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Cite This: ACS Appl. Energy Mater. 2019, 2, 413−419
Reversible High-Voltage N‑Redox Chemistry in Metal−Organic Frameworks for High-Rate Anion-Intercalation Batteries Xiaobing Lou, Fushan Geng, Bei Hu, Chao Li, Ming Shen,* and Bingwen Hu Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials Science, East China Normal University, Shanghai 200062, China
ACS Appl. Energy Mater. 2019.2:413-419. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/28/19. For personal use only.
S Supporting Information *
ABSTRACT: Because of the capacity limitation in traditional transitionmetal oxide cathodes, the design of novel cathodes is vital for the performance enhancement in lithium-ion batteries. In this work, we propose a new anion-intercalation strategy for the design of metal− organic frameworks that can be used as high-voltage cathodes in lithiumion batteries. A novel pcu-topology [Zn4O2(DAnT)3(DMF)4]·(DMF)6 MOF is synthesized with enriched nitrogen active sites and porous cage structure. When applied as cathode with LiPF6 based electrolyte, this MOF reaches the theoretical capacity of ∼60 mAh g−1 at a current density of 100 mA g−1 within high working voltage of 2.5−4.0 V versus Li/Li+. Even at a high current density of 1000 mA g−1 (∼16C), half capacity can still be obtained. The reversible conversion of N/N+ and adsorption/desorption of PF6− were also examined by a series of ex situ characterizations including XPS, XRD, and FTIR, which clearly validates our strategy that permits good electrochemical performance at relatively high working potential. KEYWORDS: metal−organic frameworks, N-redox, cathode, anion intercalation, high rate capability
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INTRODUCTION The emerging demand for abundant renewable energy calls for batteries with higher energy and power density, environmentally benignity, and longer lifetime. However, the current widely applied cathodes based on transition-metal redox couples may hardly meet these criteria in the near future. In addition to the scarce content, high cost, and high energy consumption during fabrication, the low electronic supply per unit mass (e.g., Co3+/Co4+) may restrict the further enhancement of their capacities. Compared with the transition metals, the light-element C, N, and O based redox couples exhibit high abundance and low cost, thus attracting much attention as alternate green energy resources. It is noteworthy that their higher electronic supplies (e.g., N/N+) also provide great potential for further development of capacity. By tailoring and assembling such naturally abundant elements, several kinds of important cathodes have been exploited. For example, carbonbased graphite intercalation compounds (GICs) were reported to provide intercalation sites for the electrolyte anions at a high voltage up to 5 V versus Li/Li+.1,2 However, the low capacities and enormous side reactions restrict their practical use. The Obased quinonic or carbonyl structures have been reported to maintain high capacities and good cycling stabilities.3−5 Nevertheless, the relatively low working voltage based on ntype6 also restricts their further applications. Among these cathodes, N redox couple based on p-type6 seems to be an excellent candidate for possessing both high working potential and good reversibility. To date, several © 2018 American Chemical Society
cathodes of this type have been exploited, such as conducting polymers (poly(aniline), etc.),7,8 nitroxyl polyradicals (PTMA, etc.),9−11 and organic salts.12,13 Generally, the electrochemical reaction on p-type cathode is between the neutral state (P) and the positively charged state (P+). During oxidation reaction, electrolyte anions insert to the cathode to neutralize the positive charge of P+. However, the conventional anions used in nonaqueous electrolyte are too big (for example, PF6−, 3.5 × 3.5 Å) to insert into most of those cathodes. Therefore, the resulting slow diffusion of anions throughout the electrodes is a major obstacle to their rate capabilities. To address this issue, here we propose the design of a novel metal−organic framework (MOF) based p-type cathode. MOFs assembled from metal centers and organic linkers can be tailored with certain surface area, pore size, and redox activity to meet specified applications, such as catalysis, sensing, gas storage, and energy storage.14−17 Regarding the energy storage systems, in addition to many anode application,18,19 MOFs have also been applied as cathodes.20−22 For example, MIL-53 was first employed as a cathode for Li-ion battery (LIB) by Tarascon et al.23 The Fe(II)/ Fe(III) redox couple led to a reversible capacity of 75 mAh g−1 at the voltage range of 1.5−3.5 V. Awaga and Coworkers synthesized a Cu(2,7-Anthraquinone Dicarboxylate) MOF Received: August 28, 2018 Accepted: November 29, 2018 Published: November 29, 2018 413
DOI: 10.1021/acsaem.8b01428 ACS Appl. Energy Mater. 2019, 2, 413−419
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ACS Applied Energy Materials
Figure 1. (a) 2,5-(Dianilino) terephthalic acid (H2DAnT) linker and (b) the [Zn4(μ3-O)2(COO)6(DMF)4] cluster in (c) the 6-connected pcu net topology (41263) of ZnDAnT MOF. (bond, DAnT linker; ball, [Zn4(μ3-O)2(COO)6(DMF)4] cluster). The intralayer and interlayer linkers are colored in blue and purple, respectively. (e) Structure of ZnDAnT shown along the a-axis. (d) Same structure shown with the anilino groups neglected to easily observe the voids. Hydrogen atoms are omitted for clarity in (d) and (e). Other structural parameters of ZnDAnT are displayed in Table S1. and kept at 90 °C for 24 h. The obtained yellow crystals were washed in succession with DMF and ethanol to give [Zn 4 O 2 (DAnT) 3 (DMF) 4 ] 0.5 ·(DMF) 3 (66% yield based on H2DAnT). The synthesis can be conducted with 10-fold dosage in a Teflon-lined stainless-steel autoclave. Anal. Calcd for C45H56N8O12Zn2(Mr = 1031.71): C, 32.06; H, 3.35; O, 11.39; N, 6.65%. Found: C, 32.52; H, 3.38; O, 11.52; N, 6.50%. Because of the liberation of DMF molecules in ZnDAnT upon drying, the ZnDAnT was evacuated at 110 °C for 6 h to remove most DMF and get an accurate quantity for electrode weighting before electrode fabrication. Synthesis of Lithium 2,5-(Dianilino) Terephthalate (Li2DAnT). To compare the performance of MOF and the traditional organic electrode in the common aprotic electrolyte, lithium 2,5(dianilino)terephthalate (Li2DAnT)25 was synthesized in deionized water with H2DAnT and a stoichiometric amount of lithium hydride. The obtained solution was stirred over 1 h and filtrated to obtain a yellow settled solution. The solution was then evaporated in an aircirculating oven at 80 °C and further dried under vacuum at 60 °C to obtain Li2DAnT. Crystal Structure Analysis. A single crystal of H2DAnT or ZnDAnT was selected and glued on a glass fiber before the test. Bruker APEX II diffractometer equipped with a CCD area detector and graphite-monochromatic Mo Kα radiation (λ = 0.71073 Å) was employed to collect the diffraction data of ZnDAnT at 100 K. Empirical absorption corrections were applied using the SADABS program. The structure was solved by direct methods and refined by the full-matrix least-squares method on F2. The hydrogen atoms of the coordinated and uncoordinated water molecules were located from the difference map and restrained using SHELXL DFIX and DANG instructions.26 Physical Characterizations. A Nexus670 infrared spectrometer (Nicolet) was used to perform Fourier transform infrared spectroscopy (FTIR) analysis in the wavenumber range of 4000−400 cm−1 in transmission mode. The Elementar Vario ELIII analyzer was used to analyze elemental composition. Rigaku Ultima IV X-ray Diffractometer with Cu−Kα radiation (V = 35 kV, I = 25 mA, λ = 1.5418 Å) was used to conduct X-ray powder diffraction (XRD) measurements. TG curves were recorded using a STA 449 F3 Jupiter simultaneous thermo-analyzer (NETZSCH Gerätebau GmbH) from room temper-
with anthraquinone-based redox couple, demonstrating a high capacity of 105 mAh g−1 with a voltage window of 4.0−1.7 V.24 These previous attempts suggest that MOFs hold great potential for cathodes in energy storage systems. More specifically, MOFs can be ingeniously modified with N/N+ redox-active groups, which thus permits the operation at a relatively high voltage. Moreover, by using suitable modified organic linkers, the porous structures and appropriate cages in MOFs may allow good electrolyte accessibility and fast migration and storage of large-sized anions. On the basis of these assumptions, we designed a pcutopology [Zn4O2(DAnT)3(DMF)4]·(DMF)6 MOF as an anion-intercalation cathode in this work. The electrochemical functionality of this MOF is resulted from redox-active anilino groups that reside in the skeleton. When assembled into button cells versus Li/Li+ using LiPF6-based electrolyte, the theoretical capacity of ∼60 mAh g−1 was maintained at a current density of 100 mA g−1 with a high working voltage of 2.5−4.0 V. Even at a high current density of 1000 mA g−1 (∼16C), half capacity can still be obtained, implying fast transportation of PF6− anions in MOF cages.
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EXPERIMENTAL SECTION
Purification of 2,5-(Dianilino) Terephthalic Acid (H2DAnT). The as-received 2,5-(dianilino) terephthalic acid (H2DAnT, Alfa, 97%) was recrystallized in dimethylformamide (DMF) solvent for 10 days. Single crystal diffraction (Figure S1) and 1H NMR (Figure S2) reveal the high purity of the obtained deep yellow H2DAnT@DMF crystals. Pure purple H2DAnT was collected by drying H2DAnT@ DMF at 180 °C (determined from TG results shown in Figure S1c) for 3 h. Synthesis and Activation of [Zn4O2(DAnT)3(DMF)4]0.5· (DMF)3 (ZnDAnT). In a typical synthetic process of ZnDAnT, 0.02 mmol H2DAnT and 0.1 mmol Zn(NO3)2·6H2O (Alfa, 99%) was dissolved in a mixed solvent of DMF/EtOH/H2O (1.00/0.25/0.25 mL). The solution was then sealed into a small glass bottle (3 mL) 414
DOI: 10.1021/acsaem.8b01428 ACS Appl. Energy Mater. 2019, 2, 413−419
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Figure 2. (a) Simulated and experimental XRD patterns of ZnDAnT, and experimental pattern of evacuated ZnDAnT. (b) TG curves of ZnDAnT. (c) Nitrogen adsorption−desorption isotherms of evacuated ZnDAnT. (d) Pore size distribution curves of evacuated ZnDAnT.
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ature to 800 °C with a heating rate of 10 °C min−1 under air atmosphere. 1H solution NMR spectra (∼2 mg sample dissolved in 0.5 mL DMSO-d6) were recored on a Bruker AVANCE-III 11.7 T spectrometer operating at a 1H Larmor frequency of 500 MHz. A Hitachi F-4500 fluorospectrometer was used to collect excitation and emission spectra. The electron paramagnetic resonance (EPR) spectra were recorded at room temperature on a Bruker Elexsys 580 X-band spectrometer equipped with HS (high sensitivity) cavity. The X-ray photoelectron spectroscopy (XPS) data were recorded using an AXIS Ultra DLD spectrometer (SHIMADZU Ltd., Japan) operating at 250 W. A S-4800 scanning electron microscope (Hitachi, Japan) was employed to take SEM images. For the ex-situ characterizations, the cells were firstly charged or discharged to the desired states at a current density of 100 mA g−1. Whereafter the cells were disassembled and the electrodes were washed with dimethyl carbonate (DMC) for at least three times in an argon-filled glovebox with both water and oxygen less than 0.1 ppm. Finally, the electrodes were dried in the glovebox before performing ex situ characterizations. Electrochemical Testing. All the electrochemical measurements were carried out at room temperature. The active material (evacuated ZnDAnT, weight ratio: 70%), Super-P carbon black (20%), and the polyvinylidene fluoride (PVDF) binder (10%) were homogeneously mixed in N-methyl-2-pyrrolidone solvent (NMP) and stirred for at least 3 h to produce a slurry. The obtained slurry was then coated onto Al foil and dried at 110 °C in vacuum for 12 h. The electrodes were punched into round plates with a diameter of 14.0 mm. The loading of the as-prepared electrodes is about ∼1.5 mg/cm2. 1 M LiPF6 in EC-DMC-EMC (1:1:1 in volume) was used as the electrolyte. Finally, the coin cells (CR2032) were assembled using the as-prepared cathode, a Celgard 2325 separator (diameter of 19.0 mm), a pure lithium wafer (counter electrode and reference electrode), and the electrolyte (∼50 μL per cell) in an argon filled glovebox with oxygen and water level less than 0.1 ppm. The galvanostatic cycles and rate tests were performed on a LAND 2001A battery test system in the voltage range of 2.5−4.0 V. Cyclic voltammetry (CV) measurements were carried out using an electrochemical workstation (CHI660e) at a scan rate of 0.2 mV s−1 in the voltage range of 2.5−4.0 V.
RESULTS AND DISCUSSION Structures of the MOF. We selected conventional terephthalic acid as the starting linker to construct highthroughput MOF skeleton for further introducing abundant high-voltage redox-active species. In this work, two redoxactive anilino groups were introduced to the rigid linker, leading to a larger 2,5-(dianilino)terephthalic acid (H2DAnT, shown in Figure 1a) linker which solvothermally reacted with Zn(NO3)2·6H2O to synthesize the ZnDAnT MOF. Because of the steric effect of the linker, an unusual Zn4(μ3-O)2(COO)6 cluster containing two tetrahedral and two octahedral Zn centers (Figure 1b) was formed, which is distinct from the four octahedral Zn centers in normal MOF-5 structure. The resulting ZnDAnT MOF possesses a distorted 6-connected pillared-layer network with the Schläfli symbol of 41263 (calculated using the TOPOS 4.0 program, same to MOF5).27 As shown in Figure 1d, this structure contains large intralayer voids with an average diagonal separation of ca. 18.07 Å (measured between two nearest diagonal metal clusters) and an average edge-to-edge separation of ca. 12.78 Å (measured between two nearest metal clusters from opposite edges). The layers are further pillared into a 3D framework with an interlayer spacing of 15.06 Å (Figure S3a). It is anticipated that the redox-active anilino groups laying at the edge of void space can adsorb the anions from the electrolyte that transfers through the MOF channels.28 The phase purity of ZnDAnT was proved by the negligible difference between experimental and simulated XRD patterns (Figure 2a). The two strong peaks at 5.87° ({1 0 0}, 15.06 Å) and 6.99° ({0−1 −1}, 12.64 Å) correspond well with the interlayer spacing and intralayer edge separation. The TG curve of ZnDAnT shown in Figure 2b exhibits a continuous weight loss of 31.62% below 300 °C, which can be attributed to the evaporation of coordinated DMF molecules. The residual weight loss of 49.92% until 520 °C corresponds to the 415
DOI: 10.1021/acsaem.8b01428 ACS Appl. Energy Mater. 2019, 2, 413−419
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Figure 3. (a) Cyclic voltammograms of ZnDAnT for the first cycle between a voltage of 2.5−4.0 V at a scan rate of 0.2 mV s−1. (b) XRD patterns, (c) FTIR spectra, (d) N 1s XPS spectra, (e) P 2p XPS spectra, and (f) Zn 2p XPS spectra of electrodes charged or discharged to the desired states within the first cycle. The XRD patterns of ZnDAnT, evacuated ZnDAnT, and fresh electrode were also plotted for comparison. The FTIR data of ZnDAnT, evacuated ZnDAnT, and electrolyte were also displayed to identify the peaks.
treatment, declaring the removal of most DMF molecules after heating. The fluorescent excitation and emission spectra are also displayed in Figure S5, and detailed discussions can be found in the Supporting Information. Li-Storage Performance. The redox activity of ZnDAnT was first evaluated by cyclic voltammetry as shown in Figure 3a. It can be observed that the electrochemical behavior at the first cycle is distinct from the following cycles. To investigate the redox reactions during the first cycle, several charge/ discharge states, including the pristine state, the charge to 3.8 V state (C3.8), the charge to 4.0 V state (C4.0), the discharge to 3.5 V state (D3.5), and the discharge to 2.5 V state (D2.5), were selected for studying the evolution of electrochemical processes. As discussed in the following, a series of ex situ characterizations including XRD, FTIR, and XPS were conducted for these samples. During the first anodic scan, the weak and broad peak between the states of pristine and C3.8 can be ascribed to the oxidation of N to N+, accompanied by slow diffusion of PF6− inside the MOF framework and further adsorption of PF6− by N+ to neutralize its positive charge. The followed strong peak centered at 3.9 V can be attributed to the activation of electrode with large amount of electrolyte filled into the MOF channels and the adsorption of PF6− by the residual N+. During the first cathodic scan, the two reduction peaks before and after 3.5 V can be attributed to the sequential reduction of two N+ to N and the desorption of PF6−. The second and third cycle almost coincide with each other, indicating the full activation of the electrode during the first cycle. The apparent two oxidation peaks at 3.5 and 3.9 V and two reduction peaks at 3.4 and 3.8 V in the latter two cycles can be assigned to the reversible conversion of 2N ↔ N.N+ ↔ 2N+ in DAnT linker. Figure 3b displays the ex-situ XRD spectra of the cycled electrodes. The as-prepared electrode shows indentical patterns in comparision with the evacuated ZnDAnT. However, after soaking in the electrolyte for 1 day (pristine state) the strong peak at 8.20 and 9.74° disappear, which may
decomposition of organic linkers. The ZnDAnT sample after evacuation at 110 °C gives a distinct XRD pattern (Figure 2a) because of the structure transformation from single crystals to polycrystals due to the removal of DMF molecules filled in the channels.29,30 Compared to the diffractions of the pristine ZnDAnT, the two main diffraction peaks of evacuated ZnDAnT shift to higher degree of 8.20° (10.78 Å) and 9.74° (9.08 Å), implying the lattice shrinking after solvent removal. To further determine the microstructural of evacuated ZnDAnT, nitrogen adsorption−desorption experiment was conducted. As shown in Figure 2c, a type IV isotherm with a mixed H2 and H3 type hysteresis loop reveals the coexistence of structural micropores as well as interparticle pores. The total surface area is calculated to be 52.9 m2 g−1 and the total pore volume is 0.082 cm3 g−1. The corresponding pore size distribution and microporous analysis were also calculated as plotted in Figure 2d. The micropores centered at 0.76 and 0.94 nm can be attributed the structural pores. It is reasonable that the structural pores are smaller than the lattice spacing observed from XRD patterns because of the occupation of MOF skeleton and connected anilino groups. The pores larger than 3 nm are interparticle pores induced by the collapse of MOF crystal structure after grinding and annealing. The micropore volume (0.030 cm3 g−1) is suitable for storing massive large-sized anions. Moreover, the moderate mesopore and macropore volumes are beneficial for their fast migration. The FTIR results prove the complete deprotonation of H2DAnT upon the formation of ZnDAnT (Figure S4). The characteristic bonds of H2DAnT at 1213, 1666, and 2800− 3050 cm−1 ascribed to ν(C−O), ν(C=O), and ν(OH) of nonionized −COOH disappear, while new peaks corresponding to the symmetric stretching vibrations of −COO− in ZnDAnT appear at 1369−1441 cm−1. The presence of ν(NH) bonds at ∼3350 cm−1 and v(C−N) at ∼1530 cm−1 in both H2DAnT and ZnDAnT indicate the retaining of C−NH−C linkage. The peak at 1660 cm−1 of ZnDAnT can be assigned to the ν(C=O) of free DMF. This peak disappears after thermal 416
DOI: 10.1021/acsaem.8b01428 ACS Appl. Energy Mater. 2019, 2, 413−419
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Figure 4. (a) Galvanostatic charge−discharge profiles of ZnDAnT during 2.5−4.0 V. (b) Cyclic performance of ZnDAnT and Li2DAnT at 100 mA g−1. (c) Rate performance of ZnDAnT at current densities of 100, 200, 500, and 1000 mA g−1.
process (2N → N.N+→ 2N+) is deduced from the above observations. Because the electronic state of P in PF6− is not changed during the electrochemical cycles, the chemical environment (adsorption/desorption behavior) of PF6− can be assessed by P 2p XPS data. Since the electrodes were washed for several times, all the detected signals are assigned to PF6− inside ZnDAnT framework. For the pristine sample, a single P 2p peak at 140.6 eV is detected due to penetration of free PF6− into the MOF structure. At the C3.8 state, the formation of a new peak (136.5 eV) associated with N+ signal suggests the absorption of PF6− by N+ species to neutralize the positive charge. Moreover, the low-energy peak at 141.1 eV still exists, thus clarifying the coexistence of bonded and free PF6− in MOF cages. With the rest N converting to N+ at 4.0 V, two peaks appear at high energy and no peak can be detected at low energy, which demonstrates the complete adsorbing of PF6− by N+ inside the MOF. We proposed that two PF6− adsorbed by one linker is too space-occupying for ZnDAnT MOF to accommodate more PF6−. It is also supposed that the splitting of P 2p peak is induced by the steric effect of big PF6− anion. Upon discharge, the peak at low energy reappears at D3.5 and increases in intensity at D2.5, whereas the peaks at high energy decreases. This observation implies the desorption of PF6− from the reduced N atoms. It should be noticed that residual high-energy peaks still exist at D2.5 state, which may be assigned to trapped PF6− due to the partial collapse of the framework. The five Zn 2p spectra (Figure 3f) show identical bonding energy of 1046.2 eV (2p 1/2) and 1023.1 eV (2p 3/2), suggesting that the Zn2+ is retained. Therefore, the redoxinnocent metal center is proposed to be beneficial to the stabilization of MOF structure. The above N, P, and Zn XPS analysis is consistent with the electrochemical behavior observed from CV curves, revealing the complete anion insertion process. Other ex situ XPS spectra of Li, C and F have also been collected as shown in Figure S6. It should be noticed that the Li+ is also adsorbed into the MOF skeleton as proved by Figure S6a. It is noteworthy that the presence of a broad EPR signal (Figure S7) at C4.0 state also indicates the variation of electronic state of the nitrogen atoms. We also proposed that the reduction of Li+ and the adsorption of PF6− decrease ionic conductivity of electrode. To verify this, the electrochemical impedance spectra were collected and analyzed (Figure S8).The increase in Rs and Rct at C4.0 state is indeed in good agreement with our expectation. With two active nitrogen sites in each ligand, the theoretical capacity of the evacuated ZnDAnT electrode was calculated to be 63.1 mAh g−1. To assess the effectiveness of the designed
be induced by partial penetration of the electrolyte molecules into the MOF structure. After charging to 3.8 V, a weak peak that can be indexed to the interlayer spacing emerges at ∼6.0°, due to partial reestablishment of layered structure upon solvent adsorption. At C4.0 state, the peak at ∼6.0° sharpens and a new peak corresponding to the intralayer edge separation appears at ∼6.9°, indicating further recovery of the ZnDAnT structure. At the discharge state (D3.5 and D2.5), these two peaks are still observed, suggesting that the desorption of PF6− does not destroy the MOF structure. The structure stability can also be confirmed from the retainment of several weak peaks (15.1, 17.9, and 22.1°) during charge and discharge. The variation of N−H vibration can be monitored by ex-situ FTIR analysis shown in Figure 3c. Although the electrodes were dried in glovebox for several days, the residual electrolytes inside the framework are not fully evacuated, which is proved by the presence of strong solvent peaks, e.g. 1809, 1774, 1406, 1184, 1078, 974, 847, 777, and 559 cm−1. It is noted that the peaks at 1580−1600 and 1450 cm−1 assigned to ν(C=C) of benzene ring are retained at all states, declaring the effective detection of the metal−organic coodination structure upon charge/discharge. The C−N stretching vibration peak at ∼1530 cm−1 is present in all electrodes, whereas the N−H stretching vibration peak at ∼3350 cm−1 is weakened after charge, especially at the state of C4.0, probably suggesting the limited N−H vibration in C-HN+-C species induced by the steric effect of adsorbed PF6− anions. The electronic states and chemical environment of N, P, and Zn were measured by ex-situ XPS. The N 1s spectra at pristine state can be deconvoluted into two peaks. The main peak located at 399.6 eV can be attributed to the neutral state of nitrogen, while the weak peak at 400.7 eV to the few oxidized N induced by the oxygen in air. At C3.8 state, the 2N is oxidized to N.N+, thus the weakening of the neutral-state nitrogen peak at 399.6 eV and the strengthening of oxidizedstate nitrogen peak are observed. When charged to 4.0 V, the neutral-state nitrogen almost disappears, leaving one peak at relatively high energy of 401.1 eV. This observation proves the complete oxidization of N to N+ at high voltage. It should be noted that the peaks of N+ at C3.8 state (400.4 eV, in N.N+) and at C4.0 state (401.1 eV, in 2N+) are different in energy position, demonstrating the close relationship of the two nitrogen atoms in one DAnT. The electron-deficiency state of N+ is transferred through benzene rings to the other N atoms, leading to an increase in the oxidation potential. Besides, the reverse peak shift and peak intensity evolution in XPS spectra of D3.5 and D2.5 V samples demonstrates the reversibility of such a two-step redox process. Therefore, a two-step redox 417
DOI: 10.1021/acsaem.8b01428 ACS Appl. Energy Mater. 2019, 2, 413−419
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Figure 5. Schematic illustration of the anion-intercalation behavior of ZnDAnT.
believe that the limited cycle life may result from the existing dissociation reaction between metals and linkers and the further dissolution of MOF in the electrolyte under high voltage cycling. This is proved by ex-situ 1H NMR results shown in Figure S12, from which 1H signals arised from DAnT indicate the partial dissolution of the MOF after a few cycles. We therefore anticipate that the design of stable metal−organic coordinate bonds is promising for achieving enhanced cycling stability.
MOF for anion-intercalation battery, the cycling performance was first measured at 100 mA g−1. Figure 4a shows the galvanostatic charge−discharge profiles of the first two cycles. The MOF displays an initial charge and discharge capacity of 112.8 and 59.3 mAh g−1. The large irreversible capacity and long charge platform can be mainly attributed to activation of electrode with some side reactions. The side reactions may be attributed to irreversible intercalation of PF6− induced by the gradual collapse of MOF and the decomposition of the electrolyte.31 As the electrode is fully activated during first cycle, the second cycle shows little polarization and a charge capacity of 77.1 mAh g−1 was obtained, which implies mitigated side reactions. Moreover, the charge and discharge curves can be divided into two segments by ∼30 mAh g−1, which coincides well with the CV results and the theoretical capacity maintained by one nitrogen oxidation. Owing to these side reactions, the discharge capacity is fading during cycles as shown in Figure 4b, and a capacity of ∼40.1 mAh g−1 was maintained after 100 cycles. By contrast, the Li2DAnT sample only shows a capacity of 45.0 mAh g−1 at the first cycle which gradually drops down to 6.8 mAh g−1. The low discharge capacity of Li2DAnT can mainly be attributed to the lack of micro pores (as proved in Figure S9) as well as higher solubility in organic electrolyte (as proved in Figure S10). On the contrary, the MOF show negligible solubility in aprotic solvent, thus delivering higher cycling stability. As we expected, the morphology of ZnDAnT electrode can be maintained even after 100 cycles (Figure S11). Because of the porosity in the MOF, we proposed that the electrolyte can rapidly go through MOF, leading to high rate performance. As expected, the ZnDAnT electrode displays remarkable rate performance, and discharge capacities of 53.6, 49.7, 41.3, 30.8 mAh g−1 were retained at 100, 200, 500, and 1000 mA g−1. When the current was reversed back to 500, 200, and 100 mA g−1, the discharge capacity recovers, further demonstrating the relatively high stability of MOF upon cycling. Based on these analyses, we are able to conclude the anionintercalation behavior of ZnDAnT cathode as illustrated in Figure 5. During the first charge to C4.0 state, the MOF is activated with electrolytes penetration into the pores, accompanied by the adsorption of PF6− anions in the cages and the capture by newly generated N+. Upon discharge, the N+ is reduced to N, whereas the PF6− anions are desorbed from the active sites. The reversibility of above processes thus permits working of this cathode at relatively high voltage. Further works can be conducted to improve the Coulombic efficiency and cycling performance of the MOF cathode. We
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CONCLUSIONS Targeting at anion-intercalation in metal−organic frameworks, a Zn-based 2,5-(dianilino)terephthalic MOF was designed. With the N/N+ redox couple assembled in its micropore/ mesopore structure, this MOF cathode delivers high rate capability at high operating voltage. Even at a current of 1000 mA g−1 (∼16C), half theoretical capacity of 30.8 mAh g−1 can still be maintained. A series of ex situ characterizations reveal the resulting capacity is mainly ascribed to the two-step N/N+ redox reactions and PF6− anions that are reversibly adsorbed/ desorbed from the N active sites. Further works can be done to enhance the reversible capacity and the cycling stability. We believe this work will inspire the rational design of novel functionalized MOFs for electrochemial applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01428. Structure of H2DAnT, 1H solution NMR spectra, structural diagrams, structural parameters, FTIR, fluorescent spectra, ex situ EPR spectra, ex situ XPS spectra, impedance spectra (PDF) Crystallographic data for H2DAnT (CIF) Crystallographic data for ZnDAnT (CIF)
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AUTHOR INFORMATION
Corresponding Author
*Email:
[email protected]. ORCID
Xiaobing Lou: 0000-0002-6933-9649 Chao Li: 0000-0002-0153-7825 Ming Shen: 0000-0003-1343-2761 Notes
The authors declare no competing financial interest. 418
DOI: 10.1021/acsaem.8b01428 ACS Appl. Energy Mater. 2019, 2, 413−419
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21703068, 21373086, 21872055, and 21522303) and the National High Technology Research and Development Program of China (2014AA123401). Crystallographic data for the structures reported in this article have also been deposited in the Cambridge Crystallographic Data Center (http://www.ccdc.cam.ac.uk/) with CCDC reference numbers 1565393 and 1586366 for H2DAnT and ZnDAnT, respectively.
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DOI: 10.1021/acsaem.8b01428 ACS Appl. Energy Mater. 2019, 2, 413−419
