Charge Transfer Mechanism into the Chevrel Phase Mo6

Article pubs.acs.org/JPCC

Charge Transfer Mechanism into the Chevrel Phase Mo6S8 during Mg Intercalation J. Richard,†,‡ A. Benayad,*,†,‡ J.-F. Colin,†,‡ and S. Martinet†,‡ †

Université Grenoble Alpes, F-38000 Grenoble, France CEA−LITEN, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France

‡

S Supporting Information *

ABSTRACT: We re-examine the charge transfer mechanism in the Chevrel phase Mo6S8 upon Mg intercalation and deintercalation by the mean of X-ray photoemission spectroscopy. We highlighted a two-step reversible charge transfer process involving successively axial sulfurs and Mo6 cluster redox center. In parallel, the Mg 2p and Mg KLL Auger transition evolution evidenced two insertion sites having different polarizations in accordance with previous studies. The nonconventional metal transition redox reaction, involved during Mg intercalation in Chevrel phase, might be at the origin of its excellent reversibility and intercalation kinetics.

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slow insertion, and the reversible insertion of Mg2+ in these compounds has not been demonstrated yet. Unlike close-packed oxygen anion sublattices of classical insertion material structures (layered, spinel or olivine) the Chevrel phase Mo 6 S 8 has a unique cluster structure. Synthesized for the first time in 1973 by R. Chevrel et al.11 this structure can be described as 6 molybdenum cations forming an octahedron on the faces of the cube where eight sulfur anions are occupying the corners (Figure 1a). There are two types of sulfurs with different coordinations within the Chevrel structure. Sulfur coordinated to three and

INTRODUCTION Since Sony introduced the first lithium-ion battery (LiB) in 1991,1 great academic efforts have been devoted to the development of high-performance Li-ion and other post Liion secondary batteries.2 Material size tuning from micrometer to nanometer level, electrode/electrolyte interfaces design, electrolyte stability windows adjustment, etc., are the most developed strategies to improve the performances of LiB such as capacity retention and safe operating conditions. Lithium remains the most used alkali metal element for secondary electrochemical storage devices, however some drawbacks such as the unevenly resources distribution and safety issues, pave the way toward alternative alkali metal like sodium and magnesium. For a few years, Mg batteries have gained attention due to the abundancy of magnesium (eighth most available element in the Earth’s crust), its divalent character, its eco-friendliness, and its higher volumetric capacity compared to lithium (3833 versus 2061 mAh·cm−3). The cost of input materials, 1000 dollars/ton in 2016 for MgCO3 versus 13 000 dollars/ton in 2015 for Li2CO3, contributes significantly to explore new alkali based storage materials. However, Mg-ion batteries show some major limitations such as electrolyte instability toward metallic magnesium and current collectors (typically aluminum or copper), as well as slow insertion kinetics in the common host materials.3 Most of the analogues of lithium-conventional insertion materials were studied with regard to magnesium insertion and deinsertion with little success due to the strong electrostatic interactions generated by divalent magnesium ion. Among them, we can cite V2O54 or Prussian blue analogues K0.1Cu[Fe(CN)6]0.7·3.6H2O5 that are able to insert Mg ions in aqueous electrolyte. Other insertion materials such as manganese dioxide6−8 or magnesium silicates9,10 show very © XXXX American Chemical Society

Figure 1. Structure of the Chevrel phase Mo6S8: (a) the pattern Mo6S8; (b) the three types of cavities (1, 2 and 3) surrounding the Mo6S8 unit. Received: April 27, 2017 Revised: June 21, 2017 Published: July 21, 2017 A

DOI: 10.1021/acs.jpcc.7b03979 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 2. (a) X-ray diffraction pattern and (b) SEM images of as-synthesized Mo6S8 powder.

charge.20−22 Up to our knowledge, there is no study by XPS on the electrochemical insertion mechanism of Mg ions in Mo6S8 Chevrel phase; the only XPS reported studies concern the Chevrel phase Mo6S8 and the electronic structure of chemically inserted copper and platinum.23,24 In this paper, we present a detailed XPS study on the charge transfer from Mg to Mo and S atoms in Chevrel phase Mo6S8 during the electrochemical insertion and deinsertion of Mg ions. We evidenced an insertion mechanism of Mg-ions based on a two-step reversible charge transfer process involving successively axial sulfurs and Mo6 cluster redox centers.

four molybdenum, labeled in red and blue color, respectively. These two types of sulfurs are called respectively axial and peripheral sulfurs.12 The Mo6S8 patterns are separated from each other by three types of cavities, as illustrated in Figure 1b. The cavities labeled (Cavity 1) share corners with Mo6S8 through sulfur atom, whereas cavities labeled (Cavity 2) and (Cavity 3) share edges and faces, respectively.13 In 2000, Aurbach et al.14 tested for the first time the electrochemical performances of the Chevrel phase Mo6S8 as a cathode material for Mg-ion battery and revealed an excellent cyclability with a theoretical capacity of 122 mAh.g−1 and a working potential around 1 V vs Mg2+/Mg. The insertion mechanism and the structural change involved during the electrochemical process was largely investigated by cyclic voltammetry, impedance spectroscopy,15,16 X-ray diffraction spectroscopy,13 and X-ray absorption spectroscopy.17 Two different Mg insertion sites were evidenced in Chevrel phase Mo6S8, named inner and outer rings, corresponding to the two insertion plateaus observed on the galvanostatic discharge curve. The Mg ions insertion and deinsertion induce a slight reversible structural distortion, mainly localized around the molybdenum octahedron structure. The insertion of two divalent ions in Mo6S8, involves the introduction of four additional electrons to the 20 electrons of the distorted Mo6 octahedron in order to completely fill the valence band of the Mo6S8 structure. This process gives rise to a change in the Mo− Mo distance to form regular Mo6 octahedron. Yvon et al.18 evidenced by XRD that Mo−Mo distances change during cation insertion in Chevrel phase. Only few studies are dedicated to the understanding of the charge transfer into Chevrel phase during Mg-ion insertion, and up-to-date there is no clear evidence of the involved mechanism. A recent comparison of simulation and experimental X-ray absorption near edge spectroscopy (XANES) data confirmed the electron transfer from Mg to S atoms in the Chevrel host.17,19 A direct analysis of the oxidation states of magnesium, sulfur and molybdenum probed by X-ray photoemission spectroscopy (XPS) might bring new understanding elements on the Mg ions insertion process in Chevrel phase Mo6S8. The XPS is one of the direct and nondestructive surface analyses tools that is used for battery active materials and allows highlighting the evolution of the oxidation degrees of the transition metals but also their chemical environment at different states of

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EXPERIMENTAL SECTION Synthesis. The Chevrel Phase Mo6S8 was synthesized following an adaptation of the protocol published by Choi et al.25 Stoichiometric amounts of precursors Cu (99.9%, Alfa Aesar), MoS2 (99%, Sigma-Aldrich) and Mo (99.95%, Alfa Aesar) were grounded for 20 min under argon atmosphere in a high-energy Spex Miller (SPX SamplePrep 8000-series, the grinding container and the two balls are made of zirconium nitride). Then the powder is heated in an alumina crucible in a tubular furnace under Ar flow at 900 °C for 7 days. The obtained Cu2.5Mo6S8 powder is immersed in a 6 M HCl solution and stirred for 9 days to leach the copper from the structure. The powder is then filtered and washed with deionized water and finally dried in an oven at 100 °C for 24 h. The structure of the Chevrel phase in Figures 1 and 5 is represented using the software VESTA.26 The X-ray diffraction (XRD) and the SEM images reported in Figure 2, were performed on a Θ−2Θ, Brüker AXS D8 diffractometer using a Cu Kα anticathode and by scanning electron microscopy using a LEO 1530 FE-SEM microscope equipped with a field electron gun. The X-ray photoelectron spectroscopy (XPS) analyses were performed with a ULVAC PHI 5000 VersaProbe II spectrometer using AlKα X-ray radiation (1486.6 eV). The residual pressure inside the analysis chamber was 7 × 10−8 Pa. A fixed analyzer pass energy of 23 eV was used for core level scans, leading to an overall energy resolution of 0.6 eV. Survey spectra were captured at a pass energy of 117 eV. All spectra were referenced against an internal signal, typically by adjusting the C 1s level peak at a binding energy of 284.8 eV. B


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The Journal of Physical Chemistry C The XPS spectra were fitted by using casaXPS software in which a Shirly background is assumed and the fitting peaks of the experimental spectra are defined by a combination of Gaussian (80%) and Lorentzian (20%) distributions for Mg 2p and S 2p. In the case of the Mo 3d core peak, an asymmetric distribution modulated the Voigt-like function at lower kinetic energy electrons to take into account the final state transition induced during the photoemission process in metal orbitals. The fitting curves were performed based on a reference study of Mg metal, MoS2, and Mo6S8 pristine material (see Supporting Information). Composite electrodes were prepared from a slurry composed of active material (80 wt %), carbon super P (10 wt %), and polyvinylidene fluoride (PVdF, 10 wt %) with N-methyl-2pyrrolidone (NMP) as solvent. The slurry was coated on an aluminum current collector at a wet thickness of 200 μm using a doctor blade. Three-electrodes coin-cells (CR2032 type) were assembled with the following stacking: Mo6S8 composite electrode/ Viledon separator/reference Mg foil (99.9% GalliumSource)/ Celgard separator/Mg foil counter electrode. The electrolyte is composed of 1 M Mg(ClO4)2 (99%, Sigma-Aldrich) in solution in acetonitrile (99, 9%, Sigma-Aldrich). Galvanostatic cycling tests were performed between 0.55 and 1.6 V vs Mg2+/Mg at C/20 rate and 55 °C using a VMP3 from Bio-Logic controlled via the EC-Lab software. Cycled electrodes for XPS measurements were recovered after dismantling of the coin-cells in an argon-filled glovebox. They were then rinsed with acetonitrile and dried under inert atmosphere and transferred to the XPS analysis chamber using an airtight vessel to avoid any air contamination.

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RESULTS AND DISCUSSION The as-synthesized Chevrel phase Mo6S8 was characterized by XRD and SEM and revealed a rhombohedral phase (space

Figure 4. XPS Mg 2p core level of different MgxMo6S8 electrodes.

Levi et al.13 assigned both plateaus to two types of insertion sites, the inner and outer rings, located in the cavities labeled 1 and 2 corresponding to the plateaus above 1 V and under 0.9 V vs Mg2+/Mg, respectively. In our case, the galvanostatic curve shows a slope in the range between 0.5 and 1 inserted Mg. This behavior can find origin in the reactivity of the electrolyte at the surface of the cathode during first magnesiation and/or an increase of the polarization at the end of the biphasic reaction. The electronic structure of MgxMo6S8 active material was analyzed at different insertion rates x of the first discharge and charge symbolized by the points on the galvanostatic curve in Figure 3. The XPS Mg 2p core level spectra registered at the surface of the electrodes at different states of the first cycle are displayed in Figure 4.

Figure 3. Galvanostatic discharge/charge curve of Mo6S8 vs Mg in 1 M Mg(ClO4)2/acetonitrile electrolyte at a C-rate of C/20 and 55 °C.

group R3,̅ see Figure 2a) corresponding to the PDF card 04− 003−6723. Our material consists of nugget-like particles with a size between 100 nm and 1 μm (see Figure 2b). In Figure 3, the galvanostatic discharge and charge curve obtained in three electrode coin cell at 55 °C and C-rate of C/ 20 is similar to that reported in the literature,14 and the theoretical insertion rate was reached. We can distinguish two plateaus assigned to two biphasic domains Mo6S8/Mg1Mo6S8 and Mg1Mo6S8/Mg2Mo6S8, respectively. C


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the presence of two chemical environments as observed by XPS. The Figure 5b is a projection of the Chevrel phase Mo6S8 structure on the perpendicular plan of the 3-coordinated sulfurs axis, highlighting the inner ring and the outer ring. We paid particular attention to the Mg KLL Auger transition, involving deep Mg 1s and shallow Mg 2s and Mg 2p core levels, to investigate the Mg polarization and the electronic structure in relation to local neighbor atoms (Figure 6). The Auger KLL line of Mg metal and MgO reference sample represented in Figure 6 are located at 300.8 and 306.0 eV respectively. As the Auger KL2,3L2,3 transition involves 1S, 1D, and 1P, where the most intense one is the 1D. The 1S and 1P transitions are located at ∼5 eV and ∼ −3 eV relatively to 1D transition, respectively.28 We not that in the Mg metal and MgO, the transition 1P is not observable. For the Mg1Mo6S8 phase, we observe an Mg KLL transition at 307.4 eV and a minor signal at 303.3 eV. The first signal corresponds to Mg2+ ions in a polarized environment, because the binding energy is close to the Mg KLL transition of Mg2+ ions in MgO whereas the second peak is assigned to Mg ions in quasi-metallic environment, because the binding energy is close to the Mg KLL transition of Mg2+ ions in Mg. The presence of the transition peak at 303.3 eV is related to the formation of Mg− Mg bonds occurring at the middle of the first galvanostatic plateau, equivalent to 0.5-inserted Mg. The XPS Mg KLL Auger signature provides a picture of the multiplet coupling in the final states, i.e., the extra-atomic relaxation energy and the local electronic structure of the material.29 It seems that the extraatomic relaxation of Mg ions located in the cavity 1 are different from that of Mg ions bonded to oxygen in MgO crystal. For Mg2Mo6S8 phase, the slight signal at 303.3 eV shift to 304.2 eV, indicating a change in the extra-atomic relaxation of Mg ions within the fully intercalated Chevrel phase Mo6S8. This two Mg KLL Auger transitions related to Mg inserted in the inner and outer ring underline the difference of chemical environment between cavities 1 and cavities 2 as seen before with the Mg 2p core peak analysis. To understand the charge transfer mechanism occurring during magnesiation upon the first discharge and charge, we performed a systematic Mo 3d5/2−3/2 and S 2p3/2−1/2 XPS core levels study at the surface of the electrodes at different states of the first cycle displayed in Figure 7.

Table 1. XPS Mg 2p Binding Energies (eV) in MgxMo6S8 Electrodes at Different Stages of the First Cyclea Mg 2p Mg inner ring Mg0.5Mo6S8 Mg1Mo6S8 Mg1.5Mo6S8 Mg2Mo6S8 Mg0.1Mo6S8 after discharge

51.3 (1.4) 100% 51.4 (1.5) 100% 51.4 (1.5) 60.8% 51.4 (1.5) 56.4% 51.4 (1.5) 100%

Mg outer ring − − 50.9 (1.4) 39.2% 50.9 (1.4) 43.6% −

a The Mg relative percentages of the different components are indicated as well as the full width at half maximum values in parentheses.

The Mg 2p core peaks recorded after insertion of 0.5 and 1 magnesium ion show one peak assigned to Mg2+ oxidation state. Upon Mg insertion, up to 2 magnesium ions, the Mg 2p shows an asymmetry toward lower binding energies assigned to a reduced state of Mg ions. The XPS Mg 2p core peaks evolution highlights two different environments for the inserted magnesium in the Chevrel phase Mo6S8. In the Mg insertion range between 0 < x < 1, the Mg 2p peak binding energy at 51.4 eV (Table 1), close to that reported for MgO reference sample (51.6 eV) (Figure S1), emphasizes that Mg ions are in a polarized environment. The second peak which appears when x > 1 is assigned to the magnesium in a metallic-like environment (50.9 eV) (Table 1) close to the binding energy of Mg 2p of Mg metal (50.3 eV) (Figure S1). Levi et al.27 using the potential intermittent titration technique (PITT) highlighted the slower insertion kinetic of the first inserted Mg ion compared to the second one, which seems to support our observation. The change in Mg ion polarization environment induces different insertion kinetics. At the end of the first cycle (Figure 4), the insertion/deinsertion process seems to be reversible, only few atomic percent of Mg remain trapped in the Mo6S8 host structure. According to previous works13 the cavities 1 and 2 can host six Mg insertion sites each (see Figure 5a), which corroborate

Figure 5. Structure of the Chevrel phase Mg2Mo6S8 after Mg insertion: (a) the three types of cavities (1, 2, and 3) with the magnesium insertion sites and (b) projection on the perpendicular plan of the 3-coordinated sulfurs axis. D


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The delay of the charge transfer to Mo orbitals, regarding the Mo 3d peak evolution during the first discharge, was investigated by probing the S 2p core peaks registered at the same stages of magnesiation. The S 2p3/2−1/2 XPS core peak of pristine Mo6S8 was fitted using three doublets (Figure 7): • a doublet named 4-coordinated S2‑ at binding energy of 162.3−163.4 eV, attributed to sulfur with an oxidation degree of S2− and coordinated by four molybdenum atoms (three from the same Mo6 cluster and one from neighboring cluster), labeled peripheral sulfurs • a doublet named 3-coordinated S2‑ at binding energy of 161.5−162.7 eV, attributed to sulfur with an oxidation degree of S2− and coordinated by three molybdenum atoms as close neighbors belonging to the same Mo6 cluster, labeled axial sulfurs • a doublet at higher binding energies (164.2−165.3 eV), assigned to nonstoichiometric sulfur atoms from the extreme surface. This can find origin in the instable electronic structure of Mo6S8 that lacks four electrons to fill its valence band. Therefore, the sulfurs from the extreme surface may bond together to get more stable surface electronic structure. Similar behavior was detected through S 2p3/2−1/2 core peak study of the influence of cation such as W and Ti on high sulfur content oxysulfide thin films.22 During Mg2+ ions insertion, the evolution of the S 2p core peaks occurs in two steps. At the beginning of the magnesiation, from x = 0 to 0.5 inserted Mg ion, the 3coordinated S2‑ S 2p3/2−1/2 peak contribution decreases to the benefit of 4-coordinated S2‑ S 2p core level peak, whereas the nonstoichiometric sulfur related doublet disappears. Hence, from x = 0.5 to 2 inserted Mg ions, the S 2p3/2−1/2 core peak shape remains unchanged; only the doublet of 4-coordinated S2‑ was evidenced. At the end of the first cycle, the S 2p3/2−1/2 core peak spectrum is similar to the one of the pristine electrode indicating a reversible redox process involving sulfur atoms. Regarding the Mo 3d and S 2p core level evolution during the first cycle, the magnesiation process occurs in two steps. During the magnesiation of cavity 1 (0 < Mg < 1), the charge transfer involves mainly sulfur atoms. A coupled study based on density functional theory simulation and X-ray absorption near edge spectroscopy evidenced the electron transfer to sulfur atoms during Chevrel host intercalation.17 It was reported that the cation inside the cavity 1 creates strong bonds with 3coordinated sulfurs whereas the interactions with 4-coordinated sulfurs are weak.12 The redox reaction may occur at the anions instead of the transition metals when the anion p-states arise to the top of the valence band and become dominant.30,31 During the second insertion plateau (1 < Mg < 2), the cavities 2 are filled involving a reduction of Mo3+ to Mo2+ giving rise to symmetric Mo6 octahedron rearrangement. The Mo6 cluster can easily accommodate the four electrons required to compensate the two Mg2+ by modifying their Mo−Mo interatomic distances.18 The relative high mobility of Mg ions in this range of galvanostatic magnesiation is attributed to the fast redistribution of electronic charge over the Mo6 cluster. Therefore, unlike the conventional metal ligand structure (analogous to transition metal oxide), the metal cluster shows nonconventional metal transition redox reaction. The intercalation of Mg ions engenders at first a charge transfer to sulfur

Figure 6. XPS characterization of Mg KLL Auger transition of different MgxMo6S8 electrodes and magnesium foil with native MgO layer as reference.

The Mo 3d5/2−3/2 core peaks of pristine Chevrel phase Mo6S8 can be fitted with two asymmetric doublets, assigned to the formal molybdenum oxidation states Mo3+ and Mo2+ in sulfur environment at binding energies of 228.5−231.7 and 229.2− 232.4 eV, respectively (Table 2). A third doublet positioned at a binding energy of 233.3− 236.5 eV (Table 2) is attributed to Mo6+ oxidation state in oxygen environment due to surface oxidation during sample manipulation in the air atmosphere, this contribution disappears during cycling. Upon magnesiation, up to Mg1Mo6S8, there is no evolution on the Mo 3d5/2−3/2 core peaks signature. Once the second plateau is reached (x > 1), a reduction of Mo3+ into Mo2+ is observed, the Mo3+ peak intensity decreases whereas the Mo2+ peak increases. The Mo 3d XPS spectrum recorded after one cycle is similar to that observed in the case of pristine electrode indicating a reversible redox process. E


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Figure 7. XPS Mo 3d and S 2p core peaks evolution of the Chevrel phase Mo6S8 registered at different stages of the first discharge and charge.

different polarizations. The S 2p and Mo 3d core level evolutions confirmed that the Mg insertion is a two-step process. During the first galvanostatic plateau, the electronic charge is redistributed toward axial sulfurs whereas the Mo6 cluster remains inactive. During the second plateau, the charge transfer involves Mo6 cluster redox center to stabilize the electron deficient Mo6S8 structure. The reversibility of the charge transfer process was evidenced through XPS core level analysis at the end of the charge. Our result reconfirms that Chevrel structure displays a reversible redox process even if it provides insufficient energy densities for commercial applications. This partial charge transfer inside the anionic framework and the capacity to delocalized charges inside the Mo6 cluster is an answer to the capacity of the Chevrel phase to insert/deinsert rapidly and reversibly divalent ions such as Mg2+. The methodology followed in this paper will be extended to other Chevrel phases like Mo6Se8 to have a better understanding of the anionic framework role.

atoms and then shifts the Fermi level of Mo6S8 into its pseudo gap, inducing metal to semiconductor transition. This type of redox mechanisms in two steps, where the transition metal and the anion participate to the charge transfer are unconventional, but already observed in the case of lithium insertion in titanium oxysulfide thin films.22 On the light of these results, the different mobilities of divalent Mg ion in Mo6S8, within the first and second plateau, may find origin in the polarization effects induced by sulfur anions. Once the Mg inner ring is formed, the activation energy of Mg ion seems to be lower allowing an increase of its mobility.27

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CONCLUSION

To conclude, we have used XPS to study the electronic structure change of the Mo 6 S 8 Chevrel phase upon galvanostatic magnesiation. The Mg 2p core peak and Mg KLL Auger transition evolution evidenced the presence of two magnesium insertion sites, inner and outer rings, having F


164.3 (0.76)−165.4 (0.76) 7.7%

−

−

−

S nonstoichiometric

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03979. XPS analysis of Mg 2p core peak of Mg foil with MgO native layer as reference and XPS analysis of S 2p and Mo 3d core peaks of the commercial precursor MoS2 as reference (PDF)

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162.3 (0.86)−163.4 (0.86) 59.4% 162.3 (1.2)−163.5 (1.2) 100% 162.3 (1.2)−163.5 (1.2) 100% 162.3 (1.2)−163.5(1.2) 100% 162.3 (1.2)−163.5 (1.2) 100% 162.3 (0.93)−163.4 (0.93) 76.1%

4-coordinated S2−

S 2p3/2−1/2

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AUTHOR INFORMATION

Corresponding Author

*(A.B.) E-mail: [email protected]. ORCID

A. Benayad: 0000-0002-8854-1105 Notes

ACKNOWLEDGMENTS

This work was entirely supported by the CEA-LITEN.

The relative percentages of the different doublets are indicated as well as the full width at half maximum values in parentheses.

Mg0.1Mo6S8 after discharge

Mg2Mo6S8

Mg1.5Mo6S8

Mg1Mo6S8

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REFERENCES

(1) Yoshino, A.; Sanechika, K.; Nakajima, T. Secondary Battery. U.S. Patent 4,668,595 A, 1986. (2) Larcher, D.; Tarascon, J.-M. Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2014, 7, 19−29. (3) Canepa, P.; Sai Gautam, G.; Hannah, D. C.; Malik, R.; Liu, M.; Gallagher, K. G.; Persson, K. A.; Ceder, G. Odyssey of Multivalent Cathode Materials: Open Questions and Future Challenges. Chem. Rev. 2017, 117, 4287−4341. (4) Pereira-Ramos, J. P.; Messina, R.; Perichon, J. Electrochemical Formation of a Magnesium Vanadium Bronze MgxV2O5 in SulfoneBased Electrolytes at 150°C. J. Electroanal. Chem. Interfacial Electrochem. 1987, 218, 241−249. (5) Mizuno, Y.; Okubo, M.; Hosono, E.; Kudo, T.; Oh-ishi, K.; Okazawa, A.; Kojima, N.; Kurono, R.; Nishimura, S.; Yamada, A. Electrochemical Mg2+ Intercalation into a Bimetallic CuFe Prussian Blue Analog in Aqueous Electrolytes. J. Mater. Chem. A 2013, 1, 13055. (6) Arthur, T. S.; Zhang, R.; Ling, C.; Glans, P.-A.; Fan, X.; Guo, J.; Mizuno, F. Understanding the Electrochemical Mechanism of KαMnO2 for Magnesium Battery Cathodes. ACS Appl. Mater. Interfaces 2014, 6, 7004−7008. (7) Rasul, S.; Suzuki, S.; Yamaguchi, S.; Miyayama, M. Synthesis and Electrochemical Behavior of Hollandite MnO2/acetylene Black Composite Cathode for Secondary Mg-Ion Batteries. Solid State Ionics 2012, 225, 542−546. (8) Kumagai, N.; Komaba, S.; Sakai, H.; Kumagai, N. Preparation of Todorokite-Type Manganese-Based Oxide and Its Application as Lithium and Magnesium Rechargeable Battery Cathode. J. Power Sources 2001, 97−98, 515−517. (9) Mori, T.; Masese, T.; Orikasa, Y.; Huang, Z.-D.; Okado, T.; Kim, J.; Uchimoto, Y. Anti-Site Mixing Governs the Electrochemical Performances of Olivine-Type MgMnSiO4 Cathodes for Rechargeable Magnesium Batteries. Phys. Chem. Chem. Phys. 2016, 18, 13524− 13529. (10) Orikasa, Y.; Masese, T.; Koyama, Y.; Mori, T.; Hattori, M.; Yamamoto, K.; Okado, T.; Huang, Z.-D.; Minato, T.; Tassel, C.; et al. High Energy Density Rechargeable Magnesium Battery Using EarthAbundant and Non-Toxic Elements. Sci. Rep. 2015, 4, 5622. (11) Chevrel, R.; Sergent, M.; Prigent, J. Un Nouveau Sulfure de Molybdene: Mo3S4 Préparation, Propriétés et Structure Cristalline. Mater. Res. Bull. 1974, 9, 1487−1498.

a

161.5 (0.86)−162.7 (0.86) 16.2% − 229.2 (1.0)−232.4 (1.2) 29.3%

− −

(1.2)

− (1.1)

(1.2)

−

161.5 (0.75)−162.7 (0.75) 14.4% −

233.3 (1.2)−236.5 (2) 5.3% 233.4 (1.2)−236.6 (1.2) 2.0% 233.3 (1.2)−236.5 (1.2) 2.3% −

229.2 (1.1)−232.4 23.9% 229.2 (1.1)−232.4 26.4% 229.2 (1.0)−232.4 29.5% 229.3 (1.0)−232.5 23.7% − Mg0.5Mo6S8

Mo

228.5 (0.7)−231.7 (0.9) 70.8% 228.5 (0.8)−231.7 (1.0) 71.6% 228.5 (0.9)−231.7 (1.0) 68.2% 228.5 (0.74)−231.7 (1.0) 76.3% 228.5 (0.74)−231.7 (1.0) 100% 228.5 (0.7)−231.7 (0.9) 70.7% Mo6S8 pristine

Mo

3+

Mo 3d5/2−3/2

(1.2)

Mo

6+

3-coordinated S

2−

The authors declare no competing financial interest.

2+

Table 2. XPS Mo 3d5/2−1/2 and S 2p3/2−1/2 Binding Energies (eV) in MgxMo6S8 Electrodes at Different Stages of the First Cyclea

164.2 (1.0)−165.3 (1.0) 26.2% −


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Charge Transfer Mechanism into the Chevrel Phase Mo6

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