Few-Layered Boronic Ester Based Covalent Organic Frameworks

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Few-Layered Boronic Ester Based Covalent Organic Frameworks/ Carbon Nanotube Composites for High-Performance K-Organic Batteries Xiudong Chen, Hang Zhang, Chenggang Ci, Weiwei Sun, and Yong Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00165 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Graphical Abstract The few-layered structure of COF-10 on the exterior surface of CNT is designed and exhibits ultrahigh reversible capacities as anode materials for K-organic batteries

Few-Layered Boronic Ester based Covalent Organic Frameworks/Carbon Nanotube Composites for High-Performance K-Organic Batteries Xiudong Chen,†,‡,§ Hang Zhang,†,§ Chenggang Ci,‡ Weiwei Sun,† and Yong Wang*,†

1

ACS Paragon Plus Environment

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Few-Layered Boronic Ester Based Covalent Organic Frameworks/Carbon Nanotube Composites for High-Performance K-Organic Batteries Xiudong Chen,†,‡,§ Hang Zhang,†,§ Chenggang Ci,‡ Weiwei Sun,† and Yong Wang*,† †Department

of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai, P. R. China, 200444 School of Chemistry and Chemical Engineering, Qiannan Normal College for Nationalities, Duyun, Guizhou, P. R. China, 558000

‡

Email: [email protected] Abstract: Organic electrodes for low-cost potassium ion batteries (PIBs) are attracting more interest by virtue of their molecular diversity, environmental friendliness, and operation safety. But the sluggish potassium diffusion kinetics, dissolution in organic electrolyte, poor electronic conductivity and low reversible capacities are several drawbacks compared with inorganic counterparts. Herein, the boronic ester based covalent organic frameworks (COF) material is successfully prepared on the exterior surface of carbon nanotubes (CNTs) via rational design of the organic condensation reaction and used as an anode material for PIBs. The few-layered structure of COF-10@CNT can provide more exposed active sites and fast K+ kinetic. It exhibits ultra-high potassium storage performances (large reversible capacities of 288 mAh g-1 after 500 cycles at 0.1 A g-1 and 161 mAh g-1 after 4000 cycles at 1 A g-1), which is superior to previous organic electrodes and most of inorganic electrodes. Moreover, the K-storage mechanism is proposed to be π-cation interaction between K+ and conjugated π electrons of benzene rings. Keywords: carbon nanotube, covalent organic framework, organic electrode, π-cation interaction, potassium ion batteries 2


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Lithium ion batteries (LIBs) is a type of mature energy storage system, which has been widely used in portable electronics.1 The lithium content in the earth’s crust is only 0.0017 wt%, which greatly limits the sustainable application of LIBs. Considering the resource deficiency, there is an urgent need to find alternative energy systems. Recently, potassium ion batteries (PIBs) is suggested as a new energy storage system, due to the abundant potassium resources and low electrolyte cost.2 Besides, K+ has a weaker Lewis acidity in alkali metal ions, ensuring better kinetics for PIBs.3 However, K dendrites can be formed as a more serious hazard during charging and discharging process as the result of the active potassium metal. Therefore, searching for a suitable anode material is critical for PIBs. Various carbon materials (graphite, nitrogen-doped graphene) and transition metal compounds (oxide, phosphide, selenide) have been explored.4 The theoretical capacity of commercial graphite for PIBs (279 mAh g-1) is less than LIBs (372 mAh g-1).5 Most carbon materials exhibit a capacity below 220 mAh g-1.6,7 Because of the larger potassium ions radius (1.38 Å) compared with lithium ions (0.76 Å), it can severely hinder the insertion of K+ into the electrode material and therefore result in low electrochemical activity.8,9 During the potassisation and depotassisation process, the significant volume change in the graphite electrode may cause structural destruction, leading to poor cycling stability. Moreover, the potential platform of graphite is at ~0.2 V vs. K+/K, which is very close to the deposition potential of potassium (forming K dendrites), leading to the latent safety concern.10 Therefore, the development of functional anode materials, which can accommodate large-radius potassium ions and 3


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ensure the safety application is extremely urgent.

The organic electrodes, composed of abundant light elements in nature, are recyclable, green and low-cost. More importantly, molecular design based on diverse organic functionality can be achieved compared to their inorganic counterparts.11 However, few reports have reported organic materials as anode materials for PIBs.12,13 Because most of organic materials showed some obvious shortcomings as potassium-storage materials, such as the low electronic conductivity, dissolution in organic electrolyte, and low reversible capacities.14,15 In recent years, the emerging covalent organic frameworks (COFs) is a class of two-dimensional or three-dimensional materials, which can be used as potential electrode materials for Li-ion battery applications.16-18 COFs provides designable robust network with enhanced structure stability and offers open channels for convenient transport of ions/electrons. Meanwhile, the framework construction of functional organic units in COFs can largely prevent the dissolution of organic electrode in the organic electrolyte. However, COFs have never been explored for potassium ion batteries.

From the perspective of material design, non-covalent interactions such as hydrogen bonding, ion pairing (salt bridge), cation-π and hydrophobic interactions play a leading role in many frontiers of modern chemistry. In 1981, Sunner et al.19 found that cations can combine with aromatic systems in the gas phase, leading to cation-π action. In recent years, Fang’s researchs20,21 have shown that cations such as alkali 4


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metal cation (Li+, Na+, K+) and alkaline earth metal cation (Mg2+, Ca2+) have strong cation-π interaction with two-dimensional (2D) carbon materials. Furthermore, Dougherty studied22 the interaction between monovalent cations of lithium, sodium, potassium and rubidium and the surface of benzene molecules by theoretical calculation. They found that these cations in the gas phase have strong binding force with benzene and decrease in turn according to lithium, sodium, potassium and rubidium. However, the reordering of forces in the aquatic environment makes potassium ions superior to all other ions. Therefore, it is suggested that strong π-K+ interaction between potassium ions and π-conjugated 2D COF materials with aromatic benzene rings, similar to graphene structure, could be also formed, indicating that π-conjugated 2D COF materials may have great potentials for large potassium storage capacity.

Herein, we report the controlled growth of few-layered COF-10 on CNT (COF-10@CNT) surface with a π-K+ interaction when used as an anode for PIBs. The enhanced π-π stacking in the few layered COF-10@CNT can provide more exposed active sites, shorten the ion/electron diffusion distance and enhance the insertion/extraction kinetics of K+. Meanwhile, the hierarchical pore structure of COF-10@CNT facilitates the transport of K+ and provides sufficient void space to effectively buffer the volume change during potassisation/depotassisation process. The π electron clouds of COF-10@CNT have a detected interaction with potassium ions to form π-K+, which leads to outstanding potassium storage performances (288 5


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mAh g-1 after 500 cycles at 100 mA g-1) and long stable cycling life (161 mAh g-1 after 4000 cycles at 1 A g-1).

Results and Discussion Structure and Morphology Characterization As illustrated in Figure 1, COF-10@CNT and pristine 2D COF-10 were successfully synthesized via the solvothermal-induced polycondensation of 4, 4'-biphenyldiboronic acid (BPDA) and 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (HHTP) in a mixed solvent of mesitylene and dioxane at 85 °C with/without CNT. The solid state

13C

CP/MAS NMR spectrum in Figure 2a shows five peaks corresponding to seven C atoms in COF-10, which is consistent with previous report.23 As shown in XRD patterns of COF-10 (Figure 2b), a strong diffraction peak at 2.8° can be assigned to the (100) plane23,24 along with other small peaks at 4.7, 5.6, 7.4, 9.8 and 25.7°, corresponding to the (110), (200), (210), (310) and (001) planes respectively. The peak at 25.7° assigned to the (001) plane of COF-10 cannot be observed for the COF-10@CNT because it is shadowed by a strong peak (~26.1°) for CNT. The dominated pore sizes of COF-10 (~3.06 nm) and COF-10@CNT (~3.09 nm) (Figure S1) are slightly smaller than the theoretical value (~3.2 nm),24 mainly due to the partially occupied pores caused by the staggered stacking of 2D COF layers.25 The obtained products were also characterized by Fourier transform infrared spectroscopy (FTIR, Figure S2), thermogravimetric analysis (TGA, Figure S3) and elemental analysis

(EA, Table S1) to confirm their organic structure, good thermal stability 6


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and the composition respectively.

The density functional theory (DFT) was used to construct the structure of COF-10 and calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of COF-10 (Figure 2c and S4). The HOMO and LUMO of COF-10 were calculated to be -5.76 and -2.16 eV, respectively, forming a band gap of 3.6 eV. The LUMO of COF-10 molecule is much higher than those of vitamin (-4.23 eV),13 indicating that COF-10 molecule has better K+ affinity,26 lower reduction potential, and therefore it is suitable as an anode material in PIBs. Compared with the bulk structure of the COF-10 (Figure S5), COF-10 uniformly-coated on the exterior surface of CNTs can be observed for COF-10@CNT (Figure 2d-f). The elemental mapping images and EDS spectra (Figure 2g-j and S6) indicate the uniform distribution of three elements (C, B and O) in the COF-10@CNT. A thin COF-10 layer (~6 nm) is found on CNT by HRTEM images (Figure 2k-l and S7). The interplanar distances of ~0.35 and ~0.34 nm can be assigned to the (001) plane of COF-10 and the (002) plane of CNT, respectively (Figure 2l and S7). Although these two interplanar distances are very close to each other, the COF or CNT area can be first positioned in the core-shell structure as shown in low-magnification TEM image before the final determination based on five neighbouring planes in Figure S7. In the Raman spectra (Figure S8), the intensity of the COF-10@CNT band is weaker than CNT, further indicating the uniform coating of the COF-10 on the CNT, which is in good accordance with the SEM, TEM and XPS results (Figure S9). 7


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Electrochemical performance The COF-10@CNT was fabricated as an anode for potassium ion batteries and the capacity was calculated based on the total weight of the composite. Cyclic voltammetry (CV) curves (Figure 3a) reveal a cathodic peak at 0.61 V, ascribed to the potassisation and the formation of solid electrolyte interface (SEI) films,27 and three anodic peaks at 0.81, 0.54 and 0.31 V, corresponding to the depotassisation process. Figure 3b shows that an initial charge capacity of COF-10@CNT (348 mAh g-1) is larger than COF-10 (130 mAh g-1). In comparison, the discharge/charge curves of COF-10@CNT, COF-10 and CNT for the first three cycles are shown in Figure S10a, which is in good accordance with CV results (Figure 3a and S11). The low initial Coulomb efficiency of COF-10@CNT is mainly attributed to the electrolyte decomposition and the formation of solid electrolyte interface film around this porous anode with large specific surface area, which is often observed for almost all carbon anodes6,28-33 and organic anodes27 (~20%-70%) for potassium ion batteries in the literature. There is a detected capacity fading in the initial few cycles possibly due to some unstable sites for irreversible K-storage. In the following cycles, the COF-10@CNT exhibited very stable cycling performances. The discharge-charge curves of the COF-10@CNT after 100, 200, 300, 400 and 500 cycles exhibited similar shape and reversible capacities (Figure S10b), implying stable electrode structure and excellent reversible electrochemical behaviors. The reversible capacity of COF-10@CNT retains at 288 mAh g-1 after 500 cycles at 0.1 A g-1 (Figure 3c), which is much larger than that (57 mAh g-1) for COF-10 and all previous organic anodes for PIBs (Table S2). Pristine CNT was also tested as the anode material for PIBs, and exhibited a small reversible capacity of 123 mAh g-1 after 500 cycles 8


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(Figure S12). Therefore the capacity contribution of CNT can be estimated at 24.6 mAh g-1 based on its weight percentage (20%) in the composite assuming there is no capacity change in the composite. The large reversible capacity of the COF-10@CNT composite is mainly from the COF-10 contribution. When used for Li-ion batteries, COF-10@CNT is also better than COF-10 (Figure S13-14). Both COF-10 and COF-10@CNT exhibited capacity increase during cycling. This may be attributed to the gradual activation of electrode materials during repetitive cycling due to poor electronic conductivity of organic materials. Better electrolyte infusion and decreased internal resistance during repetitive cycling may lead to more available sites for lithium ion storage. However, the Li-storage performance is not prominent compared with previous organic electrodes.16-18 Dougherty et al.34

has demonstrated that a

stronger effect between π electrons from benzene rings and K+ is observed compared to that with Li+ due to the easier solvation of Li+ compared to K+. To further evaluate high-rate cycling performance of COF-10@CNT at 1 A g-1, a stable capacity of 161 mAh g-1 was retained after 4000 cycles (Figure 3d), which is the best among all previous organic and inorganic carbon anodes for PIBs (Table S2 and Table S3). The rate capabilities of the COF-10@CNT and COF-10 electrode were further explored at varied current densities (Figure 3e). At currents of 25 and 5000 mA g-1, reversible capacities of COF-10@CNT are 330 and 68 mAh g-1, respectively, which are larger than those values (203 and 12 mAh g-1) for COF-10. The potassium storage mechanism of COF-10@CNT was investigated by XPS (Figure 4a-d) and Raman (Figure 4e-f). The C1s XPS peak of the as-prepared COF-10@CNT (Figure 4a) can be resolved into three major components for C=C (284.2 eV) and C-C (284.9 eV) of benzene ring, C-O bond (286.1 eV) linked to benzene ring.35 After the first potassisation process (Figure 4b), two strong peaks 9


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appeared at 293.1/295.8 eV, which can be assigned to the π-cation (π-K+). These new peaks clearly indicate the interaction between K+ and π electrons of the aromatic rings of COF-10.34 During the following depotassisation process, the π-cation (π-K+) is substantially weakened (Figure 4c), indicating the reversible depotassisation process. Furthermore, the area ratio of C=C and C-C groups decreases during the potassisation ascribed to the increment in the randomness of COF structure resulted from potassium process and then recovers in the following depotassisation process, indicating the reversible transformation between potassium ions and π electron clouds of the aromatic rings of COF-10.34 The high-resolution XPS spectrum of K 2p is shown in Figure 4d. There is no K-relative peak in the as-prepared COF-10@CNT electrode. After potassisation, two peaks of K 2p1/2 and K 2p3/2 appeared, which are significantly attenuated in the following depotassisation process (Figure 4d). The Raman spectrum of the as-synthesized COF-10@CNT anode during the potassisation and depotassisation process is compared in Figure 4e. The changes of the intensity ratio (ID/IG) in Figure 4f reveal that the ID/IG value is increased during potassisation process and largely recovered during the depotassisation. This should be attributed to the reversible reaction of potassium ions and sp2 carbon from the benzene ring of COF-10 (the π-K+ interaction between potassium ion and benzene rings of COF-10). HRTEM images of the COF-10@CNT anode after the first depotassisation (Figure S15) reveal an increase in the interplanar spacing of the COF-10 (~0.41 nm), which should be attributed to potassium insertion. Notably, the porous structure in the few-layered COF-10 electrode with large surface area also helps to facilitate the transportation and storage of K+ and relieve the volume change during cycling.28,36 Based on the above observations, the schematic illustration showing potassium insertion/extraction into the few-layered COF-10 electrode is proposed in Figure 4g. 10


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The CV tests at various scan rates (0.2 to 1.0 mV s-1) were investigated to get more insights on the kinetics behavior of potassium storage and excellent potassium-ion storage performances of the COF-10@CNT (Figure S16-19). The COF-10@CNT electrode exhibits capacitive behavior and surface controlled process (86.8% capacitance contribution at 1.0 mV s-1), while COF-10 displays mainly ionic diffusion controlled behavior (40.4% capacitance contribution at 1.0 mV s-1).37 These excellent electrochemical performances of the COF-10@CNT electrode (high reversible capacity and long cycle life) is benefited from its facilitated electrochemical reaction kinetics and pseudocapacitance performance.38 To further analyze the kinetics behavior, the COF-10@CNT and COF-10 anodes were characterized by the galvanostatic intermittent titration technique (GITT, Figure S20) and electrochemical impedance spectroscopy (in situ and ex situ) (Figure S21-22).39 The COF-10@CNT has a higher K+ diffusion coefficient and faster dynamic behavior than COF-10.

Conclusions In summary, a type of boronic ester based covalent organic framework is in situ grown on the exterior surface of CNT and used as the anode material for PIBs. The few-layered COF-10 on CNTs can exhibit more exposed active sites, shortened ion/electron diffusion distance and enhanced K+ insertion/extraction kinetics. The COF-10@CNT anode can achieve large capacities of 288 mAh g-1 after 500 cycles at 0.1 A g-1 and 161 mAh g-1 after 4000 cycles at 1 A g-1, which is better than previous organic anodes and most inorganic anodes for PIBs. The potassium storage mechanism of COF-10 is proposed by a detected π-cation effect raised between π 11


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electron clouds of benzene rings and K+ during potassisation/depotassisation process. This type of COF electrode could be a promising candidate with large reversible capacity and long cycle stability as green organic electrodes for high-performance PIBs.

Experimental Materials. 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (HHTP), carbon nanotubes (CNT) and 4, 4'-biphenyldiboronic acid (BPDA) were purchased from J&K scientific. Potassium bis(fluorosulfonyl)imide (KFSI), poly(vinylidene difluoride) (PVDF), ethylene carbonate (EC) and diethyl carbonate (DEC) were supplied by Do Chem. The other reagents and chemicals were all provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the chemicals mentioned above were used as received without purification.

Preparation of the COF-10@CNT and COF-10. In a typical synthesis process23 BPDA (0.50 g, 2.05 mmol), HHTP (0.44 g, 1.37 mmol), CNT (0.139 g), and 50 mL of a 1:1 v:v solution of mesitylene : dioxane were mixed in a 100 ML beaker. The weight ratio of COF-10 to CNT is ~4:1. The reaction mixture was sonicated for 30 min and then transferred to a 100 ml Teflon-lined autoclave. The Teflon-lined autoclave was heated at 85 °C for 3 days to obtain gray powders. The resulting powders were filtered off and washed with dry acetone. The powders were then immersed in acetone for 2 days and then dried at 85 °C for 12 h to afford COF-10@CNT powders. COF-10 was synthesized via the same process without the 12


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addition of CNT.

Characterizations. The structural information and morphologies of the synthesized composites were characterized using an X-ray diffractiometer (XRD, Rigaku D/max-2550 V) ， FTIR (Nicolet 380 FT-IR spectrometer, 4000-400 cm-1 region), and electron microscopes (FE-SEM, JSM-6700F; TEM, JEM-2010F). Raman spectra were collected on a laser Raman spectrometer (Renishaw in via plus). Elemental analysis of the COF-10 and COF-10@CNT were performed on an Elemental Analyzer (Vario MICRO cube). Surface area and porous properties were measured on a Micromeritics ASAP 2020 M+C analyzer. X-ray photoelectron spectrometer (XPS, PHI ESCA-5000C) was utilized to evaluate the surface compositions of the composites. The thermal stability was investigated using a thermal analyzer (TGA, NETZSCH STA 409 PG/PC) in N2. High resolution solid-state nuclear magnetic resonance (NMR) spectra were recorded at ambient temperature on a Bruker 600 MHz spectrometer using a standard Agilent magic angle spinning

(MAS)

probe

with

4

mm

(outside

diameter)

zirconia

rotors.

Cross-polarization with MAS (CP/MAS) was used to acquire 13C data at 150.21 MHz.

Density Functional Theory (DFT) calculation. DFT calculation was performed using Gaussian 09 software package.40 The geometry optimizations were carried out by means of hybrid B3LYP exchange-correlation without considering the solvation effect. The correlation consistent cc-pVDZ basis set of Dunning was added. The analysis on the highest occupied molecular orbital (HOMO) and the lowest 13


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unoccupied molecular orbital (LUMO) diagrams of COF-10 molecular was undertaken at the final geometries.

Electrochemical Tests. The anodes were prepared by mixing active materials (COF-10 or COF-10@CNT), acetylene black and poly (vinylidene difluoride) (PVDF) with a mass ratio of 7:2:1. The mass loading of active material in the electrode was about 1.5 mg cm-2. The electrolyte was composed of 1.0 M KFSI in a mixed solvent of diethyl carbonate/ethylene carbonate (DEC/EC=1:1 vol %). Coin cells (CR2032) were assembled in an argon-filled glove box for further electrochemical measurement, using potassium metal as the counter electrode and the Glass fibres (Whatman GF/D) as the separator. Electrochemical behaviors were tested at 25 oC at various current densities and a potential window of 0.005-3.0 V at LAND-CT2001 battery testing system. The capacity of the COF-10@CNT composite was calculated based on the total weight of the composite. The Galvanostatic Intermittent Titration Technique (GITT) measurements, Cyclic Voltammetry (CV, scan rates: 0.1-1.0 mV s-1) and the Electrochemical Impedance Spectroscopy (EIS) (frequency range: 105 - 0.01 Hz) were evaluated on the CHI760E and AUTOLAB PGSTAT101 electrochemical workstation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 14


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Additional, BET, FTIR, TGA, the geometry of six COF-10 monomers, SEM, TEM, EDS, HRTEM, Raman, XPS, electrochemical characterizations and additional tables (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Yong Wang: 0000-0003-3489-7672 Author Contributions § X.

D. Chen and H. Zhang contributed equally to this work.

ACKNOWLEDGMENTS The National Natural Science Foundation of China (51603119), Innovation Program (2019E00021) of Shanghai Municipal Education Commission, Science and Technology Commission of Shanghai Municipality (17010500300), and the Key Laboratory of Computational Catalytic Chemistry of Guizhou Province (Qianjiaohe KYzi[2017]013) are gratefully acknowledged for their financial supports. The authors thank Lab for Microstructure, Instrumental Analysis & Research Center, Shanghai University for materials characterization and the High-Performance Computing Center of Shanghai University for theoretical calculation.

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Figure 1. Structure of COF-10 and COF-10@CNT. (a) Synthesis of COF-10. (b) COF-10@CNT with few-layered COF-10 covered on the exterior surface of CNT.

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Figure 2. (a)

13C

CP/MAS solid-state NMR spectrum of COF-10@CNT. (b) XRD

patterns of the COF-10@CNT, COF-10 and CNT. (c) The calculated HOMO and LUMO energies of COF-10. (d) SEM image and (e, f) TEM images of COF-10@CNT. g) SEM image and the corresponding elemental mapping images of (h) carbon, (i) oxygen and (j) boron of the COF-10@CNT. (k-l) HRTEM images of COF-10@CNT. 21


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Figure 3. Electrochemical performances of COF-10@CNT and COF-10 for PIBs. (a) CV curves of COF-10@CNT at 0.1 mV s-1; (b) the first charge and second discharge profiles of COF-10@CNT and COF-10; (c) cycling performance of COF-10@CNT and COF-10 at 100 mA g-1; (d) cycling stability of COF-10@CNT at 1000 mA g-1; (e) rate capability of COF-10@CNT and COF-10.

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Figure 4. Structural evolution characterizations of COF-10@CNT anodes during the potassisation-depotassisation cycle. XPS results of C 1s: (a) the as-prepared, (b) after potassisation, (c) after depotassisation. (d) XPS results of K 2p. (e) Raman spectra, (f) the corresponding ID/IG change. (g) Proposed potassium storage mechanism illustration.

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Few-Layered Boronic Ester Based Covalent Organic Frameworks

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