Design and facile synthesis of defect-rich C-MoS2/rGO nanosheets for enhanced lithium–sulfur battery performance

Abstract

We report a simple one-step hydrothermal strategy for the fabrication of a C-MoS2/rGO composite with both large surface area and high porosity for the use as advanced electrode material in lithium–sulfur batteries. Double modified defect-rich MoS2 nanosheets are successfully prepared by introducing reduced graphene oxide (rGO) and amorphous carbon. The conductibility of the cathodes can be improved through the combination of amorphous carbon and rGO, which could also limit the dissolution of polysulfides. After annealing at different temperatures, it is found that the C-MoS2/rGO-6-S composite annealed at 600 °C yields a noticeably enhanced performance of lithium–sulfur batteries, with a high specific capacity of 572 mAh·g−1 at 0.2C after 550 cycles, and 551 mAh·g−1 even at 2C, much better than that of MoS2-S nanosheets (249 mAh·g−1 and 149 mAh·g−1) and C-MoS2/rGO-S composites (334 mAh·g−1 and 382 mAh·g−1). Our intended electrode design protocol and annealing process may pave the way for the construction of other high-performance metal disulfide electrodes for electrochemical energy storage. Introduction Lithium–sulfur (Li–S) batteries have attracted great attention because of the high energy density (2600 Wh kg−1) and specific capacity (1675 mAh·g−1), low cost, and abundant reserves of elemental sulfur [1,2]. Nevertheless, there are various technical challenges in the development of Li–S batteries. The intrinsic insulation properties of the discharge products (Li2S2 and Li2S) and sulfur result in a slow charge and discharge process and a low specific capacity [3]. Intermediate products of battery charge and discharge (Li2Sn, where 3 ≤ n ≤ 8) are soluble in the electrolyte and can also migrate to the lithium metal anode and precipitate there [4,5]. The decay of the electrochemically active lithium polysulfides causes rapid capacity degradation during charge and discharge process. In order to overcome the problems above, great efforts have been made to improve the performance of Li–S batteries, inBeilstein J. Nanotechnol. 2019, 10, 2251–2260. 2252 cluding combining conductive materials with sulfur [6-8], constructing Li2Sn blocking interlayers [9-11], and applying functional separators [12-15]. Although there are many methods, the most common strategy is to combine sulfur with various carbon materials owing to their excellent conductivity and flexible nanostructures. However, the capacity of carbon and sulfur composite cathodes generally fades rapidly during long-term cycling, because the carbon materials can provide only inferior physical adsorption to the polar Li2Sn [16]. Once Li2Sn is solvated, it dissolves easily in the electrolyte from the electrode surface and subsequently disperses. Consequently, the reutilization of Li2Sn will become very hard due to the repulsion between the nonpolar conductive surface and the polar reactants [17]. Two-dimensional layered transition metal dichalcogenides (TMDs), strong candidates in the search for energy storage and catalyst materials, can provide good performance at low cost [18-20]. In particular, MoS2 has attracted the most attention owing to the high electrochemical activity associated with the sulfur deficiencies [5]. It has been reported that MoS2 nanosheets show great performance in the hydrodesulphurization process catalyzing the formation of sulfur species [21,22], indicating a potential application for Li–S batteries. Previous simulation results show that the binding energy of the edge active sites of defect-rich MoS2 and Li2S is much greater than the binding energy of the base plane, which is very helpful for the adsorption of polysulfides [23,24]. However, the low conductivity of MoS2 often results in incomplete conversion of polysulfides. Thus, MoS2 is usually combined with carbon materials and annealing treatment is also considered [25,26]. Hence, double modification of defect-rich MoS2 nanosheets with amorphous carbon and rGO followed by thermal annealing should be a very hopeful strategy to increase the performance of sulfur cathodes. In this work, we firstly present a double carbon network modification method for defect-rich MoS2 anodes by introducing amorphous carbon and rGO via a one-step hydrothermal method. We concentrate not only on the material design based on both structure and chemical composition but also on the development of MoS2 electrocatalysts. Firstly, the MoS2 nanosheets are interconnected with rGO and then well covered by the carbon layer. This means that overall connected conductive networks are formed by the combination of amorphous carbon layer and rGO. Secondly, the existence of a great number of defects in the ultrathin MoS2 nanosheets leads to partial breaking of the catalytically inert basal planes, yielding additional active edge sites. Thirdly, annealing of the C-MoS2/rGO composites at different temperatures has also been investigated. The annealing can improve crystallinity and increase the binding energy, and also improve the stability while maintaining high specific capacity. The defect-rich C-MoS2/rGO prepared in this work can not only kinetically accelerate the sulfur redox reactions but also chemically adsorb polysulfides. In addition, rGO and carbon layer can also enhance the conductivity of C-MoS2/rGO. Therefore, the C-MoS2/rGO, as an efficient sulfur host, could exhibit excellent electrochemical performance. Experimental Preparation of defect-rich C-MoS2/rGO nanosheets For the synthesis of the C-MoS2/rGO nanosheets, 1.23 g hexaammonium heptamolybdate tetrahydrate, 0.89 g of glucose, and 2.28 g thiourea were dissolved in 25 mL of deionized water to form solution A; 0.03 g of GO was dissolved into 10 mL of deionized water by ultrasonic to form solution B. Then solution A is mixed with solution B and stirred for 30 min. Subsequently, the mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave and retained at 200 °C for 24 h. After cooling naturally, the product was washed with deionized water and absolute ethanol for several times and dried at 60 °C in vacuum. For comparison, the pristine MoS2 were synthesized using an identical process without GO or glucose. After that, the obtained composites were annealed at 400, 600 and 800 °C for 6 h in 10% H2/Ar atmosphere to improve the crystallinity. After thermal annealing, the samples were labeled as C-MoS2/rGO-4, C-MoS2/rGO-6, and C-MoS2/rGO-8, respectively. Preparation of sulfur composites MoS2-S, C-MoS2/rGO-S, C-MoS2/rGO-4-S, C-MoS2/rGO-6-S, C-MoS2/rGO-8-S composites were fabricated by melt diffusion. The mixture of sulfur and pristine MoS2, C-MoS2/rGO and annealed composites (C-MoS2/rGO-4, C-MoS2/rGO-6, C-MoS2/rGO-8) with a 3:1 mass ratio was calcined at 155 °C for 12 h in a sealed vessel. Materials characterization The structure and morphology were investigated by field-emission scanning electron microscopy (FE-SEM, INSPECT F50) and transmission electron microscopy (TEM, ZEISS Libra 200). Powder X-ray diffraction (XRD) measurements were conducted to determine the phase of the as-synthesized composites with Cu Kα radiation operated at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos AXIS Ultra DLD instrument using monochromated Al Kα X-rays as the excitation source. Raman spectra were collected using a Witec alpha 300M+ instrument with an excitation laser wavelength of 488 nm. Nitrogen adsorption–desorption isotherm measurements were conducted at 77 K using a micromeritics system (JW-BK132F). The contents of amorBeilstein J. Nanotechnol. 2019, 10, 2251–2260. 2253 phous carbon, rGO and sulfur in the samples were analyzed by thermogravimetric (TGA) on a Netzsch STA 449C analyzer in air for the amorphous carbon and rGO or in N2 atmosphere for the sulfur at a temperature ramp rate of 10 °C·min−1. Lithium polysulfide adsorption tests The concentration of the fabricated Li2S6 solution was set to 3 mM by dissolving lithium sulfide (Li2S) and sulfur with a molar ratio of 1:5 in 1,3-dioxolane (DOL) and dimethoxymethane (DME) (1:1 by volume) and then stirring for overnight in a glovebox. After that, 5 mg of pristine MoS2, C-MoS2/rGO and annealed composites were added to the Li2S6 solution (3 mL) as the adsorbents. Ultraviolet–visible (UV–vis) absorption spectra of these diluted solutions were collected using a Shimadzu UV 2550 spectrophotometer. Cell assembly and electrochemical measurements The working electrodes were prepared by casting a slurry of 80 wt % active materials (MoS2-S, C-MoS2/rGO-S, C-MoS2/ rGO-4-S, C-MoS2/rGO-6-S, C-MoS2/rGO-8-S), 10 wt % acetylene black and 10 wt % polyvinylidene fiuoride (PVDF) in N-methyl-2-pyrrolidone (NMP) on an Al foil current collector. Then, the electrodes were dried in vacuum at 60 °C for 12 h. The electrode was manufactured in a coin-type cell (CR 2032) in an argon-filled glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). The electrolyte was 1 M bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) dissolved in a mixed solution of dimethyl ether (DME) and 1,3-dioxolane (DOL) (1:1, v/v) with 2 wt % LiNO3. The recharge properties and cyclic voltammetry tests were carried out on a LAND battery cycler (CT2001A) in a voltage range of 1.7 to 2.9 V (vs Li/Li+). The specific capacity was calculated based on the mass of sulfur. Cyclic voltammetry (CV) tests were carried out between 1.6 and 2.9 V at a scan rate of 0.1 mV·s−1. The electrochemical impedance spectroscopy (EIS) measurements were achieved at the open-circuit potential between 0.01 Hz and 100 kHz. All the tests were carried out at room temperature. Results and Discussion The preparation of the C-MoS2/rGO composite is shown in Figure 1. The C-MoS2/rGO composite is synthesized by the simple and efficient hydrothermal route followed by annealing. The SEM image in Figure 2a clearly reveals the morphology of the pristine ultrathin MoS2 nanosheets. The apparent corrugations and ripples can be shown and the lateral size of the nanosheets is 200–