Imidazole-Linked Fully Conjugated Covalent Organic Framework for High-Performance Sodium-Ion Battery

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES15 Mar 2024Imidazole-Linked Fully Conjugated Covalent Organic Framework for High-Performance Sodium-Ion Battery Lu Liu†, Yu Gong†, Yifan Tong, Hao Tian, Xubo Wang, Yiming Hu, Shaofeng Huang, Weiwei Huang, Sandeep Sharma, Jingnan Cui, Yinghua Jin, Weitao Gong and Wei Zhang Lu Liu† State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 , Yu Gong† Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 , Yifan Tong Hebei Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004 , Hao Tian Hebei Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004 , Xubo Wang Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 , Yiming Hu Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 , Shaofeng Huang Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 , Weiwei Huang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Hebei Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004 , Sandeep Sharma Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 , Jingnan Cui State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 , Yinghua Jin Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 , Weitao Gong *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 and Wei Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309 https://doi.org/10.31635/ccschem.024.202403938 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Covalent organic frameworks (COFs) have garnered increasing attention as promising electrode materials for sodium-ion batteries (SIBs) due to their ordered backbones, uniform pore sizes, and high surface areas. However, they also face challenges, including low capacity, unsatisfactory rate performance, and limited cycling stability, which pose the primary obstacles to their practical use. Herein, a pyrenoimidazole-based COF, denoted as PyIm-COF, has been synthesized as a novel electrode material for high-performance SIBs using an efficient synthetic strategy. With extended fully conjugated backbones and readily accessible redox-active sites, PyIm-COF demonstrates remarkable high-rate performance, delivering around 250 m Ah g−1 and excellent cycling stability, retaining 97.2% of its capacity even after 2500 cycles at 5 A g−1, which is higher than that of most previously reported COF-based SIBs. This work provides valuable insights into the development of nitrogen-rich conjugated COF electrode materials for rechargeable SIBs. Download figure Download PowerPoint Introduction In recent years, the quest for high-performance energy storage solutions has intensified due to the growing demand for renewable energy sources and portable electronic devices.1,2 Among various energy storage technologies, sodium-ion batteries (SIBs) have emerged as a promising candidate, offering potential advantages over traditional lithium-ion batteries (LIBs). SIBs have gained attention for their lower cost, the abundance of sodium resources, and their reduced environmental impact compared to LIBs.3,4 However, it should be noted that sodium ions (Na+) have larger size and weaker binding ability than lithium ions, making it challenging for sodium ions to reversibly intercalate into graphite. Therefore, developing novel electrode materials with high capacity, stability, and sodium ion transport rates is highly desirable and represents a significant challenge for the application of SIBs. Over the past few decades, various types of inorganic electrode materials have been developed for SIBs,5–7 but they commonly face some inevitable drawbacks, such as constrained resources, environmental unfriendliness, instability resulting from volume changes, and irreversible phase transitions during charge/discharge. In contrast, organic materials have attracted significant attention due to their environmental friendliness, excellent electrochemical properties, low cost, and customizable structures.4,8,9 To date, various organic materials, including nitrogen-rich heterocycles, traditional conducting polymers, organic radical compounds, and carbonyl-containing organic polymers, have been explored as electrode materials for SIBs.10–14 Nevertheless, their disadvantages, such as relatively low capacity, electrode material expansion, and low cycling stability, remain to be overcome. It is still challenging to synthesize novel electrode materials to meet those requirements. Compared to other organic materials, covalent organic frameworks (COFs), renowned for their well-ordered conjugated backbones, uniform pore size, and substantial surface area, have emerged as a revolutionary material class in the domain of energy storage.15,16 These distinguishing characteristics position COFs as an enticing candidate for employment as electrode materials in SIBs.17–19 Moreover, 2D COFs are regarded as promising electrode materials for ion batteries.20–27 The crystalline structure generates 1D channels, which allow metal ions to pass through. The abundant polycyclic aromatic rings in COF structures stabilize the reversible intercalation of metal ions, and the insoluble highly porous frameworks facilitate ion transport during cyclic charge/discharge of ion batteries. As the electrode material for LIBs, 2D COFs have been widely studied, showing high capacity and cycling stability. However, there have only been a handful of studies with SIBs, and most are under low current. To enhance the sodium ion storage capacity of SIBs, we envision nitrogen-rich (e.g., containing imidazole moieties that can bind Na+) highly conjugated 2D COFs as promising candidates. Imidazole-linked COFs, as a class of nitrogen-rich and chemically robust COFs, are usually synthesized with three monomers, two organic monomers, and one ammonium salt.28,29 However, an accurate stoichiometric ratio between the monomers is required, which increases the difficulty in optimizing reaction conditions. Recently, a self-condensation strategy was developed to minimize this problem in imine-linked COF synthesis.30 To develop a new method to synthesize imidazole-linked COFs from ketone and aldehyde monomers with perfect stoichiometry, we envision that ketone and aldehyde functional groups can be integrated into one organic monomer, which can go through self-condensation with the ammonium salt. Utilizing this strategy, the ketone and aldehyde functional groups always have the exact 1:1 stoichiometry during the condensation process, and excess ammonium can be easily removed due to its high solubility in multiple solvents. Herein, a 2D pyrenoimidazole-based COF, denoted as PyIm-COF, was designed and synthesized through self-condensation and was employed as a novel electrode material for high-performance SIBs. On the one hand, the polycyclic aromatic pyrene moieties with extended π-conjugation would be beneficial to forming aligned porous channels via π–π stacking, thus facilitating charge transport. On the other hand, the formation of the imidazole rings consisting of nitrogen-rich heterocyclic redox-active sites would also increase the Na+ storage capacity, given the strong interaction between Na+ and nitrogen atoms. With these two positive factors combined, the PyIm-COF exhibits a remarkably high capacity of around 250 mAh g−1 and excellent cycling stability with a capacity retention of 97.2% after 2500 cycles at 5 A g−1, higher than most previously reported COFs. Our work provides a promising and facile strategy for constructing new electrode materials for high-rate performance SIBs. Experimental Methods The synthesis method of PyIm-COF 4,4′-(4,5,9,10-tetraoxo-4,5,9,10-tetrahydropyrene-2,7-diyl)dibenzaldehyde (TOTPA; 71 mg, 0.15 mmol) was mixed with ammonium acetate (62 mg, 0.80 mmol) in 0.9 mL 1,3,5-trimethylbenzene and 0.1 mL benzyl alcohol in an ampule and sonicated for 20 min. The mixture was frozen by liquid nitrogen and air, and the ampule was removed under high vacuum. Then the ampule was sealed and heated under 180 °C for 3 days. The brown solid was washed with methanol, acetone, and ethyl ether, and then the solid was washed by hot methanol in a solid phase extraction apparatus for 24 h. The solid was dried under vacuum for further use (55 mg, 78%). Results and Discussion Material characterization In this work, a new monomer TOTPA was designed and synthesized. TOTPA features four ketone and two aldehyde groups surrounding a pyrene moiety ( Supporting Information Schemes S1–S2 and Figures S8–S12). The new PyIm-COF was successfully synthesized via a self-condensation Debus–Radziszewski reaction of TOTPA and ammonium acetate in mesitylene and benzyl alcohol (9:1, v/v) at 150 °C for 3 days (Figure 1a). After washing with methanol, acetone, and ethyl ether, the crystalline brown solid was isolated in 78% yield. Figure 1 | (a) Structure and synthesis of PyIm-COF; (b) PXRD patterns of PyIm-COF, with top and side views of the corresponding refined eclipsed AA stacking structure of PyIm-COF. Download figure Download PowerPoint Powder X-ray diffraction (PXRD) spectroscopy was conducted to characterize the crystallinity of PyIm-COF. As shown in Figure 1b, PyIm-COF exhibits a strong diffraction peak at 2θ = 6.41°, a weak diffraction peak at 9.14, 12.81°, and a broad diffraction peak around 25°, which correspond to the planes (110), (200), (220), and (001), respectively. This result implied that PyIm-COF has a long-range ordered 2D planar structure with good crystallinity. Computational modeling and Pawley refinement were performed to obtain the simulated structure of PyIm-COF using the Materials Studio software package. The simulated models were constructed in the tetragonal crystal system (P4/m), with layers on the AA plane ( Supporting Information Figure S1). After the geometrical energy minimization, the optimized parameters for the unit cell were a = b = 19.69 Å, c = 3.47 Å (residuals: Rwp = 2.70%, Rp = 2.88%). By comparing the simulated PXRD patterns (AA vs AB stacking, Supporting Information Figure S1) with the experimental profile, we determined that PyIm-COF takes an eclipsed AA stacking mode with an interlayer distance of 3.47 Å. The chemical structure of the resulting PyIm-COF was characterized by Fourier transform infrared (FT-IR) spectroscopy. The FT-IR spectra of PyIm-COF and monomer TOTPA are shown in Figure 2a. FT-IR showed that typical vibration bands for the imidazole moieties at 1600 and 3410 cm−1 were clearly observed, indicating the formation of the imidazole ring. The chemical structure of PyIm-COF was also supported by the 13C cross-polarization magic angle spinning (CP-MAS) solid-state nuclear magnetic resonance (NMR) spectra (Figure 2b). The peaks at 170–160 ppm correspond to the imidazole ring carbon between two nitrogen atoms. The peaks at 144–137 ppm are attributed to the linkage carbon in aromatic rings, which are connected with the phenylene rings. The peaks at around 127 ppm correspond to those carbons on the phenylenes and the linkage carbons shared by the imidazole and pyrene moieties. Peaks from 109–120 ppm show resonance signals of the pyrene carbons. The morphology and microstructure of PyIm-COF were further explored by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). The SEM image shows that PyIm-COF is mainly composed of flake structures with lengths of 10–20 μm ( Supporting Information Figure S3a). The HR-TEM image shows the clear crystal lattice of PyIm-COF, which is reasonable for π–π stacking. In addition, the lattice constant of 0.35 nm was observed for PyIm-COF, which is consistent with the crystal plane along the (001) (Figure 2c). Figure 2 | (a) FT-IR spectra of PyIm-COF; (b) 13C CP-MAS NMR spectra of PyIm-COF; (c) HR-TEM images of the PyIm-COF captured from regions in panel (d). Scale bar 2 nm; (d) nitrogen adsorption-desorption isotherms of PyIm-COF. Download figure Download PowerPoint The permanent porosity of PyIm-COF was assessed by N2 sorption measurements at 77 K. The PyIm-COF displayed the typical type IV isotherms with a sharp N2 uptake under low relative pressures (P/P0 > 0.05), which are the typical characteristics of microporous materials and the calculated Brunauer–Emmett–Teller surface area of PyIm-COF is 248 m2 g−1 ( Supporting Information Table S1 and Figure 2d). The pore size distribution analysis revealed a major pore with a diameter of 0.75 nm in PyIm-COF, calculated using the quenched-solid density functional theory, which matches well with the theoretical pore size of 0.75 nm ( Supporting Information Figure S2). Thermogravimetric analysis showed that PyIm-COF has good thermal stability with a weight loss of 10% at 300 °C ( Supporting Information Figure S4a). The chemical stability was examined by treating PyIm-COF in different solvents for 24 h. PXRD patterns revealed that the crystalline samples remained essentially unchanged after being soaked in various solvents, including water, dimethylformamide, tetrahydrofuran, CHCl3, aqueous HCl (1 M), and aqueous NaOH (1 M) ( Supporting Information Figure S4b). These results demonstrate that PyIm-COF has superior chemical stability due to the presence of irreversible imidazole linkages. Electrochemical performance Next, we investigated the electrochemical performance of PyIm-COF in SIBs in coin cells with sodium foil as the counter electrode and 1.0 M NaPF6 in diethylene glycol dimethyl ether as the electrolyte. In the first cycle of the cyclic voltammetry (CV) curve, a distinguishable wide cathodic peak at the beginning of the first cycle and a sharp peak at 0.44 V can be assigned to the formation of a solid electrolyte interface layer. In the following cycles, the CV curves displayed significant overlap, indicating a highly reversible and stable electrochemical behavior of PyIm-COF (Figure 3a). It is also important for electrode materials to maintain their capacity when the current density increases, and this can be evaluated by the rate performance. As shown in Figure 3b, the PyIm-COF electrode exhibits specific capacities of 286, 272, 267, 243, 214, and 192 mAh g−1 at 0.2, 0.5, 1.0, 2.0, 5.0, and 10 A g−1, respectively, indicating the superior rate capability of PyIm-COF electrode. When the current density is restored from 10 to 0.2 A, the specific reversible capacity is still able to deliver 362 mAh g−1, and the coulombic efficiency reaches nearly 100%, indicating excellent Na+ storage capability of PyIm-COF in rechargeable Na+ batteries. This excellent rate performance indicates that PyIm-COF has both fast ion transportation and high electrochemical stability. Figure 3c exhibits the charge/discharge curve of the rate performance, which displays similar plateaus at different current densities, further demonstrating the outstanding reversibility and stability of PyIm-COF. Notably, after the current density was rehabilitated to 5 A g−1, the capacity of the PyIm-COF electrode remained constant at the reversible capacity of 250 mAh g−1, revealing the excellent reversibility of the PyIm-COF electrode. Furthermore, the PyIm-COF electrode shows outstanding high-rate and long-life performance with capacity retention of 97.2% after 2500 cycles, even at a high current density of 5 A g−1 (Figure 3d), indicating the high stability of the PyIm-COF electrode. In particular, the coulomb efficiency of the PyIm-COF electrode stays remarkably high, close to 100% during the whole cycling period, further confirming its excellent reversibility. It should be noted that the electrochemical performance of PyIm-COF puts it among the best performers of COF-based electrodes for SIBs thus far reported (Figure 3e and Supporting Information Table S2). Figure 3 | Electrochemical properties of PyIm-COF (a) CV curves of PyIm-COF electrode at a scan rate of 0.5 mV s−1; (b) rate performance of PyIm-COF electrode from the current density of 0.2 to 10 A g−1, then back to 0.2 A g−1; (c) charge/discharge profiles of PyIm-COF electrodes at different current densities; (d) long cycling stability of PyIm-COF electrodes at high current densities (5 A g−1); (e) the comparisons of specific capacity at different current density of PyIm-COF with other all-organic symmetric SIBs. Download figure Download PowerPoint Reaction kinetics To explore the detailed reaction kinetics of PyIm-COF, CV curves were acquired at gradually increasing scan rates from 0.2 to 1.0 mV s−1 (Figure 4a). With the increase in the scanning rate, the cathodic peaks shifted to lower potentials, and the anodic peaks shifted to higher potentials, indicating an enhanced degree of polarization. Figure 4b shows the relationship between the peak current (i) and the scan rate (ν) for PyIm-COF, from which the calculated b values were close to 1.0 for peaks 1, 2, 3 and 4, respectively. This illustrates that the Na+ storage mechanism of PyIm-COF is a fast surface-controlled pseudocapacitive process,31,32 due to the infinite diffusion of ions in the aligned channels and the high reactivity of the active sites available in PyIm-COF. Meanwhile, the proportion of pseudocapacitance in the electrochemical reaction process electrode material was calculated using the formula i(V) = K1V + K2V1/2 at various scan rates. PyIm-COF shows a higher capacitive contribution (85%) at a scanning speed of 1.0 mV s−1 ( Supporting Information Figure S5). Moreover, as the sweep rate increased from 0.1 to 1.0 mV s−1, the capacitive contributions of PyIm-COF increased from 72% to 85% (Figure 4c), indicating that pseudocapacitors play an important role in the process of charge/discharge. These results suggest a kinetically fast pseudocapacitance process in Na+ storage in the PyIm-COF battery system, leading to high-rate performance due to the efficient transport and excellent conductivity of Na+ in ordered porous channels. The electrochemical impedance spectroscopic tests were conducted before and after 100, 500, and 1000 cycles for PyIm-COF (Figure 4d). Clearly, the PyIm-COF Rct values decreased gradually as the number of cycles increased. These results indicate that PyIm-COF has a faster charge transport capability. Overall, these experimental results indicate the rapid and stable reaction kinetics of PyIm-COF. Figure 4 | (a) CV curves of PyIm-COF electrodes at different scan rates; (b) the relationship between log i and log v; (c) the pseudocapacitive and diffusion-controlled charge storage contributions at different scan rates; (d) Nyquist plots of PyIm-COF electrodes before and after cycling. Download figure Download PowerPoint Sodiation-desodiation mechanism To investigate the mechanism of sodiation-desodiation of the PyIm-COF electrode, the X-ray photoelectron spectroscopy (XPS) spectra and in-situ FT-IR tests were carried out on the electrode at different charging and discharging states of electrochemical cycling. As shown in Supporting Information Figure S7, the C1s spectrum can be deconvoluted into four peaks at 285.3, 283.6, 283.3, and 284.0 eV, corresponding to C–N, C–C, C=C, and C=N, respectively. After the sodiation process, a peak at 288.5 eV appeared, which is attributed to the C-Na interaction. In particular, a similar phenomenon was also observed in the N1s spectrum. After discharge, an N-Na peak at 397.9 eV appeared, similar to the N-Li interaction in organic LIBs.33 These results demonstrate the reaction of sodium with C=N on the imidazole rings of the PyIm-COF electrode during the sodiation process. In the in-situ FT-IR spectra, the characteristic signal of the amine group in the imidazole rings was observed at 1257 cm−1. This signal gradually intensified as the battery underwent discharge from 2.7 to 1.2 V versus Na+/Na. Conversely, during the charge process, the peak corresponding to the amine group of the imidazole rings at 1257 cm−1 diminished. These results indicate the reversible transformation of the aromatic amine group, resulting from the reaction of sodium with the C–N bonds in the imidazole rings of the PyIm-COF electrode during the sodiation process (Figure 5). Figure 5 | Galvanostatic charge/discharge curves of PyIm-COF (a); in situ FT-IR spectra of PyIm-COF at different states of charge (b). Download figure Download PowerPoint To gain more insights into the sodiation process, we next performed density functional tight binding (DFTB) calculations to investigate the potential sodiation sites.34 All the computations were performed using the DFTB+ package with a 3ob parameter set.35,36 4 × 4 × 4 k-point mesh was used for all solid-phase geometry optimizations. DFT-D4 correction was used to describe dispersion interactions ( Supporting Information Table S3).37 We started with a calculation with four charges placed on the neutral PyIm-COF unit cell; five groups of sodiation sites were then determined to be the most negatively charged sites, as shown in Supporting Information Figure S6; all sodiation sites were initially put in the middle of two layers of PyIm-COF. Among them, site 1 was the most stable, the Na+ was with the in the imidazole ring. This site was also in with the N-Na peak in The site of sodiation was determined to be site the pyrene stabilize the charges in the system during charge/discharge. These two sites for the specific capacities of 267, 243, and mAh g−1 at current densities of 1.0, 2.0, and A g−1, respectively. The capacity at low current densities still for a and sodiation After both site 1 and site 3 were site 4 out to be the stable site of The sodiation was Na+ from the there was also a in the position of in which site 2 was in and the site 4 Na+ from on top of the phenylene moieties to the phenylenes but by the on the The of the Na+ at site 4 be a result of the being and significant (Figure A in its position in a significant in interaction between has also been reported previously in an inorganic by Cui As shown in Figure the Na+ storage of PyIm-COF after cycling should be attributed to the of the With the of the more lithium ions the of PyIm-COF during which gradually the kinetics of the active Figure | sodiation sites of PyIm-COF. Na+) simulated reversible electrochemical mechanism of PyIm-COF during the process. on sodiation sites the atoms. Download figure Download PowerPoint In we designed and synthesized a new PyIm-COF, for the first through a self-condensation Debus–Radziszewski The resulting 2D COF excellent stability to the irreversible formation of imidazole linkages. the fully extended backbones and the readily accessible redox-active PyIm-COF was then employed as a novel electrode material for high-performance SIBs and a remarkably high-rate performance around 250 mAh g−1 and excellent cycling stability with a capacity retention of 97.2% after 2500 cycles at 5 A g−1, which is higher than that of most previously reported COFs. A that the extended fully conjugated COF redox-active nitrogen-rich imidazole heterocycles, and ordered aligned porous channels the potential its charge storage capability. 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