plasma treatment for 1 min showed complete wettability in LE, while the CAs against distilled water were gradually decreased from 1148 to 8.18 with plasma treatment time of 10 min (Fig. 4b). The treated PE separators exhibited lower CA than the untreated PE separator. This has a positive effect on wettability and electrochemical performance of plasma-treated PE separator, compared to untreated PE separator. The decrease in CA of treated PE separators indicates that the oxygen plasma treatment was very efficient in wettability enhancement of the surface of PE separators. Therefore, the treated PE separators have a high affinity toward LE by introducing polar groups on surface of PE separator [20], and thus interfacial adhesive properties between separator and electrode can be improved. This may contribute to the improvement in the electrochemical performance of the lithium ion cell. Fig. 5 represents the SEM images of the untreated and treated (1, 3, 5, and 10 min) PE separators. The untreated PE separator shows a typical morphology originated from wet process, which has a highly porous and uniformly interconnected pore structure (Fig. 5a). From SEM images in Fig. 5b–e, the morphological change of PE separator by plasma treatment with different time duration was observed. With increasing plasma treatment time from 1 to

Fig. 2. XPS spectra of untreated and plasma-treated PE separators with different oxygen plasma treatment time.

Fig. 3. ATR-IR spectra of untreated and plasma-treated PE separator with different oxygen plasma treatment time.

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Fig. 4. (a) Contact angles and (b) photographs of untreated and plasma-treated PE separators with different oxygen plasma treatment time.

225 10 min, the surface became rougher and thus pore structure was 226 expanded by plasma etching processing. In general, the presence of 227 the highly porous structure on the separators can lead to the 228 increase of LE uptake [4]. However, the plasma etching of the 229 separators was expected to deteriorate the mechanical strength of 230 the treated separators. The oxygen plasma treatment has a positive 231 effect to modify the hydrophobic surface of PE separator by 232 introducing the oxygen-containing functional groups via mainly 233 radical reaction. On the contrary, it also has a negative effect to 234 induce the chain scission of the polymeric surface, resulting in the 235 surface etching. Therefore, the optimal plasma treatment needs to 236 be employed by considering both the surface functionalization and 237 mechanical deterioration of PE separator. 238 The LE wettability of the separator plays an important role in 239 battery performance since the separator with good wettability can 240 readily absorb the LE and enable ion transport between electrodes 241 [18]. The performance of separator is also determined by the 242 surface morphology and pore structure of the membrane [3]. The 243 electrolyte uptake (%) values shown by the separators are 98% 244 (untreated), 125% (1 min), 138% (3 min), 200% (5 min), and 233% 245 (10 min), respectively (Table 2). The increment in LE uptake was 246 associated with improved hydrophilicity, interfacial compatibility, 247 and enlarged pore size. High porosity and interconnected pores of 248 battery separators facilitate the migration of lithium ions. The 249 porosity of PE separator was gradually increased from 31% to 46% 250 by oxygen plasma treatment for 1–10 min (Table 2). The separator 251 with high porosity and electrolyte uptake is able to trap more LE, 252 and thus have an enhanced ionic conductivity. In addition, the 253 weight loss of PE separator after oxygen plasma treatment was 254 increased up to 0.20 mg/cm2 with an increase of plasma exposure 255 time (Table 2). This increase in porosity and weight loss was 256 Q2 mainly due to physical etching by plasma (Table 3).

The separators for LIB should have enough mechanical stability to withstand the high tension during the battery assembly procedure [5], and prevent internal short-circuits caused by the rough electrode surface and growth of lithium dendrites. Fig. 6 showed the tensile strength of untreated and plasma-treated PE separators. The tensile strength of untreated PE was 169 MPa, and the tensile strength of treated PE was 95.4 MPa (1 min), 80.2 MPa (3 min), 66.8 MPa (5 min), and 36 MPa (10 min), respectively. The tensile strength of treated PE separator was gradually decreased with an increase in oxygen plasma treatment time duration. Surface morphology and interconnected pore structure of PE separator was partially destroyed by oxygen plasma treatment. By considering the above results, among plasma-treated PE separators, the PE separator with plasma treatment for 1 min was chosen as an optimum sample for electrochemical performance tests. For practical battery applications, electrochemical stability of the electrolytes within the operation voltage of the battery system is an important parameter in characterizing battery separators. Electrochemical tests were performed mainly to verify whether the plasma-treated PE separator retains better electrochemical properties than the untreated PE separator. The electrochemical stability windows of untreated and treated PE separator were determined with Li/separator/SS cells by LSV in the range of 2.5– 5.5 V at a scan rate of 1.0 mV/s (Fig. 7). The increased current in high-voltage range is generally known to be the decomposition of the LE. As shown in Fig. 7, the treated separator showed much enhanced electrochemical stability (4.5 V) than that of untreated PE separator (4.2 V). It is suggested that the oxygen plasma treatment for 1 min enhanced the electrochemical stability of the LE (1 M LiPF6 in EC/DMC), which is possibly due to the good affinity between the LE and plasma-treated PE separator. The compatibility between the separator and electrodes was investigated by EIS

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Fig. 5. SEM images of (a) untreated and plasma-treated PE separators with different time of (b) 1 min, (c) 3 min, (d) 5 min, and (e) 10 min, respectively.

289 290 291 292 293 294 295

and the obtained result is shown in Fig. 8. The EIS curve consists of a semicircle with intercepts on the high frequency region which is the bulk electrolyte resistance and the diameter of the semicircle being the interfacial resistance. It can be clearly seen that the bulk electrolyte resistance of plasma-treated as well as the untreated separators is comparable and very low. The observed interfacial resistances were 90.7 and 136.0 V for the treated and untreated PE

separators, respectively, which clearly indicates a better affinity between the plasma-treated separator and electrodes. The electrochemical performances of the coin cells containing the untreated or plasma-treated PE separator were further

Table 2 Physical properties of untreated and plasma-treated PE separators. Sample

Electrolyte uptake (%)

Porosity (%)

Weight loss (mg/cm2)

Untreated 1 min 3 min 5 min 10 min

98 125 138 200 233

31  1.7 39  2.9 43  2.6 45  7.6 46  11.4

0 0.06 0.08 0.13 0.20

Table 3 Mechanical properties of untreated and plasma-treated PE separators. Sample

Tensile strength (MPa)

Elongation (%)

Modulus (MPa)

Untreated 1 min 3 min 5 min 10 min

169.0  13.0 95.4  6.9 80.2  6.4 66.8  8.7 36.0  6.9

56.4  6.4 29.6  1.4 24.7  1.9 21.4  1.5 13.4  1.9

307  30 340  31 330  21 320  43 220  109

Fig. 6. Tensile strength of untreated and plasma-treated PE separators with different oxygen plasma treatment time.

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Fig. 7. LSV of untreated and plasma-treated PE separators with LE (Li/PE separator/ SS cell, 1 mV/s, 2.5–5.5 V).

Fig. 8. AC impedance spectra of untreated and plasma-treated PE separators (Li/PE separator/Li cell, LE, frequency range 10 mHz to 2 MHz) and the schematic of the cell configuration for AC impedance testing (inset).

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investigated with LiFePO4 cathode. As shown in Fig. 9a, Li/LiFePO4 cells with untreated and treated separators delivered initial discharge capacities of 141.2 and 155.6 mAh/g, respectively at a c rate of 0.1C. The cell with treated PE separator showed a higher capacity retention value of 88.5% (137.7 mAh/g) than the cell with untreated separator (87.5%) after 100 cycles at 0.1 C-rate (Fig. 9b). The improved capacity retention for the treated separator was probably due to the enhanced compatibility with LE and its high affinity to the electrodes. Fig. 9c showed rate capability of the Li/ LiFePO4 cells at different c rates from 0.1C to 2C. For both untreated and plasma-treated PE separators, the discharge capacity of cells gradually decreased with the increase in C-rate. Compared to the cell with untreated separator, the cell with plasma-treated PE separator showed higher discharge capacities over various discharge current densities than that with untreated PE separator. These results demonstrated a high perspective in applying the plasma-treated PE separator in high performance LIBs.

Fig. 9. (a) Charge–discharge curves (1st and 100th cycles) and (b) cycle performance of Li/separator with LE/LiFePO4 cells with untreated and plasma-treated PE separators at a current density of 0.1C, and (c) rate capability of the cells with untreated and plasma-treated PE separators at different C-rates (LE, 2.5–4.0 V, 25 8C).

Conclusions

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In this study, it was found that the plasma treatment on PE separator with oxygen gas was an effective method in improving the wettability against LE due to the introduction of large amount of functional groups. Except for the mechanical strength, essential properties of PE separator including electrolyte uptake, porosity, and electrochemical stability were increased even after plasma treatment for 1 min. The plasma-treated PE separator showed an enhanced compatibility with LE and electrodes with reduced interfacial resistance. The surface modification of the PE separator

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by oxygen plasma treatment led to the enhancement in the electrochemical performance of the LIB. Subsequently, the Li/LiFePO4 cell assembled with the treated separator exhibited better capacity retention than that with untreated separator. It is expected that the plasma-treated separator would be a promising candidate as LIB separator.

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Acknowledgements

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This work was supported by the Industrial Strategic Technology Q3 Development Program (10040033), funded by the Ministry of

Knowledge Economy (MKE), Korea and also supported by Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (No. NRF-2016R1A2A2A07005334). References [1] M. Armand, J.M. Tarascon, Nature 451 (2008) 652. [2] M.R. Palacin, Chem. Soc. Rev. 38 (2009) 2565.

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