Synthesis of novel 1–dimensional structure from Mo6S8 Chevrel phase of electrode for Mg batteries

The Chevrel phase (CP) (Mo6S8), which is used as an electrode material in Mg rechargeable batteries, has a capacity limit owing to ion insertion and trapping. To address this problem, we modify the wire structure of the CP. Mo6S3I6 nanowires, in which iodiene is substituted for Mo6S9 nanowires as infinite CP structures, can be synthesized in various ways. When synthesizing stoichiometrically, an unwanted secondary phase may appear. We solved these problems by reducing the synthesis time. Electrochemical analysis was performed using these nanowires as an active material in Mg batteries.


Introduction
Mo 6 S 8 Chevrel phase (CP) structures are widely used as a Mg electrode materials. Mg ions enter the cavities of each Mo 6 S 8 unit cell and form a ring structure weak Mg bonds. This formation reduces the diffusion rate of Mg ions in the active material during charging and discharging and limits their capacity [1][2][3][4]. As is clear from figure 1(a), the more Mo 6 S 8 unit cells are connected, the more M ions (Cr, Cu, Fe, Co, Sn, or Pb) can enter. Thus, increase in the trapping phenomenon and diffusion rate owing to the transformation to 1-dimensional (1D) Mo 6 S 6 in the structure of the Mo 6 S 8 CP (figure 1(b)) was expected [5]. 1D Mo 6 S 6 , can be synthesized through M ion removal from M x Mo 6 S 6 as it is obtained by removing metals from metal-compounds such as M x -CP. However, there are several synthesis mechanisms for this material. First, it was known as a method for synthesizing M 2 Mo 6 S 6 by initially forming M 2 MoS 4 [6] and then obtained after several sulfur addition steps. Recently, a Mg battery method for synthesizing 1D Mo 6 S 6 at high temperatures was reported [7]. However, this method is complicated and time consuming.
To address this problem, we focused on Mo 6 S 9-x I x , a 1D material with the same structure as Mo 6 S 6 that can control the electrical properties of semiconductor metals based on the availability of X [8][9][10]. Various studies on the synthesis of Mo 6 S 9-x I x are being conducted; among them, Mo 6 S 3 I 6 wires (figure 1(c)) were synthesized in a manner similar to the CP synthesis of Mo 6 S 8 ; that is, the wires were synthesized from Mo 6 S 6 I 2 CP material that replaced iodine in Mo 6 S 8 CP [8,9]. Typically, Mo 6 S 3 I 6 nanowires (NWs) are in the stoichiometric ratio shown in the following ideal reaction: However, for realistic synthesis, 6Mo (s)+3S (s)+3I 2 (s)→x Mo 6 S 3 I 6 (s)+y MoI 2 (g)+various intermediate phase. Thus, in the study on the synthesis of Mo 6 S 3 I 6 NWs, MoI 2 and Mo 6 S 6 I 2 act as seeds for the synthesis of Mo 6 S 3 I 6 NWs [7]. In addition, to prevent the explosion of the quartz ampoule owing to the high vapor pressure of iodine at high temperatures during synthesis, the precursor ratio was modified to the following reaction formula: In the above reaction, Mo 6 S 6 I 2 acts as a seed, and the final material was used to synthesize Mo 6 S 3 I 6 NWs. In addition, the most commonly used substrates have been synthesized using Mo foil, Mo wire, and quartz [11][12][13]. However, synthesis can be performed without them, and the synthesis time could be reduced. This method is expected to be applicable to various Mo 6 S 3 I 6 synthesis methods.

Experimental section
2.1. Synthesis of Mo 6 S 3 I 6 Focusing on the synthesis method of the CP, we recognized that the heat treatment time and temperature of the ternary molybdenum compound (M x Mo 6 X 8 ), depended on the properties of the M and chalcogen (X) elements and the nature of the ternary molybdenum chalcogenide. The synthesis temperature was modified [14]. The elemental 12M Mo (99.9%, 1∼3 μm Avention, 234.48 mg), 9M S (99.98%, Sigma-Aldrich, 58.76 mg), and 4M I 2 (> 99.99%, Sigma-Aldrich, 206.76 mg) were mixed with mortar and sealed in an evacuated quartz tube. First, the heat was increased from 900 to 1100°C for 48 h in a tube furnace, and fixed at 1100°C, where the NWs clearly formed. Next, heat treatment was performed starting at 72 h, and then cooled down to room temperature at 5°C min -1 , gradually reducing the heating time. The experiment was concluded by mechanically opening the ampoule (figure 2), removing the source, and peeling off the Mo 6 S 3 I 6 NWs from the quartz using pouring  [5]. (c) Hexagonal unit cell containing Mo 6 S 9-x I x nanowires, oriented parallel to the hexagonal axis and side view [8,9]. ethanol along the quartz wall. We dispersed 50 mg of Mo 6 S 3 I 6 NWs for 10 min (2s on/2s off) at 400 W in 100 ml of isopropyl alcohol (IPA) by tip sonication. Finally, bath sonication was carried out for 3 h [7, 15, 16].

Electrochemical measurement
Coin-cell was prepared for the electrochemical characterization of the Mo 6 S 3 I 6 NWs. The electrode as a current collector was prepared by coating a copper foil with mixing 50 wt% Mo 6 S 3 I 6 NWs powder of the active material, 30 wt% super-P carbon of the conducting agent, and 20 wt% polyvinylidene fluoride (PVDF) of binder dissolved in N-methyl-2-pyrrolidone (NMP). After coating, the cathodes were dried at 60°C for 12 h in a vacuum oven. The diameter and loading mass of the electrodes were 12π and approximately 0.49 mg·cm -2 , respectively. We assembled 2032 coin-cells in an Ar-filled glovebox using Mg foil (0.25 mm thick) as the counter electrode with native MgO removed by polishing and the PE-PP membrane as the separator. 0.4M (PhMgCl) 2 -AlCl 3 /tetrahydrofuran (THF) solvent as a APC electrolyte and stirred it for 1 day. A coin-cell consisting of electrodes and a sperator was filled with electorlyte (S1).

Characterization of active materials
We used a field emission scanning electron microscope (FE-SEM, Hitachi and JEOL) to observe the morphologies and sizes of the synthesized Mo 6 S 3 I 6 NWs. X-ray powder diffraction (XRD, Bruker) patterns were characteristic Cu Kα radiation sources. The vibration mode of Mo and S characterization was performed using Raman spectroscopy at λ=532 nm (WITec Project). Further, dispersions of Mo 6 S 3 I 6 bundles were prepared and measurement were performed with high-resolution transmission electron microscopy (HR-TEM, JEOL), Figure 2. Schematic of synthesis growth for Mo 6 S 3 I 6 nanowires. After synthesis of Mo 6 S 3 I 6 NWs, the reaction materials were divided between the source and transport zones in the quartz ampule. and electrochemical tests (WonATech) were conducted in the voltage range of 0.05 to 1.30 V at room temperature.

Analysis of Mo 6 S 3 I 6 nanowire
The photographic and SEM images of the synthesized material source and transport zones are shown in figure 2. Knowing that ternary molybdenum chalcogenide reacts at 800 to 1100°C, [17] we synthesized it from 900 to 1100°C. First, we confirmed the wire growth by SEM ( figure 3(a)), and nanowires seem to be highly dense, and the Mo 6 S 3 I 6 NWs and MoS 2 were synthesized at 900°C. Above 1000°C, the torsional mode of Mo as Mo6 octahedron can be observed at 94.27 cm -1 in figure 3(c) [18][19][20][21][22]. In addition, at 1000°C, the crystallinity of Mo 6 S 3 I 6 NWs were improved and characterized by XRD, SEM, and Raman spectroscopy. MoS 2 and Mo 2 S 3 phases were observed at 2θ=14.44 (MoS 2 , ICCD number: 98-002-6622) and 2θ=16.24 (Mo 2 S 3 , ICCD number: 01-081-2031), respectively, in XRD, [18,19] and 310, 378, and 407 cm -1 in Raman spectroscopy [18,20]. At 1050°C, the MoS 2 disappeared, the nanowire crystallinity improved, and the Mo 6 S 6 I 2 phase acting as a Mo 6 S 3 I 6 NW seed was observed. The Mo 6 S 6 I 2 and Mo 6 S 3 I 6 phases were synthesized only when the temperature was raised to 1100°C. In the XRD results shown in figure 3(b), unlike the Mo 6 S 6 I 2 (ICDD number: 00-038-0540) seeds in the source zone, the Mo 6 S 6 I 2 seed in the transport zone was expected to remain as several hundred nanometers in size. However, the particles were small and wire synthesis was not achieved. It was later confirmed that the Mo 6 S 6 I 2 with decreasing and Mo 6 S 3 I 6 phases were synthesized only when the temperature increased to 1100°C. To investigate the Mo 6 S 3 I 6 NWs synthesized at 1100°C, the synthesis time was varied and observed. Typically, synthesis takes approximately 72 h [7,12]. Hence, we determined whether to increase or decrease the starting time at 72 h. First, SEM was used ( figure 4(a)) to confirm the shapes of the wire after synthesis based on the time. We observed that an area resembling seeds were confirmed at the end of the wire after 72 h. The thickness of the wires was an average of 300 nm. The XRD data indicated that various phases, such as Mo 6 S 3 I 6 , Mo 6 S 6 I 2 , Mo 2 S 5 I 3 (ICCD number; 00-039-0717), and Mo 2 S 3 occurred after 72 h (green color). After 60 h, wires were synthesized. However, their thickness varied. The SEM image in figure 4(a) shows that an area resembling seeds were confirmed at the end of wires, and Mo 6 S 3 I 6 (wire phase) and Mo 6 S 6 I 2 (seed phase) occurred, which corresponds with the XRD results ( figure 4(b)). After 48 h, the average wire thickness was 200 nm (∼20 nm after dispersion), and XRD data showed that the Mo 6 S 3 I 6 wire was synthesized. Therefore, it was confirmed that the change in the wire composition occurred within 48 h. Mo 2 S 3 and Mo 2 S 5 I 3 were observed after 24 h, but decreased after 36 h, and the structural change to Mo 6 S 6 I 2 was confirmed. After 42 and 48 h, the Mo 6 S 3 I 6 phase was dominant, and it was observed that Mo 6 S 6 I 2 decreased and the crystallinity of the Mo 6 S 3 I 6 phase peaked after 48 h. In the Raman spectroscopy results, the peaks did not change. However, the peak intensities and S-breathing modes gradually decreased, contrasting the results depended on temperature. Thus, if the synthesis time is insufficient, the phase change is incomplete, whereas if it is excessive, bonds cannot be formed owing to the loss of iodine (energy instability). In the formed Mo 6 S 3 I 6 NWs, TEM showed a microstructure with an interatomic gap (0.932 nm), which is shown in figure 4(d) and S2 [7,23]. It was confirmed that MoI 2 material formed by the synthesis of Mo 6 S 3 I 6 did not appear, and that the mechanism was similar to that of CP synthesis. However, it was confirmed that Mo 6 S 3 I 6 NWs were synthesized by the new reaction formula.

Electrochemical test of Mo 6 S 3 I 6 nanowire
The cells were charged and discharged in a voltage range from 0.05 to 1.30 V versus Mg/Mg 2+ , and the constant current was set at 15 mA·g -1 (1/5 C). Then, the battery characteristics were investigated. In figure 5(a), the Mo 6 S 8 CP, which was used as a cathode in Mg batteries, exhibits two plateaus owing to the change in the mechanism of divalent Mg and does not confirm the change in mechanism. In addition, although the theoretical specific capacity per unit weight of the active Mo 6 S 3 I 6 material was∼75 mAh·g -1 , the specific capacity confirmed in the initial discharge was 219.4 mAh·g -1 , which is double the improved electrical conduction of Mo 6 S 3 I 6 NW discharge capacity when using a Mo 6 S 8 CP (∼100 mAh·g -1 ). The specific capacity of typical insertion anode materials is lower than 100 mAh·g -1 , [24] which is smaller than the specific capacity (>300 mAh·g -1 ) of metal alloy anodes. Nanowire materials were first studied a lot as an anode material in lithium-ion batteries. Among them, various materials such as CNT [25] and Si [26] are used, and the potential of Mo-S-I nanowires has also been shown through simulations [27]. However, despite the high theoretical capacity of MIBs, insertion-type anode materials, such as layered-Na 2 Ti 3 O 7 [28] and Li 4 Ti 5 O 12 , [29,30] have been experimentally identified as alternatives. Nevertheless, they have a low diffusion kinetic energy owing to the strong electrostatic interaction between divalent Mg 2+ and their anions/cations [31]. Hence, in figure 5(b), a voltage of 0.14 V was obtained in the first cycle and that of 0.07 V in the second cycle. As shown in figure 5(c), the capacity rapidly decreased from the second cycle, theoretical values at the 10th cycles, and capacity of∼44.5 mAh·g -1 is maintained without loss after 25 cycles. The coulombic efficiency remained above 90%. Further, these capacities were confirmed by varying current density, as shown in figure 5(d). In the insertion mechanism of Mo 6 S 8 , ions interact with both the surface of the CP and the electrolyte salt (Mg x Cl y ) and solvent (THF). Cations such as solvated MgCl + and Mg 2 Cl 3 + are attracted to the negatively charged surface of the CP, which in turn weakens their interactions with solvent molecules, promoting the Mg dissolution process. Here, Mg ions bind to sulfur ions, and Clbonds move to the Mo surface. Moreover, it does not interfere with Mg insertion because the solvated MgCl + and Mg 2 Cl 3 + interact [32]. Based on this principle, we used a suitable electrolyte (0.4 M APC), and expected more Mg intercalation because sulfur and iodine ions have negative charge. Therefore, the capacity of Mo 6 S 3 I 6 NWs is similar to that of insertion cathode materials (Na 2 Ti 3 O 7 , Li 4 Ti 5 O 12 ), even though their potentials are widely different. To improve the electrochemical properties of NW, further investigation is required.

Conclusions
We synthesized and analyzed Mo 6 S 3 I 6 NWs and found that second phases such as MoI 2 appear at 900°C, and synthesis was performed under various conditions. Mo 6 S 3 I 6 NWs were synthesized with MoS 2 and Mo 2 S 3 phases below 1000°C, and the Mo 6 S 6 I 2 seed phase remained up to 1050°C. We employed SEM, XRD, and Raman spectroscopy to confirm that the crystallinity of the nanowires improved at 1100°C. At 1100°C, various compounds were formed before Mo 6 S 3 I 6 NWs formed. The synthesis of Mo 6 S 3 I 6 NWs was optimized at 1100°C for 48 h. The nanowire was used as an active material and the electrochemical test showed a specific capacity of 219.4 mAh·g -1 at 0.07 V. We showed that Mo 6 S 3 I 6 NWs are suitable as MIB anodes and can be used with highvoltage cathode materials (MoS 2 [33], WSe 2 [34], and Ti 3 C 2 T x [35][36][37][38]) in APC electrolytes. Mo 6 S 3 I 6 are expected to be utilized in various applications [39][40][41][42][43][44].