Ion Conduction in Polyelectrolyte Covalent Organic Frameworks

specific advantages, such as high energy density, compatibility with lithium salts .... The coordination interaction between Li+ and functional groups...
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Ion Conduction in Polyelectrolyte Covalent Organic Frameworks Qing Xu, Shanshan Tao, Qiuhong Jiang, and Donglin Jiang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03814 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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Journal of the American Chemical Society

Ion Conduction in Polyelectrolyte Covalent Organic Frameworks Qing Xu†, Shanshan Tao†, Qiuhong Jiang†, and Donglin Jiang†* †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543

Supporting Information Placeholder ABSTRACT: Covalent organic frameworks (COFs) with ordered one-dimensional channels could offer a predesigned pathway for ion motion. However, implanting salts into bare channels of COFs gives rise to a limited ion conductivity. Here, we report the first example of polyelectrolyte COFs by integrating flexible oligo(ethylene oxide) chains onto the pore walls. Upon complexation with lithium ions, the oligo(ethylene oxide) chains form a polyelectrolyte interface in the nanochannels and offer a pathway for lithium ion transport. As a result, the ion conductivity was enhanced by more than three orders of magnitude compared to that of ions across the bare nanochannels. The polyelectrolyte COFs promoted ion motion via a vehicle mechanism and exhibited enhanced cycle and thermal stabilities. These results suggest that the strategy for engineering polyelectrolyte interface in the 1D nanochannels of COFs could open a new way to solid-state ion conductors.

Poly(ethylene oxide) (PEO)-based electrolytes are attracting increasing attention for lithium ion batteries (LIBs) owing to their specific advantages, such as high energy density, compatibility with lithium salts, safety, and easy assembly.1 However, PEO hardly meets implementation requirement because it is highly crystalline at low temperature and forms rigid structure that greatly impedes ion motion.1d Various attempts have been made to address this issue by developing PEO-grafted copolymers and PEO-crosslinked networks.1e,2 Nevertheless, these polyelectrolytes lack pathways for quick ion diffusion. On the other hand, porous materials have been used to improve the stability of LIBs by suppressing the growth of lithium dendrites, through either electrode modification with porous compounds or loading liquid electrolytes into pores to form solid-like electrolytes.3 However, these approaches basically could not resolve the leak and safety problems of liquid electrolytes. Thus, design of solid-state polyelectrolytes with well-defined porous structures would offer a platform that can provide ion transport pathway to solve the issue of PEO-based polyelectrolytes and simultaneously address the drawback of liquid electrolytes. Unfortunately, it is difficult to dissociate ionic bonds and to transport lithium ions even if lithium salts are loaded within the bare pores of a porous material. Design of solid-state polyelectrolytes based on porous compounds is still to be well explored. Covalent organic frameworks (COFs) consist of ordered organic skeletons and pores.4-10 By virtue of the diversity of topology design diagram, the availability of building units, and the accessibility of linkages, various COFs with different topologies, skeletons, and pores have been designed and synthesized. The capability of designing porous channels renders COFs able to design outstanding properties ranging from heterogeneous catalysis to

gas adsorption and energy storage.10 The aligned one-dimensional (1D) channels are accessible to proton carries11 and lithium salts,12 thus making COFs attractive for designing ion-conducting polyelectrolytes. Herein, we report the first example of solid-state polyelectrolyte COFs that enable facilitated lithium ion conduction by designing a polyelectrolyte interface within the 1D channels to assist ionic bond dissociation and to offer a pathway for ion transport.

Chart 1. Polyelectrolyte COFs for lithium ion conduction. (A) Structural representation of Li+@TPB-DMTP-COF with bare pore walls that lack polyelectrolyte interface. (B) Structural representation of Li+@TPB-BMTP-COF with oligo(ethylene oxide) chains on pore walls to form a polyelectrolyte interface. TPB-DMTP-COF and TPB-BMTP-COF were synthesized by condensation of 1,3,5-tri(4-aminophenyl)benzene (TPB) with 2,5dimethoxyterephthalaldehyde (DMTP) or 2,5-bis ((2methoxyethoxy) methoxy)terephthalaldehyde (BMTP) under solvothermal conditions in yields of 82% and 78%, respectively (Chart 1, Supporting Information). TPB-DMTP-COF has methoxy groups on the edge phenyl units, while TPB-BMTP-COF holds flexible oligo(ethylene oxide) chains on the channel walls. From the element analysis, the C, H, and N contents are close to the theoretical values of TPB-BMTP-COF (Table S1). From the nitrogen sorption isotherm curve (Figure 1A, blue curve), TPB-DMTP-COF exhibited a Brunauer–Emmett–Teller (BET) surface area of 2658 m2 g–1, a pore size of 3.26 nm, and a pore volume of 1.34 cm3 g–1 (Figure 1B), respectively.13 TPBBMTP-COF has a BET surface area of 1746 m2 g–1 (Figure 1C, red curve), a pore size of 3.02 nm, and a pore volume of 0.96 cm3 g–1 (Figure 1D), respectively. From the powder X-ray diffraction (PXRD) patterns, TPBDMTP-COF exhibited peaks at 2.76°, 4.82°, 5.60°, 7.42°, 9.70°, and 25.2°, which were assigned to the (100), (110), (200), (210), (220), and (001) facets, respectively (Figure 2, blue curve). TPB-

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DMTP-COF adopts an eclipsed AA stacking model.11a The corresponding PXRD peaks of TPB-BMTP-COF (Figure 2, red curves) slightly shifted to 2.82°, 5.04°, 5.73°, 7.45°, 9.97°, and 25.7°, respectively. The experimentally observed curve agree well with the Pawley-refined PXRD curve as confirmed by their negligible difference with the RWP and RP values of 6.32% and 4.56%, respectively (Figure S1; Tables S2 and S3).

Figure 1. Porosity. (A) Nitrogen sorption isotherm profiles of TPB-DMTP-COF (blue) and Li+@TPB-DMTP-COF (yellow). (B) Pore size (black) and pore size distribution (red) profiles of TPBDMTP-COF. (C) Nitrogen sorption isotherm profiles of TPBBMTP-COF (red) and Li+@TPB-BMTP-COF (black). (D) Pore size (black) and pore size distribution (red) profiles of TPBBMTP-COF. The thermal instability of traditional polyelectrolytes is a critical issue. From the thermogravimetric analysis (TGA) curves, TPB-DMTP-COF and TPB-BMTP-COF under nitrogen did not show decomposition before 400 and 300 °C, respectively (Figure S2), which are much higher than traditional PEO-based polyelectrolytes. TPB-DMTP-COF and TPB-BMTP-COF were loaded with LiClO4 through solution diffusion method to yield Li+@TPBDMTP-COF and Li+@TPB-BMTP-COF, respectively (Supporting Information). According to X-ray photoelectron spectroscopy analysis (Figure S3), the contents of Li+ in Li+@TPB-DMTPCOF and Li+@TPB-BMTP-COF are 5.1 wt% and 4.3 wt%, respectively, which are close to their loading contents of 5.0 wt% and 4.6 wt%. The Li+@TPB-DMTP-COF (Figure 1A, yellow curve) and Li+@TPB-BMTP-COF samples (Figure 1C, black curve) possess the BET surface areas of only 20 m2 g–1. In the PXRD patterns, the peaks owing to the (100) facets disappeared, while new peaks appeared at 21°, 23°, and 31° that were assigned to the (101), (110), and (201) faces of LiClO4 (Figure S4).14 From the Field-emission scanning electron microscopy (FE-SEM) images, the macroscopic morphology of Li+@TPB-DMTP-COF and Li+@TPB-BMTP-COF are the same as those of TPB-DMTP-COF and TPB-BMTP-COF. Moreover, no free LiClO4 particles were observed on the COF surface (Figure S5). Infrared spectrum (Figure S6) further suggests the presence of lithium salts in COFs. The FE-SEM elemental mapping analysis revealed that LiClO4 was uniformly distributed in Li+@TPB-DMTP-COF (Figure S7) and Li+@TPB-DMTP-COF (Figure S8). The coordination interaction between Li+ and functional groups was verified by electronic absorption spectroscopy. Li+@TPBDMTP-COF exhibited a red-shifted shoulder at 565 nm (Figure

S9A, red curve), while Li+@TPB-BMTP-COF showed a more intense shoulder peak (Figure S9B, red curve). These new shoulders reflect the charge transfer from the electron-rich methoxy group and ethylene oxide chain to the electron-deficient Li+.15 Li+@TPB-DMTP-COF and Li+@TPB-BMTP-COF did not show decomposition up to 100 °C under nitrogen as revealed by the TGA measurements (Figure S10). After loading with LiClO4 and keeping in the air for over one month, the TPB-DMTP-COF and TPB-BMTP-COF upon washing LiClO4 exhibited the BET surface areas of 2450 and 1600 m2 g–1, respectively (Figure S11) with slightly decreased pore volume (Figure S12). From the PXRD curves, the crystallinity of COFs is well retained (Figure S13). These results further verified the exceptional long-term stability of the COFs in the presence of lithium salts. The ion conductivity was measured by using COF pellets and alternating-current impedance spectroscopy. The Nyquist plots of Li+@TPB-DMTP-COF under nitrogen atmosphere were obtained from 40 to 90 °C (Figure 3A; Figure S14). The plots of the real component (Z) versus the imaginary component (Z″) of the complex impedance function displayed a semicircular curve followed by a spike.16 The resistances of Li+@TPB-DMTP-COF were evaluated to be 1.59 × 106, 3.21 × 105, and 4.02 × 104 Ω at 40, 60, and 80 °C, respectively (Figure 3 A-C). These results gave rise to the ion conductivities of 1.36 × 10–7, 6.74 × 10–7, and 5.37 × 10–6 S cm–1 at 40, 60, and 80 °C, respectively. The Nyquist plots of Li+@TPB-BMTP-COF were obtained under the same condition as those of Li+@TPB-DMTP-COF (Figure 3D-F; Figure S15). Compared to Li+@TPB-DMTP-COF, the corresponding ion conductivities were evaluated to be 6.04 × 10–6, 2.85 × 10–5 and 1.66 × 10–4 S cm–1, which were 44, 42 and 30 times higher than those of Li+@TPB-DMTP-COF. Notably, the ion conductivity at 90 °C reaches 5.49 × 10–4 S cm–1. These results are remarkable because the ion conductivity of PEO-Li+ complex is only 8.0 ×10–8 S cm–1 at 40 °C (Table S4).1f Thus, the polyelectrolyte chains aligned on the pore walls of the COFs greatly promote the ion motion. We measured the conductivity of TPB-BMTP-COF in the absence of LiClO4 at the same temperature range and did not observe conductivity (Figure S16). Thus, the COF skeleton itself is an electric insulator.

Figure 2. Crystallinity. PXRD curves of TPB-DMTP-COF (blue) and TPB-BMTP-COF (red). Compared to Li+@TPB-DMTP-COF, the greatly enhanced ion conductivity observed for Li+@TPB-BMTP-COF originates from the presence of dense oligo(ethylene oxide) chains that form a polyelectrolyte interface in the channels upon complexation with lithium ions. This built-in interface facilitates the dissociation of ionic bond and offers a pathway for lithium ion transport between the neighboring oligo(ethylene oxide) chains. These results indi-

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Journal of the American Chemical Society cate that engineering a polyelectrolyte interface in the channels is of substantial importance for designing ion-conducting COFs. As a control, TPB-TP-COF with bare pore walls was synthesized (Figure S17). TPB-TP-COF owing to the lack of resonance effect on the phenyl linker yields low crystallinity and low porosity.11b TPB-TP-COF exhibited a BET surface area of 145 m2 g–1, a pore size of 3.3 nm, and a pore volume of 0.1 cm3 g–1 (Figure S18) with weak PXRD signals (Figure S19). The ion conductivity of Li+@TPB-TP-COF was 3.25 × 10–9, 4.21×10–8, and 2.27 × 10–7 S cm–1 at 40, 60, and 80 °C, respectively (Figure S20). Comparted to Li+@TPB-DMTP-COF, the extremely low ion conductivity of Li+@TPB-TP-COF is resulted from its low porosity that greatly decreases the content of lithium salts, while the methoxy groups of Li+@TPB-DMTP-COF help the dissociation of ionic bonds of LiClO4. Remarkably, the ion conductivity of Li+@TPB-BMTPCOF at 40 °C is enhanced by 1858 fold upon integrating the oligo(ethylene oxide) chains onto the pore walls of the same COF skeleton.

3510 Ω (Figure S22E), giving rise to a conductivity of only 6.85 ×10–5 S cm–1. This instability reflects that the physically implanted polyelectrolytes are not strong enough to reach a stable polyelectrolyte interface owing to the easy outflow of the polyelectrolytes from the pores at high temperature; covalently integrating oligo(ethylene oxide) chains on the pore walls to engineer a polyelectrolyte interface is essential for achieving a stable performance.

Figure 4. Arrhenius Plots. Temperature dependencies of ion conductivities of Li+@TPB-DMTP-COF (black) and Li+@TPBBMTP-COF (red).

Figure 3. Impedance spectroscopy. Nyquist plots of Li+@TPBDMTP-COF (A-C) and Li+@TPB-BMTP-COF (D-F) measured at 40, 60, and 80 °C, respectively. The cyclic stability of polyelectrolytes at high temperature is another important issue. The cyclic stability of Li+@TPB-BMTPCOF was measured at 90 °C for 24 h (Figure S21). After 24 h, the resistance of Li+@TPB-BMTP-COF exhibited no changes, and the corresponding ion conductivity was retained at 5.49 ×10–4 S cm–1. To investigate the mechanism of lithium ion transport in the COFs, ion conductivities were plotted as a function of temperature. As shown in Figure 4, Li+@TPB-DMTP-COF exhibited a typical Arrhenius-type linear curve (black curve) and the activation energy (Ea) was calculated as high as 0.96 eV. By contrast, the Ea of Li+@TPB-BMTP-COF was decreased to be 0.87 eV (red curve). On the other hand, the Ea of Li+@TPB-TP-COF was 1.05 eV (Figure S19D). The lowest activation energy observed for Li+@TPB-BMTP-COF suggests that the oligo(ethylene oxide) chains in the nanochannels assist the dissociation of ionic bond of LiClO4 and upon complexation with lithium ions2a form a polyelectrolyte phase that offers a pathway to promote ion transport across the channels. Such a built-in polyelectrolyte interface is not available for TPB-DMTP-COF and TPB-TP-COF. According to the Ea value, the ions transport across the COF channels via a vehicle mechanism. We loaded a complex (Li+PEO400) of LiClO4 and polyethylene glycol (Mn = 400) into the channels of TPB-DMTP-COF to prepare Li+PEO400@TPB-DMTP-COF as a control of Li+@TPBBMTP-COF. The resistance changed from 3020 to 1945, 1418, and 1120 Ω at 40, 50, 60, and 70 °C, respectively (Figure S22AD). During the heating process, the resistance increases and does not yield a constant at 80 °C. Moreover, after a continuous measurement at 70 °C for 1 h, the resistance increased significantly to

In summary, we reported the first example of polyelectrolyte COFs by integrating oligo(ethylene oxide) chains onto the channels walls to create a polyelectrolyte interface upon complexation with lithium ions. The covalent integration of ethylene oxide chains allows for the engineering of a solid-state polyelectrolyte that can combine thermal, long-term, and cycle performance stabilities. More remarkably, the engineered polyelectrolyte helps to dissociate the ionic bond of lithium salts upon complexation with lithium ions, offers a pathway for ion conduction, and promotes the ion transport through a low energy-barrier vehicle mechanism. These results are of substantial importance to the field of solidstate polyelectrolytes. We anticipate that the present strategy also opens a new aspect of COFs for designing ion conductors.

ASSOCIATED CONTENT Supporting Information Supporting information is available free of charge via the Internet at http://pubs.acs.org. Materials and methods, supporting figures, supporting tables, and supporting references.

AUTHOR INFORMATION Corresponding Author

*[email protected] ORCID Donglin Jiang: 0000-0002-3785-1330 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT D.J. appreciates the start-up grant of NUS (R-143-000-A28-133).

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