Thin Coating of Microporous Organic Network Makes a Big Difference

Oct 9, 2017 - Thin Coating of Microporous Organic Network Makes a Big Difference: Sustainability Issue of Ni Electrodes on the PET Textile for Flexibl...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36936-36943

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Thin Coating of Microporous Organic Network Makes a Big Difference: Sustainability Issue of Ni Electrodes on the PET Textile for Flexible Lithium-Ion Batteries Chang Wan Kang,†,⊥ Jaewon Choi,†,⊥ Yoon-Joo Ko,‡ Sang Moon Lee,§ Hae Jin Kim,§ Jong Pil Kim,*,∥ and Seung Uk Son*,† †

Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea Laboratory of Nuclear Magnetic Resonance, The National Center for Inter-University Research Facilities (NCIRF), Seoul National University, Seoul 08826, Korea § Korea Basic Science Institute, Daejeon 34133, Korea ∥ Korea Basic Science Institute, Busan 46742, Korea ‡

S Supporting Information *

ABSTRACT: Poly(ethylene terephthalate) fibers (PET-Fs) were coated with microporous organic networks (MONs) by the Sonogashira coupling of tetra(4-ethynylphenyl)methane with 1,4-diiodobenzene. Ni was deposited on the PET-F@ MON via electroless deposition. Interestingly, although Ni on the PET-F showed a sharp decrease in conductivity in repeated bending tests, the PET-F@MON@Ni showed excellent retention of conductivity. We suggest that thin MON layers play roles of an efficient binder for Ni attachment to fibers and a structural buffer for the relaxation of bending strain. The positive effect of MON was supported by scanning electron microscopy studies of the PET-F@Ni or PET-F@MON@Ni retrieved after 2000 bending numbers. Although Ni on the PET-F showed severe detachment after bending tests, PET-F@MON@Ni retained the original morphologies. The pouch cells of lithium-ion batteries fabricated using PET-F@MON@Ni as the current collectors showed excellent performance against bending. KEYWORDS: microporous organic network, Sonogashira coupling, flexible device, lithium-ion battery, current collector

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been engineered for use as advanced platforms.39−43 However, inorganic materials can gradually detach from organic platforms in the bending of devices because the organic nature of platforms is intrinsically different from that of inorganic metals. In our own tests, Ni metals deposited on PET films showed gradual detachment in repeated bending, resulting in a sharp decrease in the electrochemical performance. Thus, to overcome this problem, efficient materials lubricating and binding the contact of metals with organic platforms44,45 are required. Considering structural flexibilities,33,46 high surface areas, and microporosities of MONs, the incorporation of MON as lubricating44,45 or binding materials between metals and organic platforms may result in unprecedented sustainability. Our research group has explored sustainable and flexible current collectors.38 Carbon materials and organic conducting polymers have been tested as flexible current collectors.47,48 However, their conductivities are limited. The engineered metal

INTRODUCTION Microporous organic networks (MONs) are a recent class of materials.1−8 They can be prepared by the coupling reactions of rigid organic building blocks.1−8 The properties of MONs are comparable to nonporous polymers and covalent organic frameworks (COF).9,10 Similar to the conventional nonporous polymers, the MONs usually have amorphous characteristic and show relatively facile engineering.11−27 Our research group has shown various engineering approaches of MONs based on template methods.24−27 The key features of MONs are their high surface areas and microporosities.1−8 The COFs having crystallinity also show microporosities and high surface areas. However, their engineering is relatively limited compared to that of MONs, despite recent advances.28−32 We have explored new application fields of MONs in which both their microporosities and noncrystalline properties are beneficial.33 Organic polymeric materials are expected to have flexibility and have been applied as platforms for the fabrication of flexible devices such as flexible lithium-ion batteries.34−43 For example, poly(ethylene terephthalate) (PET) films have been used as platforms for the deposition of metallic materials.34−38 Moreover, PET microfibers (PET-Fs) and their textiles have © 2017 American Chemical Society

Received: August 22, 2017 Accepted: October 9, 2017 Published: October 9, 2017 36936

DOI: 10.1021/acsami.7b12653 ACS Appl. Mater. Interfaces 2017, 9, 36936−36943

Research Article

ACS Applied Materials & Interfaces materials such as metal foams have been tested as flexible current collectors,49−53 exhibiting high conductivities but limited sustainability against repeated bending tests. Metal− polymer composites can be optimal materials in the aspects of both conductivity and flexibility.34−39,54 In our exploration for sustainable current collectors, we found the dramatically positive effect of MONs in the binding metallic Ni with PET platforms. In this work, we report the preparation of PET-F@ MON@Ni and its excellent sustainability against repeated bending due to potential lubricant44,45 and binder roles of MON coating, compared to that of PET-F@Ni.

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nium bromide (10 mg, 0.065 mmol) and NaOH (0.30 g, 7.5 mmol) were dissolved in a mixture of methanol (40 mL) and 1,4-dioxane (20 mL). The PET-F@MON plate was added to this mixture and heated at 60 °C for 6 h. The resulting MON tubes were retrieved by filtration, washed with methylene chloride, methanol, distilled water, and acetone, and dried at 80 °C under vacuum. Synthesis of PET-F@MON@Ni and PET-F@Ni. SnCl2 (3.4 g, 18 mmol) was dissolved in a mixture of distilled water (880 mL) and 37% HCl solution (20 mL). Methanol was added until the total volume became 1 L to form the SnCl2 stock solution. PdCl2 (0.10 g, 0.56 mmol) was dissolved in a mixture of distilled water (880 mL) and 37% HCl solution (20 mL). Methanol was added until the total volume became 1 L to form the PdCl2 stock solution. NiCl2·6H2O (9.2 g, 39 mmol), sodium citrate dehydrate (4.7 g, 16 mmol), NH4Cl (10 g, 187 mmol), and NaH2PO2·H2O (8.0 g, 76 mmol) were dissolved in distilled water (500 mL). To this solution, 28% NH4OH solution (10 mL) was added. Distilled water was added to this solution until the total volume became 1 L to form the NiCl2 stock solution. The PETF@MON plate or PET-F plate was cut to the size of 2.5 cm × 3.0 cm. The plate was added to the SnCl2 stock solution (50 mL) in a 70 mL vial and sonicated for 10 min at room temperature. Then, the plate was added to the PdCl2 stock solution (50 mL) in a 70 mL vial for 20 min (without sonication) at room temperature. Then, the plate was added to the NiCl2 stock solution (50 mL) in a 70 mL vial at 54 °C for 10 or 20 or 30 min. The PET-F@MON@Ni and PET-F@Ni plates were washed with distilled water and acetone and dried using N2 flushing. Procedure of Bending Tests. The sheet resistance (Ω/□) of the PET-F@MON@Ni and PET-F@Ni plates in Figure 4a were measured using a four-point probe sheet resistance meter. The sheet resistances of 10 parts of each sample were measured, and the data were statistically treated. The resistance for the bending angle tests in Figure 4b were measured 10 times, and the data were statistically treated. For the data in Figure 4c,d, the two end parts of PET-F@MON@Ni and PET-F@Ni plates were connected to copper plates and fixed with clips. One bending number in Figure 4c,d consisted of changing the bending angles from 180° to 0, 180, 360, and then 180°. The potential in the range of +1.0 to −1.0 V was applied. The resistances were calculated from the slopes obtained from the plots of voltage versus current. Fabrication of Pouch Cells. The PET-F@MON@Ni-30 and PET-F@Ni with a size of 1.25 cm × 3 cm were used as the current collectors. Ni tabs (0.4 cm × 5 cm, Wellcos Co.) were attached to current collectors using a conductive epoxy paste (Chemtronics). The electrodes were dried at 50 °C for 1 h in an oven. To prepare anode materials, graphite (artificial graphite, GS Caltex Co., 80 mg), Super P carbon black (10 mg), and poly(vinylidene difluoride) (PVDF, 10 mg, 13% PDVP in N-methyl-2-pyrrolidone (NMP)) were mixed in Nmethyl-2-pyrrolidone (NMP, 0.10 g). This slurry was coated on one side of the current collector using a spatula. The electrode was dried at 50 °C in an oven for 12 h. Al-laminated film (packing material, Wellcos Co.), with a size of 5.5 cm × 7 cm, was folded in half. A graphite/current collector/Ni tab was attached to the laminated Al film, as a sealer (T-230 K, 220 V, 60 Hz, Wellcos Co.). The polypropylene separator (5.5 cm × 3.5 cm) was loaded on the working electrode. In a glovebox, a Li plate (1 cm × 2 cm) was adhered to the Ni tap (0.4 cm × 5 cm). The Li/Ni tap was loaded on the separator. The electrolyte solution (0.50 mL) of 1 M LiPF6 in 1:1 v/v ethylene carbonate/dimethyl carbonate was added. The pouch cell with a size of 5.5 cm × 3.5 cm was sealed using a sealer. The loading amount of graphite on the current collector was measured as 2.1 mg/cm2. The cycling studies were conducted using a WBCS3000 automatic battery cycler system (WonAtech Co.). The pouch cells were discharged from an open-circuit voltage to 1 mV and cycled between 1 mV and 1.8 V at a current density of 50 mA/g.

EXPERIMENTAL SECTION

General Information. Scanning electron microscopy (SEM) images and energy-disperse X-ray spectra (EDS) were obtained using a field emission scanning electron microscope (JSM6700F). The Kr and N2 adsorption−desorption isotherm curves at 77 K were obtained at the Korea Basic Science Institute (Daejeon, South Korea) using a Micromeritics ASAP2020. The pore size analysis using Kr and N2 isotherm curves were conducted on the basis of the Horvath− Kawazoe (HK) and density functional theory (DFT) method, respectively. Powder X-ray diffraction (PXRD) patterns were obtained using Rigaku MAX-2200 (filtered Cu Kα radiation). The solid-state 13 C NMR spectroscopy (CP-TOSS) was conducted using a 500 MHz Bruker ADVANCE II NMR spectrometer at the NCIRF of Seoul National University, utilizing a 4 mm magic angle spinning probe. Infrared (IR) absorption spectroscopy was performed using a Bruker VERTEX 70 FT-IR spectrometer. The sheet resistance was measured using a four-point probe of an FPP-2000 sheet resistance meter (Dasoleng Co. Ltd). The resistance in the bending tests was obtained via the analysis of voltage−current plots, which were obtained using a Keithley 2400 power supply. The electrochemical studies were performed using a WonAtech ZIVESP electrochemical workstation. Synthesis of PET-F@MON. A nonwoven PET textile plate (diameter of PET fibers: 13−14 μm, weight: 18 g/m2, thickness: 0.1 mm, Toray Advanced Materials Korea Inc.) was cut to the size of 2.5 cm × 6.0 cm. The plate was sonicated in acetone (100 mL) for 10 min at room temperature and dried under air. The plate was sonicated in 1.2 M HCl solution (500 mL) for 1 h, washed with distilled water (300 mL) and acetone (100 mL), and dried at 80 °C in an oven for 3 h. The plate was added to a mixture of triethylamine (35 mL) and toluene (15 mL). (PPh3)2PdCl2 (2.5 mg, 3.6 μmol) and CuI (0.70 mg, 3.7 μmol) were dispersed in toluene (5 mL). This solution was added to the reaction mixture containing the PET plate. The reaction mixture was stirred at 800 rpm for 1.5 h. Tetra(4-ethynylphenyl)methane55 (15 mg, 36 μmol) and 1,4-diiodobenzene (24 mg, 72 μmol) were dissolved in toluene (3 mL). This solution was added to the reaction mixture containing the PET plate. The reaction mixture was stirred for 30 min at 800 rpm at room temperature. The reaction mixture was heated at 90 °C for 20 h without stirring. When the reaction mixture was stirred during the heating, the homogeneous coating could not be obtained because of the separated formation of MON particles. After being cooled to room temperature, the resulting PET-F@MON plate was retrieved and sonicated in methylene chloride, acetone, and ethanol for 5 min each. Then, the PET-F@MON plate was added to distilled water (500 mL) and sonicated for 24 h. After being washed with ethanol, the PET-F@MON plate was dried under vacuum. When we reduced the amount of building blocks to half, the MON coating was incomplete. When we increased the amount of building blocks by two times, the MON coating was contaminated with MON particles (Figure S1 in the Supporting Information, SI). Through the measurement of weight changes of materials before and after MON coating, the content of MON coating in the PET-F@MON was calculated as ∼3.87 wt %, corresponding to ∼2.8% deposition of building blocks to the PET-F (excess building blocks formed the separated MON particles in the reaction mixture). To confirm the MON coating, the inner PET fibers of PET-F@ MON were etched by the following procedure.55 Tetramethylammo-

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RESULTS AND DISCUSSION Figure 1 shows the synthetic schemes for PET-F@MON@Ni and PET-F@Ni. A porous textile plate56 consisting of PET fibers was added to a mixture of tetra(4-ethynylphenyl)36937

DOI: 10.1021/acsami.7b12653 ACS Appl. Mater. Interfaces 2017, 9, 36936−36943

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Figure 1. Synthetic schemes for PET-F@MON@Ni and PET-F@Ni and photographs of PET-F and PET-F@MON.

Figure 2. SEM images of (a−c) PET-F, (d−f) PET-F@MON, (g) PET-F@Ni-10, (h) PET-F@Ni-20, (i) PET-F@Ni-30, (j) PET-F@ MON@Ni-10, (k) PET-F@MON@Ni-20, and (l) PET-F@MON@ Ni-30.

methane55 and 1,4-diiodobenzene. Through the Sonogashira coupling, MON coating was formed on the surface of each PET fiber.16,19,20,57−59 The white textile plate turned bright yellow through the incorporation of MON coating (refer to photographs in Figure 1). Metallic Ni was introduced on the PETF@MON through the electroless deposition39 to form PETF@MON@Ni. As a control material, PET-F@Ni was prepared by the electroless Ni deposition on the textile plate without MON coating. According to scanning electron microscopy (SEM), the PET fibers had an average thickness of 13.7 ± 0.5 μm and a smooth surface (Figure 2a−c). After coating of PET fibers with MONs, the thickness of PET-F@MON slightly increased to 14.4 ± 0.6 μm, with small islands of MON on the surface (Figure 2d,e). Through intensive investigation on PET-F@MON by SEM, we could find the stripped MON from PET-F, indicating the thickness of MON on the PET-F as ∼350 nm. (Figure 2f) The homogeneous coating of MON on the PET fibers was further confirmed by the SEM analysis on the tubular MONs prepared by the removal of the inner PET from PET-F@MON via hydrolysis of the ester bonds.60 (Refer to the Experimental Section and Figure S1 in the SI.) The electroless Ni deposition on PET-F or PET-F@MON resulted in the significant increase of fiber thickness, depending on the deposition time. The PET-F@Ni materials obtained with Ni deposition time of 10, 20, and 30 min were denoted as PET-F@Ni-10, PET-F@Ni-20, and PET-F@Ni-30, respectively. As deposition time increased from 10 min to 20 and 30 min, the thickness of PET-F@Ni increased from 14.3 ± 0.8 μm to 15.2 ± 1.0 and 16.9 ± 1.3 μm, respectively (Figure 2g− i). The Ni thicknesses in PET-F@Ni-10, PET-F@Ni-20, and

PET-F@Ni-30 were ∼0.6, ∼1.5, and ∼3.2 μm, respectively (Table S1 and Figure S2 in the SI). Although PET-F@Ni showed quite a smooth surface, the surface of PET-F@MON@ Ni was relatively rough (Figure 2j−l). The thicknesses of PETF@MON@Ni-10, PET-F@MON@Ni-20, and PET-F@ MON@Ni-30 were 15.1 ± 0.6, 15.9 ± 0.7, and 17.7 ± 1.1 μm, respectively. The Ni thicknesses in PET-F@MON@Ni-10, PET-F@MON@Ni-20, and PET-F@MON@Ni-30 were ∼0.7, ∼1.5, and ∼3.3 μm, respectively. Actually, the thicknesses of Ni in the PET-F@Ni were similar to those of PET-F@MON@Ni with the same Ni deposition time. The analysis of the Kr adsorption isotherm curve showed that the surface areas of PET-F increased from 0.1 to 36 m2/g with the formation of micropores through the MON coating (Figure 3a). The tubular MONs obtained by the base-catalyzed PET etching from PET@MON showed a high surface area of 970 m2/g and microporosity (Vmicropore: 0.32 cm3/g, Figure 3b,c). The powder X-ray diffraction (PXRD) studies showed that the MON layer has amorphous characteristics, which is the conventional property of the MONs prepared by the Sonogashira coupling in the literature1−8,57−59 (Figure S3 in the SI). The chemical component of MON coating was further characterized by solid-state 13C nuclear magnetic resonanace (NMR) spectroscopy. As shown in Figure 3d, 13C peaks of alkynes and benzyl carbon appeared at 91 and 64 ppm, respectively. The 13C peaks of aromatic groups appeared at 122, 131, 137, and 146 ppm. These observations indicate that the 36938

DOI: 10.1021/acsami.7b12653 ACS Appl. Mater. Interfaces 2017, 9, 36936−36943

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showed excellent conductivities. As the amounts of Ni increased, sheet resistances gradually decreased (Figure 4a).

Figure 4. (a) Sheet resistance of PET-F@Ni and PET-F@MON@Ni. (b) Bending angle-dependent conductivity retention of PET-F@ MON@Ni. Conductivity retention tests of (c) PET-F@Ni and (d) PET-F@MON@Ni against repeated bending. Bending angles are defined in (b). One bending number is defined in (d).

The average sheet resistances of PET-F@Ni-10, PET-F@Ni20, and PET-F@Ni-30 were measured as 0.90 ± 0.24, 0.26 ± 0.05, and 0.16 ± 0.04 Ω/□, respectively. As expected, the conductivity of PET-F@Ni increased with increasing Ni content. The sheet resistances of PET-F@MON@Ni-10, PET-F@MON@Ni-20, and PET-F@MON@Ni-30 were similar to the corresponding PET-F@Nis and measured as 0.57 ± 0.09, 0.29 ± 0.04, and 0.14 ± 0.03 0.1 Ω/□, respectively (Figure 4a and Table S1 in the SI). The conductivities of PETF@Ni and PET-F@MON@Ni are much superior to those of the carbon or conducting polymer-based current collectors47,48 and comparable to the metallic ones in the literature.54 At first, we investigated the conductivity retention of PETF@MON@Ni depending on bending angles (the bending angles are defined in Figure 4b). As shown in Figure 4b, PETF@MON@Ni showed nearly same conductivities at any bending angle. Next, we tested the conductivity retention of PET-F@Ni and PET-F@MON@Ni against repeated bending. We changed the bending angles of PET-F@Ni and PET-F@ MON@Ni from 180° to 0, 360, and then 180°, corresponding to one bending test number (Figure 4d). As shown in Figure 4c, the PET-F@Ni-30, PET-F@Ni-20, and PET-F@Ni-10 sharply lost their conductivity after 370, 518, and 1052 bending numbers, respectively. It is noteworthy that as the amount of Ni in PET-F@Ni increased, the retention performance of conductivity became worse. Considering that the amount of

Figure 3. (a) Kr isotherm curves at 77 K and the pore size distribution diagram (based on the HK method) of PET-F and PET-F@MON. (b) N2 isotherm curves at 77 K and pore size distribution diagram (based on DFT method), (c) SEM image, and (d) solid-state 13C NMR spectrum of MON in PET-F@MON. (e, f) PXRD patterns of PETF@Ni and PET-F@MON@Ni.

MONs were formed by the Sonogashira coupling of the used building blocks.17 The infrared (IR) absorption spectroscopy of the tubular MONs showed the main stretching peaks of C−H and CC at 819 and 1509 cm−1, respectively, matching well with that of MON materials prepared by the Sonogashira coupling of the same building blocks61,62 (Figure S4 in the SI). The electroless Ni deposition of PET-F and PET-F@MON was characterized by X-ray diffraction studies.63 As shown in Figure 3e,f, the PXRD patterns of PET-F@Ni and PET-F@MON@Ni showed clear metallic Ni peaks at 44, 52, and 76°, corresponding to the (111), (200), and (220) peaks of the face-centered cubic Ni metals (JCPDS#87-0712), respectively. As deposition time increased, the observed intensity of PXRD peaks increased, supporting the increase of Ni amount in PETF@Ni and PET-F@MON@Ni. The additional diffraction peaks at 17, 23, and 26° correspond to PET (Figure S3 in the SI). We studied electrochemical properties of PET-F@Ni and PET-F@MON@Ni. The PET-F@Ni and PET-F@MON@Ni 36939

DOI: 10.1021/acsami.7b12653 ACS Appl. Mater. Interfaces 2017, 9, 36936−36943

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ACS Applied Materials & Interfaces

SEM images, compared with original ones (Figure S5 in the SI). It is surprising that the thin MON coating resulted in a dramatic difference in the conductivity retention of PET-F@ MON@Ni in repeated bending, compared to that of PET@Ni. Considering the microporosity of MON materials, we suggest that the MON coating may serve as an efficient binder for the more efficient attachment of Ni on the PET platforms. Moreover, we suggest that the MON coating can serve as a structural buffer in repeated bending tests due to its microporous and amorphous nature. These are conventional functions of organic lubricant materials.44,45 We have devised model studies to understand the underlying principles on the role of MON layers in the conductivity retention of PET-F@MON@Ni against bending. Because we could not come up with suitable microscopic model studies, we devised macroscopic ones. First, we prepared two samples: Cu plate on PET film (Cu/PET, Figure 6a,b) and Cu plate on a

Ni in PET-F@Ni should be sufficiently increased to achieve high conductivity, the poorer conductivity retention of PETF@Ni-30 compared to PET-F@Ni-10 and PET-F@Ni-20 is actually desperate. In contrast, all the PET-F@MON@Ni showed the excellent conductivity retention against 2000 bending numbers (Figure 4d and Table S1 in the SI). To elucidate the observed difference in the conductivity retention performance, we investigated the PET-F@Ni and PET-F@MON@Ni materials retrieved after bending tests. Figure 5a−f shows the SEM images of PET-F@Ni-10 (Figure

Figure 5. SEM images of (a, d) PET-F@Ni-10, (b, e) PET-F@Ni-20, (c, f) PET-F@Ni-30, (g, j) PET-F@MON@Ni-10, (h, k) PET-F@ MON@Ni-20, and (i, l) PET-F@MON@Ni-30 retrieved after the repeated bending tests. Figure 6. Macroscopic model studies for the roles of MON in PETF@MON@Ni. Photographs of Cu plate/PET film before bending (a) after 20 bending numbers (b) and Cu plate/polymer sponge/PET film before bending (c) after 20 bending numbers (d). The Cu plate in (b) was broken. Photographs of (f) PET film, (g) Ni/PET film obtained through the electroless Ni deposition, (h) polymer sponge, (i) Ni/ polymer sponge obtained through electroless Ni deposition, and (e) corresponding cartoons.

5a,d), PET-F@Ni-20 (Figure 5b,e), and PET-F@Ni-30 (Figure 5c,f) recovered after 1740, 1190, and 730 bending numbers, respectively. The broken Ni layers on the PET fibers were significantly observed in the center region (bent part) of PETF@Ni materials. As the amount of Ni in PET-F@Ni increased, the damage on the Ni layer became more serious. We think that this damage originates from the intrinsic difference in the chemical nature of the Ni layers from organic PET materials and the resulting inefficient contact. The repeated bending induced the detachment of Ni from the smooth surface of PET fibers. This may be a common reason for the decrease in the electrochemical performance of flexible metal−organic composite electrodes in repeated bending tests. In contrast, the PEF-F@MON@Ni recovered after 2000 bending numbers showed nearly no damage in the SEM images (Figure 5g−l). In control tests, the PET-F@MON recovered after 2000 bending numbers also showed no changes in the

polymer sponge on PET film (Cu/P-S/PET, Figure 6c,d). According to the bending tests for 10 samples each of which were conducted by five independent persons, the Cu plate on Cu/PS-S/PET was broken after 45 ± 7 bending numbers, whereas that of Cu/PET was broken after 22 ± 7 bending numbers. This indicates that the P-S layer plays a positive role in the retention of Cu in bending. A careful investigation on the bending state revealed that the P-S layer can relax the bending strain of Cu due to its structural flexibility and the permissible 36940

DOI: 10.1021/acsami.7b12653 ACS Appl. Mater. Interfaces 2017, 9, 36936−36943

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profiles matched well with those of conventional lithium-ion batteries containing graphite as an electrode material67,68 (Figure S6 in the SI). The pouch cell containing PET-F@ MON@Ni-30 as the current collector showed an average discharge capacity of 360 mAh/g at a current density of 50 mA/ g and stable cycling performance with capacity changes within ±24 mAh/g in bent and flat states (Figure 7b,c). In comparison, the pouch cell containing PET-F@Ni-30 as the current collector showed unstable performance in bent and flat states and a gradual decrease in the capacities from 212 to 89 mAh/g at a current density of 50 mA/g (Figure 7b,c). Interestingly, as shown in Figure 7d, a significant amount of detached black Ni or graphite powders was observed in the transparent pouch cell of PET-F@Ni-30 retrieved after repeated bending. In comparison, such powders were not observed in the pouch cell of PET-F@MON@Ni-30. We think that the unstable performance of the pouch cell of PET-F@Ni30 is attributed to the gradual detachment of Ni from PET-F and the followed loss of graphite. These observations indicate that the sustainability of current collectors is critical for the stable operation of flexible lithium-ion batteries.

moving of Cu into P-S layer through the deformation of the P-S layer (Figure 6e). Second, as shown in Figure 6f,g, the Ni electrolessly deposited on the pristine PET film could not be adhered efficiently, possibly due to the intrinsic difference of metallic Ni from organic PET polymers. In contrast, the Ni deposited on the porous sponge showed efficient adhesion of Ni due to enhanced surface area and excellent conductivity retention (at least for 7000 bending numbers) against bending (Figure 6h,i). Thus, the roles of porous sponge layers can be understood as the relaxing bending strain of the deposited metals and the inducing efficient adhesion of metals on polymer films (Figure 6e). Next, we tested the electrochemical performance of PET-F@ Ni-30 and PET-F@MON@Ni-30 as the current collectors64 for flexible lithium batteries. Flexible pouch-type lithium-ion batteries65,66 were fabricated using a laminated Al film or a transparent polyethylene film as the packing material. The commercial graphite and lithium metal were used as the electrode materials. Graphite (loading amount: 2.1 mg/cm2) was deposited on the current collectors of PET-F@Ni-30 and PET-F@MON@Ni-30. Then, the cells were assembled in a glovebox (see detail procedures in the Experimental Section). As shown in Figure 7a, the pouch cells containing PET-F@ MON@Ni-30 current collectors maintained battery performance with a sustained bulb light in repeated bending. The charge/discharge process of the pouch cells was cycled in the voltage range of 1.8 V−1 mV (vs Li/Li+). The charge/discharge

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CONCLUSIONS This work shows that MON materials can be applied as new lubricant and binder materials between organic polymers and inorganic metals.69 The uniform coating of MON materials on PET fibers was engineered by the Sonogashira coupling of organic building blocks. The MON coating showed a high surface area, microporosity, and amorphous nature. Ni was deposited on the PET-F@MON by electroless deposition. As the control material, Ni was deposited on PET fibers to form PET-F@Ni. Through screening the deposition time, the amount and thickness of Ni were controlled. Interestingly, as the amount of Ni increased, the conductivity retention performance of PET-F@Ni against repeated bending became worse. In contrast, the PET-F@MON@Ni showed excellent conductivity retention. The thin MON coating resulted in the enhanced performance of PET fibers as platforms for flexible lithium-ion battery devices. On the basis of the SEM analysis of PET-F@Ni and PET-F@MON@Ni recovered after the repeated bending tests and model studies, the roles of MON coating were suggested as an efficient binder of Ni to platform and a structural buffer to relax the bending strain due to the amorphous and microporous structure of MON. We believe that various more fibers can be coated with MON to generate unprecedented platform materials for flexible devices.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12653. Additional SEM images of tubular MON obtained through etching of inner PET-F in PET-F@MON, cross-sectional SEM images of PET-F@Ni and PET-F@ MON@Ni, PXRD patterns and IR spectra of PET-F, PET-F@MON, and tubular MON, and charge− discharge profiles of pouch cells (PDF)

Figure 7. (a) Photographs of flexible lithium-ion batteries using PETF@MON@Ni-30 as the current collectors. (b) Cycling stability of pouch-type lithium-ion batteries (current density: 50 mA/g) fabricated using PET-F@MON@Ni-30 or PET-F@Ni-30 as the current collectors and Coulombic efficiencies of the pouch cell of PET-F@ MON@Ni-30. (c) Photographs of the pouch cells of PET-F@MON@ Ni-30 at bent and flat states and (d) transparent flexible pouch cells of PET-F@MON@Ni-30 and PET-F@Ni-30 after bending cycles.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.P.K.). *E-mail: [email protected] (S.U.S.). 36941

DOI: 10.1021/acsami.7b12653 ACS Appl. Mater. Interfaces 2017, 9, 36936−36943

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Seung Uk Son: 0000-0002-4779-9302 Author Contributions ⊥

C.W.K. and J.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2016R1E1A1A01941074) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning and the grants CAP-1502-KBSI (R&D Convergence Program) of National Research Council of Science & Technology (NST) of Korea.

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DOI: 10.1021/acsami.7b12653 ACS Appl. Mater. Interfaces 2017, 9, 36936−36943