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Condens. Matter 2019, 4, 53 2 of 8 the intercalation of Li and Na into a 1T-MoS2 matrix for various concentrations. For low Na or Li concentration, AXMoS2 (A = Li, Na) shows good conductivity with a relatively large electrode voltage. At x = 1, 1T-MoS2 develops a band gap concomitant with the appearance of a Jahn–Teller distortion driven by changes in d-electron count, which can be mitigated via disorder to restore the good conductivity of the anode. For x > 1, the anode is always found to be metallic, and here we introduce a Na multilayer model to understand this high-capacity regime. 2. Materials and Methods Computational—The ab initio calculations were performed using the pseudopotential projected augmented wave method [9] implemented in the Vienna ab initio simulation package [10,11] with an energy cutoff of 400 eV for the plane-wave basis set. Exchange-correlation effects were treated using the generalized gradient approximation (GGA) [12], and van der Waals corrections were included using the DFT-D2 method of Grimme [13], where a 14 × 14 × 7 Γ-centered k-point mesh was used to sample the Brillouin zone. A large enough vacuum of 15 Å in the z direction was used to ensure negligible interaction between the periodic images of the Na layer doubled MoS2 thin film. All the structures were relaxed using a conjugate gradient algorithm with an atomic force tolerance of 0.01 eV/Å and a total energy tolerance of 10−6 eV. Experimental—MoO3 (18 mg, Fisher Scientific, Hampton, NH, USA), thioacetamine (21 mg, Sigma-Aldrich, St. Louis, MO, USA), and urea (0.15 g, Sigma-Aldrich, USA) were dissolved in ethanol (15 mL) and stirred for 1 h. Then the solution was transferred to a Teflon-lined stainless-steel autoclave. The autoclave was kept in a furnace for 16 h at 200 ◦C. After cooling to room temperature, the product was washed with ethanol a few times and dispersed in ethanol for future use. Structural and Physical Characterization—The morphology of the as-prepared MoS2 was characterized by scanning electron microscopy (SEM) (Hitachi S4800) and transmission electron microscopy (TEM) (JEOL 1010). Raman spectroscopy was carried out on a Thermo Scientific DXR with 532 nm laser excitation. The set up for the in-situ Raman is a three-electrode cell with optical window from EL-CELL. Electrochemical Measurement—Standard CR2025-type coin cells were assembled to measure the electrochemical performance of the as-synthesized metallic MoS2. The metallic MoS2 electrodes were directly assembled into the coin cells in an argon filled glove-box, using 1M NaPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 vol/vol) as an electrolyte. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were measured using a biologic SP-150. Galvanostatic charge–discharge tests were performed on a multichannel battery testing system (Land CT2001A). 3. Results and Discussion Figure 1a shows a schematic of the sodiation–desodiation process of a 1T-MoS2 electrode prepared following the method described in Ref. [8]. The morphology of this electrode has been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1b,c is the SEM images of 1T-MoS2, which show the presence of lamellar structures with a vertically arrayed structure. The formation of an open structure of 1T-MoS2 nanosheets favors the accessibility of the Na ions. Moreover, the TEM image (Figure 1d) shows that 1T-MoS2 nanosheets are uniformly distributed in agreement with the SEM results (Figure 1d). Finally, Figure 1e provides a high resolution TEM image of the atomic lattice.PDF Image | Understanding Phase Stability of Metallic 1T-MoS2 Anodes for Sodium-Ion Batteries
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