<i>N</i> -Heterocycles Extended π-Conjugation Enables Ultrahigh Capacity, Long-Lived, and Fast-Charging Organic Cathodes for Aqueous Zinc Batteries

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES6 Oct 2022N-Heterocycles Extended π-Conjugation Enables Ultrahigh Capacity, Long-Lived, and Fast-Charging Organic Cathodes for Aqueous Zinc Batteries Huiling Peng, Jin Xiao, Zhonghan Wu, Lei Zhang, Yaheng Geng, Wenli Xin, Junwei Li, Zichao Yan, Kai Zhang and Zhiqiang Zhu Huiling Peng State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Jin Xiao School of Science, Hunan University of Technology, Zhuzhou 412007 , Zhonghan Wu Frontiers Science Center for New Organic Matter, Renewable Energy Conversion and Storage Center (RECAST), Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071 , Lei Zhang State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Yaheng Geng State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Wenli Xin State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Junwei Li State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Zichao Yan State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Kai Zhang Frontiers Science Center for New Organic Matter, Renewable Energy Conversion and Storage Center (RECAST), Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071 and Zhiqiang Zhu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 https://doi.org/10.31635/ccschem.022.202202276 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The aqueous zinc-organic battery is a promising candidate for large-scale energy storage. However, the rational design of advanced organic cathodes with high capacity, long lifespan, and high rate capability remains a big challenge. Herein, we propose that extending the π-conjugation by N-heterocycles can provide more active sites, lead to insolubility, and facilitate charge transfer, thus boosting the overall electrochemical performance of organic electrodes. Based on this concept, a novel organic compound, dipyrido[3ʹ,2ʹ:5,6;2″,3″:7,8]quinoxalino[2,3-i]dipyrido[3,2-a:2ʹ,3ʹ-c]phenazine-10,21-dione (DQDPD), has been rationally designed and evaluated as the cathode for aqueous zinc batteries. Excitingly, DQDPD shows a record high capacity (509 mAh g−1 at 0.1 A g−1, corresponding to a record-breaking energy density of 348 Wh kg−1), excellent cycling stability (92% capacity retention after 7500 cycles at 10 A g−1), and fast-charging capability (161 mAh g−1 at 20 A g−1). Our work offers new ideas in the molecular engineering of organic electrodes for high-performance rechargeable batteries. Download figure Download PowerPoint Introduction Rechargeable aqueous batteries (ABs) employing low-cost, safe, and eco-friendly aqueous electrolytes have attracted increasing research interest for large-scale energy storage applications.1–5 Among various AB systems, aqueous zinc batteries (AZBs) stand out since the zinc metal anode has the characteristics of high theoretical specific capacity (820 mAh g−1), low redox potential (−0.76 V vs standard hydrogen electrode), resource abundance, and good compatibility with water.6–11 Currently, the cathodes for AZBs mainly rely on inorganic compounds, such as transition-metal oxides/sulfides and Prussian blue analogs, most of which suffer from poor cyclability and slow kinetics due to the irreversible structural distortion and dissolution of active materials.12–14 In addition, these inorganic cathodes usually contain nonrenewable resources and environmentally hazardous elements, making them unsuitable for green and sustainable development.15–17 Compared to inorganic materials, organic materials with the merits of resource renewability, environmental benignity, and synthetic availability are more in line with the requirements for sustainable development.18–22 Moreover, the electrochemical performance of organic electrodes, such as theoretical capacity, cycle stability, and rate capability, can be subtly tailored via molecular design,23–29 which offers great opportunities for developing high-performance AZBs. Nevertheless, most organic electrodes, especially small molecules, suffer from limited cycle stability and rate capability, mainly due to their relatively high solubility in the electrolytes and low electrical conductivity.30–32 Although these issues can be partially resolved by molecular engineering and electrolyte modification,33–37 the rational design of high capacity, fast-charging, and long-lasting organic electrodes is still a challenging but ultimately rewarding pursuit. Extending the π-conjugation of organic electrodes represents one promising strategy to improve their electrochemical performance.38–41 Theoretically, the enlarged π-conjugation can enhance the intermolecular interactions and promote the orderly arrangement of organic molecules, which would not only facilitate the charge transfer but also reduce the solubility, thus simultaneously improving the rate capability and cycle stability.42–44 However, the reported extended π-conjugated systems always involve numerous inactive constituents, such as benzene, naphthalene, and other rigid structures, which inevitably reduce the theoretical capacity.38,39 For instance, benzoquinone (BQ), the smallest quinone molecule, with a high theoretical capacity of 496 mAh g−1 cannot be directly used as electrode material due to the sublimation/dissolution issue. In this case, various larger conjugated analogs, such as 1,4-naphthoquinone (1,4-NQ), 9,10-anthraquinone (AQ), and 5,7,12,14-pentacenetetrone (PT), have been designed to gain improved cycle stability and rate performance, but all of which come at the expense of decreased theoretical capacity (Figure 1a).45,46 Therefore, designing novel extended π-conjugated building units that could simultaneously enhance the capacity, cycle stability, and rate capability should be of great importance for exploiting high-performance organic electrodes. Figure 1 | Molecular design of high-performance organic cathodes. (a) Extending the π-conjugation from BQ to NQ, AQ and PT decrease the theoretical capacity. (b) Extending the π-conjugation of BQ by N-heterocycles gives TAPQ and DQDPD, both of which feature higher theoretical capacity than that of BQ. (c) HOMO/LUMO energy levels and energy gaps (ΔEH–L) of BQ, TAPQ, and DQDPD. Download figure Download PowerPoint Here, we proposed that extending the π-conjugation of the organic electrode with redox-active N-heterocycles provides more active sites, facilitates charge transport, and decrease solubility, thus simultaneously enhancing capacity, accelerating reaction kinetics, and prolonging cycle life. As a proof of concept, a novel BQ derivative with extended N-heterocyclic-conjugated structure, dipyrido[3ʹ,2ʹ:5,6;2″,3″:7,8]quinoxalino[2,3-i]dipyrido[3,2-a:2ʹ,3ʹ-c]phenazine-10,21-dione (DQDPD), was rationally designed (Figure 1b), which holds an extremely high theoretical capacity of 519 mAh g−1, even higher than that of BQ. Moreover, the N-heterocycle-extended π-conjugation endows DQDPD with layer-by-layer stacking mode, rodlike morphology, insolubility in water, and relatively high electrical/ionic conductivity, which enable it to deliver unprecedented electrochemical performance as the cathode for AZBs, including an ultrahigh reversible capacity (509 mAh g−1 at 0.1 A g−1), superior rate performance (161 mAh g−1 at 20 A g−1), and outstanding cycle stability (92% capacity retention after 7500 cycles at 10 A g−1). Particularly, both the capacity (509 mAh g−1) and energy density (348 Wh kg−1) of DQDPD set new records for organic cathodes in AZBs. Moreover, the DQDPD cathode also demonstrated excellent performance in the soft-package cell, showing great potential for practical applications. The H+/Zn2+ co(de)insertion mechanism of DQDPD has also been comprehensively disclosed by combining experimental characterizations and theoretical calculations. Our work affords new insights for the molecular engineering of organic electrode materials, which should have important implications for advancing the practical application of rechargeable organic batteries. Experimental Methods Materials synthesis Synthesis of tetra(phthalimido)-benzoquinone Tetra(phthalimido)-benzoquinone (TPB) was prepared according to a previous report with slight modifications.47 Under N2 atmosphere, 12.3 g of tetrachloro-p-benzoquinone (Macklin, 98.0%) and 37.2 g of potassium phthalimide (Macklin, 98.0%) were added to 250 mL of acetonitrile (ACN, Heowns) at 80 °C, and then the mixture was stirred for 24 h. After cooling to room temperature, the products were filtered and washed with N,N-Dimethylformamide (Aladdin, 99.5%), and boiling water five times. The obtained samples were suspended in 150 mL of boiled ethanol and then vacuum filtered. After being dried in a vacuum oven at 105 °C for 12 h, 30 g of brown-yellow TPB powder was obtained (yield 80%). Synthesis of tetramino-benzoquinone (TABQ) The synthesized TPB (14.66 g, 20 mmol) was transferred into a 500 mL round bottom flask, into which 200 mL of hydrazine hydrate (Macklin, 80.0 wt %) was added. After being kept at 60 °C for 12 h, purple TABQ was obtained (yield 52%). Synthesis of DQDPD TABQ (84.08 mg, 0.5 mmol) and 1,10-phenanthroline-5,6-dione (PTD, 210 mg, 1 mmol) were added to acetic acid (50 mL) in a three-mouth flask, and then the flask was evacuated and refilled with Ar three times, followed by refluxing for 24 h. After cooling to room temperature, the precipitate was washed with 1-methyl-2-pyrrolidinone (NMP) five times and then dried under vacuum to give a dark brown powder (yield 80%). Synthesis of 5,7,12,14-tetraaza-6,13-pentacenequinone (TAPQ) TAPQ was synthesized according to a literature report.48 2,5-dihydroxy-1,4-benzoquinone (1.40 g, 10 mmol, Macklin) and o-phenylenediamine (4.33 g, 40 mmol, Macklin) were mixed uniformly using the mortar. The mixture was heated at 180 °C for 5 h in a tube furnace under argon atmosphere. After cooling to room temperature, the mixture was filtered and washed several times successively with deionized water and acetone and then dried at 80 °C in vacuum for 24 h to give deep purple 5,14-dihydroquinoxalino[2,3-b]phenazine (DHTAP) powder (yield 90%). Then, 12 mL H2SO4 (98%) was diluted with 50 mL of deionized water, in which the as-prepared DHTAP (1.00 g, 3.5 mmol) was added. After gradually adding K2Cr2O7 (4.12 g, 14 mmol, Kermel) as an oxidant, the mixture was heated at 80 °C for 4 h and then poured into 100 mL of ice water. The product was then filtered, subsequently washed with deionized water and acetone, and dried at 80 °C in vacuum for 18 h to give brown TAPQ powder (yield 70%). Materials characterizations Fourier transform infrared spectroscopy (FT-IR) was recorded using KBr pellets on a Bruker TENSOR II (FTS6000, Bruker, Germany) in the wavenumber range of 400–4000 cm−1. Raman spectra were recorded using a Raman microscope (Renishaw plc, Gloucestershire, England) with a 532 nm diode laser. The morphologies of the electrodes were investigated using field emission scanning electron microscopy (SEM, TESCAN MIRA3, Tescan, Czech Republic) along with energy-dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) images were acquired on Talos f200i (Thermo Fisher, Massachusetts, United States) with an electron acceleration energy of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an X-ray photoelectron spectrometer (ESCALAB Xi+; Thermo Fisher, Waltham, United States) under a vacuum of 8 × 10−10 Pa. All of the binding energies were referenced to the C 1s peak at 284.6 eV. X-ray diffraction (XRD) patterns were conducted by the Bruker D8 ADVANCE (Bruker, Germany) with Cu Kα (λ = 0.154 nm) radiation. The 1H and 13C NMR were conducted on a 400 MHz NMR spectrometer (Bruker ADVANCE III HD, Bruker, Billerica, United States). Thermogravimetric analysis (TGA) was carried out on a TGA/DSC3+ (DSC = differential scanning calorimetry) thermal analysis system (STA 700, Hitachi, Tokyo, Japan). Inductively coupled plasma emission spectrometry (ICP-OES) was characterized by the Agilent 5110 (Agilent, New York, United States). Elemental analyses including C, H, O, and N were checked by an elementar vario el III. For the ex situ FT-IR, Raman, XPS, XRD, and SEM measurements, the cells were cycled to a certain state of charge at 0.2 A g−1, and then disassembled to get the used electrode. Before each test, the cycled electrodes were washed with ethanol and vacuum dried at 40 °C. Electrochemical measurements To prepare the DQDPD electrode, DQDPD, conductive carbon, and poly(vinylidene difluoride) binder were mixed in NMP in a weight ratio of 6:3:1. The slurry was cast onto a tantalum foil by using the doctor blade technique, which was then dried under vacuum at 80 °C for 24 h. The electrode plate was 10 mm in diameter and 1–2 mg cm−2 of active load. The electrochemical performance of the DQDPD in different electrolytes (i.e. 1 M ZnSO4 in water, 1 M ZnSO4 + 2 M Na2SO4 in water, 1 M Zn(OTf)2 in ACN) were investigated by using CR2032-type coin cells containing the Zn foil as the anode and a glass fiber membrane as the separator (GF/D Whatman). Galvanostatic charge/discharge (GCD) test was performed on the LAND-CT2001A battery instrument in the voltage range of 0.15–1.5 V. Cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) were conducted using a Squidstat Plus electrochemical workstation. The EIS was performed in a frequency range of 105–0.1 Hz with an amplitude of 10 mV of alternating current. All the electrochemical tests were conducted at room temperature. For the soft-package cell, the DQDPD cathode and Zn foil were cut to a size of 3 cm × 4 cm, respectively. The mass loading of DQDPD material was about 12∼13 mg. Whatman glass fiber in a size of 3.2 cm × 4.2 cm was used as the separator and 1 M ZnSO4 + 2 M Na2SO4 aqueous solution was used as the electrolyte. For the CV test in 0.05 mM H2SO4 and 1 M ZnSO4 + 2 M Na2SO4 aqueous solution using typical three-electrode systems, the DQDPD cathode was used as the working electrode, Ag/AgCl was used as the reference electrode, and a titanium sheet was used as the counter electrode. Computational method Density functional theory (DFT) calculations were carried out with the Materials Studio dmol3 software package to investigate the reactivity of DQDPD. The geometry optimization and electronic properties were calculated using the first-principles density functional with B3LYP function and double numerical plus polarization basis set.49–51 In all calculations, the spin was unrestricted. The convergence is reached when the residual forces on each atom are less than 0.001 Ha/Å, and the change of total energy was less than 10−6 Ha. The isosurface figures of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and electrostatic potential (ESP) are shown by Material Studio.49 The isovalue is 0.03 e/Bohr3 for all HOMO and LUMO figures. Result and Discussion To demonstrate the superiority of the N-heterocycles extended π-conjugation in boosting the electrochemical performance of organic electrodes, the smallest quinone, BQ, was chosen as the starting molecule. Two BQ derivatives with an extended N-heterocyclic conjugated system, TAPQ and DQDPD were designed for comparison (Figure 1b). Interestingly, the theoretical capacity increased on the order of BQ (496 mAh g−1) < TAPQ (515 mAh g−1) < DQDPD (519 mAh g−1), which is contrary to the previous strategy that extending the π-conjugation always decreases the theoretical capacity.38,39 This is reasonable since the redox-active C=N group in N-heterocycles could also contribute to the theoretical capacity. Figure 1c displays the LUMO and HOMO of these three molecules. Accordingly, the bandgap decreases in the order of BQ (3.904 eV) > TAPQ (3.637 eV) > DQDPD (3.613 eV), which means that extending the π-conjugation can also increase the conductivity.38,42 Moreover, the extension of the π-conjugated system is expected to decrease the solubility and facilitate the charge transfer, which is beneficial to improving the cycle stability and rate capability (discussed later).42–44 Based on the above analysis, DQDPD with the larger N-heterocyclic conjugated system should provide much better performance compared to BQ and TAPQ. In fact, BQ is not suitable for use as an electrode in AZBs due to serious sublimation and dissolution. TAPQ is much more stable and has recently been explored as the cathode material for AZBs.48 However, it only achieved a capacity of 443 mAh g−1 (86% of its theoretical capacity) at 0.05 A g−1 in the voltage range of 0.15–1.5 V versus Zn2+/Zn, and the capacity dropped quickly during cycling. Therefore, DQDPD was selected as a model in this study. Synthesis and characterizations DQDPD was synthesized by a one-step solvothermal condensation reaction of TABQ with PTD in acetic acid at 120 °C for 24 h (Figure 2a), which gives a high yield of ∼80% (see Supporting Information Figures S1–S3). The successful synthesis of DQDPD was confirmed by FT-IR (see Supporting Information Figure S4a,b), NMR (see Supporting Information Figure S4c–f), and mass spectrum (see Supporting Information Figure S5). Elemental analysis (see Supporting Information Table S1) manifested that the accurate composition of the as-prepared DQDPD should be C30H12O2N8·H2O, suggesting the presence of crystal or/and adsorbed water. This is consistent with the thermogravimetric analysis (see Supporting Information Figure S6) showing that DQDPD experienced ∼4% weight loss below 350 °C. Nevertheless, the degradation of DQDPD mainly occurs at above 550 °C, implying its outstanding thermal stability. Figure 2 | Synthesis and characterizations of DQDPD. (a) Schematic of the synthetic route for DQDPD, (b) XRD pattern, (c) SEM image, (d) TEM image, (e–f) EDS elemental mapping, and (g) HRTEM image of DQDPD. Download figure Download PowerPoint The structure and morphology of the as-prepared DQDPD were also studied. The XRD result indicates that the obtained DQDPD shows high crystallinity, which originates from its large π-conjugation that induces a layer-by-layer molecular arrangement.38 Accordingly, the peak of 28.11° should correspond to the π–π stacking construction with a d-spacing of 3.21 Å (Figure 2b).52 The SEM analysis reveals that DQDPD shows rodlike morphology assembled by various nanosheets (Figure 2c), consistent with the TEM image (Figure 2d). The EDX elemental mapping images the of C, and (Figure In addition, with a diameter of nm were in the TEM image (Figure which with the XRD The π–π stacking could provide a the rodlike structure could the which could DQDPD with fast-charging the solubility and electrical of DQDPD were The of the other conjugated analogs, BQ from Macklin) and TAPQ (see Supporting Information Figures and are also shown for Excitingly, DQDPD is in water and most organic and (see Supporting Information Figure which be from the π–π intermolecular interactions and hydrogen In BQ is in water TAPQ is in water but in most organic which the of the enlarged π-conjugation in the Moreover, the electrical in the order of BQ × < TAPQ × < DQDPD × (see Supporting Information Figure with the by the bandgap calculations. As a in to its high theoretical capacity, DQDPD also layer-by-layer stacking mode, rodlike morphology, insolubility in water, and high electrical conductivity, all of which it a promising organic electrode for AZBs. Electrochemical performance The electrochemical performance of DQDPD was evaluated by using a with a zinc metal as the counter electrode. the electrolyte used in this work was 1 M ZnSO4 + 2 M Na2SO4 in water. The of Na2SO4 to the Zn anode (see Supporting Information Figure and the added not in the redox reaction of DQDPD (see Supporting Information Figure The redox of the DQDPD electrode in 1 M ZnSO4 + 2 M Na2SO4 was investigated by CV Figure shows the three CV of the DQDPD cathode at a rate of 0.1 mV which three and and three and suggesting the reaction The cycles are different from the which is to be the of the electrode Figure shows the of DQDPD in the voltage range of 0.15–1.5 V at 0.1 A The and capacity were 500 and mAh g−1, a high of the capacity reached mAh g−1 in the cycle of its theoretical specific an unprecedented energy density of 348 Wh on the mass of active the capacity and energy density of DQDPD of all reported organic cathodes in AZBs (Figure which a good to the extended N-heterocyclic conjugated system that offers redox-active C=N Figure 3 | Electrochemical performance of the DQDPD cathode in AZBs. (a) CV of DQDPD at a rate of 0.1 mV (b) of DQDPD for the five cycles at 0.1 A g−1 in the voltage range of 0.15–1.5 V. (c) of DQDPD cathode with reported organic cathodes in AZBs. (d) of BQ, TAPQ, and DQDPD electrodes at 0.1 A performance of TAPQ and DQDPD at different of the rate performance of DQDPD with reported organic cathodes for AZBs. (g) cycling stability of DQDPD at 10 A transfer impedance of TAPQ and DQDPD of the soft-package at 0.1 A stability of the soft-package at 2 A shows the of the soft-package the of the electronic performance of soft-package in the range of A Download figure Download PowerPoint As the large π-conjugation of also to insolubility in water, which cycling. As shown in Figure a capacity of mAh g−1 was after cycling at 0.1 A g−1 for 100 corresponding to a capacity retention of cycled at 0.5 A g−1, it an capacity of mAh g−1 with capacity retention after 500 cycles (see Supporting Information Figure should be out that in previous the good cycle performance of aqueous batteries was under high A g−1), which the energy Therefore, the excellent cycle stability of DQDPD obtained at such low is for practical applications. As the cycle stability of BQ and TAPQ in 0.15–1.5 V at 0.1 A g−1 was also (Figure Supporting Information Figure TAPQ much better cycle stability compared to BQ, but its capacity also kept during cycling to mAh g−1 after 100 which is consistent with the previous report.48 This is reasonable TAPQ and its product are still in water (see Supporting Information Figure BQ, TAPQ, and DQDPD the of extending the π-conjugation of organic electrodes in enhancing the cycling Figure the rate performance of TAPQ and DQDPD at various BQ was not since its cycle stability is the specific of DQDPD reached and mAh g−1 at the of and 10 A g−1, much higher than that of TAPQ obtained at the current. at an ultrahigh of 20 A g−1 for a DQDPD could also a reversible capacity of mAh g−1 (see Supporting Information Figure Moreover, the capacity was to mAh g−1 when the density to 0.1 A g−1, implying the excellent of DQDPD to under extremely rate capability of the DQDPD cathode with reported aqueous batteries (Figure DQDPD cycling at 5 A g−1, of its capacity after cycles (see Supporting Information Figure at a high density of 10 A g−1, a capacity retention of was achieved after 7500 which is to (Figure The fast-charging capability of DQDPD should be to its layer-by-layer molecular rodlike morphology, and relatively high electrical conductivity, which electron and To the of the extended π-conjugation on reaction kinetics, EIS and measurements were carried As in Figure DQDPD shows a charge transfer impedance than that of TAPQ which should be to the higher electronic of DQDPD. In addition, the of DQDPD during the and is in the range of one order of larger than of TAPQ (see Supporting Information Figure In fact, the of DQDPD is even better compared to most of the reported electrode materials for AZBs (see Supporting Information Table that the extension of π-conjugation facilitates both electron and which is for developing organic cathodes. The electrochemical properties of DQDPD to its in soft-package Excitingly, the soft-package also an ultrahigh capacity of mAh g−1 at 0.1 A g−1, to of its theoretical capacity (Figure the density increased to 2 A g−1, a capacity of mAh g−1 was still along with a capacity retention of after cycles (Figure As a the soft-package could the of an electronic of Figure The rate performance of this soft-package was also reversible of and mAh g−1 at 5 A g−1, (Figure Supporting Information Figure To the DQDPD electrode unprecedented electrochemical performance in both and