Thin Coating of Microporous Organic Network Makes a Big Difference

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12653 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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ACS Applied Materials & Interfaces

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, NCIRF, Seoul National University, Seoul 08826, Korea ∞ Korea Basic Science Institute, Daejeon 34133, Korea ∂ Korea Basic Science Institute, Busan 46742, Korea §

ABSTRACT: Polyethylene terephthalate fibers (PET-Fs) were coated with microporous organic networks (MONs) by the Sonogashra coupling of tetra(4-ethynylphenyl)methane with 1,4-diiodobenzene. Ni was deposited on the PET-F@MON via electroless deposition. Interestingly, while Ni on the PET-F showed a sharp decrease in conductivity in repeated bending tests, the PETF@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 SEM studies of the PET-F@Ni or PET-F@MON@Ni retrieved after 2000 bending numbers. While 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  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 MONs, despite of recent advances.28-32 We have explored new application fields of MONs in which both their microporosities and non-crystalline 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. 35-44 For example, polyethylene terephthalate (PET) films have been used as platforms for the deposition of metallic materials. 35-39 Moreover, PET microfibers (PET-Fs) and their textiles have been engineered for use as advanced platforms. 40-44 However, inorganic materials can gradually detach from organic platforms in the bending of devices because organic nature of platforms is intrinsically different from 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 platforms45-46 are required. Considering structural flexibilities33-34, high surface areas, and microporosities of MONs, the incorporation of MON as lubricating45-46 or binding materials between metals and organic platforms may result in unprecedented sustainability. Our research group has explored sustainable and flexible current collectors.39 Carbon materials and organic conducting polymers have been tested as flexible current collectors. 47-48 However, their conductivities are limited. The engineered metal materials such as metal foams have been tested as flexible current collectors,49-53 exhibiting high conductivities but limited sustainabilities against repeated bending tests. Metalpolymer composites can be optimal materials in the aspect of both conductivity and flexibility.35-40,54 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 PETF@MON@Ni and its excellent sustainability against repeated bending due to potential lubricant45-46 and binder roles of MON coating, compared to PET-F@Ni.  EXPERIMENTAL SECTION General Information. Scanning electron microscopy (SEM) images and energy disperse X-ray spectra (EDS) were obtained using a FE-SEM (JSM6700F). The Kr and N2 adsorption-desorption isotherm curves at 77K were obtained at the Korea Basic Science Institute (Daejeon, South Korea) using a Micromeritics ASAP2020. The pore size analysis using Kr and

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N2 isotherm curves were conducted based on the HorvathKawazoe (HK) and density functional theory (DFT) method, respectively. Powder X-ray diffraction (PXRD) patterns were obtained using a Rigaku MAX-2200 (filtered Cu-K radiation). The solid state 13C-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 absorption spectroscopy (IR) was performed using a Bruker VERTEX 70 FTIR spectrometer. The sheet resistance was measured using a four point probe of a 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 m, 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 80oC 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 mol) and CuI (0.70 mg, 3.7 mol ) 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(4ethynylphenyl)methane55 (15 mg, 36 mol) and 1,4diiodobenzene (24 mg, 72 mol) 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 90oC 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 measurment of weight changes of materials before and after MON coating, the contents of MON coating in the PET-F@MON was calculated as ~3.87w%, 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 PETF@MON were etched by the following procedure.55 Tetramethylammonium 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 PETF@MON plate was added to this mixture and heated at 60 oC for 6 h. The resulting MON tubes were retrieved by filtration, washed with methylene chloride, methanol, distilled water, and acetone and dried at 80oC under vacuum.

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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. PET-F@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 sonnication) at room temperature. Then, the plate was added to the NiCl2 stock solution (50 mL) in a 70 mL vial at 54oC for 10 min or 20 min or 30 min. The PETF@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 resitance meter. The sheet resistances of 10 parts of each sample were measured and the data were statistically treated. The resistance for the bending anlge tests in Figure 4b were measured 10 times and the data were statistically treated. For the data in Figures 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 Figures 4c-d consisted of changing the bending angles from 180o to 0o, 180o, 360o, and then 180o. The potential in the range of +1.0 ~ -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 50oC 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 polyvinylidene fluoride (PVDF, 10 mg, 13% PDVP in NMP) were mixed in N-methyl-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 50oC 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. Graphite/current collector/Ni tab was attached to the laminated Al film using as a sealer (T-230K, 220 V, 60 Hz, Wellcos Co.). The polypropylene separator (5.5 cm  3.5 cm) was loaded on the working electrode. In a glove box, a Li plate (1 cm  2 cm) was adhered to 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 (EC/DMC) 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 dis-


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charged 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.  RESULTS and DISCUSSION Figure 1 shows the synthetic schemes for PET-F@MON@Ni and PET-F@Ni. A porous textile plate57 consisting of PET fibers was added to a mixture of tetra(4ethynylphenyl)methane55 and 1,4-diiodobenzene. Through the Sonogashira coupling, MON coating was formed on the surface of each PET fiber.16,19-20,58-60 White textile plate turned bright yellow through the incorporation of MON coating. (Refer to photographs in Figure 1) Metallic Ni was introduced on the PET-F@MON through the electroless deposition40 to form PET-F@MON@Ni. As a control material, PET-F@Ni was prepared by the electroless Ni deposition on the textile plate without MON coating.

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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) PETF@MON@Ni-10, (k) PET-F@MON@Ni-20, and (l) PETF@MON@Ni-30.

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

According to scanning electron microscopy (SEM), the PET fibers had an average thickness of 13.7 ± 0.5 m and smooth surface. (Figures 2a-c) After coating of PET fibers with MONs, the thickness of PET-F@MON slightly increased to 14.4 ± 0.6 m with small islands of MON on the surface. (Figures 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.56 (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 m to 15.2 ± 1.0 and 16.9 ± 1.3 m, respectively. (Figures 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 m respectively. (Table S1 and Figure S2 in the SI) While PET-F@Ni showed a quite smooth surface, the surface of PET-F@MON@Ni was relatively rough. (Figures 2j-l) The thicknesses of PETF@MON@Ni-10, PET-F@MON@Ni-20, and PETF@MON@Ni-30 were 15.1 ± 0.6, 15.9 ± 0.7, and 17.7 ± 1.1 m, 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 m 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 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). (Figures 3b-c) The powder X-ray diffraction (PXRD) studies showed that the MON layer has amorphous characteristics, which is the con-



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ventional property of the MONs prepared by the Sonogashira coupling in the literature.1-8,58-60 (Figure S3 in the SI) (a)

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the observed intensity of PXRD peaks increased, supporting the increase of Ni amount in PET-F@Ni and PETF@MON@Ni. The additional diffraction peaks at 17, 23, and 26o 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 showed excellent conductivities. As the amounts of Ni increased, sheet resistances gradually decreased. (Figure 4a) (a)

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

The chemical component of MON coating was further characterized by solid state 13C nuclear magnetic spectroscopy (NMR). 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 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 blocks.61-62 (Figure S4 in the SI) The electroless Ni deposition of PET-F and PET-F@MON was characterized by XRD studies.63 As shown in Figures 3e-f, the PXRD patterns of PET-F@Ni and PET-F@MON@Ni showed clear metallic Ni peaks at 44, 52, and 76o, corresponding to the (111), (200), and (220) peaks of the face centered cubic Ni metals (JCPDS#87-0712), respectively. As deposition time increased,

Figure 4. (a) Sheet resistance of PET-F@Ni and PETF@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 Figure 4b. One bending number is defined in Figure 4d.

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, PETF@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 PET-F@Ni and PET-F@MON@Ni are much superior to 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 PETF@Ni and PET-F@MON@Ni against repeated bending. We changed the bending angles of PET-F@Ni and PETF@MON@Ni from 180o to 0, 360, and then 180o, correspond-


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ing 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 Ni in PET-F@Ni should be sufficiently increased to achieve high conductivity, the poorer conductivity retention of PET-F@Ni30, 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. Figures 5a-f show the SEM images of PET-F@Ni-10 (Figures 5a,d), PET-F@Ni-20 (Figures 5b,e), and PET-F@Ni-30 (Figures 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 PET-F@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. (Figures 5g-l) In control tests, the PET-F@MON recovered after 2000 bending numbers also showed no changes in the 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 PETF@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.45-46

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Figure 5. SEM images of (a,d) PET-F@Ni-10, (b,e) PET-F@Ni20, (c,f) PET-F@Ni-30, (g,j) PET-F@MON@Ni-10, (h,k) PETF@MON@Ni-20, and (i,l) PET-F@MON@Ni-30 retrieved after the repeated bending tests.

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, Figures 6a-b) and Cu plate on a polymer sponge on PET film (Cu/P-S/PET, Figures 6c-d). According to the bending tests for 10 samples each of which were conducted by 5 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 moving of Cu into P-S layer through the deformation of the P-S layer. (Figure 6e) Second, as shown in Figures 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. (Figures 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)



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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@Ni-30 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.

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PET-F @Ni-30

Figure 6. Macroscopic model studies for the roles of MON in PET-F@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 Figure 6b 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.

Next, we tested electrochemical performance of PET-F@Ni30 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 packing materials. 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@Ni30. Then, the cells were assembled in a glove box. (See detail procedures in the Experimental Section) As shown in Figure 7a, the pouch cells containing PETF@MON@Ni-30 current collectors maintained battery performance with a sustained blub 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 profiles matched well with those of conventional lithium ion batteries containing graphite as an electrode material.67-68 (Figure S6 in the ESI) The pouch cell containing PETF@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. (Figures 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 mAh/g to 89 mAh/g at a current density of 50 mA/g. (Figures 7b-c) Interestingly, as shown in Figure 7d, a significant amount of detached black Ni or graphite powders was observed in the

Detached Ni pieces

(c)

(d)

PET-F @MON @Ni-30

PET-F @Ni-30

Figure 7. (a) Photographs of flexible lithium ion batteries using PET-F@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 PETF@Ni-30 after bending cycles.



CONCLUSION 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. Thin MON coating resulted in the enhanced performance of PET fibers as platforms for flexible lithium ion battery devices. Based on the SEM analysis of PET-F@Ni and


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PET-F@MON@Ni recovered after the repeated bending tests and the 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 more various fibers can be coated with MON to generate unprecedented platform materials for flexible devices.  SUPPORTING INFORMATION Additional SEM images of tubular MON obtained through etching of inner PET-F in PET-F@MON, cross section 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. This material is available free of charge via the Internet at http://pubs.acs.org.  AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Author Contributions ‡These authors contributed equally

Notes The authors declare no competing financial interests.

 ACKNOWLEDGMENT This work was supported by 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-15-02-KBSI (R&D Convergence Program) of National Research Council of Science & Technology (NST) of Korea.

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cells with PET-F@MON-Ni-30 showed poor performance at high current densities, they showed excellent recovery of capacities at low current densities. (Figure S10 in the SI) (65) Marinaro, M.; Yoon, D.; Gabrielli, G.; Stegmaier, P.; Figgemeier, E.; Spurk, P. C.; Nelis, D.; Schmidt, G.; Chauveau, J.; Axmann, P.; Wohlfahrt-Mehrens, M. High Performance 1.2 Ah Sialloy/Graphite/LiNi0.5Mn0.3Co0.2O2 Prototype Li-Ion Battery. J. Power Sources 2017, 357, 188-197. (66) Liao, J. –Y.; Oh, S. –M.; Manthiram, A. Core/Double-Shell Type Gradient Ni-Rich LiNi0.76Co0.10Mn0.14O2 with High Capacity and Long Cycle Life for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 24543-24549. (67) Wang, X.; Xing, L.; Liao, X.; Chen, X.; Huang, W.; Yu, Q.; Xu, M.; Huang, Q.; Li, W. Improving Cyclic Stability of Lithium Cobalt Oxide Based Lithium Ion Battery at High Voltage by Using Trimethylboroxine as an Electrolyte Additive. Electrochim. Acta 2015, 173, 804-811. (68) Cao, X.; Li, Y.; Li, X.; Zheng, J.; Gao, J.; Gao, Y.; Wu, X.; Zhao, Y.; Yang, Y. Novel Phosphamide Additive to Improve Thermal Stability of Solid Electrolyte Interphase on Graphite Anode in Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2013, 5, 11494-11497. (69) The copper also could be incorporated into PET-F@MON by the electroless deposition. In addition, other fibers could be coated by the synthetic method of this work. (Figure S11 in the SI)



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Table of Contents

Lubricant & Binder Roles

Amorphous Nature & Microporosity MON Enhanced Sustainability against Bending

VS

PET-F@MON@Ni


PET-F@Ni

10

Thin Coating of Microporous Organic Network Makes a Big Difference

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