Solvent-Free, Single Lithium-Ion Conducting Covalent Organic

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Solvent-Free, Single Lithium-Ion Conducting Covalent Organic Frameworks Kihun Jeong, Sodam Park, Gwan Yeong Jung, Su Hwan Kim, Yong-Hyeok Lee, Sang Kyu Kwak, and Sang-Young Lee J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00543 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Journal of the American Chemical Society

Figure 1. (a) Conceptual illustrations of ion transport phenomena in the porous crystalline ion conductors: previous approaches (top) and this study (bottom). (b) Chemical structure of lithium sulfonated COF (TpPaSO3Li). 229x535mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. Theoretical model structure of TpPa-SO3Li depicted along (a) c- and (b) b-axes, in which hydrogen atoms of hydrocarbons are omitted for clear representation. (c) Experimental and simulated PXRD patterns. (d) TEM image (inset shows high magnification view). (e) Nitrogen gas sorption isotherms measured at 77 K. (f) Saturation recovery plot from MAS 7Li NMR spectra (inset shows 7Li NMR spectrum). 303x449mm (300 x 300 DPI)


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Figure 3. (a) Arrhenius plot for ionic conductivity of TpPa-SO3Li (insets show photo images of the selfstanding pellet). (b) tLi⁺ values of TpPa-SO3Li and previously reported porous crystalline ion conductors (cationic COF/LiTFSI,14 spiroborate-linked COF/PVdF/PC,17 Cu azolate MOF/LiBF4/PC,22 Cu3(btc)2 MOF/LiClO4/PC,19 and Cu porphyrin–Zr cluster-based MOF/Li0.2EMIm0.8TFSI;20 TFSI– =

bis(trifluromethanesulfonyl)imide, btc3– = benzene-1,3,5-tricarboxylate, EMIm+ = 1-ethyl-3methylimidazolium). (c) Theoretical elucidation of anisotropic Li-ion migration behaviors inside the pore (top) with corresponding energy diagrams (bottom). The initial, intermediate, transition, and final states are abbreviated as IS, IM, TS, and FS, respectively. The atomic colors are same as those shown in Figure 2a, except for the green colored O atoms of keto groups that are bound to Li-ions. 301x399mm (300 x 300 DPI)



Figure 4. (a) Galvanostatic Li plating/stripping profile of the Li/Li symmetric cell containing TpPa-SO3Li (inset shows schematic illustration of the cell configuration). (b) Change in interfacial resistance (RInt) of the cell during the cycling test (inset shows EIS profiles at cycling time of 0, 80, and 320 h, along with an associated equivalent circuit).38 (c) SEM image of the Li metal electrode surface after the cycling test (320 h). 300x281mm (300 x 300 DPI)


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Solvent-Free, Single Lithium-Ion Conducting Covalent Organic Frameworks Kihun Jeong,†,⁋ Sodam Park,†,⁋ Gwan Yeong Jung,‡ Su Hwan Kim,‡ Yong-Hyeok Lee,† Sang Kyu Kwak,*,‡ and Sang-Young Lee*,† †Department

of Energy Engineering and ‡Department of Chemical Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea Supporting Information ABSTRACT: Porous crystalline materials such as covalent organic frameworks and metal–organic frameworks have garnered considerable attention as a promising ion conducting medium. However, most of them additionally incorporate lithium salts and/or solvents inside the pores of frameworks, thus failing to realize solid-state single lithium-ion conduction behavior. Herein, we demonstrate a lithium sulfonated covalent organic framework (denoted as TpPa-SO3Li) as a new class of solvent-free, single lithium-ion conductors. Benefiting from the well-designed directional ion channels, high number density of lithium-ions, and covalently tethered anion groups, TpPa-SO3Li exhibits an ionic conductivity of 2.7 × 10–5 S cm–1 with a lithium-ion transference number of 0.9 at room temperature, and an activation energy of 0.18 eV without additionally incorporating lithium salts and organic solvents. Such unusual ion transport phenomena of TpPaSO3Li allow reversible and stable lithium plating/stripping on lithium metal electrodes, demonstrating its potential use for lithium metal electrodes.

number (tLi⁺) of porous crystalline ion conductors.14,19,22 For example, Chen et al. reported a solvent-free cationic COF that incorporates Li salt.14 Although it showed a high ionic conductivity (σ = 2.09 × 10–4 S cm–1 at 70 oC), the weak binding between the cationic framework and free anions resulted in an (a)

Previous approaches Mobile Li-ion Li+

–

Li+

Li+

–

–

Solvent molecule Porous crystalline ion conductors containing Li salt (left) and/or solvent (right) Mobile anion

This study Covalently tethered anion

Mobile Li-ion –

Li+

–

Li+

INTRODUCTION With the advent of ubiquitous energy era, which will find the widespread use of smart portable electronics, electric vehicles (EVs), and grid-scale energy storage systems (ESSs), the demands for high-energy density power sources with reliable electrochemical performance and safety are rapidly growing.1,2 Among numerous next-generation battery systems explored to date, all-solid-state lithium (Li) batteries3,4 and Li metal batteries57 have been extensively investigated as promising candidates for post Li-ion batteries. Such a great interest in these batteries inspires the relentless pursuit of solid-state Li-ion conductors as a key-enabling technology.3,8 Previous studies on the solid-state ion conductors have focused on inorganic sulfides/oxides8-10 or polymer-based11-13 ones. In addition to these traditional approaches, a new concept of solidstate Li-ion conductors based on porous crystalline materials such as covalent organic frameworks (COFs)14-18 and metal–organic frameworks (MOFs)19-26 has been pioneered as an appealing alternative due to their directional ion conduction pathways through the ordered pores and versatile structural design. However, most of the previously reported porous crystalline ion conductors have additionally incorporated Li salts and/or solvents into the pores of frameworks to enable ion transport (Figure 1a, top),14-26 thus failing to achieve solid-state single Li-ion conduction behavior. Recently, introducing Lewis acidic frameworks has been suggested to enhance Li-ion transference

–

Li+

Solvent-free, single Li-ion conducting COF

(b)

H O N

O

SO3Li

O HN

N H O

O

O

SO3Li

NH

HN

O

O LiO3S

LiO3S

NH O

O

HN

11.8 Å O O HN

NH SO3Li

H N

NH O O

N O H

O

SO3Li

O O

TpPa-SO3Li

Figure 1. (a) Conceptual illustrations of ion transport phenomena in the porous crystalline ion conductors: previous approaches (top) and this study (bottom). (b) Chemical structure of lithium sulfonated COF (TpPa-SO3Li).


RESULTS AND DISCUSSION The stepwise synthetic procedure of TpPa-SO3Li is schematically provided in Scheme S1 (see also the Experimental details in the Supporting Information). First, a sulfonic acid COF (denoted as TpPa-SO3H) was prepared based on a previous report,32 via a solvothermal reaction between 1,3,5-triformylphloroglucinol (Tp) and 1,4-phenylenediamine-2-sulfonic acid (Pa-SO3H) in a solvent mixture (1,4-dioxane/mesitylene = 1/4 (v/v)) with 6 M acetic acid (HOAc). The obtained TpPa-SO3H was reacted with LiOAc to exchange its protons with Li-ions, yielding the reddish powders of TpPa-SO3Li. The successful cation exchange reaction was quantitatively confirmed by the inductively coupled plasma optical emission spectroscopy (ICP-OES) measurement for Li content (2.33 (calcd.) and 2.31 wt% (found), Table S1). The ketoenamine linkage formation32-34 in TpPa-SO3Li (Figure S1a) was confirmed by the characteristic Fourier transform infrared (FT-IR) spectrum (1575 (C=C stretch) and 1240 cm–1 (C–N stretch), Figure S1b). This result was further verified by the cross polarization magic angle spinning 13C nuclear magnetic resonance (CP-MAS 13C NMR) measurement, showing the characteristic peak at 184 ppm ascribed to the carbon atom of keto (–C=O) group (Figure S1c). The porous crystalline structure of TpPa-SO3Li was elucidated. The structural model (see the Density functional theory (DFT) calculations, Table S2, and S3 in the Supporting Information) shows that the hexagonal pores are stacked along caxis with slight slippage from the eclipsed configuration, in which Li-ions are located near oxygen (O) atoms of the sulfonate and keto groups (Figure 2a). Two types of optimized Li-ion geometries with similar thermodynamic stability are shown in Figure S2. In addition, the interplanar π–π stacking distance was estimated to be 3.4 Å (Figure 2b). The powder X-ray diffraction

(a)

(b)

3.4 Å

: Li :H :C :N :O :S

a

a b

b

c

c

(d)

(c) Experimental Simulated

2 nm

3.4 Å

×10 24

28

10 nm

5

10 15 20 25 30 35 2 (deg.)

(e)

(f) 300

Adsorption Desorption

T1 = 2.76 ± 0.06 s

200

I/I0

unsatisfactory tLi⁺ value of 0.61. Meanwhile, anionic frameworks have been explored to afford high tLi⁺ (e.g., up to 0.8),17,21 yet they still incorporated organic solvents in order to secure reliable ionic conductivity. Note that the presence of freely mobile anions and organic solvents in the electrolytes tends to cause unwanted interfacial side reactions with electrodes.27-29 This problem becomes more serious in Li metal electrodes. In specific, the freely mobile anions and solvent molecules trigger ionic concentration gradient and non-uniform Li plating/stripping on Li metal surface, giving rise to uncontrolled mossy and dendritic Li growth.5-7,30,31 Therefore, development of advanced single Li-ion conductors without freely mobile anions and solvents, which can address the longstanding issues of conventional electrolytes described above and eventually bring exceptional improvements in the cell performance, is urgently needed. Herein, we present a lithium sulfonated COF (denoted as TpPa-SO3Li) as a new class of solvent (and mobile anion)-free, single Li-ion conductors (Figure 1a, bottom). The chemical structure of TpPa-SO3Li is rationally designed to construct an anionic framework with well-defined directional ion channels (Figure 1b). In specific, the hexagonal pore arrays with ketoenamine linkage are vertically stacked, in which sulfonates are covalently tethered to enable single Li-ion conduction. Moreover, monoaromatic building blocks are used to form small-sized pores, thus affording high number density of Li-ions. The resultant TpPa-SO3Li exhibits exceptional ion conduction characteristics (σ = 2.7 × 10–5 S cm–1, tLi⁺ = 0.9 at room temperature, and activation energy (Ea) of 0.18 eV), thereby enabling highly stable Li plating/stripping on Li metal electrodes that lies far beyond those attainable with the previously reported porous crystalline ion conductors.

Intensity (a.u.)

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100 20

0 0.0 0.2 0.4 0.6 0.8 1.0 P/P0

0

10

0 ppm

-10

-20

50 100 150 200 250 300 Time (s)

Figure 2. Theoretical model structure of TpPa-SO3Li depicted along (a) c- and (b) b-axes, in which hydrogen atoms of hydrocarbons are omitted for clear representation. (c) Experimental and simulated PXRD patterns. (d) TEM image (inset shows high magnification view). (e) Nitrogen gas sorption isotherms measured at 77 K. (f) Saturation recovery plot from MAS 7Li NMR spectra (inset shows 7Li NMR spectrum).

(PXRD) pattern shows the characteristic diffraction peaks at 2θ = 4.6 and 26.2 deg. that are assigned to the (100) and (001) planes, respectively, which appears similar to the simulated pattern obtained from the structural model (Figure 2c). The peak broadening and difference in the peak intensity ratio between the experimental and simulated patterns may be due to the small-sized TpPa-SO3Li powders (Figure S3) and deviation from perfect crystalline structure, which appear consistent with the previously reported results.14,32 Meanwhile, no significant difference in the PXRD patterns (Figure S4) was observed between TpPa-SO3Li and TpPa-SO3H (precursor), revealing that the framework structure is not disrupted after the cation exchange reaction. The well-ordered crystalline structure of TpPa-SO3Li was further verified by the transmission electron microscopy (TEM) image (Figure 2d). From the d-spacing distribution (Figure S5), the average interplanar distance was measured to be 3.4 Å (inset of Figure 2d), which is consistent with the PXRD result and the theoretical value. This structural feature allows adjacent alignment of sulfonate and keto groups, which thus could facilitate Li-ion migration along their O atoms. In addition, the nitrogen gas sorption isotherms show the porous structure with a


Journal of the American Chemical Society Brunauer–Emmett–Teller (BET) surface area of 348 m2 g–1 (Figure 2e). A small pore size of 11.8 Å was observed (Figure S6) compared to those of previously reported COFs,35,36 which may contribute to increasing the number density of Li-ions. The MAS 7Li NMR spectrum of TpPa-SO3Li shows a singlet at –0.3 ppm (inset of Figure 2f), indicating that Li-ions experience an identical environment. To evaluate the mobility of Li-ions, saturation recovery experiments were conducted at room temperature and the normalized intensities (I/I0) from the multiple spectra were plotted as a function of time (orange dots in Figure 2f; see also the Experimental details in the Supporting Information). From this analysis, the spin–lattice relaxation time was measured to be T1 = 2.76 ± 0.06 s, which appears comparable to the previously reported value for highly mobile Li-ions in a boroxine-linked COF (specifically, LiClO4-containing COF-5; T1 = 1.91 ± 0.05 s).18 This result demonstrates the fast migration of Li-ions inside the well-ordered pores of TpPa-SO3Li. To examine the Li-ion conduction behavior of TpPa-SO3Li, its self-standing pellet was prepared using a cold-pressing method (inset of Figure 3a). The obtained pellet showed the pinhole-free and dense morphology with ca. 50 µm thickness (Figure S7). The ionic conductivity was evaluated at varied temperatures (from room temperature to 110 ºC) using an electrochemical impedance spectroscopy (EIS) analysis (Figure S8). The Arrhenius plot

(b)

-3.5

This study

-4.0

Ea = 0.18 eV

Ref. 14 Ref. 17

-4.5

Ref. 22

-5.0 -5.5

Ref. 19 Ref. 20

5 mm

2.6

2.8

3.0

3.2

3.4

0.4

0.6

1000/T (K-1)

(c)

View along View along z-axis x-axis

0.8

1.0

tLi⁺

Axial pathway IS

Planar pathway

IM1

IM2

FS

IS

IM

FS

z x

y

y z

x TS2

40 E (kcal mol-1)

Solvent-containing

log (S cm-1)]

(a)

Solvent-free

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TS1

30 IM

20 10 0

TS1 IS

TS3

TS2 IM1

Em = 31.6 kcal mol–1

IM2

FS

Em = 7.6 kcal mol–1

IS

Reaction coordinate

FS

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shows a proportional increase of logarithmic ionic conductivity with a rise in temperature, yielding Ea = 0.18 eV (Figure 3a). This remarkably low Ea value appears similar to those of previously reported porous crystalline ion conductors,14-19,21-26 verifying the presence of directional ion conduction pathways in TpPa-SO3Li. A notable achievement of TpPa-SO3Li is an ionic conductivity of σ = 2.7 × 10–5 S cm–1 at room temperature without any additional incorporation of Li salts or organic solvents. This value appears comparable to those of propylene carbonate (PC) solventcontaining ion conductors based on anionic frameworks (e.g., σ = 3.05 × 10–5 S cm–1 for a spiroborate-linked COF/polyvinylidene fluoride (PVdF)/PC composite17 and σ = 5.7 × 10–5 S cm–1 for a tetrahedral aluminate MOF/PVdF/PC composite21 at room temperature). To demonstrate the single Li-ion conduction behavior of TpPaSO3Li, its tLi⁺ was examined at room temperature using a potentiostatic polarization method.37 TpPa-SO3Li showed a high tLi⁺ value of 0.9 (Table S4 and Figure S9), exhibiting the predominant contribution of Li-ions to the ionic conductivity. Notably, this tLi⁺ value is significantly higher than those of the previously reported porous crystalline ion conductors (Figure 3b), underscoring the structural superiority (in particular, the immobilized sulfonate groups) of TpPa-SO3Li. The details on the comparison with other porous crystalline ion conductors are provided in Table S5, with a focus on the use of additional components, σ, tLi⁺, and Ea values. This unique Li-ion conduction phenomena of TpPa-SO3Li were theoretically elucidated by DFT calculations. In specific, the Li-ion migration inside the pores was investigated by comparing migration barriers (Em) at rate-determining steps in two types of anisotropic pathways that are parallel and perpendicular to the axially stacked pores (denoted as axial and planar pathways, respectively; Figure 3c). At the initial and final states (IS and FS), the major (red circled) one of two types of the optimized Li-ion geometries (Figure S2) was chosen. For both pathways, the hopping of Li-ion occurs along O atoms with the aid of cation– dipole interaction. A notable finding is that the Li-ion migration in the axial pathway requires a much lower migration barrier (Em = 7.6 kcal mol–1, Figure 3c, left) compared to that in the planar one (Em = 31.6 kcal mol–1, right). This preferred axial Li-ion migration could be attributed to the shorter hopping distances, which is promoted by the O atoms of keto groups (green colored) that are adjacently aligned in the axial pathway (Figure S10). Furthermore, these O atoms, in combination with the O atoms of sulfonates, considerably enhance the thermodynamic stability of Li-ion intermediates (IM1 and IM2) in the axial pathway. Meanwhile, the IM in the planar pathway, to which only two O atoms of a sulfonate are bound, is thermodynamically unstable. Note that the other possible planar pathway (toward the opposite direction) also requires long-distance hopping, resulting in a high migration barrier (Em = 41.1 kcal mol–1, Figure S11). These theoretical results correspondingly demonstrate the directional Liion conduction along the stacked pores of TpPa-SO3Li, in which the O atoms of keto groups play a viable role. In addition to the Li-ion conduction behavior, the thermal and electrochemical stabilities were investigated. The thermogravimetric analysis (TGA) curve shows that TpPa-SO3Li is thermally stable up to ca. 200 ºC under a nitrogen atmosphere (Figure S12). From a linear sweep voltammetry (LSV) measurement, the electrochemical stability window of ca. 4 V was observed for TpPa-SO3Li (Figure S13). Future works will be devoted to further widening the electrochemical stability window of single Li-ion conducting COFs. The applicability of TpPa-SO3Li as a new solid-state electrolyte for Li metal electrodes was examined using Li/Li

Figure 3. (a) Arrhenius plot for ionic conductivity of TpPaSO3Li (insets show photo images of the self-standing pellet). (b) tLi⁺ values of TpPa-SO3Li and previously reported porous crystalline ion conductors (cationic COF/LiTFSI,14 spiroboratelinked COF/PVdF/PC,17 Cu azolate MOF/LiBF4/PC,22 Cu3(btc)2 MOF/LiClO4/PC,19 and Cu porphyrin–Zr cluster-based MOF/Li0.2EMIm0.8TFSI;20 TFSI– = bis(trifluromethanesulfonyl)imide, btc3– = benzene-1,3,5tricarboxylate, EMIm+ = 1-ethyl-3-methylimidazolium). (c) Theoretical elucidation of anisotropic Li-ion migration behaviors inside the pore (top) with corresponding energy diagrams (bottom). The initial, intermediate, transition, and final states are Plus Environment ACS Paragon abbreviated as IS, IM, TS, and FS, respectively. The atomic colors are same as those shown in Figure 2a, except for the green colored O atoms of keto groups that are bound to Li-ions.

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symmetric cell configuration (inset of Figure 4a). The galvanostatic Li plating/stripping on the Li metal electrodes was repeatedly conducted at a current density of 10 µA cm–2 for 4 h per each cycle. The symmetric cell showed a stable and reliable Li plating/stripping behavior for over 320 h without appreciable

Voltage (V vs. Li/Li+)

(a) 0.4

Li metal

0.2

All experimental procedures are provided in the Supporting Information. 0.1 0.0 -0.1 300

-0.2 -0.4 0

40

80

(b) 15 12

-Z'' (k)

12 9

9

120

QTotal

QInt

RTotal

RInt

6

6

0 0

160 Time (h)

ASSOCIATED CONTENT 305

200

310

240

315

280

320

320

3

3

6 9 Z' (k)

(c)

AUTHOR INFORMATION Corresponding Authors

12

20 µm

0

80

160 240 Time (h)

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details, DFT calculation details, tables, and supporting data (PDF)

0h 80 h 320 h

3

0

structural uniqueness, in combination with the absence of freely mobile anions and solvents, allowed TpPa-SO3Li to show the exceptional ion conduction characteristics (σ = 2.7 × 10–5 S cm–1, tLi⁺ = 0.9 at room temperature, and Ea = 0.18 eV), eventually contributing to the stable cycling of Li plating/stripping on Li metal electrodes. This study provides a new electrolyte strategy for next-generation batteries (in particular, recently spotlighted all-solid-state and Li metal batteries) that are in urgent need of high-performance solid-state single-ion conductors.

EXPERIMENTAL SECTION

TpPa-SO3Li

0.0

RInt (k)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


320

*[email protected] (S.K.K.) *[email protected] (S.-Y.L.) ORCID

Figure 4. (a) Galvanostatic Li plating/stripping profile of the Li/Li symmetric cell containing TpPa-SO3Li (inset shows schematic illustration of the cell configuration). (b) Change in interfacial resistance (RInt) of the cell during the cycling test (inset shows EIS profiles at cycling time of 0, 80, and 320 h, along with an associated equivalent circuit).38 (c) SEM image of the Li metal electrode surface after the cycling test (320 h).

Sang Kyu Kwak: 0000-0002-0332-1534 Sang-Young Lee: 0000-0001-7153-0517

Author Contributions ⁋K.J.

and S.P. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT increase and irreversible fluctuation of overpotential (Figure 4a). This decent cyclability was verified by monitoring the change in interfacial resistance (RInt) of the cell as a function of cycling time (Figure 4b). The increase of RInt was retarded during the cycling, indicating the good interfacial stability of TpPa-SO3Li with Li metal electrodes. This result was confirmed by the clean and smooth surface of the Li metal electrode after the cycling test (Figure 4c). Notably, the random Li deposition was hardly observed, which may reveal that TpPa-SO3Li allows uniform Liion flux toward the Li metal electrodes. In addition, the ordered structure of TpPa-SO3Li was not disrupted after the cycling test (Figure S14), demonstrating the good structural durability. As already described in the introduction part, formidable challenges facing Li metal electrodes mostly arise from the unwanted interfacial side reactions with freely mobile anions and solvent molecules.5-7,30,31 The Li/Li symmetric cell test results described above demonstrate the promising potential of TpPa-SO3Li as a solvent-free, single Li-ion conductor for Li metal electrodes.

CONCLUSION In summary, we have demonstrated for the first time the solventfree, single Li-ion conducting COF. The lithium sulfonated COF (TpPa-SO3Li) is designed to provide the well-defined ion channels, high number density of Li-ions, and covalently tethered anion groups. The directional Li-ion conduction through the axially stacked pores of TpPa-SO3Li, along with the important role of O atoms of keto groups, was theoretically elucidated. Such

This work was supported by Basic Science Research Program (2018R1A2A1A05019733, 2018M3D1A1058624, and 2017R1D1A1B03033699) and Wearable Platform Materials Technology Center (2016R1A5A1009926) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education and the Ministry of Science, ICT and Future Planning. This work was also supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFCMA1702-04. S.K.K. acknowledges the financial support from the NRF grant funded by the Korea government (MSIT) (NRF2018M1A2A2063341) and computational resources from UNISTHPC.

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Solvent/mobile s anion-free, single Li-ion conducting COF


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Solvent-Free, Single Lithium-Ion Conducting Covalent Organic

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