Chinese Chemical Letters  2018, Vol. 29 Issue (8): 1313-1316   PDF    
Synthesis of MXene-supported layered MoS2 with enhanced electrochemical performance for Mg batteries
Min Xua, Na Baia, Hong-Xia Lib, Cong Hua, Jing Qia, Xing-Bin Yanb    
a School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China;
b Laboratory of Clean Energy Chemistry and Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Abstract: In this paper, the petal-like MoS2/MXene composite has been successfully synthesized by one-step hydrothermal method. With the combination of few-layer MoS2 nanosheets and the high conductive MXene substrate, the composite exhibits enhanced capacities of 165 mAh/g at 50 mA/g and outstanding rate performance of 93 mAh/g at 200 mA/g as a cathode material of Mg batteries. Simultaneously, MXene with high conductivity and abundant surface functional groups is proved to be a promising substrate for the wider design of high performance electrode materials.
Keywords: Substrate     MXene     MoS2     Cathode     Mg batteries    

Rechargeable metal batteries have attracted increasingly attention because of its supernal energy density [1-4]. One of them is the well-known Li based batteries (LBs). However, several drawbacks such as the inherent safety problems of forming undesirable dendrites on the surface of Li anode and the shortage of lithium resources greatly hinder the further development of LBs [5]. Therefore, some research efforts have shifted to develop Magnesium metal batteries (MBs), which are notable for their lack of dendrite formation during cycling [6-9]. In addition, compared to the Li-based materials, the Mg-based materials are much cheaper owing to their earth abundance. Moreover, Mg delivers a higher volumetric capacity than Li (3833 mAh/cm3 for Mg and 2046 mAh/cm3 for Li) due to its divalent property. However, MBs are far away from widespread commercial application. One of the major obstacles is that the divalent Mg2+ cations have a strong electrostatic interaction with the host materials, which leads to sluggish diffusion kinetics and low reversible capacity [10]. Besides, the incompatibility between Mg anode and electrolytes hinders the reversible Mg plating and stripping [11-13]. More important, only a few cathode materials can work in the Mg cell system [8, 14, 15].

One of them is layer-structured Molybdenum disulfide (MoS2), which has weak interlayers van der Waals interaction and large interlayer spacing. MoS2 has been proved to be the suitable host for the diffusion and storage for Mg2+ cations. However, the reported MoS2 based cathodes show either the moderate capacities or poor cycling stability [16]. To improve the diffusion kinetics of Mg cations in MoS2 host, the graphene-like 2D materials, MXene, with the excellent physicochemical properties [17-20], have been utilized in this work. The general formula of MXene is Mn+1XnTx (n = 1, 2, 3), where M represents an early transition metal and X is carbon or nitrogen; Tx stands for the various surface functional groups (e.g., -O, -OH, -F). The combination of both the outstanding electrical conductivity and hydrophilic surface makes MXene the promising matrixes for supporting the active MoS2 host. Delaminated Ti3C2Tx, the most studied MXene up to now, was chosen as the typical representative of MXene.

In this paper, the MoS2/Ti3C2Tx composite was prepared by onestep hydrothermal method. As cathodes of MBs, MoS2/Ti3C2Tx composite exhibited an enhanced capacity of 165 mAh/g at 50 mA/g, compared to other reported MoS2-based cathodes [16, 21, 22]. Based on the presented results, MXene is proved to be the promising substrate for the design of functional materials for charge storage.

The synthesis Ti3C2Tx flakes references our previous work [15]. In the terms of brief, Ti3AlC2 powders were etched at 40 ℃ for 45 h in LiF and HCl. After etching, the resulting sediments were washed several times through centrifugation (3500 rpm, 5 min for each cycle) and decanting. Next, the clay-like sediments were collected and then dispersed in distilled water. After sonication for 1 h in ice bath and then centrifugation for 1 h at 3500 rpm, the delaminated Ti3C2Tx flakes were obtained by collecting the supernatant.

Typically, 0.15 g Na2MoO4·2H2O were dissolved into 40 mL Ti3C2Tx supernatant with stirring and sonication for 5 min. Then 0.4 g NH2CSNH2 was added. After another 5 min, the mixture were transferred into 50 mL stainless steel autoclave and maintained at 210 ℃ for 24 h. The MoS2/MXene composite was obtained after washed and collected by vacuum filtration, and then dried in air at 60 ℃ for at least 12 h. The pure MoS2 was prepared by the same procedure, but the deionized water instead of Ti3C2Tx supernatant. MXene supernatant alone was also subjected to the hydrothermal process and then vacuum filtration was carried out to obtain paperlike H-MXene.

MoS2 and MoS2/MXene composite were characterized by XRD (Rigaku D/Max-2400, Japan) diffractometer using Cu Kα radiation (45 kV and 40 mA) and step scan 0.02°, 3°–80° of 2θ range and step time of 0.5 s. The morphology of the samples were characterized using a field emission scanning electron microscopy (FESEM, JSM 6701F, JEOL, Japan) and transmission electron microscopy (TEM, JEOL 2100 FEG). MoS2 and MoS2/MXene were analyzed by applying an XPS (ESCALAB210, VG, UK), calibrated by using C 1s at 284.6 eV. Raman spectra of MoS2 and MoS2/MXene were recorded using a microRaman spectroscope (JY-HR800, the excitation wavelength of 532 nm).

The electrochemical measurements were performed in a coin cell (CR2032) that assembled in an argon-filled glove box. MoS2 and MoS2/MXene cathode was prepared by maxing 80 wt% active material, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride with the aid of N-methy pyrrolidinon. The mixture was spread on Cu foil and dried at 110 ℃ for 12 h under vacuum. Paperlike H-MXene was cut into ~0.6 × 0.6 cm2 squares and directly used as the cathode in Mg betteries cell. Mg disc was polished bright, cut into square pieces and used as the anode. Polypropylene membranes were employed as the separators. The APC electrolyte synthesized referring to our previous work was used as the electrolyte [15]. The charging/discharging tests of coin cells were measured using a CT2001A cell test instrument (LAND Electronic Co.) in the voltage range of 0.01–2.0 V (vs. Mg/Mg2+).

In Fig. 1A, the FESEM images of MoS2 displayed tightly stacked nanosheets with the thickness of around 35 nm. The nanosheets became thinner (~7.5 nm) and petal-like after the incorporation of Ti3C2Tx nanosheets (Fig. S1 in Supporting information), as shown in Fig. 1B. The morphology difference before and after introducing Ti3C2Tx nanosheets can be more obviously observed in their transmission electron microscopy (TEM) images in Fig. 2, which shown the stacked MoS2 nanosheets (Fig. 2A) and the fluffy and petal-like MoS2/Ti3C2Tx nanosheets (Fig. 2B). The high-resolution TEM (HRTEM) images displayed in Figs. 2C and D indicated the same layer distance of MoS2 nanosheets (~0.65 nm) in bare MoS2 and MoS2/Ti3C2Tx composite. However, the thickness of MoS2 layers in MoS2/Ti3C2Tx was much lower than that in bare MoS2. The hybrid with anopenpetal-like structure not only offers more exposedactive sites for Mg-ions storage, but also provides more channels to increased Mg-ions diffusion kinetics [21, 23-25]. Moreover, MXene with the outstanding electrical conductivity facilitates electrons transportation during the battery operation, which is greatly beneficial to the electrochemical performance [22]. EDX in Fig. 2E and EDS in Fig. S2 (Supporting information) including Ti element indicated that the mixture are made up of MoS2 nanosheets and Ti3C2Tx nanosheets. The atomic ratio of Mo/S was very close to 1:2, confirming the formation of stoichiometric MoS2.Atthesametime, it was calculated that the content of Ti3C2 is 4.77 wt%. These results further proved that layered MoS2 was grown on the surface of MXene and the stack of each other was restrained.

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Fig. 1. FESEM images of MoS2 (A) and MoS2/Ti3C2Tx composite (B). The inserts show the corresponding magnifications.

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Fig. 2. TEM images of (A) MoS2 and (B) MoS2/Ti3C2. HRTEM images of (C) MoS2 and (D) MoS2/Ti3C2. (E) EDX of MoS2/Ti3C2 composite in (B), the presence of Cu is attributed to the microgrids for supporting samples.

Fig. 3A compared the X-ray diffraction (XRD) patterns of MoS2 and MoS2/Ti3C2Tx. The peaks of MoS2 were indexed to a hexagonal phase (JCPDS No. 37-1492). For MoS2/Ti3C2Tx, the hybrid basically retained the position of the diffraction peak of MoS2, implying the consistent MoS2 phase. The (002) peaks at 13.6° of two samples demonstrated a d-spacing of 0.65nm. The slight enhanced peak of (002) and the new diffraction peak around 25° of MoS2/Ti3C2Tx composite were attributed to the layered MXene (Fig. S3 in Supporting information). However, two strong peaks of MXene at 6.5° and 27.6° disappeared, which indicated that MXene flakes were well separated by MoS2 nanosheets instead of stacking together.

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Fig. 3. XRD pattern (A) and Raman spectra (B) of MoS2 and MoS2/Ti3C2.

Fig. 3B exhibited the Raman spectra of the as-prepared samples. Two peaks corresponded to the E2g1 (in-layer displacements of Mo and S atoms) and A1g (out-of-layer symmetric displacements of sulfur atoms along the c-axis) vibrational modes of hexagonal MoS2 [26]. Compared to pure MoS2, the decrease of ratio between A1g and E2g1 for MoS2/Ti3C2Tx composite indicated the decreasing number of MoS2 layers, as observed in SEM.

The Mo 3d X-ray photoelectron (XPS) spectra of MoS2 and MoS2/Ti3C2Tx composite were compared in Fig. 4. The two peaks at 228.9eV and 232.0 eV were the doublet Mo 3d5/2 and Mo 3d3/2 of Mo4+. However, the peaks of MoS2/Ti3C2Tx composite shifted to 229.2eV and 232.3eV, the binding energy increased by 0.3 eV. The increased binding energy after adding MXene indicates the improvement of electronegativity around Mo element. It demonstrated not just simple physical mixing but chemical bonding between MoS2 and MXene [21].

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Fig. 4. XPS of Mo 3d of MoS2 and MoS2/Ti3C2Tx composite.

For H-MXene electrode, negligible capacity was displayed in magnesium battery (Fig. S4 in Supporting information). Fig. 5A showed the cycling performance of MoS2 and MoS2/Ti3C2Tx composite at 50mA/g. Pure MoS2 delivered a capacity of only 62mAh/g. MoS2/Ti3C2Tx composite achieved a higher capacity of 165 mAh/g and maintained a discharge capacity of 108 mAh/g after 50 cycles. The obtained capacities are higher than other reported MoS2–based materials [16, 21, 22]. The significant increase of magnesium ions storage capacity after incorporating MXene could be ascribed to the open structure with few-layer MoS2 nanosheets and high conductivity of MXene, which offers more paths and active sites for Mg2+ insertion/extraction.

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Fig. 5. (A) Cycle performance and (B) charge-discharge curves of magnesium batteries utilizing MoS2 and MoS2/Ti3C2 electrodes after stabilization (firstly at 0.02A/g for 5 cycles and then at 0.02 A/g for 5 cycles).

Fig. 5B showed the charge-discharge curves of MoS2 and MoS2/Ti3C2Tx composite between the voltages of 0.01–2.0 V at 50mA/g. The charge-discharge curves were similar to that of capacitive behavior materials. We conjectured that the dominant energy storage mechanism for MoS2/MXene electrode could be insertion pseudo-capacitance, which suggest that charge is stored at the electrode/electrolyte interface [26]. The Coulombic efficiencies of both were close to 100%.

MoS2/Ti3C2Tx composite electrode also displayed excellent rate performance, as can be seen in Fig. 6. The capacity maintained 140 mAh/g, 93 mAh/g and 53 mAh/g at large current density of 100 mA/g, 200 mA/g and 500 mA/g, respectively. The attenuation of the capacity in the first ten cycles was possibly due to Mg2+ ions are easily trapped at some active sites for the stronger electrostatic interactions between Mg2+ and materials.

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Fig. 6. Race capacities at different current densities of MoS2/Ti3C2 composite.

In conclusion, we have successfully synthesized MoS2/MXene composite by one-step hydrothermal method. On account of the petal-like structure of the composite, the capacity of electrode is significantly improved. For the first time, MoS2/MXene composite delivers a high reversible capacity of 165 mAh/g at 50mA/g and outstanding rate properties of 93 mAh/g at 200mA/g. MXene with high conductivity and abundant hydrophilic functional groups on surface is proved to be a promising substrate for the wider design of high performance electrode materials.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.04.023.

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