Surface Modification of Polyethylene Separator for Li-Ion Batteries via Imine Formation

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Introduction
Recently, climate change due to fossil fuel consumption has become a serious issue around the world; thus, the use of renewable energy and energy storage devices has accelerated.Among the energy storage devices commonly used, rechargeable Li-ion batteries (LIBs) are considered the most important power sources owing to their high energy density, excellent cycling performance, and reasonable cost [1][2][3].LIBs mainly consist of an anode, cathode, electrolyte, and separator.Although the electrode materials for the anode and cathode can determine the energy density of the batteries, the electrolyte and separator also play important roles in operating the electrochemical systems.The separator prevents shortcircuiting through the direct electron transport between the anode and cathode while allowing fast Li-ion transportation through the facile penetration of the liquid electrolyte [4].To perform this role, porous and insulating polymer materials such as polyolefin are used.The typical material is polyethyl-ene (PE), which has great advantages such as high mechanical strength, wide electrochemical stability windows, low cost, and good processability for the mass production of porous thin films.However, pristine PE materials exhibit low thermal stability and poor wettability to the organic liquid electrolyte due to its intrinsic hydrophobic property [5][6][7][8].
Much research has already been devoted to addressing the issues limiting PE materials as efficient LIB separators.The development strategies can be divided into three categories: the grafting or coating of different types of polymers, the coating of inorganic materials, and surface modification by physical and chemical methods.The grafting of hydrophilic monomers on the PE surface has been reported in the literature in which monomers such as glycidyl methacrylate, acrylonitrile, polyethylene glycol, or methyl acrylate were grafted onto the PE separator film to induce hydrophilic properties using various techniques [9][10][11][12][13].In other studies, hydrophilic polymers such as polyvinylidene fluoride-hexafluoropropylene copolymer, polyimide, and poly(methyl methacrylate) were coated on the PE surface [14][15][16][17].Examples of inorganic material coating include studies in which inorganic particles such as Al 2 O 3 , TiO 2 , and SiO 2 were coated onto the PE film to provide surface affinity with liquid electrolytes and to improve the thermal resistance [18][19][20][21].Organic-inorganic hybrid coating has also been performed to easily achieve this aim [22][23][24][25].Finally, surface modification of PE films by physical treatment has been conducted through methods such as plasma and electron beam irradiation [26,27].In addition, it has been demonstrated that the PE surface can be changed into a hydrophilic surface through chemical treatment [28][29][30][31][32].The grafting and coating methods of polymer and/or inorganic materials have proven to be effective in improving the properties of the PE separator for LIBs.However, these methods can lead to some drawbacks that include high costs, low energy density, and increased internal resistance caused by the introduction of complex processes and additional material [5,6].Chemical treatment, on the other hand, boasts advantages of low cost owing to the simple process and facile surface modification without property degradation [6][7][8].
In this paper, we propose a facile chemical modification process via imine group formation.In general, the formation of polar functional groups such as hydroxyl, carboxyl, and amine on the surface of a PE film is commonly known to be advantageous in improving the electrolyte wettability of the PE film.Although the imine functional group can also play a role in modifying the PE separator, the chemical formation of the imine group on the PE separator surface and its effect have not been well investigated.The imine formation by the carbonyl-amine reaction is also well known [33][34][35], but implementing the reaction in the surface treatment of PE separators is introduced for the first time in this study.First, PE was pretreated by KMnO 4 and HCl etching to produce a carbonyl group (C=O).Then, the carbonyl group was reacted with amine (R-NH 2 ) in a poly(ethylene glycol) bis(amine) solution to form imine (R-N=C) on the PE surface.Through these simple water-based solution processes, imine groups were abundantly produced, and the hydrophilic property was introduced on the PE surface.The chemical reaction process for the surface modification through the formation of imine was examined by analysis.The physical, thermal, and electrochemical characteristics of the surface-modified PE separator were evaluated, and the advantageous effect of the chemical treatment was demonstrated.

Experimental
2.1.Materials Synthesis.To generate oxygen functional groups on the polyethylene (PE) separator (16 μm, Tonen F16BME), a pretreatment was performed.A piece of PE was immersed in 0.2 M KMnO 4 solution (Daejung, 99.3%) and maintained at 25 °C for 72 h.Then, the PE membrane was washed with deionized water, ethanol, and acetone several times to remove impurities and nonreacting chemicals.After thorough drying at 60 °C for 12 h, the membrane was etched in a 4 M HCl solution for 24 h to remove the manganese oxide formed by the KMnO 4 pretreatment.Then, the PE was washed and dried with the same process again.For the surface treatment, 2.0 g of poly(ethylene gly-col) bis(amine) (Sigma-Aldrich, M w 3,400) was dissolved in a pH 5.0 solution (200 mL).Then, the PE piece was dipped into this solution at 25 °C for 24 h.Afterward, it was thoroughly washed and dried once more.

Material Characterization.
Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7401F) was conducted to observe the morphology of the PE separator.Fourier transform infrared (FT-IR, Bruker VERTEX 70) spectroscopy was employed to examine the molecular bond structure of the surface-modified PE membrane.X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Al Kα radiation) was used to characterize the chemical bonding structure of the PE surface.To compare the wettability of the pristine and treated PE separators, the contact angle (CA, DSA30S, KRÜSS GmbH) was measured after dropping 1 M of LiPF 6 in an ethylene carbonate (EC)/diethyl carbonate (DEC) solution (3 : 7 volume ratio, Panax Etec) on the surface.The electrolyte uptake was calculated by measuring the weight of the separators with a fixed area before and after immersion in the liquid electrolyte.Equation (1) was used.

Electrolyte uptake
where W i is the initial weight of the PE separator and W f is the final weight of the PE separator during the immersion test.The thermal stability of the pristine and I-PE samples was evaluated by differential scanning calorimetry (DSC, Mettler Toledo STARe system).

Electrochemical Measurements.
Electrochemical impedance spectroscopy (EIS, Biologic VSP) was performed on the cells consisting of a stainless steel (SS) plate/separatorelectrolyte/SS plate structure at a frequency range from 0.1 MHz to 100 MHz at an amplitude of 5 mV.From the EIS Nyquist plots, the x-axis intercept was measured and represents the electrolyte resistance (R).After measuring the thickness (d) and area (A) of the separators in the cells, the ionic conductivity was calculated using the following equation: For the electrochemical cycling tests, coin-type cells (CR2032) were assembled in an argon-filled glove box.In the half cells, the pristine or treated PE separator was sandwiched between a Li metal foil anode and a LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathode with 1 M LiPF 6 in an EC/ DEC (3 : 7 volume ratio) solution as the electrolyte.The cathode was prepared by doctor-blading slurries consisting of the active material (NCM811, 92 wt%), a polyvinylidene fluoride binder (4 wt%), and a conducting carbon agent (Super P, 4 wt%).The cells were galvanostatically tested within the potential window of 2.75-4.2V (vs.Li + /Li) at specific currents of 20-400 mA g -1 (1C =200 mA g -1 ).

2
International Journal of Energy Research Figure 2 presents the surface treatment process used to generate the surface functional groups on the PE separator.The oxidation was first performed in a KMnO 4 solution as reported in literature [36].When the PE separator was immersed in the solution, it was oxidized by producing the C=O bond.Then, MnO 2 was chemically attached to the bond.The digital photographs show the change in the separator color into light brown after MnO 2 was formed on the To examine the chemical bond states of the PE separators, FT-IR and XPS spectroscopies were employed.Figure 3 shows the FT-IR spectra of the separators at each step of the treatment.Figure S1 presents the spectra for a broad wavenumber range.After MnO 2 formation and etching (E-PE), the C=O bonds were observed at 1713 cm -1 , indicating that the C=O bonds (carbonyl group) were well generated as expected [37][38][39].After treatment in the poly(ethylene glycol) bis(amine) solution, a peak was detected at 1630 cm -1 in the FT-IR spectrum, which is attributed to the C=N bond in the imine group [37][38][39].In addition, the broad peak at 1103 cm -1 is attributed to the C-N (R-N) bond in the imine group [40][41][42].It was confirmed from these results that the imine group was formed on the surface of the PE separators through the reaction.Figure S2A shows the XPS survey spectra of the separators at each process step.Mn 2p signals were observed after the pretreatment in the KMnO 4 solution, and they disappeared after etching.Figure S2b shows the O 1s core-level spectra of the treated PE separators.After the pretreatment in the KMnO 4 solution, a deconvoluted subprofile centered at 529.2 eV was observed, which could be assigned to the Mn-O bond.After etching, the subprofile was not detected.The FE-SEM image of the separator pretreated in the KMnO 4 solution is shown in Figure S3.MnO 2 particles were clearly observed as reported [36].Figure S4 presents the FE-SEM image and the corresponding EDS elemental mapping results of the E-PE sample after etching.Signals for Mn were not observed.These findings indicate that MnO 2 was formed and then removed through the pretreatment and etching process [36].

Results and Discussion
To examine the surface property changes of the PE separators as a result of the surface treatment, the electrolyte uptake of the separators and the contact angle between the separator and electrolyte were measured, the results of which are shown in Figure 4(a).The calculated electrolyte uptake of the pristine PE, E-PE, and I-PE separators was 54.2%, 54.2%, and 87.5%, 4 International Journal of Energy Research respectively.The contact angles decreased from 26.5 °(pristine PE) to 25.8 °(E-PE) and 11.5 °(I-PE) as the surface modification proceeded.In this study, we introduced the imine group through the reaction between the carbonyl and amine groups onto the PE surface, which greatly enhanced the electrolyte wettability.The imine group formation further increased the hydrophilic property by changing the surface energy, and thus, the electrolyte uptake and contact angle measurement results showed further improvement.Figure 4(b) shows the digital photos of the separators over time after dropping the electrolyte.As the treatment proceeded from pristine PE to E-PE and I-PE, the wetted portion of the separators was observed to increase at each time step.This result proves that the electrolyte wettability was enhanced with the surface treatment process.Figure S5a demonstrates the digital of the separators after heating at 150 °C for 30 min.It was observed that the shape of the I-PE separator was well maintained after the heat treatment.Figure S5b shows the DSC profiles of pristine and I-PE separators.The peak for melting shifted rightward a little after the treatment (I-PE).It was confirmed that the thermal resistance of the separators improved with the formation of the C=O and C=N bonds.Figure 5(a) displays the Nyquist plots of the cells with the pristine and I-PE separators.Using the x-axis intercept data, the resistance values were converted into ionic conductivities of the liquid electrolyte and are shown in Figure 5(b).The ionic conductivity increased from 3:08 × 10 -4 S cm -1 to 3:55 × 10 -4 S cm -1 after surface modification with the imine groups.This indicates the surface-modified separator with improved electrolyte wettability can promote the penetration of the electrolyte and the Li ions can be more quickly transported through the electrolyte and separator between both electrodes.To calculate the Li + transference number in the cells with the pristine and I-PE separators, the chronoamperometry profiles were obtained at a constant polarization voltage of 5 mV (see Figure S6 with detailed description for the measurement) [43,44].The Li + transference numbers were calculated to be approximately 0.33 and 0.35 for the cells with the pristine and I-PE separators, respectively.It indicates that the surface treatment affects the Li + transference number positively.To confirm the effect of enhanced ionic conductivity and Li + transference number after the treatment, the rate capability of the half cells employing the pristine and I-PE separators was compared, and the results are shown in Figure 6.The capacity was measured based on the weight of the cathode active material (NCM811).The rate performance was tested from 0.1C to 2C.At low rates between 0.1 and 0.2C, the capacity difference was negligible.However, the cell with the I-PE separator exhibited much higher capacities at high rates of 0.5C, 1C, and 2C.More specifically, this cell exhibited a capacity retention of 72.8% at 2C, compared to the capacity at 0.1C, while the cell with the pristine PE separator exhibited a capacity retention of 45.8% under the same conditions.This improved rate performance can be attributed to the surface modification.After the imine formation, the separator surface became further hydrophilic.Therefore, electrolyte penetration was facilitated, and Li transference number and ionic conductivity were improved.As a result, the rate performance of the cell with the surface-modified separator was greatly enhanced.
To further examine the effect of surface modification on the PE separators, electrochemical tests were performed using the half cells with the pristine and I-PE separators.The voltage profiles of the cells for the first cycle are compared in Figure 7  profiles for the NCM811 cathode were observed, indicating that the cell worked properly.Figure 7(c) shows the cycling performance of the cells employing the two separator samples at 1C.The cell with I-PE exhibited higher capacity and improved retention.It is confirmed that the surface-treated PE separator is better than the pristine PE at high rate cycling.
After the high rate cycling, the digital photographs of both separators are given in Figure S7.It was observed that the separators maintained the shapes well after the cycling.These results reveal that surface treatment with imine groups on a PE separator can improve the rate performance of the cell without any capacity loss or adverse effects on the cells.

Conclusions
We developed a simple chemical treatment method to modify the PE surface.First, the separator film was pretreated in a KMnO 4 solution and etched with hydrochloric acid to produce a carbonyl group (C=O).Then, the carbonyl functional group reacted with an amine group (R-NH 2 ) in a poly(ethylene glycol) bis(amine) solution, resulting in the formation of an imine functional group (R-N=C).Through this chemical surface modification approach, the PE separator was transformed into a hydrophilic material, thereby increasing the electrolyte wettability and ionic conductivity.In addition, the thermal stability of the PE separator was improved.
The electrochemical test results exhibited that the cell with the I-PE separator demonstrated an improved rate capability at a high rate of 2C.It was confirmed that the chemical treatment on the PE surface did not have any adverse effects on the physical and electrochemical properties of the PE separator.It is expected that this surface treatment approach can be implemented in the surface modification of polyolefinbased separators for rechargeable battery applications.

Figure 1 (
Figure 1(a) illustrates the concept of the surface modification implemented in this study.Using a chemical surface treatment, we created an imine group (C=N bond) on the PE surface without any morphological changes to the membrane bulk structure.More specifically, the carbonyl group (C=O) and amine group (R-NH 2 ) react to produce the imine group (R-N=C) and H 2 O [33-35].As a result, the C=O bond can be replaced by the C=N bond with the nucleophilic addition of amine.To examine the morphology of the separators, FE-SEM was used.Figures 1(b) and 1(c) show the FE-SEM images

Figure 1 :
Figure 1: (a) Schematic illustration of the surface modification process of the PE separator and FE-SEM images of the (b) pristine and (c) final surface-treated PE samples.

Figure 2 :Figure 3 :
Figure 2: Schematic of the change in the surface functional groups on the PE separator with digital photographs after the chemical surface treatments.
(a), and the profiles of the cell with the I-PE separator for various cycle steps are shown in Figure 7(b).Typical

Figure 7 :
Figure 7: Electrochemical properties: (a) voltage profile of the cells with the pristine PE and I-PE for the first cycle at 0.1C; (b) voltage profiles of the cells with I-PE for various cycles at 0.1C; (c) cycle performance of the cells at 1C.