Journal of Power Sources 453 (2020) 227904

Available online 19 February 20200378-7753/© 2020 Elsevier B.V. All rights reserved.

A carboxylate group-based organic anode for sustainable and stable sodium ion batteries

Chao Luo *, John J. Shea , Jinghao Huang Department of Chemistry and Biochemistry, George Mason University, Fairfax, VA, 22030, USA

H I G H L I G H T S

� Trisodium 1,2,4-benzenetricarboxylate is developed as a new organic anode material. � The new organic anode exhibits exceptional performance in Na-ion batteries. � Three carboxylate groups stabilize the organic anode. � The stable chemical structure and interface contribute to the high performance.

A R T I C L E I N F O

Keywords: Sodium ion batteries Organic materials Anode Carboxylate group High sustainability Trisodium 1,2,4-benzenetricarboxylate

A B S T R A C T

The carboxylate group-based organic electrode materials are promising anodes for rechargeable sodium ion batteries (SIBs) due to the low cost, abundance, environmental benignity and high sustainability of organic materials. Here, we compare the electrochemical performance of three carboxylate group-based organic elec-trode materials in SIBs, and identify that the two carboxylate groups in the para position of the benzene ring are electrochemical active centers to reversibly react with Na-ions and electrons. The carboxylate group-based anode, trisodium 1,2,4-benzenetricarboxylate (TBC), exhibits exceptional electrochemical performance in SIBs, in terms of high specific capacity (195 mAh g� 1) and long cycle life (600 cycles). The third carboxylate group in TBC enhances the sodiation/de-sodiation potentials, and reduces its solubility in the electrolyte, resulting in a highly stable organic anode. The low cost, high sustainability, high capacity and high cyclic stability of TBC render it a promising anode material for SIBs.

1. Introduction

Sodium ion batteries (SIBs) are promising alternatives to lithium ion batteries (LIBs) due to the low cost, abundance, and high sustainability of sodium resources [1,2]. However, the 1.4 times larger ion size of Naþ

than Liþ induces larger volume change upon sodiation/de-sodiation process and much more complicated intercalation chemistry in SIBs [3]. Most of exceptional anodes in LIBs do not exhibit good electro-chemical performance in SIBs. For example, the commercial anode, graphite, does not extend its high capacity (>300 mAh g� 1 in LIBs) and long cycle life (>500 cycles) in LIBs to SIBs [4,5]. As a consequence, it is pivotal to search for high performance anodes for SIBs. Up to date, there are five major types of anodes in SIBs, including alloys, metal oxides, metal sulfides, carbon materials and organic compounds [6–15]. The bulk alloys, metal oxides and metal sulfides suffer from large volume expansion during sodium insertion process, resulting in low capacity

and fast capacity loss. Reducing the size to nanoscale enhances the specific capacity and cycle life of alloys, metal oxides and metal sulfides, but sacrifices the volumetric capacity and increases the material syn-thetic cost. Hard carbon anodes show high capacity and long cycle life in SIBs, but the discharge plateau of hard carbon is very close to the sodium metal deposition potential, resulting in a potential safety concern for the thermal runaway caused by the sodium dendrite growth. Organic anode materials with the advantages of good structural tunability and abun-dant structural diversity are promising for high performance SIBs due to the low cost, abundance, high sustainability and superior electro-chemical performance [16–21].

A decade ago, Armand et al. reported carboxylate group based organic anodes (di-lithium terephthalate and di-lithium trans-trans- muconate) for LIBs, providing a new type of promising organic anodes for rechargeable batteries [22]. Since then, considerable research efforts were devoted to exploring carboxylate group based organic anodes due

* Corresponding author. E-mail address: cluo@gmu.edu (C. Luo).

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Journal of Power Sources

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https://doi.org/10.1016/j.jpowsour.2020.227904 Received 5 January 2020; Received in revised form 7 February 2020; Accepted 13 February 2020

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to the low cost, abundance, high sustainability, low reaction potential and high capacity of these organic anodes [23–28]. The electrochemical performance of carboxylate group-based anodes in LIBs is extended to SIBs. Hu et al. prepared disodium terephthalate (Na2C8H4O4) as a high-performance anode material for SIBs, and used atomic layer deposition method to modify the surface of the anode to further improve the rate capability and cycle life of the Na2C8H4O4 anode [29]. Amine et al. and Hong et al. also reported sodium terephthalate as a low cost and high-performance anode material for SIBs [30,31]. A variety of disodium terephthalate derivatives have been reported to compare the electrochemical behaviors of disodium terephthalate with various functional groups. Though great achievements have been obtained for the carboxylate group based organic anodes, it is still challenging to obtain a low potential (<1.0 V) and long cycle life (>500 cycles) organic anode for SIBs.

In this work, a carboxylate group-based organic material with three carboxylate groups attached to a benzene ring (Trisodium 1,2,4-benze-netricarboxylate, TBC) was prepared and used as an anode in SIBs. Two carboxylate groups at para position (Fig. 1a) act as active centers to reversibly react with sodium ions, and the third carboxylate group re-duces the solubility of TBC in the electrolyte and enhances the reaction potential of the anode, improving the cyclic stability and safety of the organic anode. The other two carboxylate group-based organic anodes, di-sodium terephthalate (DST, Fig. 1b) and di-sodium phthalate (DSP, Fig. 1c), were used as controls to compare the electrochemical perfor-mance and exploit the reaction mechanism of the carboxylate group- based organic anodes. It was discovered that DST is electrochemically active, but DSP is inactive in SIBs, demonstrating that the two carbox-ylate groups at para position rather than ortho position in TBC are the electrochemical active centers for the reversible sodiation/de-sodiation (Fig. 1d). In SIBs, TBC exhibits an initial capacity of 195 mAh g� 1 with a pair of redox plateaus centered at 0.55 V, and retains a reversible ca-pacity of 137 mAh g� 1 after 600 cycles, corresponding to a very slow capacity decay rate of 0.0496% per cycle. The exceptional electro-chemical performance of TBC renders it a promising anode material for stable and sustainable SIBs.

2. Experimental

2.1. Material synthesis

1,2,4-Benzenetricarboxylic acid, terephthalic acid, and phthalic acid were purchased from Sigma Aldrich and used as received. Trisodium 1,2,4-benzenetricarboxylate (TBC), di-sodium terephthalate (DST) and di-sodium phthalate (DSP) were prepared as follow: 1,2,4-Benzenetri-carboxylic acid, terephthalic acid, or phthalic acid were dispersed in ethanol with sodium hydroxide powders in 5% excess. The solution was stirred at room temperature for 24 h, then the solution was filtered to collect the precipitates. The precipitates (TBC, DST, and DSP) were

washed with ethanol and dried in the vacuum oven at 100 �C overnight. The products were white powders.

2.2. Material characterizations

X-ray diffraction (XRD) pattern was recorded by Bruker Smart1000 (Bruker AXS Inc., USA) using CuKα radiation; Raman measurements were performed on a Horiba Jobin Yvon Labram Aramis using a 532 nm diode-pumped solid-state laser, attenuated to give ~900 μW power at the sample surface; Fourier transform infrared spectroscopy (FTIR) was recorded by NEXUS 670 FT-IR Instrument; SEM images were taken by Hitachi SU-70 analytical ultra-high resolution SEM (Japan).

2.3. Electrochemical measurements

The organic compounds were mixed with carbon black and PVDF binder to form a slurry at the weight ratio of 60:30:10. The electrode was prepared by casting the slurry onto copper foil using a doctor blade and dried in a vacuum oven at 100 �C overnight. The slurry coated on copper foil was punched into circular electrodes with an area mass loading of ~1.5 mg cm� 2. Coin cells for sodium ion batteries were assembled with sodium metal as the counter electrode, 1 M NaPF6 in dimethoxyethane (DME) electrolyte and Celgard®3501 (Celgard, LLC Corp., USA) as the separator. Electrochemical performance was tested using Arbin battery test station (BT2000, Arbin Instruments, USA). Cyclic voltammograms were recorded using Gamry Reference 3000 Potentiostat/Galvanostat/ ZRA with a scan rate of 0.1 mV s� 1. Impedance analysis was also per-formed by Gamry Reference 1010 Potentiostat/Galvanostat/ZRA.

3. Results and discussion

Three carboxylate group-based organic anode materials, TBC, DST and DSP, were prepared by neutralizing the corresponding carboxylic acid with sodium hydroxide in ethanol. These organic anode materials were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and scanning elec-tron microscopy (SEM). As shown in the XRD patterns (Fig. 2a, S1a and S2a), TBC displays an amorphous structure, while DST and DSP have crystal structures. In FTIR (Fig. 2b), TBC shows two sharp peaks at 1562 cm� 1 and 1378 cm� 1, representing the asymmetric and symmetric stretching vibrations of the carboxylate group, respectively [32]. The small and broad peak at ~3350 cm� 1 is attributed to the water molecule, which is probably absorbed by TBC from the air. Analogous asymmetric

Fig. 1. The molecular structure of (a) TBC, (b) DST, and (c) DSP; (d) The sodiation/de-sodiation mechanism of TBC.

Fig. 2. Material characterizations for TBC. (a) XRD pattern; (b) FTIR spectrum; (c) Raman spectrum; (d) SEM image.

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and symmetric stretching peaks of the carboxylate group are also observed in the FTIR spectra (Figs. S1b and S2b) of DST and DSP, demonstrating the presence of carboxylate groups in all these organic anode materials. To further confirm the presence of carboxylate groups, we conduct Raman measurements to detect asymmetric and symmetric stretching modes of the carboxylate group in these organic anodes. As shown in Fig. 2c, the sharp peaks at 1603 cm� 1 and 1441 cm� 1 in the Raman spectrum for TBC represent asymmetric and symmetric stretch-ing modes of the carboxylate group, which are also observed in DST (Fig. S1c) and DSP (Fig. S2c). The morphology of TBC, DST and DSP was studied by SEM. All the materials consist of micro-sized particles. The particle size of TBC is in the range from 1 to 5 μm (Fig. 2d), while DST and DSP show larger particle size from 5 to 10 μm (Figs. S1d and S2d). Therefore, the material characterizations confirm that carboxylate groups are presented in TBC, DST and DSP, all of which consist of micro-sized particles.

The electrochemical performance of TBC, DST and DSP were measured in coin cells with sodium metal as the counter electrode. As shown in Fig. 3a, TBC delivers an initial charge capacity of 195 mAh g� 1

with a pair of charge/discharge plateaus centered at 0.55 V. The discharge/charge plateaus of TBC are located at 0.43 V and 0.67 V, respectively, corresponding to the cathodic and anodic peaks at 0.37 V and 0.67 V in the cyclic voltammograms (Fig. 3b). The first cycle Coulombic efficiency (CE) of TBC anode is 58.1%. The low initial CE of the TBC anode is due to the large irreversible discharge capacity in the first cycle. The growth of solid electrolyte interface (SEI) consumes Naþ

and contributes to the irreversible discharge capacity. In addition, the initial CE of carbon black is only 33.1% in Fig. S4, which is much lower than the initial CE of the TBC anode. Therefore, there are three possible methods to enhance the initial CE of the TBC anode: (1) optimizing the organic electrolyte by using fluorinated solvents such as fluoroethylene carbonate and 3,3,3-fluoroethylmethyl carbonate, which facilitate the formation of stable NaF-based SEI layer; (2) depositing an artificial SEI layer on the surface of the TBC anode to mitigate the formation of SEI from the decomposition of organic electrolyte in the first cycle; (3) reducing the content of carbon black in the anode. DST with two carboxylate groups at para position shows a higher initial charge

capacity of 272 mAh g� 1 due to the lower molecular weight of DST than TBC, and the charge/discharge plateaus of DST are centered at 0.4 V (Fig. S3a). However, DSP with two carboxylate groups at ortho position only delivers an initial charge capacity of 37 mAh g� 1, which is contributed by the conductive carbon black in the anode. The galva-nostatic charge/discharge curves in Fig. S4 show that carbon black de-livers a reversible capacity of ~50 mAh g� 1 at 50 mA g� 1 in SIBs. The low capacity of DSP anode demonstrates that it is electrochemically inactive in SIBs. The charge/discharge behaviors of TBC, DST and DSP demonstrate that the two carboxylate groups at the para position are electrochemically active, while the two carboxylate groups at the ortho position are electrochemically inactive, probably due to the large steric hindrance during the sodiation of two carboxylate groups at the ortho position. In addition, one of the major challenges for the organic anode is the low electronic conductivity. Therefore, 30 wt% conductive carbon black is used in the TBC anode to increase its conductivity. To further improve the electronic conductivity of the TBC anode, there are three approaches: (1) using high conductivity and large surface area graphene or carbon nanotube to partially replace carbon black in the anode; (2) decreasing the particle size of TBC from microscale to nanoscale, which increases the surface area of TBC particles and improves the intimate contact between TBC and conductive carbon; (3) producing carbon coated TBC particles to generate high conductivity TBC/carbon com-posite [33]. The electrochemical performances of these organic anodes are further evaluated by the long-term cycling test. While DST has a high initial capacity of 272 mAh g� 1, we can see in Fig. S3c that it suffers from fast capacity loss, dropping down to 113 mAh g� 1 after 200 cycles. The cycle life of DSP is much worse that it only retains a reversible capacity of 14 mAh g� 1 after 50 cycles (Fig. S3d). To improve the cycle life of carboxylate group-based organic anodes, we introduce a third carbox-ylate group to generate TBC and further decrease its solubility in the liquid electrolyte. In Fig. 3c, TBC retains a reversible capacity of 137 mAh g� 1 after 600 cycles, corresponding to a very slow capacity decay rate of 0.0496% per cycle. In addition, the Coulombic efficiency of TBC is close to ~100% upon long-term cycling. Due to the superior cycle life of TBC, we further evaluate its rate capability at various current den-sities in Fig. 3d. When the current density increases from 50 mA g� 1 to

Fig. 3. Electrochemical behaviors of TBC in sodium ion batteries. (a) Galvanostatic charge and discharge curves at 50 mA g� 1; (b) Cyclic voltammogram; (c) Cycle life with Coulombic efficiency at 50 mA g� 1; (d) Rate capability at various current densities.

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400 mA g� 1, the capacity decreases from 191 mAh g� 1 to 53 mAh g� 1. The reversible capacity recovers to 183 mAh g� 1 immediately after the current density decreases back to 50 mA g� 1, demonstrating the robust reaction kinetics. Therefore, the electrochemical performances of TBC, DST and DSP demonstrate that the third carboxylate group in TBC remarkably enhances its cycle life in SIBs. In the XRD patterns (Fig. 2a, S1a and S2a), TBC shows an amorphous structure, while the other two control samples, DST and DSP, show crystal structures. Since the amorphous TBC shows better electrochemical performance than the crystalline DST and DSP, it demonstrates that the crystalline structure does not determine the electrochemical performance of organic elec-trode materials.

Due to the exceptional electrochemical performance of TBC in SIBs, the reaction kinetics was investigated by cyclic voltammetry (CV) measurements at various scan rates and electrochemical impedance spectroscopy (EIS). As shown in Fig. 4a, TBC anode was cycled with various scan rates from 0.1 mV s� 1 to 1.0 mV s� 1. The linear fit of natural logarithm relationship of scan rate and peak current is displayed in Fig. 4b. The ion diffusion dominates the sodiation/de-sodiation of TBC anode if the b value is closed to 0.5, while the capacitive behavior dominates the redox reaction if the b value is closed to 1. Since the slope values of cathodic and anodic peaks are 0.6846 and 0.69041, respec-tively, the reaction kinetics of TBC anode exhibits partial capacitive behavior, which is beneficial to the high rate capability [34]. EIS anal-ysis is carried out to analyze the interface resistance of the TBC anode, which represents by the depressed semi-circle in Fig. 4c. The interfacial impedance of the TBC anode after 1st charge/discharge cycle is ~250 Ohm, which gradually reduces to ~180 Ohm after 10 cycles. The decrease of the interfacial impedance from the 1st cycle to 10th cycle demonstrates the stable solid electrolyte interface layer on the TBC anode upon cycling, which contributes to the stable cycle life of TBC in SIBs. The CV and impedance results confirm the good reaction kinetics and stable interface layer of TBC anode in SIBs.

The structure and interface of the TBC anode before and after cycling were studied by FTIR and X-ray photoelectron spectroscopy (XPS). As shown in Fig. S5, FTIR peaks of the TBC anode do not change in the pristine and cycled electrodes, demonstrating that the structure of TBC remains upon cycling. The stable chemical structure contributes to the long cycle life of TBC anode in SIBs. The interface between TBC anode and the electrolyte was checked by using XPS. In the pristine TBC anode, the C 1s peak at 284.2 eV for the graphitic carbon is used as a reference in XPS (Fig. 5a). The C 1s peak at 291 eV stands for the carbon in C–F group, while the C 1s peaks at 286.2 eV and 288.4 eV stand for the oxidized carbon (C–O/C––O). In the XPS spectrum for F 1s (Fig. 5b), the strong peak at 688 eV represents the fluorine in the C–F group, coming from the polyvinylidene fluoride (PVDF) binder. After 1 discharge/ charge cycle, the oxidized carbon peaks become stronger (Fig. 5c), while a new F 1s peak at 685.2 eV represents fluorine in NaF, demonstrating the presence of C–O/C––O groups and NaF in the SEI layer. The stable SEI layer, which is also confirmed by the EIS result, contributes to the long cycle life of TBC anode. Therefore, TBC with three carboxylate groups is a promising anode material for sustainable and stable SIBs.

4. Conclusion

In conclusion, an organic electrode material (TBC) with three carboxylate groups is reported as a promising anode for sustainable and stable SIBs. Compared to the organic anodes with two carboxylate groups (DST and DSP), the third carboxylate group in TBC not only decreases the solubility of the organic material in liquid electrolyte to improve its cycle life, but also enhances the reaction potential in SIBs to mitigate the safety concern. TBC delivers an initial charge capacity of 195 mAh g� 1 with a pair of charge/discharge plateaus centered at 0.55 V, and retains a reversible capacity of 137 mAh g� 1 after 600 cycles, corresponding to a very slow capacity decay rate of 0.0496% per cycle. The reaction kinetics, structure and interface of the TBC anode are

Fig. 4. Reaction kinetics of TBC. (a) CV of TBC at various scan rates; (b) The ln relationship of peak current and scan rate for TBC; (c) EIS analysis of TBC electrode at 1st cycle and 10th cycle.

Fig. 5. XPS spectra for the TBC electrode before cycling (a) C 1s, (b) F 1s; XPS spectra for the TBC electrode after cycling (c) C 1s, (d) F 1s.

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studied by using CV, EIS, FTIR and XPS, proving that the stable chemical structure and interface layer contribute to the exceptional electro-chemical performance of TBC anode in SIBs. Therefore, this work con-firms that carboxylate group-based organic materials are promising anodes for sustainable and stable SIBs.

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.

CRediT authorship contribution statement

Chao Luo: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Supervision, Project admin-istration, Funding acquisition. John J. Shea: Investigation, Writing - review & editing. Jinghao Huang: Investigation, Writing - review & editing.

Acknowledgements

This work was supported by the George Mason University, College of Science Postdoctoral Fellowship and COS-VSE Seed Award No. 183904.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227904.

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