Accumulation of Glassy Poly(ethylene oxide) Anchored in a Covalent

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Accumulation of Glassy Poly(ethylene oxide) Anchored in Covalent Organic Framework as Solid-state Li+ Electrolyte Gen Zhang, You-lee Hong, Yusuke Nishiyama, Songyan Bai, Susumu Kitagawa, and Satoshi Horike J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07670 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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Journal of the American Chemical Society

Accumulation of Glassy Poly(ethylene oxide) Anchored in Covalent Organic Framework as Solid-state Li+ Electrolyte Gen Zhang,† You-lee Hong,‡ Yusuke Nishiyama,‡,¶ Songyan Bai,§ Susumu Kitagawa,*† and Satoshi Horike*,†,§,|| †Institute

for Integrated Cell-Material Sciences-Vidyasirimedhi Institute of Science and Technology Research Center, Institute for Advanced Study, and §AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEMOIL), National Institute of Advanced Industrial Science and Technology (AIST), Yoshida-Honmachi, Sakyo-ku, Kyoto 6068501, Japan. ‡RIKEN CLST-JEOL Collaboration Center, Tsurumi, Yokohama, Kanagawa 230-0045, Japan ¶JEOL

RESONANCE Inc., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan. of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. ||Department

ABSTRACT: Design of molecular structures showing fast ion conductive/transport in solid state has been a significant challenge. Amorphous or glassy phase in organic polymers works well for fast ion conductivity, because of their dynamic and random structure. However, the issues about these polymers has been the difficulty to elucidate the mechanisms of ion conduction, and thus low designability. Furthermore, the amorphous or glassy state of ion conductive polymers often confronts the problems of structural/mechanical stabilities. Covalent organic framework (COF) are an emerging class of crystalline organic polymers with periodic structure and tunable functionality, which exhibit potential as a unique ion conductor/transporter. Here, we describe the use of COF as a medium for all-solid-state Li+ conductivity. A bottom-up self-assembly approach was applied to covalently reticulate the flexible, bulky, and glassy poly(ethylene oxide) (PEO) moieties that can solvate Li+ for fast transport by their segmental motion in the rigid two-dimensional COF architectures. Temperature-dependent powder X-ray diffraction and thermogravimetric analysis showed the periodic structures are intact even above 300 C, and differential scanning calorimetry and solid-state NMR revealed the accumulated PEO chains are highly dynamic and exhibit a glassy state. Li+ conductivity was found to depend on the dynamics and length of PEO chains in the crystalline states, and solid-state Li+ conductivity of 1.33  10−3 S cm−1 was achieved at 200 C after LiTFSI doping. The high conductivity at the specified temperature remains intact for extended periods of time as a result of the structure robustness. Furthermore, we demonstrated the first application of a COF electrolyte in an all-solid state Li battery at 100 C.

INTRODUCTION The design of efficient ion conductive/transport pathways in solid state has been a challenge for the development of functions of electrolyte for battery/fuel cell, electrocatalysis, and gas sensing for instances. Li+,1 Na+,2 H+,3-4 and recently H− have been highlighted as important carriers for these applications.5 In general, organic polymer-based ion conductivity requires structural dynamics and high concentration of ion hopping sites. To accumulate both dynamic and highly concentrated functional groups for ion conduction, amorphous or glassy phases are preferred.6-10 On the other hand, amorphous polymers for ion conductivity still present some issues that need to be addressed; thus, their random and heterogeneous structure prevents the precise design and understanding of structures. Moreover, their thermal and mechanical stabilities often experience a trade-off of high ion conductivity. Covalent organic frameworks (COFs) are an emerging class of crystalline materials that are constructed by linking organic building blocks via covalent bonds.11-17 The chemistry of COF has mainly focused on the construction of diverse porous structures and their accompanying functions such as separation

and catalysis. Recently, several studies on ion conductivity using COF have been reported.18-24 However, most of these works require solvent support to achieve efficient ion conductivity, and temperature below 100 C are needed in all cases, an inherent limitation of conventional liquid electrolyte system. To overcome this drawback of conventional amorphous polymers, and explore the new applications with solid-state molecular ion conductor, the defined COF architecture constitutes an attractive option due to its structural designability and integrity. In this context, we have incorporated poly(ethylene oxide) (PEO) chains into the inner space of twodimensional (2D) COFs, thereby demonstrating rational construction of Li+ conduction pathway in their crystalline states. Unlike conventional amorphous Li+ conductive organic polymers, we could discuss Li+ conduction mechanism with related to the crystal structures by X-ray and solid-state NMR, and the COF-based Li+ conductors represent exceptionally high thermal/mechanical stability. PEO is a bulky and flexible functional group that can solvate Li+ for fast transport by their segmental motion.25-29 Amorphous PEO polymers have been mainly studied for high Li+ conductivity, whereas reports on Li+ transport on crystalline PEO structures are scarce.30-32 To introduce dense and flexible PEO groups into COFs while

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retaining their crystallinity, we applied a direct self-assembly approach that involves pre-organization of the functional groups into the building blocks.33-35 Other approaches that consist of post-synthetic modification or physical impregnation

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of PEO polymer into a porous material have been used,36-38 but these often result in low concentration of such bulky groups and insufficient thermal/chemical/mechanical stability.

Figure 1. (a) Synthesis of COF-PEO-x (x = 3, 6, 9). (b) Observed PXRD pattern (i), refined modeling profile (ii), simulated PXRD pattern for an eclipsed structure (iii), difference between the observed and refined PXRD patterns (iv) for COF-PEO-3. (c) PXRD patterns of COFPEO-x (x = 3, 6, 9). (CuK) = 1.5418 Å.

RESULTS AND DISCUSSION Three PEO-functionalized hydrazone-linked COFs were synthesized under solvothermal conditions (Figure 1a) by the condensation of 1,3,5-triformylbenzene with three PEO-based hydrazide monomers of different length, i.e., PEO-x (x = 3, 6, or 9, depending on the number of PEO units). The corresponding COFs, denoted as COF-PEO-x (x = 3, 6, or 9) hereafter, were produced as microcrystalline powders insoluble in common organic solvents. The crystallinity of all COF-PEOx was evaluated by powder X-ray diffraction (PXRD, Figures 1b and 1c). We used the analogous COF-42, which was constructed from the same monomers but without PEO, for comparison.12 The result of the Pawley refinement of the pattern of COF-PEO-3 was in good agreement with the experimentally observed pattern, with negligible difference (wRp = 3.18% and Rp = 5.09%). The lattice model of COF-PEO-3 was simulated by using the Materials Studio suite of program, which yielded an eclipsed structure with the optimized parameters of a = b = 25.550 Å and c = 3.774 Å for the unit cell with the space group

of P3, which is identical to that of COF-42. The simulated PXRD of COF-PEO-3 with an eclipsed structure was consistent with the experimental data, whereas that with a staggered stacking structure did not match the observed data (Figure S1). As can be seen in Figure 1b, COF-PEO-3 show the highest peak at 3.66°, which is attributable to the (100) diffraction. On the other hand, the peaks observed at 7.2, 9.4, 12.6, and 26.0° correspond to the (200), (210), (220), and (001) diffractions, respectively. No diffraction peaks attributable to the starting materials PEO-3 or 1,3,5-triformylbenzene were observed (Figure S2), indicating the complete formation of crystalline frameworks. The PXRD patterns for COF-PEO-6 and COFPEO-9 show broader peaks (Figure 1c), but the position of the (100) and (200) peaks is the same as for COF-PEO-3, which supports the formation of 2D layer structures. In contrast, the peaks corresponding to (001), which represent the stacking of layers along the c axis, shifted to lower 2θ angle as the PEO chain length increases, with the concomitant decrease in the crystallinity due to the expansion of the layer–layer packing. The bilayer distances from the (001) peaks were estimated to be


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3.8, 4.0, and 4.2 Å for each COF-PEO-x (x = 3, 6, 9). Further characterization of the structures by solid-state NMR, FT-IR, and elemental analysis is discussed below.

temperature PXRD pattern (Figure 4c). The observed peaks remained unaltered in the range of −123 °C to 117 °C, suggesting that the overall crystalline structure underwent negligible thermal expansion. The structural integrity upon heating/cooling is also an advantage for a solid-state ion conductor.

Figure 2. SEM images of (a) COF-PEO-3, (b) COF-PEO-6, (c) COF-PEO-9, (d) COF-PEO-3-Li, (e) COF-PEO-6-Li, and (f) COFPEO-9-Li.

All COF-PEO-x were subjected to scanning electron microscopy (SEM), whose corresponding images revealed sphere morphologies (Figures 2a–2c). The results of the elemental analyses of all COF-PEO-x were in good agreement with the expected chemical formulas (Table S2). The Fourier transform infrared (FT-IR) spectra of all COF-PEO-x exhibited vibration bands at 1684 cm−1, which can be assigned to the −C=N− stretching (Figure 3a). The concomitant disappearance of the NH2 stretching vibrations (3376 and 3323 cm−1) characteristic of the hydrazide monomer PEO-x and the C=O stretching vibration at 1692 cm−1 of 1,3,5-triformylbenzene (Figure S7) confirmed the successful formation of imine bonds. In addition, the similarity of these FT-IR spectra with that of COF-42 supports the formation of a 2D network with PEO chains.12 Meanwhile, the high thermal stability of all COFPEO-x was demonstrated by thermogravimetric analysis (TGA), which showed no weight loss up to 340 °C (Figure 3b). N2 gas sorption experiments at 77 K (Figure 3c) indicated the virtual lack of porosity. The BET surface areas SBET for COF-PEO-x (x = 3, 6, 9) were determined to be 13, 4, and 5 m2 g−1, respectively, whereas that of COF-42 was 748 m2 g−1 (Figure S14). This result suggests that the inner space of COF architectures was fully occupied by the PEO chains. This high concentration of PEO should be beneficial for the Li+ conductivity. Note we tried to observe lattice structures of COF-PEO-x by transmission electron microscope (TEM) under the previously-successful condition,38 but because of electron beam damage, they were not successful. The thermal behavior of the PEO-x monomers and the corresponding COF-PEO-x was characterized by differential scanning calorimetry (DSC, Figures 4a and S15–17) under Ar atmosphere. The DSC analysis of PEO-3 showed a melting point at 89 °C (Figure S15a), while PEO-6 and PEO-9 are yellow viscous liquids at 25 °C that exhibited endothermic glass transitions (Tg) at −54 °C and −58 °C (Figures S16a and S17a), respectively. As shown in Figure 4a, COF-PEO-3 did not give rise to any endothermic or exothermic peak in the measured temperature region, whereas COF-PEO-6 and COF-PEO-9 afforded Tg at −57 °C and −62 °C, respectively, which are comparable with those of the monomers. This suggests that the dynamics of the PEO assemblies in COF-PEO-6 and COFPEO-9 can be regarded as quasi-liquid state. To investigate the temperature dependence of the structural change of COF-PEOx, we selected COF-PEO-6 and measured its variable-

Figure 3. (a) FT-IR spectra of COF-PEO-x (x = 3, 6, 9) and COF42. (b) TGA profiles measured at 10 °C min−1. (c) N2 adsorption isotherm at 77 K for COF-PEO-x (x = 3, 6, 9).

Figure 4. DSC profiles of (a) COF-PEO-x (x = 3, 6, 9) and (b) COF-PEO-x-Li (x = 3, 6, 9) in the first heating process. Heating rates are 10 °C min−1. Tg is highlighted by arrows. (c) Temperaturedependent PXRD of COF-PEO-6. Wavenumber λ is 0.999273(2)



Å. (d) PXRD patterns of (i) COF-PEO-9, (ii) COF-PEO-9-Li, and (iii) COF-PEO-9-Li after treatment with 18-crown-6-ether.

To investigate the molecular motion of COF-PEO-x, solidstate 1H NMR spectra were measured from −20 C to 70 C. The 1D 1H spectra of COF-PEO-6 depicted in Figure 5a shows well separated 1H signals for the frameworks (CH, 7.5 ppm and NH, 11 ppm) and the PEO chains (3.3 ppm). With increasing temperature, narrower 1H linewidth was observed for the PEO signals, whereas those of the frameworks remained unchanged. This temperature dependence is indicative of the mobility of the PEO chains within the rigid framework, since the 1H signal broadening most likely stem from the averaging of the 1H–1H dipolar interactions as a result of molecular motion. The contribution of the 1H–1H dipolar interactions on the 1H linewidth was evaluated by 1H spin–spin relaxation (T2) measurement (Figure 5c and S19). The rigid framework gave rise to short T2 values of 0.5~1 ms regardless of the temperature, which is typical for rigid organic solids.39-41 On the other hand, T2 for the PEO chain dynamics increased with temperature, agreeing with Tg = −57 C.

Figure 5. Variable-temperature 1D 1H spectra of (a) COF-PEO-6 and (b) COF-PEO-6-Li. T2 of framework and PEO chain for (c) COF-PEO-6 and (d) COF-PEO-6-Li as a function of temperature. 2D 1H DQ/SQ spectra of (e) COF-PEO-6 and (f) COF-PEO-6-Li at 40 C. Correlation are highlighted as red arrows.

Having confirmed the dynamic motion of the PEO chains and the high thermal stability without volume change, we investigated the Li+ conductivity in solid samples. Powders of all COF-PEO-x were immersed into a THF solution of LiTFSI (TSFI− = bis(trifluoromethane)sulfonimide), filtered, and washed with fresh solvents. The samples were dried under vacuum to obtain solvent-free powder samples. The as-prepared Li+ doped samples are hereafter denoted as COF-PEO-x-Li (x = 3, 6, 9). Elemental analysis revealed that the O/Li ratios (wt%) for COF-PEO-x-Li were 8.8/1.0, 7.5/1.0, and 5.9/1.0, respectively (Table S2). The sample with the longer chain

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possesses larger amount of Li+ due to the higher number of solvating sites of PEO. The FT-IR spectra of all COF-PEO-xLi exhibited strong stretching vibrations at 1200 and 611 cm−1 (Figure S9) that can be assigned to CF3 and SO2 from TFSI−, respectively, (Figure S10), which is indicative of the LiTFSI doping into the frameworks. Other peaks are identical to those of COF-PEO-x, indicating that the layer structures remained unaltered after the doping. Slight changes in the morphologies of all COF-PEO-x-Li compared with those of COF-PEO-x can be seen in the corresponding SEM images (Figures 2d–2f). Thus, although the surface roughness increased by doping, the spherical shape and size of particles remain unchanged. Importantly, the PXRD analysis of all COF-PEO-x-Li revealed their amorphous nature (Figure 4d and S5). The introduction of bulky TFSI− and multi-site interaction (solvation) of Li+ and PEO groups might decrease the long-range periodicity of the layers, according to previous reports.14 To investigate the integrity of the layer structures of COF-PEO-x-Li, LiTFSI was removed by the solid–liquid reaction of COF-PEO-x-Li with 18-crown-6-ether, which strongly captures Li+ ion in THF. The complete removal of LiTSFI from COF-PEO-x-Li was confirmed by FT-IR (Figure S11), and the recovered samples showed the same PXRD pattern as the respective COF-PEO-x (Figure 4d and S6). The recovery of crystallinity indicates that the 2D layer structures were preserved after LiTFSI doping. Several COFs were reported to show structural flexibility upon guest inclusion, and the observation of reversible crystal-toamorphous in COF-PEO-x is related to the behavior.42-43 TGA analysis revealed that all COF-PEO-x-Li are thermally stable up to 340 °C, indicating that LiTFSI doping does not affect the thermal stability of the frameworks. The Tg values obtained from DSC for COF-PEO-6-Li and COF-PEO-9-Li, 27 °C and 32 °C, respectively, were higher than those for COF-PEO-6 and COF-PEO-9 (Figure 4b), which is most likely due to the complexation of Li+ with polar PEO units by intra or inter chain interactions decreasing the chain mobility. Dynamics of COFPEO-6-Li were verified by 1D 1H spectra (Figure 5b) and T2 measurement (Figure 5d) as a function of temperature. The signals of the 1D 1H spectra for COF-PEO-6-Li were slightly downfield shifted compared with COF-PEO-6, with the protons of the framework resonating at 8 (CH) and 11.5 ppm (NH) and those of PEO at 3.3 and 3.5 ppm. The splitting of the PEO peak could be due to the different chemical environment induced by the interaction between Li+ and the PEO chain. Meanwhile, line narrowing and elongated T2 of the PEO signals above Tg supports the chain dynamics, whereas significant line broadening and a T2 plateau that occur at and below Tg indicates the suppression of chain dynamics. This also supports the insufficient mobility of PEO chains to lead low Li+ conductivity at room temperature. Figure 5e and 5f show 1H single quantum (SQ) / double quantum (DQ) spectra for COF-PEO-6 and COFPEO-6-Li respectively. The measurement provides the information of distance correlation less than 4 Å, and we elucidated PEO chains mostly locate nearby the 2D networks even after LiTFSI insertion. To evaluate the Li+ conductivities of COF-PEO-x-Li, powder samples were mechanically pressed into solid pellets and measured by AC impedance spectroscopy under Ar atmosphere from 30 °C to 200 °C (Figure 6a). The Nyquist plot of each COF-PEO-x-Li showed a semicircle at high frequency and a linear tail at low frequency, which can be attributed to the blocking effect at the electrode typical of ionic conductivity. The conductivity was found to increase upon heating, affording


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values of 6.25  10−10, 7.94  10−6, and 1.23  10−4 S cm−1 at 30 °C, 100 °C, and 160 °C, respectively, for COF-PEO-6-Li. The increase of PEO chain motion with increasing temperature as confirmed by the T2 NMR experiment supports that the conductivity increase is related to the dynamics of the PEO chains. A dependence of the conductivity on the chain length was observed; the conductivities of COF-PEO-3-Li, COF-PEO6-Li, and COF-PEO-9-Li at 200 °C were determined to be 9.72

 10−5, 3.71  10−4, and 1.33  10−3 S cm−1, respectively (Figure 6b). COF-PEO-9-Li retained long-term Li+ conductivity at 200 °C (Figure 6c), which is representative of the high thermal stability of the structure and the conductivity. Nonlinear profiles of conductivity as a function of temperature are normally found for quasi-liquid PEO and other polymer electrolytes.44-47

Figure 6. (a) Nyquist plot for COF-PEO-9-Li at 200 °C. (b) Li+ conductivities of COF-PEO-x-Li (x = 3, 6, 9) as a function of temperature in the range of 30 °C to 200 °C. (c) Time course Li+ conductivity of COF-PEO-9-Li at 200 °C. (d) Ten cycles of cyclic voltammetry measurements on Li/COF-PEO-9-Li/stainless steel at 100 °C under 10 mV s−1. (e) Charge–discharge curves of an all-solid-state battery of LiFePO4/COF-PEO-9-Li/Li cell at a current intensity of 3.0 mAg−1 and cut-off voltage of 2.5 V to 4.2 V at 100 °C. (f) SEM images of the pellets for (left) as-synthesized and (right) milled COF-PEO-9-Li.

As a reference, we also tested COF-42 following the same procedure.12 The conductivity of COF-42-Li was 1.77  10−8 S cm−1 at 200 °C (Figure S25), which is 5 orders of magnitude lower than that of COF-PEO-9-Li. The result confirms that the PEO chains play a crucial role in the solvation of Li+ for fast transport. Generally, liquid electrolytes for Li secondary batteries do not work above 80 °C, whereas the development of high temperature batteries above 100 °C is important for other technologies such as space applications. The sufficient Li+ conductivity and thermal/mechanical stabilities observed for COF-PEO-9-Li encouraged us to evaluate its applicability as solid electrolyte for solid-state rechargeable Li battery at 100 °C.48-52 To evaluate the electrochemical stability of COFPEO-9-Li, cyclic voltammetry measurements were performed on Li/COF-PEO-9-Li/stainless steel with a CR2032 coin cell at 100 °C (10 mV s−1, Figure 6d). We observed electrochemical stability in the range of 2.4 - 4.2 V versus Li+/Li. We also

measured linear scanning voltammetry (LSV) on a same coin cell at 100 °C (1 mV s−1, Figure S26).53 We observed electrochemical stability up to 5.2 V versus Li/Li+, confirming that the sample possesses a wide electrochemical window. An all-solid-state battery consisting of a LiFePO4 cathode, COFPEO- 9-Li, and a Li metal anode was fabricated. The charge– discharge test was performed at a current density of 3.0 mAg−1 in a voltage range of 2.5–4.2 V at 100 °C (Figure 6e). The first attempt using the as-synthesized COF-PEO-9-Li only afforded a discharge capacity of 30 mAh g−1 (Figure S27). Since the SEM image of the pellet shows an uneven and rough surface morphology (Figure 6f, left), which does not provide an optimal electrolyte and electrode interface, we pulverized the particles by using a mixer mill to obtain smaller particles. Figure 6f (right) displays the smooth surface with smaller particles (2 μm size in average) of the milled sample. The battery with the milled sample exhibited a much better performance at 100 °C, as shown in Figure 6e. The discharge profile showed over 120



mAh g−1 of capacity with a clear plateau at 3.35 V, and the discharge efficiency after 10 cycles was maintained above 95%. Considering the theoretical averaged voltage of 3.45 V and the specific capacity of 169 mAh g−1 in this half cell system, we observed overcharge behavior. This is because of low rate and resultant partial reduction of Fe3+ in LiFePO4 at the interface of electrolyte-electrode. This is often observed in other organic polymers-LiFePO4 systems and chemical/engineering optimization is required to reduce the overcharge.54-56 Although the tested current density was small, the demonstration in allsolid-state at 100 C proves that COF-PEO-x-Li can act as solid-state Li electrolytes having sufficient thermal/chemical/mechanical properties.

CONCLUSION We have developed a bottom-up self-assemble approach for the accumulation of dynamic PEO groups with high concentration into the networks of imine-bonded 2D covalent organic frameworks (COFs) in crystalline state. They acquired both structural periodicity and glassy-state PEO dynamics, which were comprehensively characterized by X-ray diffraction, TGA/DSC, and solid-state NMR. The dynamic, glassy PEO moieties anchored in COFs contribute to the fast Li+ conductivity in all-solid state, and one of the structures having highest density accumulation of PEO units (COF-PEO-9-Li) showed over 10−3 S cm−1 at 200 °C while maintaining the original periodic structure and electrochemical stability. Demonstration of all-solid-state Li batteries proved the COFPEO-9-Li works as a "pure" solid-state ion conductor/transporter for the application of energy devices. Our design highlights the great potential of the COF architecture as a platform for accumulation of functional groups with dynamics for various ions conductivity and transportation in the solid state.

ASSOCIATED CONTENT Supporting Information Experimental details and additional characterizations (PXRD, TG/DTA, DSC, FT−IR, 1H NMR, solid-state NMR, N2 adsorption, SEM, AC impedance and charge-discharge profiles). 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 The manuscript was written through contributions of all authors.

Funding Sources The work was supported by the Japan Society of the Promotion of Science (JSPS) for a Grant-in-Aid for Scientific Research (B) (JP 18H02032) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Strategic International Collaborative Research Program (SICORP), and Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) from the Japan Science and Technology, Japan.

ACKNOWLEDGMENT G. Z. thanks to JSPS Postdoctoral Fellowships for Research in Japan. We thank Dr. Shogo Kawaguchi for his help of synchrotron PXRD measurements at SPring-8 BL02B2 beamline, and Dr. Jet-

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Sing M. Lee for helpful discussion on electrochemical measurements.

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Accumulation of Glassy Poly(ethylene oxide) Anchored in a Covalent

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