Exfoliated MoS2 and MoSe2 Nanosheets by a Supercritical Fluid Process for a Hybrid Mg-Li-Ion Battery

Abstract

The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS₂ and MoSe₂ nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS₂ and MoSe₂ nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg-Li-ion battery as a proof of functionality. The MoS₂ and MoSe₂ nanosheets exhibited the specific capacities of 81 and 55 mA h g^-1, respectively, at a current rate of 20 mA g^-1.

Figure 1

Exfoliation of TMDs by SCF processing. The photographs of (a) bulk MoS₂ crystals, (b) bulk crystals in DMF, (c) SCF reactor, (d) designed tube furnace (AKICO, Japan) with controlled temperature and pressure, and (e) resultant MoS₂ nanosheets dispersed in DMF. (f) Schematic illustration for the mechanism of exfoliation of TMDs by SCFs.

Figure 2

Electron microscopy characterization of MoS₂ nanosheets. (a) Low-magnification image. (b) Typical TEM image. (c) HRTEM image of nanosheets with six layers. The expanded layers are indicated by the white arrows (the inset corresponding to FFT pattern clearly showing hexagonal symmetry). (d) HRTEM image of a thin MoS₂ nanosheet showing the layer edges indicated by the dark arrows. (e) HRTEM image of a thin sheet clearly illustrates the hexagonal structure. (f) Atomic-resolution HAADF-STEM image of the monolayer in the bottom part. The superimposed atomic arrays on the inset indicate the locations of atoms.

Figure 3

Electron microscopy characterization of MoSe₂ nanosheets. (a) Low-magnification image. (b,c) HRTEM images of nanosheets with four layers and two layers. HRTEM images of a thin MoSe₂ nanosheet showing layer edges indicated by the white arrows showing layer edges. (d) HRTEM image of a thin nanosheet clearly illustrates the hexagonal structure. (e) Typical electron-diffraction pattern clearly showing hexagonal symmetry. (f) HAADF-STEM image of the monolayer in the top part with a structural model overlaid.

Figure 4

(a,d) AFM images of MoS₂ and MoSe₂ nanosheets. (b,e) Corresponding height profiles obtained along the lines indicated in the AFM images. (c,f) Thickness distribution histograms of the nanosheets, as estimated from AFM analysis.

Figure 5

XPS spectra of the bulk and exfoliated MoS₂ and MoSe₂ showing (a) Mo 3d, S 2s and (b) S 2p and Se 3d core level peak regions.

Figure 6

Electrochemical performances of the synthesized MoS₂ nanosheets in hybrid Mg–Li-ion batteries tested in the potential range of 0.2–2.2 V vs Mg/Mg²⁺. (a) Typical charge/discharge profiles at a current rate of 20 mA g^–1 and (b) CV curves of the cell containing MoS₂ nanosheets.

Figure 7

Electrochemical performances of the synthesized MoSe₂ nanosheets in hybrid Mg–Li-ion batteries tested in the potential range of 0.2–2.2 V vs Mg/Mg²⁺. (a) Typical first charge/discharge profiles at a current rate of 20 mA g^–1 and (b) CV curves of the cell containing MoS₂ nanosheets.