Ultrathin Cobaltosic Oxide Nanosheets as an Effective Sulfur

Publication Date (Web): August 30, 2016. Copyright © 2016 American Chemical Society. *E-mail: [email protected]. This article is part of the New Materi...
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Ultrathin Cobaltosic Oxide Nanosheets as an Effective Sulfur Encapsulation Matrix with Strong Affinity Toward Polysulfides Hongqiang Wang,† Tengfei Zhou,† Dan Li,† Hong Gao,† Guoping Gao,§ Aijun Du,§ Huakun Liu,† and Zaiping Guo*,†,‡ †

Institute for Superconducting & Electronic Materials and ‡School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong, Wollogong, New South Wales 2522, Australia § School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane City, Queensland 4001, Australia S Supporting Information *

ABSTRACT: Two-dimensional ultrathin cobaltosic oxide nanosheets with numerous geometrical holes were synthesized by the hydrothermal method, and further used as an effective encapsulation matrix for sulfur and polysulfides in lithium−sulfur batteries. The cobaltosic oxide/sulfur nanosheet composite electrode exhibits high Coulombic efficiency (99%), a suppressed shuttle effect, and a reversible capacity of 656 mA h g−1 at 0.2 C after 200 cycles, with small capacity fading of 0.219% per cycle, whereas its carbon−sulfur electrode counterpart only retains a capacity of 386 mA h g−1 after 100 cycles. The improved performance is attributed to the strong chemical interaction between polysulfides and cobaltosic oxide, and its facile ionic transport and enhanced reaction kinetics, which can effectively control the diffusion of polysulfides and keep them within the cathode region, leading to good electrochemical stability. KEYWORDS: cobaltosic oxide, ultrathin nanosheet, encapsulation matrix, lithium−sulfur battery, affinity to polysulfides, DFT calculations

1. INTRODUCTION The rechargeable lithium−sulfur (Li−S) battery is regarded as one of the most promising energy storage systems because sulfur cathode has a number of significant advantages, such as high energy density and high theoretical specific capacity (2567 Wh kg−1 and 1675 mA h g−1, respectively), and it is also lowcost, with resources that are abundant in nature.1−4 To realize the practical application of Li−S batteries, however, several huge challenges need to be overcome, mainly arising from the poor conductivity of sulfur and its discharge product and the large volume changes caused by the conversion from sulfur to lithium sulfides during the discharge and charge process, as well as the dissolution of the polysulfide intermediates and the related shuttle effect.5−8 These issues result in low utilization of sulfur, low Coulombic efficiency, and fast capacity fading.9−11 In the past few years, many strategies have been adopted to address the issues mentioned above. The mainstream methods have been focused on the design of nanostructured cathodes, physically confining the sulfur within nanopores or highsurface-area host materials, such as micro/mesoporous carbon, 7,12−16 carbon nanotubes, 17,18 hollow carbon spheres,19−21 and graphene.22−26 Significant capacity improvement can be obtained when these materials are used as the lithium polysulfide host. The sulfur cathode still suffers from capacity decay, however, especially under long-term cycling, © XXXX American Chemical Society

because of the weak physical affinity between the polysulfides and the porous host matrix. Alternatively, chemical strategies involving functional groups and special chemical structures should be a more efficient way to capture polysulfides because of their good chemical stability. More recently, it was noted that metal oxides, such as TiO2, Ti4O7, SiO2, and MnO2, are thought to bind lithium polysulfides with chemical bonds that would keep them within the cathode region, thus improving the cycling stability of Li−S batteries.27−32 Herein, we report a new kind of sulfur host material, ultrathin cobaltosic oxide (Co3O4) nanosheet, possessing a highly active two-dimensional (2D) surface with numerous geometrical holes. The ultrathin nanosheet structure could guarantee high levels of sulfur deposition, as well as high contact area and highly active electrochemical reaction sites with sulfur. Furthermore, the strong chemical interaction between the Co3O4 nanosheets and the polysulfides could effectively control the diffusion of polysulfides and thus improve the electrochemical stability of the cathode. Special Issue: New Materials and Approaches for Beyond Li-ion Batteries Received: June 30, 2016 Accepted: August 24, 2016

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2.4. Electrochemical Measurements. The Co3O4−S composite was mixed with carbon black and poly(vinylidene difluoride) (PVDF; 7:2:1 by weight) in N-methyl-2-pyrrolidone (NMP) to form the slurry. The as-prepared electrodes dried at 50 °C for 24 h. For comparison, carbon−sulfur (C−S) electrode and Co3O4/S mixture electrode (mixed with commercial sulfur without heating treatment) containing the same sulfur content as in the Co3O4−S electrode were also prepared. The sulfur mass loading of the electrodes was between 0.6 and 1.0 mg cm−2. The CR2032 coin cells were assembled in a glovebox filled with argon. The electrolyte was 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in a mixture of 1,3dioxolane (DOL) and dimethoxyethane (DME) (1:1 by volume) containing LiNO3 (2 wt %). The electrochemical performance of the coin cells was tested within the voltage window of 1.8−2.6 V on a battery test system (LAND CT2001A).

2. EXPERIMENTAL SECTION 2.1. Preparation of Ultrathin Co3O4 Nanosheets. First, 6.3 g of Co(NO3)2·6H2O was dissolved in 70 mL of methanol. After the Co(NO3)2·6H2O was dissolved completely, 10.5 mL of benzyl alcohol and 0.5 g of urea were added to the mixture. After stirring for another 1 h, the solution was put into a Teflon-lined stainless steel autoclave and heated to 180 °C for 20 h. The product was washed with anhydrous ethanol and deionized (DI) water several times before drying at 60 °C overnight. A lilac-colored powder was obtained, which was subsequently calcined in air with a ramp rate of 5 °C/min to 400 °C for 4 h. 2.2. Synthesis of Cobaltosic−Oxide−Sulfur (Co3O4−S) Composite. The synthesis progress of the cobaltosic-oxide−sulfur composite is illustrated in Figure 1a. First, nanosized sulfur particles

3. RESULTS AND DISCUSSION The morphology of as-prepared Co3O4 nanosheets was observed by SEM and TEM (Figure 1b, c), showing the twodimensional lamellar structure of the Co3O4 nanosheets. The Co3O4 nanosheets have a lateral size of several micrometers (Figure S1) and possess numerous geometrical holes (BET surface area of 80.35 m2 g−1, Figure S2), whereas the thickness of the nanosheets is less than 10 nm, which guarantees high levels of sulfur deposition and high contact areas with sulfur and lithium polysulfides during cycling. To maximize the contact area, the Co3O4 nanosheets were first predispersed by sonication before being mixed with sulfur nanoparticles. The content of sulfur was determined to be nearly 60% by TGA in the composite, as shown in Figure 1d. After the sulfur was loaded on the Co3O4 nanosheets, no obvious change could be observed in terms of morphology, and the composite still kept the nanosheet structure with geometrical holes (Figure 1d, e, and Figure S3). The element mapping results (Figure 1f) reveal that the sulfur is homogeneously coated on the surfaces of the Co3O4 nanosheet after melt diffusion. As shown in Figure 2a, the XRD pattern of the Co3O4 nanosheets displays low-intensity peaks, which match well with cubic spinel Co3O4 (JCPDS card No. 71−0816).33,34 The peaks of pure elemental sulfur are attributed to an orthorhombic structure (JCPDS card No. 08−0247). As for the Co3O4−S composite, some diffraction peaks of elemental S have become

Figure 1. (a) Schematic illustration of synthesis of Co3O4−S composite; (b) SEM and (c) TEM images of ultrathin Co3O4 nanosheets; (d) SEM image, with the inset showing the TGA curve of the composite, and (e) TEM image of as-prepared Co3O4−S nanosheet composite; (f) elemental mapping images of oxygen, sulfur, and cobalt.

were prepared by mixing sodium thiosulfate (0.51 g) with HCl (0.556 mL) and polyvinylpyrrolidone (0.034 g) in 100 mL of DI water. Afterward, the Co3O4 nanosheets and sulfur nanoparticles in a weight ratio of 3:7 were dispersed in 50 mL of DI water by sonication to obtain a homogeneous suspension. The suspension was filtered and dried at 50 °C, and then heated to 165 °C in a sealed tube for 20 h to obtain the Co3O4−S composite. 2.3. Characterization. The structures of the resultant materials were characterized by powder X-ray diffraction (XRD, MMA GBC, Australia) and Raman spectroscopy (JOBIN YVON HR800). Thermogravimetric analysis (TGA, METTLER TOLEDO) was used to determine the sulfur content with a heating rate of 5 °C/min in argon. The morphology of the materials was studied using fieldemission scanning electron microscopy (FESEM, JEOL JSM-7500FA) and transmission electron microscopy (TEM, JEOL 2011). Nitrogen adsorption−desorption measurement for Co3O4 nanosheets was carried out at 77 K.

Figure 2. (a) XRD patterns and (b) Raman spectra of the Co3O4, S, and Co3O4−S composite. B

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Figure 3. (a) Cyclic voltammograms of the Co3O4−S electrode at a scan rate of 0.1 mV/s; (b) 1st and 100th voltage profiles of C−S and Co3O4−S electrodes; (c) cycling stability of C−S and Co3O4−S electrodes at 0.2 C; (d) rate performance of C−S and Co3O4−S electrodes.

cycling performance. After 200 cycles, the capacity of Co3O4−S electrode remains 656 mA h g−1, corresponding to a small capacity fading of only 0.219% per cycle, which is much higher than that of C−S electrode (386 mA h g−1 after 100 cycles) and the Co3O4/S mixture electrode (596 mA h g−1 after 100 cycles, Figure S5). In addition, at the higher current rate of 0.6 C, the Co3O4−S electrode can retain capacity of 572 mA h g−1 after 300 cycles (Figure S6). On the other hand, the Co3O4−S electrode delivers a higher initial Coulombic efficiency (93.5%), which approaches above 99% after several cycles. The Coulombic efficiency of the C−S cells only reaches around 97%, however, demonstrating enhanced sulfur retention in the Co3O4−S electrode. This means that Co3O4 nanosheet has strong polysulfide-absorbing capability, which means that it can chemically bind with the polysulfides and keep them within the cathode region, leading to good cycling stability. To better understand the absorption mechanism of Co3O4 nanosheet, we used ab initio simulations performed in the framework of density functional theory (DTF) to confirm the interaction between Li2S and Co3O4 (Figure 4, calculation details in the Supporting Information). This analysis reveals a high binding energy of 5.58 eV between Li2S and Co3O4, which is significantly higher than the binding energy (0.29 eV)

weak and even disappeared, suggesting that sulfur exists in small size particles with good dispersion in the Co3O4 nanosheet matrix. In addition, Raman spectra of Co3O4, S, and Co3O4−S composite were collected to further investigate their structure and are presented in Figure 2b. Elemental sulfur exhibits typical Raman peaks, corresponding to the S−S bond,35 and the peaks at 480 and 680 cm−1 for the Co3O4 are assigned to the Eg and A1g mode vibrations, respectively.36 No Co3O4 peaks can be observed in the Co3O4−S composite, however, which may be due to high intensity of the sulfur peak and overlapping by it. Cyclic voltammetry (CV) on the Co3O4−S electrode was performed in the voltage range from 1.8 to 2.6 V. Typical CV curves of the sulfur cathode are displayed in Figure 3a. Two clear cathodic peaks were observed: one is around 2.3 V and reflects the reduction of sulfur (S8) to soluble lithium polysulfides (Li2Sn, 4 < n < 8), whereas the other peak at 2.05 V is related to further reduction of polysulfides to Li2S2 or Li2S. In the subsequent anodic scan, the corresponding two anodic peaks around 2.4 V are assigned to the conversion of lithium sulfides to polysulfides, and ultimately to sulfur. These observations conform well to the plateaus in the charge− discharge curves in Figure 3b. Moreover, the Co3O4−S electrode shows stable overlapping in the subsequent cycles in terms of peak positions and areas, which indicates good electrochemical reversibility and stability. Figure 3c shows the cycling stability of the C−S and Co3O4−S electrodes tested at 0.2 C. The Co3O4−S cathode delivers initial discharge capacity of 1167 mA h g−1, which is slightly lower than that for the C−S cathode (1194 mA h g−1) because of the poor conductivity of the Co3O4 nanosheets compared to carbon black. The reason can be explained from the electrochemical impedance spectra (EIS) of the fresh C−S and Co3O4−S electrodes (Figure S4). The Co3O4−S electrode exhibits higher resistance than the C− S electrode, leading to a relatively lower initial discharge capacity. Nevertheless, the Co3O4−S sample exhibits superior

Figure 4. Ab initio simulations showing the most stable binding configurations of Li2S with Co3O4: (a) side view and (b) top view. C

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Figure 5. CV curves at different scan rates of (a) C−S and (b) Co3O4−S electrodes; peak currents of (c) the cathodic reaction (S8−Li2S4), and (d) the anodic reaction (Li2S4−S8) versus V0.5 s−0.5 and the corresponding linear fits.

between Li2S and graphene,6 and it is also higher than the 2.99 eV between Li2S and TiS2.37 This strong binding affinity toward lithium polysulfides can effectively reduce the polysulfide diffusion into the electrolyte and thus eliminate the shuttle effect to a large extent, contributing to a more stable cycling performance. The shuttle factor for the Co3O4−S electrode is 0.198, much lower than that for the C−S electrode (0.464, see Table S1). Figure 3d shows the rate performances of the C−S and Co3O4−S electrodes. Even at 5 C, the discharge capacity of the Co3O4−S composite electrode can maintained over 350 mA h g−1. The good rate performance of the Co3O4−S electrode is attributed to the strong interaction between the Co3O4 and the polysulfides, as well as its facile ionic transport and enhanced reaction kinetics. Therefore, CV measurements were conducted under different scanning rates from 0.1 to 0.8 mV/s in order to investigate the lithium diffusion properties (Figure 5a, b). The lithium-ion diffusion coefficient (DLi+) can be calculated based on the Randles−Sevcik equation in eq 1: i p = 0.4463nF

nFD AC v RT

Co3O4 nanosheets have better polysulfide-trapping capability, therefore, to a large extent, minimizing the dissolution of polysulfides into the electrolyte and the consequent increasing viscosity. The Co3O4−S electrode shows better diffusion properties (cathodic reaction: 4.25 × 10−9 cm2 s−1, and anodic reaction: 1.41 × 10−8 cm2 s−1), which contributes to the rate capability.

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CONCLUSIONS In summary, an ultrathin cobaltosic oxide/sulfur nanosheet composite with 60 wt % S was prepared and further evaluated as sulfur cathode material. Density functional theory calculations revealed the strong chemical binding interaction between the lithium polysulfides and the cobaltosic oxide nanosheet host, which acts as an inhibitor to restrain the dissolution and diffusion of the polysulfides, leading to stable electrochemical performance and suppression of the shuttle effect. The cobaltosic oxide/sulfur nanosheet composite delivered a high capacity of 656 mA h g−1 after 200 cycles at 0.2 C, with capacity decay as small as 0.219% per cycle and a low shuttle factor of 0.198.

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(1)

Where ip represents the peak current, n is the number of electrons, F is the Faraday constant, R is the gas constant, T is the temperature, A is the surface area of the electrode, D is the diffusion coefficient, C stands for the concentration of lithiumions in the electrolyte, and v is the voltage scanning rate. According to the respective plots of the peak currents of the cathodic reaction (S8−Li2S4) and the anodic reaction (Li2S4− S8) versus the square root of the scan rate and the corresponding linear fits (Figure 5c and 5d), the C−S electrode shows low lithium ion conductivity, implying poor Li2Sn capture capability and a significant amount of high viscosity Li2Sn dissolved in the electrolyte, causing low ion diffusivity, as confirmed by the EIS spectra of the cycled C−S and Co3O4−S electrodes (Figure S4). The charge transfer resistance, Rct, of the Co3O4−S cell remains unchanged after 50 cycles, whereas Rct of the C−S cell increases significantly. This indicates that

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07961. SEM and N2 sorption isotherms of Co3O4 nanosheets; TEM of Co3O4−S composite; EIS spectra of the C−S and Co3O4−S electrodes before and after cycling; cycling performance of Co3O4/S mixture electrode and Co3O4− S electrode (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. D

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ACKNOWLEDGMENTS This work was supported by an Australian Research Council (ARC) Discovery project (DP1094261). Furthermore, the authors are grateful to Dr. Tania Silver for critical reading of the manuscript.

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