Electrochemical Zinc-Ion Intercalation Properties and Crystal

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Electrochemical Zinc-Ion Intercalation Properties and Crystal Structures of ZnMo6S8 and Zn2Mo6S8 Chevrel Phases in Aqueous Electrolytes Munseok S. Chae, Jongwook W. Heo, Sung-Chul Lim, and Seung-Tae Hong* Department of Energy Systems Engineering, DGIST, Daegu 42988, South Korea S Supporting Information *

ABSTRACT: The crystal structures and electrochemical properties of ZnxMo6S8 Chevrel phases (x = 1, 2) prepared via electrochemical Zn2+-ion intercalation into the Mo6S8 host material, in an aqueous electrolyte, were characterized. Mo6S8 [trigonal, R3̅, a = 9.1910(6) Å, c = 10.8785(10) Å, Z = 3] was first prepared via the chemical extraction of Cu ions from Cu2Mo6S8, which was synthesized via a solid-state reaction for 24 h at 1000 °C. The electrochemical zinc-ion insertion into Mo6S8 occurred stepwise, and two separate potential regions were depicted in the cyclic voltammogram (CV) and galvanostatic profile. ZnMo6S8 first formed from Mo6S8 in the higher-voltage region around 0.45−0.50 V in the CV, through a pseudo two-phase reaction. The inserted zinc ions occupied the interstitial sites in cavities surrounded by sulfur atoms (Zn1 sites). A significant number of the inserted zinc ions were trapped in these Zn1 sites, giving rise to the first-cycle irreversible capacity of ∼46 mAh g−1 out of the discharge capacity of 134 mAh g−1 at a rate of 0.05 C. In the lower-voltage region, further insertion occurred to form Zn2Mo6S8 at around 0.35 V in the CV, also involving a two-phase reaction. The electrochemical insertion and extraction into the Zn2 sites appeared to be relatively reversible and fast. The crystal structures of Mo6S8, ZnMo6S8, and Zn2Mo6S8 were refined using X-ray Rietveld refinement techniques, while the new structure of Zn2Mo6S8 was determined for the first time in this study using the technique of structure determination from powder X-ray diffraction data. With the zinc ions inserted into Mo6S8 forming Zn2Mo6S8, the cell volume and a parameter increased by 5.3% and 5.9%, respectively, but the c parameter decreased by 6.0%. The average Mo−Mo distance in the Mo6 cluster decreased from 2.81 to 2.62 Å. Some examples include α-MnO2,7 λ-MnO2,8 and todorokitetype MnO2,9 in which Zn2+ ions are reversibly intercalated into tunnels of the structures in aqueous 1 M ZnSO4 or Zn(NO3)2 electrolytes. Rhombohedral zinc hexacyanoferrate [Zn3Fe2(CN)12 or ZnHCF] has also been shown to intercalate Zn2+ ions into large open sites of the porous three-dimensional (3D) framework formed by the linkage of FeC6 octahedra and ZnN4 tetrahedra via CN ligands in an aqueous 1 M ZnSO4 electrolyte.10 Chevrel phases, MxMo6T8 (M = metal; T = S, Se, or Te), are attractive host materials for electrochemical multivalent-ion intercalation because of their interesting chemical intercalation capabilities of various guest elements such as magnesium, calcium, strontium, barium, chromium, manganese, iron, cobalt, nickel, zinc, etc.11−31 Among them, the ZnxMo6S8 phase (0 ≤ x ≤ 2), in particular, has been noticed as a potential electrode material for zinc-ion batteries because Mo6S8 is known to intercalate zinc ions electrochemically.22−25 Schöllhorn et al. first described the electrochemical preparation of Zn2Mo6S8 from Mo6S8 in aqueous zinc-containing electrolytes.22 A significant change in the unit-cell parameters between

1. INTRODUCTION Rechargeable batteries have recently gained much attention of late because of the popularization of electronic devices, electric vehicles, and energy-storage systems for smart grids that utilize renewable energy generation such as solar and wind power. Thus far, lithium-ion batteries (LIBs) have been the most prevalent technology for mobile applications such as cellular phones and tablet PCs and even for electric vehicles, especially in terms of energy and power densities.1−3 There are, however, increasing concerns regarding the safety of lithium-based batteries4 and the area preponderance of lithium resources.5 Moreover, there are significant demands for higher energy densities and lower costs than LIBs can provide. Over the past few years, in order to seek solutions for such problems, there has been increasing interest in post-LIBs, such as Li−air, Li−S, sodium-ion, and multivalent-ion batteries.3,6 Zinc-ion batteries have received attention as one type of multivalent-ion battery because of their potential applications in large-scale energy storage systems. Zinc has various merits, including safety, abundance, low cost, and environmental friendliness. Zinc metal also has a large theoretical capacity of 820 mAh g−1. There are, however, only a few host compounds that demonstrate electrochemical intercalation of zinc ions. © XXXX American Chemical Society

Received: October 13, 2015

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DOI: 10.1021/acs.inorgchem.5b02362 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Observed and calculated X-ray Rietveld refinement profile of Mo6S8, recorded at 25 °C. Red points: experimental data. Green line: calculated data. Pink line: difference. Black bars: Bragg positions. (b) FE-SEM image of the Mo6S8 particles. graphite dispersion (DAG EB-012, Acheson Industries, Inc.) onto 20 μm stainless foil (SUS-316L, Wellcos Co.). Active materials (Mo6S8) of about 7.8 mg were loaded on each electrode with an area of 1.53 cm2. Zinc disks of 1 mm thickness were used as counter electrodes, and zinc rods of 3.18 mm diameter were used as reference electrodes. A glass fiber was used as a separator (GF/A, Whatman) and a 0.1 M aqueous zinc sulfate solution (0.1 M ZnSO4 solution; Fluka Analytical) as the electrolyte. The potentiostatic and galvanostatic measurements were conducted with EC-Lab software on a Biologic VMP3 multichannel potentiostat (Biologic Science Instruments SAS). The phases of ZnMo6S8 and Zn2Mo6S8 were obtained as the galvanostatic reduction products of Mo6S8 in the zinc sulfate solution. 2.2. Structural Analysis. The powder XRD data were collected at 25 °C on an X-ray diffractometer (Rigaku Miniflex 600) with a Cu Xray tube (λ = 1.5418 Å), a secondary graphite (002) monochromator, and an angular range of 10° ≤ 2θ ≤ 85° for general purposes. To attain the samples needed to refine and determine the crystal structures of ZnMo6S8 and Zn2Mo6S8, a PANalytical Empyrean X-ray diffractometer was used with a Cu Kα1 X-ray (λ = 1.5406 Å) with a Ge(111) monochromator, a position-sensitive PIXcel3D 2 × 2 detector, an angular range of 5° ≤ 2θ ≤ 150°, a step of 0.013000°, and a total measurement time of 12 h at room temperature. The crystal structures of Mo6S8 and ZnMo6S8 were refined using the powder profile refinement program GSAS.35 The initial structural models of Mo6S836 and ZnMo6S819 were adopted from previous reports. Determination of the crystal structure of Zn2Mo6S8 was performed using a combination of the program GSAS and the singlecrystal structure refinement program CRYSTALS37 in a manner similar to that described in our previous report.32 For a 3D view of the Fourier density maps, MCE38 was used. The powder sample used to determine the crystal structure of Zn2Mo6S8 also contained the ZnMo6S8 phase. Thus, Rietveld fitting was performed for the ZnMo6S8 phase, while Le Bail fitting was carried out for the Zn2Mo6S8 phase with a ZnMo6S8like structural model in which only the cell parameters are slightly different from those of ZnMo6S8. The structure factors were extracted and then were used as input data for the program CRYSTALS. Then, the position of the inserted zinc site was determined from a Fourier difference synthesis map, completing the structural model of Zn2Mo6S8. Finally, Rietveld refinement was applied.

the two phases before and after cathodic reduction was evidenced as zinc intercalation. Later, Gocke et al.23 performed more thorough galvanostatic experiments in aprotic as well as aqueous electrolytes, observing two potential plateau regions, one for 0 ≤ x ≤ 1 and the other for 1 ≤ x ≤ 2 in ZnxMo6S8. The results, combined with structural data, suggested that ZnMo6S8 and Zn2Mo6S8 might be line phases.23 Knowledge of the precise crystal structures of the intercalation compounds provides an important basis for understanding these materials with respect to ion diffusion, stability, energetics, and battery characteristics. Levi et al.19 determined the positions of the intercalated zinc ions in the crystal structure using X-ray and neutron Rietveld refinement methods for thermally synthesized ZnMo6S8 but not for Zn2Mo6S8. It has been noted that Zn2Mo6S8 seems to be obtained only via the topotactic insertion reaction but not by direct synthesis at high temperatures,19 and its crystal structure has not yet been determined. In this study, we report the synthesis of the host material Mo6S8, the electrochemical insertion and extraction properties of Zn2+ in ZnxMo6S8 (0 ≤ x ≤ 2) in aqueous electrolytes, and finally the structural analyses of the electrochemically obtained line phases of ZnMo6S8 and Zn2Mo6S8. In order to locate the inserted zinc positions, the technique of structural determination from powder X-ray diffraction (XRD) data was applied.32

2. EXPERIMENTAL SECTION 2.1. Synthesis and Electrochemical Characterization. Mo6S8 (Chevrel phase) was synthesized according to the previously reported method.33,34 Cu2Mo6S8 was first prepared via solid-state synthesis from a stoichiometric mixture of copper (99.7%, Sigma-Aldrich), molybdenum (99.9%, Sigma-Aldrich), and sulfur (99.98%, SigmaAldrich) at 1050 °C for 48 h in a vacuum-sealed fused-silica tube. Then, Mo6S8 was obtained via copper-ion extraction (oxidation) from Cu2Mo6S8 by stirring the powder overnight in a 1 M aqueous solution of FeCl3, followed by filtration with aspiration, washing with distilled water, and finally drying at 100 °C in a vacuum oven for 24 h. Morphological and elemental analyses were carried out using highresolution field-emission scanning electron microscopy (FE-SEM; Hitachi SU-8020, Japan) with an energy-dispersive X-ray spectrometry (EDX) attachment. Three electrode beaker-type cells were used for all of the electrochemical experiments. The working electrode consisted of Mo6S8 powder, conductive carbon (Super P carbon black, Timcal Graphite & Carbon), and a poly(vinylidene difluoride) binder (W#1300, Kureha Co.) (8:1:1 weight ratio), which were mixed and dispersed in N-methyl-2-pyrrolidone and cast onto thin carbon-coated stainless foil. The thin carbon coating (∼4 μm) was made by coating a

3. RESULTS AND DISCUSSION 3.1. Synthesis of Mo6S8. As presented in Figure 1a, the phase of the synthesized Mo6S8 was confirmed with the powder X-ray Rietveld refinement, and no impurities were observable. The particle sizes were in the range of 200−900 nm, as shown in the SEM image (Figure 1b). The crystal structure of Mo6S8 was trigonal with the space group R3̅, and the lattice parameters were refined as a = 9.1910(6) Å and c = 10.8785(10) Å. The refined atomic parameters and selected interatomic distances B


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Inorganic Chemistry

Figure 2. (a) CV of the Mo6S8 electrode at 0.05 mV s−1 in a 0.1 M ZnSO4 aqueous electrolyte (pH 5.2). (b) Initial discharge and charge curves of the Zn/Mo6S8 cell at a rate of 1/20 C (6.4 mA g−1) in the voltage range of 0.25−1.0 V versus zinc metal.

Figure 3. (a) Reversible charge/discharge curves of the Zn/Mo6S8 cells in the voltage range of 0.25−1.0 V at different rates . (b) Cycle performances of the Zn/Mo6S8 cell at various C rates in a 0.1 M ZnSO4 aqueous electrolyte: 1/20 C (6.4 mA g−1), 1/4 C (32 mA g−1), 1/2 C (64 mA g−1), and 1 C (128 mA g−1).

where the n C rate is defined as the rate of charge (or discharge) at which the charge (or discharge) is completed in (1/n) hours. Here, the 1 C rate was defined as 128 mA g−1 because the theoretical capacity of the four-electron reaction from Mo6S8 to Zn2Mo6S8 is 128 mAh g−1. The slightly greater capacity than the theoretical one might have been due to some minor, unidentified side reactions, probably including the reduction of water. The multiple, closely fitted peaks around 0.45 V and the single peak around 0.35 V in the CV corresponded to the sloppy and flat profiles in the galvanostatic profile, respectively. The decreased current of the oxidation step (or the second cycle) corresponded to the first cycle’s irreversible capacity loss. The greater polarization of the highervoltage region than that of the lower-voltage one in the CV was also in accordance with the results observed in the galvanostatic profile. Figure 3a shows the discharge/charge profiles of the second cycle at various current densities. The reversible capacity decreased more at higher C rates, as generally expected. The discharge capacity of 88 mAh g−1 at 0.05 C decreased to 57 mAh g−1 at 1 C. It is, however, interesting to note that the decrease was caused mainly by the decreased capacity in the higher-voltage region above 0.45 V. An increased polarization was also observed with higher C rates, as generally expected, but this increase was more severe in the higher-voltage region. Noting that the polarization was insignificant in the lowervoltage region around 0.35 V even at higher C rates, the ohmic resistance of the cell componentsthe effect of which should

are summarized in Tables S1 and S2 and are in good agreement with previous reports.36 3.2. Electrochemical Syntheses and Characterization of ZnMo6S8 and Zn2Mo6S8. Figure 2a presents the cyclic voltammogram (CV) of the Mo6S8 electrode in the aqueous zinc electrolyte in which the initial open-circuit voltage of the Mo6S8 electrode was 0.92 V (vs Zn2+/Zn), and a negative sweep was first applied down to 0.05 V with a scan rate of 0.05 mV s−1. This figure shows two distinguishable sets of quasireversible reduction and oxidation peaksone set is broad and the other is relatively sharpin the higher- and lower-voltage regions, respectively. Such quasi-reversible peaks indicated the rechargeable electrochemical zinc insertion and extraction reactions, expressed as x Zn 2 + + Mo6S8 + 2x e− ⇄ ZnxMo6S8

It is worth noting that the cathodic peak in the higher-voltage region shifted its maxima from ∼0.45 to ∼0.50 V and demonstrated significantly reduced currents in the second and later cycles in which the peak shape and the current remained almost unchanged. In addition, the broad peak could result from closely positioned double (or multiple) peaks. The observations of the CV were consistent with those of the galvanostatic reduction profile shown in Figure 2b in which the two voltage regions are clearly divided, one with a sloppy profile in the higher-voltage region (0.45−1.0 V) and the other with a single plateau in the lower-voltage region (∼0.35 V). The first discharge capacity was 134 mAh g−1 at a 0.05 C rate, C


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Figure 4. (a) Ex situ XRD patterns of the ZnxMo6S8 composite electrodes that were electrochemically prepared in the zinc cells. (b) XRD measurement points of ZnxMo6S8 during the discharge/charge process in the galvanostatic profile.

Figure 5. Powder X-ray Rietveld refinement profile for ZnMo6S8 (n = 2.2) recorded at 25 °C. Red points: experimental data. Green line: calculated data. Pink line: difference. Black bars: Bragg positions.

octahedral Mo6 cluster, and eight sulfur atoms are face-capped on top of the eight trigonal faces of the octahedron. Six of the eight sulfur atoms are also bonded axially to a molybdenum atom of a neighboring cluster, resulting in the 3D network crystal structure creating cavities surrounded by sulfur atoms. The ex situ powder XRD patterns of the ZnxMo6S8 electrodes were recorded with respect to the number (n) of electrons (0 ≤ n ≤ 4.16) employed during the galvanostatic reduction and oxidation of the first cycle, as shown in Figure 4. In principle, the number x of the divalent zinc ions inserted into Mo6S8 should be half that of the electrons (n). However, according to structural and elemental analyses and the fact that the observed discharge capacity was slightly greater than the theoretical one (134 vs 128 mAh g−1 at 0.05 C), such an exact relationship did not hold for our experiments; x was generally slightly less than n/2 because of a small degree of an unidentified side reaction,

be common to the entire voltage regionmight not have been the main cause of the greater polarization in the higher-voltage region. Thus, presumably, the significant difference in the capacity reduction and the polarization increase between the two voltage regions resulted from the intrinsic properties of the host material. The differences in the crystallographic zinc insertion sites for the two regions might have been partially responsible. The cycle performances with various C rates are shown in Figure 3b. The first-cycle irreversibility of 46 mAh g−1 at 0.05 C suggested that some of the inserted zinc ions were trapped in the host structure. The nature of the inserted zinc ion positions in the host materials was probed through the structural analyses described below 3.3. Structural Analyses of ZnMo6S8 and Zn2Mo6S8. The Chevrel phase Mo6S8 is a molybdenum cluster compound with metal−metal bondings. Six molybdenum atoms form an D


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Inorganic Chemistry

were 1.076(11), 1.838(18), and 2.130(20) Å, all of which were too short for metal−metal distances. Thus, only one out of the six positions, if any were filled, should be occupied at a given time, limiting the maximum occupancy to 1/6. Any other possible position was not evidenced by careful examination of the electron density maps obtained via the Fourier and difference-Fourier syntheses. The reduction profile from n = 2.2 to 4.16 showed a plateau that indicated a two-phase reaction of ZnMo6S8 and Zn2Mo6S8. According to the shape of the reduction profile and the number of electrons employed, the sample n = 3.2, falling in the middle of the plateau, was expected to be a mixture of two end phases, n = 2.2 (ZnMo6S8) and n = 4.16 (Zn2Mo6S8). However, only the phase n = 2.2 (ZnMo6S8) was still indicated within the XRD pattern. The sample n = 4.16, just after the end of the plateau, was expected to be a pure phase of Zn2Mo6S8, but it was instead a mixture of the two phases, ZnMo6S8 and Zn2Mo6S8. The instability of the most reduced phase Zn2Mo6S8 has also been reported previously;23 it was partially oxidized when exposed to air, similar to our observations. It is most probable that the Zn2Mo6S8 phase was oxidized in air to form the more stable ZnMo6S8 and amorphous ZnO during the process when the cell was disassembled, and the electrode was washed and then analyzed with the diffractometer. The crystal structure of Zn2Mo6S8, especially the Zn2 position, was determined for the first time in this study. The Zn2 position was located via Fourier synthesis mapping from the powder XRD data, as shown in Figure 7. The Rietveld refinement profile of Zn2Mo6S8 is shown in Figure 8. The refined parameters are summarized in Table 1, and some interatomic distances are listed in Table S7. The interatomic distances from a Zn2 site to the neighboring Zn2 sites or already occupied Zn1 sites are too close [1.42(7) and 1.78(5) Å, respectively], thus also limiting the maximum occupancy of the Zn2 sites to 1/6. The variations of the cell parameters and unit-cell volumes are shown in Figure 9a, and the average interatomic distances of Mo−Mo are shown in Figure 9b. With insertion of the zinc ions into the host material from Mo6S8 into ZnMo6S8 and Zn2Mo6S8, the unit-cell volumes increased by 1.5% and 5.3%, respectively, but the cell parameters (a and c) were anisotropically changed; the a parameter increased by 5.9% (from 9.191 to 9.730 Å), but the c parameter decreased by 6.0% (from 10.879 to 10.311 Å). It was more interesting to observe the variations of the interatomic distances. The greater the insertion of zinc ions, the more the molybdenum metals were reduced, resulting in stronger Mo−Mo bonding in the Mo6 cluster with a decrease in the average interatomic distance from 2.81 to 2.62 Å.

as discussed in the preceding section. The identification of this side reaction is under investigation and will not be discussed in this report because it is beyond its scope. The initial stage of reduction (0 ≤ n ≤ 0.24) seemed to be a single-phase-reaction region, given the very sloppy discharge curve and the fact that the XRD pattern of the sample of n = 0.24 was just slightly modified from that of Mo6S8 (n = 0) in both the peak intensities and positions. For the subsequent reduction range (0.24 ≤ n ≤ 2.2), the profile depicted a gradual slope; it did not seem to be a simple, single-phase-reaction region, probably instead involving a small degree of a two-phase reaction. The XRD pattern of the intermediate sample n = 1.08, for example, contained a major phase that was similar but different from that of the sample n = 0.24. A careful examination of the pattern also revealed a minor second phase that was similar to that of the sample n = 2.2. The XRD pattern of the sample n = 2.2, just after the abrupt voltage drop at the end of the higher-voltage region (0.45−1.0 V), obviously showed the single-phase ZnMo6S8. The elemental mapping with EDX indicated a uniform distribution of zinc in the particles of the sample n = 2.2 (Figure S3), which was absent in the sample n = 0. Elemental analyses from the results of the inductively coupled plasma (ICP) and EDX (Tables S3 and S4) also supported the zinc composition. The crystal structure of ZnMo6S8 was confirmed via the X-ray Rietveld refinement technique, as shown in Figure 5. The initial structural model was adopted from the previous report [a = 9.545(2) Å and c = 10.296(2) Å],19 and the refined results are summarized in Table S5. The important interatomic distances are listed in Table S6. The refined parameters of ZnMo6S8 prepared electrochemically at room temperature were quite close to those of the material synthesized via a hightemperature solid-state reaction.19 The zinc positions were occupied in disordered positions, forming a six-membered ring, which is the same as the previous report, but the positions were shifted from the initial positions by ∼0.5 Å. In the process of refinement, we have suspected possible false minima of the refinement; thus, a very careful examination was made to validate convergence of the refinement to locate the zinc position. The slight difference might be due to the difference in the samples: our sample did not contain any impurity noticeable in the XRD, while the sample used in the previous report contained impurities of 6% Mo6S8, 4% Mo2C, ∼2% ZnS, and 0.5% MoC.19 The unit-cell volume was increased by 1.5% from Mo6S8 to ZnMo6S8 as the cell was expanded in the ab plane (a from 9.191 to 9.512 Å) but compressed along the c axis (from 10.879 to 10.311 Å). It should be noted that the zinc ions were statistically distributed with a partial occupancy of 1/6. The Zn1 positions generated by the symmetry formed a six-membered ring, as shown in Figures 6 and 7. The interatomic Zn1−Zn1 distances

4. CONCLUSIONS The crystal structures and electrochemical properties of the Chevrel phases ZnxMo6S8 (x = 1, 2), prepared via the electrochemical Zn2+ ion intercalation into Mo6S8 in an aqueous electrolyte, were characterized. The crystal structures of Mo6S8 and ZnMo6S8 were refined using X-ray Rietveld refinement techniques, while that of Zn2Mo6S8 was determined and refined for the first time in this study using the technique of structure determination from powder XRD data. The process of the electrochemical zinc-ion insertion into the host material Mo6S8 occurred stepwise, involving two clearly separate potential regions, as observed in the CV and galvanostatic profile. The higher-voltage region corresponded

Figure 6. Local structure around the Zn1 positions in ZnMo6S8. E


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Inorganic Chemistry

Figure 7. (001) sections of (a) the observed Fourier map at z = 0.029 and (b) the difference-Fourier maps at z = 0.127 of Zn2Mo6S8 at the refinement stage prior to the location of the Zn2 atoms. The map width is 7.8 Å, and the center of the map is the Zn1 position at (0, 0, 0.029). The Zn2 positions lie above and below the plane of part a, denoted as Zn2a and Zn2b, respectively. The minimum and maximum contour levels are (a) 1.1 and 19.6 e Å−3 for the Fourier maps and (b) 1.1 and 3.5 e Å−3 for the difference-Fourier maps, respectively. The contour levels are adjusted unequally for clarity.

Figure 8. Powder X-ray Rietveld refinement profile of Zn2Mo6S8 (n = 4.16), recorded at 25 °C. It shows a mixture of ZnMo6S8 and Zn2Mo6S8. Red points: experimental data. Green line: calculated data. Pink line: difference. Black bars: ZnMo6S8 Bragg positions. Red bars: Zn2Mo6S8 Bragg positions.

Table 1. Crystallographic Data and Rietveld Refinement Results for Zn2Mo6S8 by Powder XRD Data: Atomic Coordinates, Site Occupancies, Isotropic Displacement Parameters, and Reliability Factors at Room Temperaturea

a

atom

cryst syst space group lattice param, volume, Z x

y

Mo S1 S2 Zn1 Zn2

0.0145(5) 0.3174(12) 0.0000 0.715(6) 0.170(4)

0.1623(4) 0.2860(14) 0.0000 0.4705(30) 0.279(4)

trigonal R3̅ (No. 148) a = 9.7295(2) Å, c = 10.2215(3) Å, V = 837.97(3) Å3, Z = 3 z Wyckoff occupancy 0.3950(3) 0.3997(11) 0.2013(22) 0.3300(35) 0.8773(33)

18f 18f 6c 18f 18f

1.0000 1.0000 1.0000 0.1667 0.1667

Uiso 0.011(1) 0.022(3) 0.073(12) 0.023(10) 0.031(11)

Rp = 0.107, Rwp = 0.143, Rexp = 0.064, R(F2) = 0.12204, χ2 = 5.062.

F


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Figure 9. (a) Evolution of the unit-cell parameters (a = b and c) in ZnxMo6S8 (x = 0, 1, 2) upon zinc intercalation. (b) Average interatomic distances of Mo−Mo. (2) Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Angew. Chem., Int. Ed. 2008, 47, 2930−2946. (3) Amine, K.; Kanno, R.; Tzeng, Y. MRS Bull. 2014, 39, 395−401. (4) Lee, K. T.; Jeong, S.; Cho, J. Acc. Chem. Res. 2013, 46, 1161− 1170. (5) Komaba, S.; Murata, W.; Ishikawa, T.; Yabuuchi, N.; Ozeki, T.; Nakayama, T.; Ogata, A.; Gotoh, K.; Fujiwara, K. Adv. Funct. Mater. 2011, 21, 3859−3867. (6) Tarascon, J.-M. Philos. Trans. R. Soc., A 2010, 368, 3227−3241. (7) Xu, C.; Li, B.; Du, H.; Kang, F. Angew. Chem., Int. Ed. 2012, 51, 933−935. (8) Yuan, C.; Zhang, Y.; Pan, Y.; Liu, X.; Wang, G.; Cao, D. Electrochim. Acta 2014, 116, 404−412. (9) Lee, J.; Ju, J. B.; Cho, W. I.; Cho, B. W.; Oh, S. H. Electrochim. Acta 2013, 112, 138−143. (10) Zhang, L.; Chen, L.; Zhou, X.; Liu, Z. Adv. Energy Mater. 2015, 5, 1400930. (11) Umarji, A. M.; Rao, G. V. S.; Janawadkar, M. P.; Radhakrishnan, T. S. J. Phys. Chem. Solids 1980, 41, 421−429. (12) Alekseevskii, N. E. Cryogenics 1980, 20, 257−265. (13) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 407, 724−727. (14) Geng, L.; Lv, G.; Xing, X.; Guo, J. Chem. Mater. 2015, 27, 4926−4929. (15) Lancry, E.; Levi, E.; Gofer, Y.; Levi, M.; Salitra, G.; Aurbach, D. Chem. Mater. 2004, 16, 2832−2838. (16) Cheng, Y.; Parent, L. R.; Shao, Y.; Wang, C.; Sprenkle, V. L.; Li, G.; Liu, J. Chem. Mater. 2014, 26, 4904−4907. (17) Levi, E.; Gershinsky, G.; Aurbach, D.; Isnard, O.; Ceder, G. Chem. Mater. 2009, 21, 1390−1399. (18) Levi, E.; Mitelman, A.; Aurbach, D.; Isnard, O. Inorg. Chem. 2007, 46, 7528−7535. (19) Levi, E.; Gershinsky, G.; Aurbach, D.; Isnard, O. Inorg. Chem. 2009, 48, 8751−8758. (20) Johnson, D. C.; Tarascon, J.-M.; Sienko, M. J. Inorg. Chem. 1985, 24, 2598−2604. (21) Cho, J.-H.; Aykol, M.; Kim, S.; Ha, J.-H.; Wolverton, C.; Chung, K. Y.; Kim, K.-B.; Cho, B.-W. J. Am. Chem. Soc. 2014, 136, 16116− 16119. (22) Schöllhorn, R.; Kümpers, M.; Besenhard, J. O. Mater. Res. Bull. 1977, 12, 781−788. (23) Gocke, E.; Schramm, W.; Dolscheid, P.; Schöllhorn, R. J. J. Solid State Chem. 1987, 70, 71−81. (24) Seghir, S.; Boulanger, C.; Diliberto, S.; Potel, M.; Lecuire, J.-M. Electrochim. Acta 2010, 55, 1097−1106. (25) Seghir, S.; Stein, N.; Boulanger, C.; Lecuire, J.-M. Electrochim. Acta 2011, 56, 2740−2747. (26) Boulanger, C.; Lecuire, J. M. Electrochim. Acta 1987, 32, 345− 348. (27) Boulanger, C.; Lecuire, J. M. Electrochim. Acta 1988, 33, 1561− 1565.

to the formation of ZnMo6S8 from Mo6S8, occurring at a potential of around 0.45−0.50 V in the CV. The inserted zinc ions occupied the interstitial Zn1 sites in cavities surrounded by sulfur atoms, generating a ring of six crystallographically equivalent positions close to each other, thus limiting the maximum occupancy to 1/6. A significant portion of the inserted zinc ions were trapped in the Zn1 sites, giving rise to the first-cycle irreversible capacity of ∼46 mAh g−1 out of the discharge capacity of 134 mAh g−1 at 0.05 C. The lower-voltage region defined the further reduction to form Zn2Mo6S8 from ZnMo6S8, occurring at a potential of around 0.35 V in the CV and involving a two-phase reaction. The inserted zinc ions at this stage occupied the Zn2 sites that also generated six equivalent positions in the cavity, limiting its maximum occupancy to 1/6. The electrochemical insertion and extraction into the Zn2 sites seemed to be relatively reversible and fast. This paper presents detailed structural analyses and electrochemical characteristics of the intercalation reaction of divalent Zn2+ ions into the Chevrel phase Mo6S8. The results should be an important basis for understanding the electrochemical intercalation reaction of zinc as well as other divalent ions into various host materials, promoting a further investigation of new intercalation chemistry of divalent ions.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02362. Detailed crystallographic information and elemental analyses of the EDX and ICP results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82 53 785 6409. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DGIST R&D Program of the Ministry of Science, ICT and Future Planning of Korea (No. 1501-HRLA-01).

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