Materials Today Energy
Volume 17, September 2020, 100454
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Dipotassium terephthalate as promising potassium storing anode with DFT calculations

https://doi.org/10.1016/j.mtener.2020.100454Get rights and content

Highlights

  • CNT embedded dipotassium terephthalate (K2Tp) nanosheets are upcycled from PET plastic using microwave process.

  • The inclusive contact between the CNT and K2Tp nanosheets leads to superior cycling and C-rate behavior.

  • DFT calculations investigate K-ion storage mechanism and its potential bonding sites in K2Tp.

Abstract

We report combined experimental and theoretical studies of dipotassium terephthalate as anode material for K-ion battery application. Both pristine and carbon nanotube-containing dipotassium terephthalate materials are synthesized using an ultrafast microwave-assisted method. We found that carbon nanotube (CNT)-containing dipotassium terephthalate composite electrode delivers an initial reversible capacity of 247 mAh/g with 78% capacity retention, whereas pristine K2Tp deliveries an initial reversible capacity of 212 mAh/g with 53% capacity retention at the end of 100 cycles, at a current density of 50 mA/g. The density functional theory-based calculations have shown superior stability of dipotassium terephthalate lattice against the intercalation of potassium atoms. The quantum calculations are also employed to unravel the specifics of intercalation energetics of potassium ions into dipotassium terephthalate lattice. The intercalation and de-intercalation potentials of K-ions are obtained using cyclic voltammetry and compared with theoretical calculations under the assumption of a two-electron transfer reaction. The results of this study lay the ground for understanding the electrochemical processes involved in the operation of an organic anode in K-ion batteries, and thereby can facilitate the design of optimal organic molecular crystal-based electrode materials for battery applications.

Graphical abstract

Dipotassium terephthalate (K2Tp) nanosheets are upcycled from waste PET plastic using ultrafast microwave process to study the K-ion storage behavior.

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Introduction

Due to raising public awareness of the climate change, researchers are looking for alternative energy sources to lessen our dependence on oil and gas. The recent growth of battery use in mobile technology and electric vehicles (EVs) requires development of novel energy storage devices suitable for these and other applications. [1]. Currently, lithium-ion batteries (LIBs) are considered as the best technology in the market for their application in electronic devices and EVs due to their efficient energy storing properties [2]. However, the inherent limitation of materials to store energy and concerns for the resource affordability of Li and Co limits their large-scale storing applications [3]. Alternatively, sodium- and potassium-ion batteries (NIBs and KIBs) are gaining increasing interest due to the high abundance of the constituent materials and competitive electrochemical redox potential (such as −3.04 V for Li+/Li; −2.71 V for Na+/Na and −2.94 V for K+/K) [4]. Moreover, the KIBs are considered as a potential alternative to LIBs due to their attractive electrochemical properties, such as a higher redox potential than Na and higher C-rate performance in comparison with Li- and Na-ion counterparts (because of their smaller effective solvated ion-radius in electrolyte) [5]. However, the lack of detailed investigation of optimal anode materials raises safety concerns and limits the practical application of KIBs.

Graphite/carbon and alloy-based materials have been explored as the anode in KIBs [[5], [6], [7], [8], [9], [10]]. The intercalation of K-ions into graphite occurs via stage-three KC36 → stage-two KC24 → stage-one KC8 process [6]. However, the large ionic radius of K-ion results in 61% volume expansion during intercalation into graphite anode [8]. Moreover, the electrochemical performance of the Sn-carbon composite demonstrates a reversible capacity of about 150 mAh/g within the voltage range of 2–0.01 V [9]. So it is highly desired to design a feasible anode for KIBs, which can accommodate substantial volume changes as well as operate at a safer potential.

Recently organic-based materials have been shown to be more suitable for battery applications due to their low cost, operating potential, sustainability, environmental benignity, and recyclability [[11], [12], [13], [14], [15], [16]]. For example, Chen et al. have successfully demonstrated the use of organic pigment, 3,4,9,10-perylene–tetracarboxylic acid–dianhydride (PTCDA) as a cathode and has opened the door for further research into organic materials for the KIBs [17]. Afterward, dipotassium terephthalate (K2Tp) have been investigated as anode for KIBs, which can reversibly intercalate/deintercalate two K-ions with the suitable operating potential near 0.5 V, able to reduce dendrite formation, leading to enhanced safety [[18], [19], [20], [21]]. Also, Lei et al. demonstrated a reversible capacity of 229 mAh/g for K2Tp anode in ether-based electrolyte [18]. However, poor electronic conductivity of these materials limits its electrochemical performance in terms of specific capacity and C-rate performance.

Herein, we report the synthesis of carbon nanotubes (CNTs) embedded in K2Tp nanosheets (abbreviated as CNT-K2Tp) with improved capacity retention and enhanced C-rate performance as an anode in K-ion battery. The CNT-K2Tp hybrids are synthesized by ultrafastmicrowave technique using the waste polyethylene terephthalate (PET) as precursors. The reaction of conversion of PET into dipotassium terephthalate is accomplished within 4 min, in a typical household type microwave oven setup. The application of microwave technique in organic synthesis has gained a wider attention in compare to the conventional reaction method (or conventional depolymerization), which involves severe reaction conditions, such as high temperature/pressure, prolonged reaction time, and requires a catalyst [22,23]. The physical and morphological characterization of both pristine K2Tp and CNT-K2Tp hybrid are accomplished with the aid of XRD, FTIR, Raman, TGA, BET, and FESEM, TEM, respectively. A systematic study, aimed at developing a theoretical understanding of the properties and processes taking place during the active electrochemical operation of the dipotassium terephthalate material is performed. Further, the equilibrium atomic configurations have been employed to study the electron transfer reactions and consequent micro-structural changes taking place in K2Tp molecule geometries due to two-electron transfer reactions and K-atoms attachments. The electronic structure calculations have shown possibly metallic (or semi-metallic) behavior of potassium terephthalate with intercalated K-atoms. In this study, we have not only upcycled the waste PET plastics to electrochemically active functional anode materials, using an ultrafast microwave process, but also performed comprehensive theoretical studies to get insight into the mechanism of reaction of the K-ion with dipotassium terephthalate anode material.

Section snippets

Results and discussion

The XRD, Raman and FTIR spectroscopy are carried out to confirm the chemical structure of the as-synthesized carbon nanotube integrated dipotassium terephthalate (CNT-K2Tp) and pristine dipotassium terephthalate. The overall synthesis process of pristine K2Tp and CNT-K2Tp hybrid is shown in Schematic 1. The saponification method of the PET polymer into dipotassium terephthalate, and ethylene glycol is presented by the following reaction [24,25].

The ester group present in the PET polymer

Conclusion

A systematic study, which includes both experimental measurements and theoretical calculations, was carried out to investigate the electrochemical behavior of dipotassium terephthalate in K-ion batteries. Pristine K2Tp and CNT-K2Tp hybrid are successfully upcycled from PET polymer with the aid of the ultrafast microwave synthesis. The process leads to the formations of phase pure materials, as confirmed by XRD and FTIR spectroscopy. FESEM images show that K2Tp nanosheets are self-assembled to

Synthesis of CNT-K2Tp hybrid

A microwave-assisted method was used to synthesize the CNT- K2Tp hybrid. Initially, 1.92 gm of waste PET flakes and 1.12 gm of potassium hydroxide were added into 50 mL of ethylene glycol. Then the reaction mixture is irradiated with microwave source (at 700 W) four times, with 1-min exposing time each time. In another beaker, 250 mg of functionalized CNTs were dispersed in ethanol using ultra-sonication. The transparent ethylene glycol reaction mixture was added dropwise to the ethanol

CRediT author statement

The work was carried out under the supervision of Dr. Vilas Pol. Ms. Macaggi helped Mr. Sourav to synthesize the materials. Dr. Qi, and Dr. Wang carried out the TEM characterization. Mr Sourav Ghosh, carried out all other experimental works including material and electrochemical characterizations and analyses. Dr. Makeev and Dr. Rajput performed the density functional theory calculations. Mr. Ghosh, Dr. Makeev, Dr. Rajput, Dr. Martha and Dr. Pol wrote the complete manuscript. Dr. Pol and Dr

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

SG acknowledges SERB, DST, Govt. of India, for the fellowship under the Overseas Visiting Doctoral Fellowship (OVDF) scheme with funding no. SB/S9/Z-03/2017-VIII (2018–19) to research at Purdue University. Purdue authors thanks to the Davidson School of Chemical Engineering for the financial support to pursue experimental work.

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