CoS₂ nanoparticles–graphene hybrid as a cathode catalyst for aprotic Li–O₂ batteries

Abstract

Cobalt sulfides have been demonstrated to possess high oxygen reduction activity in fuel cells with aqueous solutions. Herein, we report the use of a CoS₂ nanoparticles–graphene (CoS₂/RGO) hybrid as a cathode catalyst for aprotic lithium–oxygen (Li–O₂) batteries, where the CoS₂/RGO was prepared by a facile hydrothermal method. The resulting cells exhibited low discharge/charge overpotentials and a high rate capability, which make the metal sulfide catalysts promising cathode materials for Li–O₂ batteries.

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1. Introduction

Rechargeable aprotic lithium–oxygen (Li–O₂) batteries have attracted tremendous attention in recent years due to their potential high energy densities (a theoretical specific energy of ∼3500 W h kg_cell⁻¹, considering the weight of lithium and oxygen in the battery only).^1–5 In contrast to conventional intercalation batteries, Li–O₂ batteries comprise a porous cathode that allows ambient oxygen to be reduced to form solid oxide products (Li₂O₂ and/or intermediate LiO₂) upon discharging, which in principle reversibly evolve oxygen upon charging. This reaction occurs at a thermodynamic potential of 2.96 V:^6–9

2Li⁺ + O₂ + 2e⁻ ↔ Li₂O₂ (E_eq. = 2.96 V)

However, the commercial application of Li–O₂ batteries faces many challenges, for example, low round-trip efficiency and rate capability, as well as poor cycling stability. All these factors rely heavily on the cathode materials, where the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) take place.^1–5 Therefore, various kinds of materials have been extensively examined as cathode catalysts for Li–O₂ batteries, such as carbons,^10–15 precious metals,^16–21 metal oxides,^22–32 metal carbides,^33–35 metal nitrides^36–39 and impressive soluble catalysts.^40–43 Carbon nanomaterials have the advantages of good conductivity, large specific surface area, light weight, low cost and environmental friendliness and have been considered excellent candidates as cathode catalysts or catalyst supports for Li–O₂ batteries. In addition, metal sulfides have been demonstrated to possess high oxygen reduction activity in fuel cells with aqueous solutions.^44–48 Among all the chalcogenides of non-precious metals, cobalt sulfides showed the highest activity as an ORR catalyst.⁴⁶ To our knowledge, very few investigations have been carried out using cobalt sulfides as cathode catalysts in aprotic Li–O₂ batteries.⁴⁹ CoS₂, as an active and low-cost electrocatalyst, can enlarge the family of cathode catalysts for Li–O₂ batteries.

Herein, we report the use of a CoS₂ nanoparticles–graphene (CoS₂/RGO) hybrid as a cathode catalyst for aprotic Li–O₂ batteries, where the CoS₂/RGO, i.e., CoS₂ nanoparticles supported on reduced graphene oxide (RGO), was prepared by a facile hydrothermal method. As a result, cells with a CoS₂/RGO hybrid cathode and a lithium perchlorate–dimethylsulfoxide (LiClO₄–DMSO) electrolyte exhibited a low discharge overpotential of less than 0.15 V and a high rate capability; the discharge plateau potential only decreasing 0.17 V from 2.82 V at a rate of 0.05 A g⁻¹ to 2.64 V at a rate of 0.5 A g⁻¹. This encouraging performance confirms the metal sulfide catalysts as promising cathode materials for Li–O₂ batteries, although their cell cyclability needs further improvement.

2. Experimental

2.1 Synthesis of CoS₂/RGO hybrid

Graphene oxide was purchased from Graphene Supermarket (https://graphene-supermarket.com/Highly-Concentrated-Graphene-Oxide-175-ml.html) made by a Hummers method. In a typical synthesis of the CoS₂/RGO hybrid, ∼20 mg graphene oxide was sonicated in 60 mL deionized water for 10 min, followed by the addition of 0.5 mmol cobalt acetate (Co(Ac)₂) (>98%, Sigma-Aldrich) and 5 mL of 1 M thioacetamide (TAA) (99%, Sigma-Aldrich). The mixture was vigorously stirred at 90 °C in an oil bath for 8 h. The suspension was centrifuged and repeatedly washed with water to remove the unreacted residue, and then a precipitate was obtained. The precipitate was redissolved in 80 mL deionized water. The suspension (80 mL) was heated to and maintained at 180 °C for 5 h in a 100 mL autoclave. After cooling to room temperature, the sample was collected, centrifuged, washed with water, and finally lyophilized to obtain solid CoS₂/RGO hybrid catalysts.

2.2 Characterization

X-ray diffraction (XRD, Philips X'pert Pro X-ray diffractometer, Cu K_α1 radiation of 1.54056 Å), scanning electron microscopy (SEM, JSM-6700F at 10 kV), transmission electron microscopy (TEM, JEOL-2010 operating at 200 kV), and X-ray photoelectron spectroscopy (XPS) were used for sample characterization. The electrodes after discharging and charging were washed with acetonitrile and subsequently dried in a glovebox to be further characterized. Thermogravimetric analysis (Discovery TGA) was performed at a heating rate of 10 °C min⁻¹ from room temperature to 900 °C under an air gas flow. N₂ adsorption/desorption isotherms were measured on a NOVA 2200e at 77 K. The specific surface area was calculated using the BET (Brunauer–Emmett–Teller) method based on the adsorption data. Fourier transform infrared spectroscopy (FTIR) was carried out on a Varian 3100 Excalibur series.

2.3 Electrochemistry

Working electrodes were prepared by casting a slurry containing the active material (90 wt%) and polytetrafluoroethylene (PTFE, 10 wt%) dissolved in isopropanol onto a carbon paper (TGP-H-060) substrate, which was then dried in an oven at 110 °C for 12 h. The mass loading of active material was ∼0.8–1.0 mg cm⁻². Coin cells (2032) with homogeneously distributed holes on the positive side were assembled in an Ar-filled glovebox with the oxygen and humidity levels both below 0.1 ppm. Lithium metal foil was used as the counter electrode and glass fiber membranes as the separator. Solutions of 1.0 M lithium perchlorate–dimethylsulfoxide (LiClO₄–DMSO) or 1.0 M lithium trifluoromethanesulfonate–tetraethylene glycol dimethyl ether (LiCF₃SO₃–TEGDME) were used as the electrolyte. DMSO and TEGDME were purchased from Sigma-Aldrich and contained water contents of ≤0.20% and ≤1.0% (data from the Product Specification Data Sheet), respectively, as measured by the Karl Fischer method. The obtained cell was transported to a home-made pressure-tight glass container, which was then refilled with pure oxygen. The charge–discharge behavior was characterized in a LAND multichannel battery testing unit at different constant current rates (A g⁻¹). Cyclic voltammograms were obtained in the range of 2.0–4.5 V at a scan rate of 1.0 mV s⁻¹ with an AUTOLAB electrochemical station.

3. Results and discussion

3.1 Characterizations of CoS₂/RGO hybrid

Cobalt sulfide nanoparticles were synthesized on RGO sheets by a facile hydrothermal method via a low-temperature solution-phase reaction followed by a high-temperature hydrothermal step, as reported in the literature.⁴⁶Fig. 1 and 2 show structural and morphological characterizations of the obtained CoS₂/RGO hybrid. The XRD pattern reveals that the cobalt sulfide nanoparticles on the graphene sheet possess a typical cubic CoS₂ phase (JCPDS 00-41-1471) (Fig. 1a). The mass loading of CoS₂ in the CoS₂/RGO hybrid was about 78 wt%, as determined by thermogravimetric analysis (Fig. 1b). There were several stages of weight loss, which could be attributed to different phase changes in air. The weight loss from 500 °C to 600 °C was due to the burning of graphene and corresponded to ∼22 wt%.⁵⁰ SEM and TEM images clearly illustrate that the CoS₂ nanoparticles were homogeneously dispersed on RGO sheets with crystal sizes ranging from ∼40 nm to ∼240 nm and an average size of ∼120 nm (Fig. 2a–c and S1†). In high-resolution TEM images (Fig. 2d and e), the typical lattice fringes of 0.272, 0.221, and 0.169 nm correspond to the (200), (211), and (311) spacing of CoS₂ nanoparticles, respectively, which is consistent with the crystal structure from the XRD pattern (Fig. 1a). The energy-dispersive X-ray (EDX) spectrum demonstrates that the hybrid mainly consisted of cobalt, sulfur and carbon elements (Fig. S2a†). The specific surface areas of the CoS₂/RGO hybrid and RGO were about 27 and 434 m² g⁻¹, respectively, as measured by the N₂ adsorption/desorption method (Fig. S2b†). The CoS₂ nanoparticles in combination with the conductive RGO nanosheets could enhance the reaction activity, benefit gas diffusion and facilitate the deposition of discharge products.

Fig. 1 (a) X-ray diffraction pattern and (b) thermogravimetric analysis curve of the CoS₂/RGO hybrid.

Fig. 2 Morphological characterizations of the CoS₂/RGO hybrid. (a) Typical SEM image. (b) High-resolution SEM image. (c) Typical TEM image. (d and e) High-resolution TEM images.

3.2 Li–O₂ cell performance of CoS₂/RGO hybrid electrodes

The electrochemical performance of Li–O₂ cells with CoS₂/RGO hybrid cathodes and LiClO₄–DMSO electrolytes was examined (Fig. 3). The applied current rates (A g⁻¹) and specific capacities (mA h g⁻¹) were calculated based on the total mass of active materials (CoS₂ and RGO). Cyclic voltammetry (CV) was first employed to investigate the catalytic activity of the CoS₂/RGO hybrid in the ORR and OER (Fig. 3a). In comparison with bare RGO without CoS₂ nanoparticles and commercial Vulcan XC-72 carbon, the CoS₂/RGO hybrid exhibited a higher ORR onset potential and an obviously higher OER current peak at ∼3.75 V, which indicates higher catalytic activity in the ORR and OER. Typical charge–discharge profiles at a low current rate of 0.1 A g⁻¹ are shown in Fig. 3b. The average discharge potential measured on the CoS₂/RGO hybrid was 2.81 V (vs. Li⁺/Li), which represents a low discharge overpotential of 0.15 V. This indicates higher activity in the ORR compared with some other metal oxides.^51–54 The charging profile exhibited two plateaus at ∼3.5 V and 4.2 V, which could be ascribed to the oxidation of discharge products (intermediate LiO₂ and/or Li₂O₂) and the further oxidation of Li₂O₂, as well as partial decomposition of the electrolyte, respectively.^55–57 The first charge potential of the CoS₂/RGO hybrid was much lower than those of RGO and XC-72, suggesting higher catalytic activity in the OER, which is consistent with the above results of CV (Fig. 3a). Upon charging, a first discharge capacity of 82.1% was obtained for the CoS₂/RGO hybrid, which shows good reaction reversibility (Fig. 3b and S3†). Based on the aforementioned CV and first full discharge–charge results (Fig. 3), the CoS₂/RGO hybrid exhibited higher catalytic activity in the ORR and OER in comparison with bare RGO, which demonstrates the significant role of CoS₂ nanoparticles in the hybrid in lowering discharge/charge overpotentials.

Fig. 3 Comparison of the electrochemical performance of Li–O₂ cells with CoS₂/RGO, RGO and XC-72 cathodes and LiClO₄–DMSO electrolytes. (a) CV curves at a scan rate of 1.0 mV s⁻¹. (b) First full discharge–charge curves in the range of 2.3–4.3 V at a current rate of 0.1 A g⁻¹.

It was necessary to verify that the measured performance at a rate of 0.1 A g⁻¹ and the first full discharge–charge states arose from the formation and decomposition of Li₂O₂. Firstly, as shown in Fig. 4a, the XRD pattern confirms the formation of Li₂O₂ upon discharging. The strong peaks at 32.9° and 35.0° agree well with documented diffraction peak positions of Li₂O₂ (JCPDS 74-0115). In contrast to the standard card, the peak at 32.9° is stronger than that at 35.0°, which could be due to an overlap with the diffraction peak of CoS₂ at 32.3°, which is similar to a previous report.²⁴ Upon recharging, the Li₂O₂ diffraction peak disappeared. Secondly, XPS measurements also confirm the presence of Li₂O₂ from Li 1s spectra (Fig. 4b). Upon discharging, a peak centered at ∼54.7 eV appeared, which can be assigned to Li in Li₂O₂.^57,58 There was a very weak and wide peak at ∼55.0 eV upon recharging, which could be ascribed to slight residual Li₂O₂ and Li₂CO₃ (∼55.5 eV), which was due to decomposition of the electrolyte or side reactions of carbon and discharge products.^59,60 Thirdly, SEM was used to examine the morphology of the CoS₂/RGO electrodes in different states (Fig. 4c and d and S4†). It can be seen that the electrode was covered by nanosheet-shaped Li₂O₂ upon discharging. This kind of morphology is consistent with observations by others^61–64 and could be affected by different current densities or trace amounts of water content.^9,61 The disappearance of Li₂O₂ after subsequent recharging indicates good charging efficiency in the first cycle. The results are in good agreement with the above XRD and XPS results (Fig. 4a and b).

Fig. 4 Characterization of the CoS₂/RGO electrode at a rate of 0.1 A g⁻¹ and the first full discharge–charge states. (a) XRD patterns. (b) XPS spectra of Li 1s. All the binding energies are referenced to C 1s at 284.6 eV. (c and d) SEM images of the electrode (c) after discharging and (d) after recharging.

By using widely adopted practices, we limited the capacity (to, e.g., 500 mA h g⁻¹) to investigate the rate capability and cyclability of the Li–O₂ cell with CoS₂/RGO cathodes, as shown in Fig. 5 and S5.† It can be seen that the Li–O₂ cell displayed excellent rate capability (Fig. 5a). The discharge plateau potential only decreased by 0.17 V from 2.82 V at a rate of 0.05 A g⁻¹ to 2.64 V at a rate of 0.5 A g⁻¹, and the charge potential also only increased very slightly. Although the overpotentials greatly decreased at different current rates (Fig. 3b and 5a), the anticipated cycling stability was barely maintained (Fig. 5b). When the discharge potential dropped below 2.0 V, we considered that the cell had failed. In fact, in the initial few cycles the discharge potential was steady during high-level cycling at a rate of 0.2 A g⁻¹ and a limited capacity of 500 mA h g⁻¹ (Fig. 5b); however, after cycling for 18 cycles, the potential suddenly dropped to near 2.0 V and the cell was going to fail. Even with an increase in the current rate, the cyclability of Li–O₂ cells still did not improve (Fig. S5†). The poor cycling performance could have arisen from the indecomposable nature of the side products (e.g., Li₂CO₃) during charging/discharging, as revealed by FTIR measurements after the tenth charging process (Fig. S6†). It is mentioned that the discharge capacity and cyclability of the CoS₂/RGO hybrid are inferior to those of reported Co₃S₄ nanosheets,⁴⁹ which could be ascribed to the different electronic structures of the sulfides, which resulted in different catalytic activity in the ORR and OER.^65,66 Further effort will be devoted to investigating the contribution of the electronic structures of different cobalt sulfides such as CoS, Co_1−xS, CoS₂, Co₃S₄, Co₉S₈, etc., to the catalytic activity of Li–O₂ batteries in combination with theoretical calculations.

Fig. 5 Performance of Li–O₂ cells with CoS₂/RGO cathodes and LiClO₄–DMSO electrolytes. (a) Rate capability at different current rates (A g⁻¹) and a limited capacity of 500 mA h g⁻¹. (b) Discharge–charge curves at a rate of 0.2 A g⁻¹ and a limited capacity of 500 mA h g⁻¹.

It has been reported that a DMSO solvent with a high Gutmann donor number (DN) facilitates the growth of Li₂O₂ in the electrolyte and thereby enables higher capacities.⁷ However, a DMSO solvent has been observed to be less stable than a low-DN solvent.^67–69 While trying to improve the cycling stability, we further investigated the performance of Li–O₂ cells with CoS₂/RGO cathodes and another electrolyte solution, i.e., 1.0 M lithium trifluoromethanesulfonate–tetraethylene glycol dimethyl ether (LiCF₃SO₃–TEGDME) (Fig. S7†). In comparison with the Li–O₂ cell based on the LiClO₄–DMSO electrolyte, the Li–O₂ cell with the LiCF₃SO₃–TEGDME electrolyte demonstrated a higher ORR overpotential upon discharging and no obvious charge plateau was observed. As a result, the cycling stability was not improved. The different performances with different types of electrolytes indicate that the electrocatalytic mechanisms of the CoS₂/RGO hybrid as a cathode in Li–O₂ cells are different, which is similar to other reports.^34,70 This is possibly due to the high ionic conductivity and good oxygen dynamics of the DMSO-based electrolyte.⁷⁰

4. Conclusions

In summary, we prepared a CoS₂/RGO hybrid by a facile hydrothermal method and first investigated its performance as a cathode catalyst in aprotic Li–O₂ batteries. Cells with a CoS₂/RGO hybrid cathode and a LiClO₄–DMSO electrolyte exhibited a low discharge overpotential of less than 0.15 V and an excellent rate capability, which make the metal sulfide catalysts promising cathode materials for Li–O₂ batteries. The cells can be operated for 20 cycles at a discharge/charge capacity of 500 mA h g⁻¹ and a current rate of 0.2 A g⁻¹. Future research needs to be directed towards further improvements in the cell cyclability.

Acknowledgements

The authors acknowledge the financial support from the Singapore MOE grant R143-000-593-112.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00723f

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