Porous polyimide framework based on perylene and triazine for reversible potassium-ion storage

Jialing Wu ab, Sijia Di b, Wei Huang b, Yunling Wu b, Qiliang Huang b, Xuan Zhao b, Xiaohan Yu b, Mochun Zhang b, Hualin Ye *c and Yanguang Li *ab
aMacao Institute of Materials Science and Engineering, Macau University of Science and Technology, Taipa 999078, Macau SAR, China. E-mail: yanguang@suda.edu.cn
bInstitute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, 215123, China
cDepartment of Chemical and Biomolecular Engineering, National University of Singapore, 119260, Singapore. E-mail: hualinye@u.nus.edu

Received 9th June 2021 , Accepted 12th August 2021

First published on 13th August 2021


Abstract

Potassium-ion batteries have attracted considerable attention as an emerging energy-storage solution due to the abundance of potassium resources. The current development of potassium-ion batteries is, however, largely impeded by the lack of high-capacity and cyclable electrode materials. Herein, we report that porous polyimide frameworks (PIFs) based on perylene and triazine are a promising anode material for potassium-ion batteries. Thanks to the high-density electroactive imide groups and their robust molecular structure, our PIF delivers a large reversible specific capacity of ∼190 mA h g−1 at 0.2 A g−1, a good rate performance (∼115 mA h g−1 at 2 A g−1) and long cycling stability (83% capacity retention after 1000 cycles at 5 A g−1). These metrics render PIFs a highly promising anode material for potassium-ion batteries for future large-scale energy-storage applications.


Introduction

Lithium-ion batteries (LIBs) as an effective electrochemical energy-storage (EES) technology have dominated the energy-storage market over the past few decades.1 Unfortunately, the ever-increasing energy demand and the growing price of battery raw materials have led to concern about the sustainability of LIBs.2–4 The search for post-LIB technologies such as sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) has therefore gained a great deal of attention.5–7 Recently, KIBs have quickly emerged by virtue of their similar electrochemistry to LIBs and the more abundant potassium resources in the Earth's crust relative to lithium.8–11

However, the development of KIBs is still in its infancy and is largely hindered by the lack of suitable electrode materials. The search for anode materials with suitable potassiation potentials is important in order to circumvent the use of the highly reactive K metal. To be considered as a favorable candidate, KIB anode materials should possess a large reversible capacity (preferentially >150 mA h g−1), a good rate capability (a decent capacity at >2 A g−1) and a long cycle life (>80% after 500 cycles). Until now, there have been few anode materials that can simultaneously reach these target values.12–14 Carbonaceous materials have a low cost and good electronic conductivity, but unfortunately they suffer from a low reversible capacity due to the difficulty in intercalating large-size K+ ions.15–17 By contrast, alloying-type materials such as Bi and P have a higher capacity (>300 mA h g−1) but are short of cycling stability due to their large volume change upon cycling.18–20

Compared with inorganic materials, organic materials with structural diversity and tunability provide unique opportunities for electrochemical energy storage.21–26 They function based on the redox reaction of electroactive groups without significant bond breaking and structural rearrangement. Their flexible organic skeletons can also better accommodate large-size K+ ions. Among them, perylene-type molecules such as perylenetetracarboxylic dianhydride (PTCDA) have recently drawn considerable attention for use in KIBs due to their large theoretical capacities.27–29 Unfortunately, their practical performance has not lived up to the expected potential. Ji et al. observed that PTCDA suffered from an irreversible structure change upon discharge to 0.2 V, and thus delivered only ∼10 mA h g−1 in the subsequent cycles.30 Cai et al. found that PTCDA had a severe dissolution problem in the electrolyte during battery cycling, and hence it suffered rapid capacity fading.31 To this end, we propose that the polymerization of PTCDA can be a solution to increase its structural stability and suppress its solubility in battery electrolytes. It is demonstrated here that the condensation reaction between PTCDA and triazine gives rise to porous polyimide frameworks (PIFs) that can serve as an efficient host material for reversible K+-ion storage. The product exhibits a large reversible capacity, good rate performance and, most impressively, a long cycling stability of up to 1000 cycles at 5 A g−1.

Experimental section

Chemicals

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA, Shanghai D&B, ≥98%), melamine (MA, Sinopharm, ≥99%), zinc acetate (Zn(OAc)2, Macklin, ≥99.99%) and imidazole (Sigma-Aldrich, ≥99.5%) were purchased and used without further purification.

Preparation of PIF

PIF was prepared by the condensation reaction between PTCDA and melamine (MA) using zinc acetate (Zn(OAc)2) as the catalyst in molten imidazole. Specifically, PTCDA (1.5 mmol, 594 mg) and Zn(OAc)2 (2.5 mmol, 470 mg) were first dissolved in imidazole (5 g) in a round-bottom flask and filled with nitrogen. The solution was then heated at 180 °C in an oil bath for 30 min. Subsequently, MA (1.2 mmol, 151 mg) was added to the reaction solution and vigorously stirred at 180 °C for another 48 h to complete the condensation reaction. After naturally cooling back to room temperature, the solid precipitate was collected by filtration, washed with 1 M HCl, ethanol and deionized water, and finally freeze-dried to yield a dark brown powder.

Structural characterizations

Powder X-ray diffraction (XRD) was performed using a PANalytical X-ray diffractometer at a scan rate of 0.05° s−1. Scanning electron microscopy (SEM) images were taken using a ZeissGemini500 scanning electron microscope. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) elemental mapping were carried out using a Talos 200X transmission electron microscope at an accelerating voltage of 200 kV. Fourier transform infrared spectra (FT-IR) were measured using a Bruker Vertex 70 spectrometer. Solid-state 13C cross-polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) spectra were collected using a 400 MHz Bruker Avance III solid-state NMR spectrometer equipped with a standard 4 mm MAS double-resonance probe.

Electrochemical measurements

To prepare the working electrodes, PIF powder, Super-P carbon black and polyvinylidene fluoride (PVDF) binder in the weight ratio of 6[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]1 were dispersed in N-methyl-2-pyrrolidone (NMP). The slurry was uniformly cast onto a copper foil, and vacuum-dried at 80 °C for 12 h. The foil was then punched into 1 cm2 disks. The areal loading of active electrode material was controlled to be ∼1 mg cm−2. CR2032 coin cells were fabricated in an Ar-filled glove box by pairing the working electrode, a Celgard 2340 polymer membrane, and a potassium anode. The electrolyte was 0.5 M KPF6 in a mixed solvent of 1,3-dioxolane (DOL) and dimethoxyethane (DME) with a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio by volume. The volume of electrolyte used in the coin cell was around 100 μL. Galvanostatic charge/discharge profiles were measured in the potential range of 0.1–2.8 V (vs. K+/K) using NEWARE multichannel battery testers. Cyclic voltammetry (CV) was conducted using a CHI660 potentiostat at a scan rate of 0.1 mV s−1 between 0.2 and 2.4 V.

Results and discussion

In this study, PIF was synthesized via the condensation reaction between PTCDA and MA in molten imidazole with zinc acetate as the catalyst (Fig. 1a).32Fig. 1b presents the X-ray diffraction (XRD) pattern of the as-prepared PIF in comparison with those of the PTCDA and MA precursors. After polymerization with MA, the diffraction peaks of PTCDA disappear, while new peaks emerge at 11.7°, 25.7° and 27.6°. In particular, the signal at 27.6° is assigned to the π–π stacking of the perylene backbones in PIF.33 Fourier transform infrared (FT-IR) spectra were measured to reveal the molecular structure of PIF (Fig. 1c). It is worth noting that the vibration peak of the carbonyl groups (C[double bond, length as m-dash]O) in PIF (1690 cm−1) shifts toward a lower wavenumber relative to that in PTCDA (1760 cm−1), in good agreement with previous observations.32–35 The signal intensity of the amino groups at ∼3461 cm−1 and 3409 cm−1 is significantly attenuated in PIF, which indicates a high degree of polymerization. The characteristic vibrations of perylene at ∼1600 cm−1 and triazine at ∼1500 and 1400 cm−1 are also observed, which supports their incorporation into the final product.36–38 Similar conclusions can also be garnered from 13C cross-polarization magic angle spinning (CP-MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy. As shown in Fig. 1d, the two peaks at 165 ppm and 162 ppm are assignable to the carbon atoms in the triazine ring and the carbonyl groups of PIF, respectively. The multiple peaks between 132 ppm and 120 ppm are contributed by carbon atoms in the aromatic rings of perylene.36,37,39 All these spectroscopic results corroborate the successful imidization reaction between PTCDA and MA in PIF.
image file: d1qm00843a-f1.tif
Fig. 1 Synthesis and spectroscopic characterization of PIF. (a) Schematic synthetic procedure of PIF; (b) XRD pattern and (c) FT-IR spectrum of PIF in comparison with those of PTCDA and MA; (d) 13C NMR spectrum of PIF.

Our product was next studied by microscopic characterization. Scanning electron microscopy (SEM) imaging shows that PIF roughly has a rod shape with a length ranging from 200 nm to 2 μm (Fig. 2a). Transmission electron microscopy (TEM) images from different angles reveal that these rods are porous and poorly crystalline (Fig. 2b and c). Moreover, elemental mapping using energy dispersive spectroscopy (EDS) supports the uniform spatial distribution of C, N and O species throughout our sample, and thereby demonstrates the uniform chemical composition of our PIF (Fig. 2d).


image file: d1qm00843a-f2.tif
Fig. 2 Microscopic characterizations of PIF. (a) SEM image, (b and c) TEM images and (d) EDS elemental (C, N and O) mapping images.

The electrochemical performance of PIF as a KIB anode material was evaluated using standard CR2032 coin cells by pairing it with a metallic K disk and filling with 0.5 M KPF6 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) DOL/DME as the electrolyte. First of all, the solubility test demonstrates that PIF has diminished solubility in the electrolyte (Fig. S1, ESI). This immediately highlights the advantage of our polymerized electrode material versus small molecules. Fig. S2 (ESI) presents the CV curves of PIF for the first three cycles. During the cathodic scan, the curve exhibits two broad peaks at 1.59 V and 1.11 V (versus K+/K, and the same hereinafter), indicative of the multistep potassiation of our PIF.34 During the reverse anodic scan, a multitude of peaks at 1.35 V and 1.74–2.08 V appear, corresponding to the reverse oxidation process of PIF. Importantly, the CV curves after the initial cathodic scan completely retrace each other, which supports the good stability and reversibility of PIF during cycling.

The reversible electrochemistry of PIF is also demonstrated from galvanostatic charge/discharge testing in the potential range between 0.2 V and 2.8 V at 0.2 A g−1. It is noted that the initial discharge profile is slightly different from those of ensuing cycles, presumably owing to the formation of a solid–electrolyte interface (SEI) (Fig. 3a).46,47 Upon discharge, PIF delivers a large capacity of ∼190 mA h g−1 at 0.2 A g−1 (Fig. 3b). At the end of 200 cycles, it retains a specific capacity of 142 mA h g−1, which is larger than those of many other anode materials such as graphite and hard carbon (Fig. 3d and Table S1, ESI).40,42,44,48–51 Structural analysis of PIF recovered after the cycling test indicates no noticeable change in its molecular structure (Fig. S3, ESI). By comparison, PTCDA shows a significantly lower specific capacity (16 mA h g−1) and a poorer cycling stability due to its dissolution in the electrolyte (Fig. S4, ESI). Moreover, we investigated different electrolyte formulations, and find that PIF demonstrates the best electrochemical performance in 0.5 M KPF6/DOL/DME (Fig. S5, ESI).


image file: d1qm00843a-f3.tif
Fig. 3 Electrochemical performance of PIF as a KIB anode material. (a) Galvanostatic charge–discharge curves and (b) corresponding cycling stability of PIF at 0.2 A g−1; (c) rate capability of PIF; (d) comparison of our PIF with representative KIB anode materials from the literature;40–45 and (e) long-term cycling stability of PIF at 5 A g−1.

Besides the high specific capacity and cycling stability, PIF also exhibits a good rate capability as shown in Fig. 3c. At the current rates of 0.2, 0.5, 1 and 2 A g−1, high specific capacities of ∼170, ∼150, ∼130 and ∼115 mA h g−1 are still delivered, respectively. When the current is reverted stepwise back to 0.2 A g−1, the specific capacity was recovered to 167 mA h g−1. More importantly, the electrode stability is not compromised at high current rates. When cycled at 5 A g−1, PIF delivers an initial specific capacity of 100 mA h g−1 that is sustained at 83 mA h g−1 after 1000 cycles (Fig. 3e). Its corresponding Coulombic efficiency stays close to 100% after the first several cycles. Such excellent cycling stability has rarely been realized for organic anode materials of KIBs to the best of our knowledge.28,52 The combination of large specific capacity, great rate capability and excellent cycling stability thereby renders our PIF a highly promising candidate as a KIB anode material.

Furthermore, we studied the reaction kinetics of our PIF. Fig. 4a illustrates its CV curves at different scan rates from 0.1 to 0.8 mV s−1. Two pairs of cathodic and anodic peaks (R1, R2, and O1, O2, respectively) are used to analyze the reaction kinetics. Their peak currents (i) are not proportional to either the scan rate (v) or the square root of the scan rate (v1/2), indicating that the redox reaction consists of both faradaic (diffusion-controlled) and non-faradaic (surface-controlled) contributions. Fig. 4b plots the logarithm of the peak current (log[thin space (1/6-em)]i) with respect to the logarithm of the scan rate (log[thin space (1/6-em)]v). Fitting the plots gives rise to slope values between 0.8 and 0.9. These demonstrate that the electrochemical storage of K+ ions in PIF proceeds mainly through a surface-controlled mechanism, in line with its porous nature and nanoscale structure.53Fig. 4c and d summarize the quantified pseudocapacitive contributions at different scan rates. The proportion of pseudocapacitive capacity increases with the scan rate, as expected. It is believed to be responsible for the excellent rate capability and cycling stability observed for our PIF.


image file: d1qm00843a-f4.tif
Fig. 4 Electrochemical kinetics analysis of PIF for K-ion storage. (a) CV profiles of PIF at different scan rates. (b) Plots of log(peak current) versus log(scan rate). (c) Normalized percentage of pseudocapacitive capacity at different scan rates and (d) the corresponding CV profiles with the blue region representing the pseudocapacitive contribution.

Conclusion

In summary, we here reported a porous polyimide framework as a high-performance anode material for KIBs. The product was prepared via a facile condensation reaction between PTCDA and MA. It contained high-density electroactive imide functionalities for K+-ion storage, and had diminished solubility in the electrolyte. When used as a KIB anode material, PIF delivered a large reversible capacity (∼190 mA h g−1 at 0.2 A g−1), a good rate capability (∼115 mA h g−1 at 2 A g−1) and an excellent cycling stability (1000 cycles at 5 A g−1), which outperformed most existing organic anode materials of KIBs to the best of our knowledge. Kinetics analysis indicated that the great electrochemical performance had a pseudocapacitive origin, presumably due to its porous nature and nanoscale structure. Our study here once again demonstrates the great potential of organic materials for electrochemical applications.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We acknowledge support from the National Natural Science Foundation of China (22005209 and 51972219), China Postdoctoral Science Foundation (2020M681702), the Collaborative Innovation Center of Suzhou Nano Science and Technology, the 111 Project and Joint International Research Laboratory of Carbon-Based Functional Materials and Devices.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00843a

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