Chevrel Phase Research Articles

Charge Transfer Mechanism into the Chevrel Phase Mo6S8 during Mg Intercalation

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.

Chevrel Phase Mo6 T8 (T = S, Se) as Electrodes for Advanced Energy Storage.

With the large-scale applications of electric vehicles in recent years, future batteries are required to be higher in power and possess higher energy densities, be more environmental friendly, and have longer cycling life, lower cost, and greater safety than current batteries. Therefore, to develop alternative electrode materials for advanced batteries is an important research direction. Recently, the Chevrel phase Mo6 T8 (T = S, Se) has attracted increasing attention as electrode candidate for advanced batteries, including monovalent (e.g., lithium and sodium) and multivalent (e.g., magnesium, zinc and aluminum) ion batteries. Benefiting from its unique open crystal structure, the Chevrel phase Mo6 T8 cannot only ensure rapid ion transport, but also retain the structure stability during electrochemical reactions. Although the history of the research on Mo6 T8 as electrodes for advanced batteries is short, there has been significant progress on the design and fabrication of Mo6 T8 for various advanced batteries as above mentioned. An overview of the recent progress on Mo6 T8 electrodes applied in advanced batteries is provided, including synthesis methods and diverse structures for Mo6 T8 , and electrochemical mechanism and performance of Mo6 T8 . Additionally, a briefly conclusion on the significant progress, obvious drawbacks, emerging challenges and some perspectives on the research of Mo6 T8 for advanced batteries in the near future is provided.

Systematic Investigation into Mg2+/Li+ Dual-Cation Transport in Chevrel Phases Using Computational and Experimental Approaches

Computational and experimental investigations into the Li+, Mg2+, and Mg2+/Li+ dual-cation transport properties within the Chevrel phase Mo6S8 have been performed. Five representative paths were selected for 3D diffusion, and their corresponding energy barriers were determined. Based on density functional theory calculation results, we reveal phenomena of the cation trapping, sluggishness of Mg2+ ion transport, and synchronized movement of inserted cations induced by repulsive interactions. The computational results were further validated by cyclic voltammetry carried out at ambient to high temperatures, from which apparent diffusion constants and activation energies for each case were determined. We found broad agreement between the theoretical and experimental results and suggest an optimum scenario for charge–discharge processes within the dual-cation hybrid system.

XPS Comparative Analysis of the Chevrel Phase Mo6S8 and the Analogous Mo6Se8 during Mg Insertion

Since the earlier works in 2000 by Aurbach et al 1, magnesium-ion batteries emerged as an alternative to lithium-ion batteries thanks to many advantages: magnesium is the 5th most abundant metal in the earth crust, its divalent character and its ionic radius similar to lithium ionic radius gives it a volumetric capacity higher than lithium (3833 Ah/L and 2046 Ah/L respectively). Eventually the absence of dendritic growth on metallic magnesium is a plus in term of security. Several cathode materials were proposed for Mg-ions battery, nevertheless the reference positive electrode material in Mg-Ion technology remained the Chevrel Phase with the formula Mo6S8. The Chevrel phase, synthesized by R. Chevrel et al 2 in 1974, was described as an octahedral cluster of molybdenum located in a cube of sulfur that forms a Mo6S8 pattern. Structural analysis3 (X-Ray Diffraction) showed the existence of 12 insertions sites localized inside a cube of sulfur surrounded by Mo6S8 clusters. The electrochemistry highlighted 2 types of insertion sites able to host Mg2+ at different potentials. Mg trapping into the Chevrel Phase was also noticed but curiously this phenomenom doesn’t occur in the analogous phase Mo6Se8 4 thanks to the lower ionicity of Se compared to S. The anionic framework plays a major role in the redox process during Mg insertion, this study is a comparative analysis of Mo6S8 and analogous Mo6Se8 using electrochemistry and X-ray Photoemission Spectroscopy to get a direct observation of the redox mecanism during charge/discharge of the material. We performed ex situ XPS core peak analyses on Mo6S8 and Mo6Se8 analogous electrodes at different states of charge to probe the molybdenum, the sulfur and the selenium oxydation states and the environment (polarized or metallic) of inserted magnesium ions. These results will be discussed based on a step-by-step XPS study before and after cycling, and will be correlated with the electrochemical performances.

Transition Metal Sulfides As Cathode Materials for Rechargeable Aluminum Ion Battery

Rechargeable aluminum battery system is very intriguing due to the following reasons: First of all, aluminum has high capacity due to its trivalency. And aluminum is very cheap since it is the most abundant metal element in earth’s crust.1 As a result, rechargeable aluminum battery can be very promising in large scale energy storage application. One of the main reasons that hinders the development of rechargeable aluminum battery is the lacking of electrolyte that can enable facile deposition and dissolution of aluminum in the anode side. On the other hand, facile electrochemical deposition and dissolution of aluminum can be achieved in room temperature ionic liquid (molten salt) synthesized by mixing aluminum chloride (AlCl3) with organic salts such as 1-butylpyridinium chloride, 1-ethyl-3-methylimidazolium chloride, etc. at a certain ratio.2,3 In a previous research, our group proposed Chevrel phase Mo6S8 as the first conventional intercalation type cathode material.4 The logic of choosing transition metal sulfide instead of transition metal oxide as cathode material for aluminum ion battery is very important. Due to the strong coulombic effect, the energy barrier of multivalent ions transportation in the crystal structure is very high.5 Thus, a softer anionic framework is needed. Sulfide has a much lower electronegativity than oxide, which makes transition metal sulfide a very promising cathode candidate for rechargeable aluminum ion battery. To further validate this assumption, we synthesized nano sized layered TiS2 and cubic Ti2S4 as well as investigated their electrochemical properties as cathode materials for ionic liquid electrolyte based rechargeable aluminum ion battery at both room temperature and 50 °C. We further confirmed the aluminum intercalation in the TiS2 and Ti2S4 crystal structure using ex-situ XRD and XPS. The proposed titanium sulfide cathode materials showed decent reversible capacity and a higher working potential. In order to further understand the mechanism of Al intercalation into Chevrel Phase Mo6S8, high resolution TEM is used to directly visualize the crystal structure of AlxMo6S8 from different discharged and charged stages. Powder XRD Rietveld refinement is also undertaken to provide support from a theoretical simulation perspective. 1. Li, Q.; Bjerrum, N. J. J. Power Sources 2002, 110, 1. 2. Endres, F. ChemPhysChem 2002, 3, 144. 3. Jiang, T.; Chollier Brym, M. J.; Dube, G.; Lasia, A.; Brisard, G. M. Surf. Coat. Technol. 2006, 201, 1. 4. Geng, L.; Lv, G.; Xing, X. Guo, J. 2015, 27, 4926−4929. 5. Rong, Z.; Malik, R.; Canepa, P.; Sai Gautam, G.; Liu, M.; Jain, A. Persson, K.; Ceder, G. Chem. Mater., 2015, 27 (17), pp 6016–6021.

Battery Functionality and Reactivity of a Cathode for Magnesium Ion Battery

Post lithium ion battery is highly demanding energy storage for smaller scale as stationary use to greater scale as grid storage, and vehicle use as well. Magnesium ion battery with metal magnesium as an anode has been investigated over decades because of its high theoretical capacity of magnesium (3833 mAh cm3) and relatively lower potential (0.7 V vs lithium) for naturally abundant mineral resources in “multivalent” metals with less risk on forming dendritic deposits [1]. After over a decade of finding of stable and reversible electrolyte based on Grignard agent such as APC [2], our finding of magnesium monocarborane [3] opened next door toward high energy density by its wider potential window up to 3.7 V and high current density more than 1mS cm-1, without corrosive issue which is intrinsic to conventional electrolytes containing chlorine. Although of many efforts for electrolytes and electrodes, the battery system has lack of cathode capable of providing high potential and / or capacity with stable performance over numbers of cycles in charge and discharge, to takeover Li ion battery. Recently some challenges proved custom approach for magnesium ion effective to certain extent; eg. C60 [4] works as a mimic of cluster aspect of highly reversible Chevrel Mo6S8, and spinel Ti2S4 [5] presents the design of host framework with “soft” anion and ionic path. In this study we approach to new insights into active materials by means of charge transfer. Interfacial analysis such as electrochemical impedance, XPS, etc., to the most established cathode, Chevrel phase Mo6S8 [1] in contact with MMC electrolyte gives some key factors of highly reversible cathode. Besides, our new approach for a cathode active material and its battery performance will be introduced for the next step of the reaction design.

Thermodynamic Origin of Irreversible Magnesium Trapping in Chevrel Phase Mo6S8: Importance of Magnesium and Vacancy Ordering

The rechargeable magnesium battery is an alternative to current Li-ion technologies, potentially offering a higher energy density by employing metal magnesium as an anode material. The realization of a magnesium battery, however, is hampered by the barrier to finding a cathode material that reversibly stores and releases Mg ions through electrochemical intercalation. Understanding the underlying mechanism that prevents successful Mg intercalation is now crucial for the design and exploration of new cathode candidates. Our work reports the critical effect of a thermodynamic factor, the ordering of Mg and interstitial vacancies, on the activity of a Chevrel phase Mo6S8 cathode. Specifically, we demonstrate that the irreversible trapping in the Chevrel phase at low Mg concentrations is thermodynamically driven by the ordering of Mg and vacancies instead of being kinetic in origin. Through a combination of nudged elastic band, geometry-confined static calculation, and ab initio molecular dynamics methods, we ...

Non-Grignard and Lewis Acid-Free Sulfone Electrolytes for Rechargeable Magnesium Batteries

A major challenge for developing rechargeable Mg-ion batteries (MIB) is the lack of suitable electrolytes. We report herein dialkyl sulfones as non-Grignard and Lewis acid-free MIB electrolytes. In particular, a dipropyl sulfone (DPSO)/tetrahydrofuran (THF) (1/1, v/v) solution with MgCl2 salt exhibits high ionic conductivity (1.1 mS cm–1 at 30 °C), Mg cycling efficiency (>90%), and anodic stability (ca. 3.0 V vs Mg). As evidenced by single crystal X-ray diffraction analysis, a novel [Mg(DPSO)6]2+ cation complex balanced by two [MgCl3(THF)]− anions is identified in the DPSO/THF solution. The DPSO/THF electrolyte also enables excellent cycle performance (>300 cycles) of a Chevrel phase Mo6S8 cathode and displays a decent compatibility with an organic cathode (3,4,9,10-perylenetetracarboxylic dianhydride, PTCDA). Along with the superior electrochemical properties of the DPSO/THF electrolyte, its innate chemical stability and eco-friendly nature make it a promising MIB electrolyte.

Direct Investigation of Mg Intercalation into the Orthorhombic V2O5 Cathode Using Atomic-Resolution Transmission Electron Microscopy

Batteries based on Mg metal anode can promise much higher specific volumetric capacity and energy density compared to Li-ion systems and are, at the same time, safer and more cost-effective. While previous experimental reports have claimed reversible Mg intercalation into beyond Chevrel phase cathodes, they provide limited evidence of true Mg intercalation other than electrochemical data. Transmission electron microscopy techniques provide unique capabilities to directly image Mg intercalation and quantify the redox reaction within the cathode material. Here, we present a systematic study of Mg insertion into orthorhombic V2O5, combining aberration-corrected scanning transmission electron microscopy (STEM) imaging, electron energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDX) analysis. We compare the results from an electrochemically cycled V2O5 cathode in a prospective full cell with Mg metal anode with a chemically synthesized MgV2O5 sample. Results suggest that the electroche...

Effect of 1-allyl-1-methylpyrrolidinium chloride addition to ethylmagnesium bromide electrolyte on a rechargeable magnesium battery

The ionic liquid 1-allyl-1-methylpyrrolidinium chloride was investigated as an additive to an ethylmagnesium bromide Grignard solution, for use as an advanced electrolyte in rechargeable Mg batteries. Electrochemical studies showed that the addition of the ionic liquid significantly enhanced the anodic limit of the Grignard solution by about 1V and improved the coulombic efficiency and the kinetics of Mg deposition-stripping reactions on the negative electrode. We have also demonstrated that the Grignard solution containing the ionic liquid works well as an electrolyte in a prototype cell consisting of a Mo6S8 Chevrel phase cathode and a Mg metal anode, where the original Grignard electrolyte does not function properly.

Hybrid density functional theory modeling of Ca, Zn, and Al ion batteries using the Chevrel phase Mo6S8 cathode.

Hybrid density functional theory (DFT) is used to study the Chevrel phase Mo6X8 (X = S, Se, Te) as a promising cathode material intercalated with various metal ions (M = Li, Na, Be, Mg, Ca, Sr, Ba, Zn, Al). Electronic properties and voltages are calculated for each case. Ca ions are predicted to produce a voltage output ranging from 1.8-2.1 V, comparable to the voltage calculated for Li ions while providing two electrons per transferred ion. The highest voltage is determined to result when the chalcogen X in Mo6X8 is S, over Se or Te. Additionally, a comparison of the local-density approximation (LDA), the Perdew-Burke-Ernzerhof (PBE), the Hubbard U corrected GGA-PBE (PBE+U), the meta-GGA modified Becke-Johnson (mBJ), and the hybrid Heyd-Scuseria-Ernzerhof (HSE) functionals are made. The electronic structure determined with HSE is taken as the most reliable, and PBE and LDA can provide reasonable approximations. The PBE+U approach yields an erroneous band gap and should be avoided. The voltages calculated with HSE are in excellent agreement with available experimental data.

ChemInform Abstract: Express and Low‐Cost Microwave Synthesis of the Ternary Chevrel Phase Cu2Mo6S8 for Application in Rechargeable Magnesium Batteries.

AbstractCu2Mo6S8 is prepared by solid state microwave‐assisted reaction of MoS2, Mo, and Cu in stoichiometric quantities using a graphitized quartz tube in a carbon powder filled Al2O3 crucible covered by mineral wool (1200 W, 400 s) followed by rapid cooling in H2O.

Magnesium Insertion into the Chevrel Phase Mo6S8: XPS Analysis

Since the earlier works in 2000 by Aurbach et al 1, magnesium-ion batteries emerged as an alternative to lithium-ion batteries thanks to many advantages: magnesium is the 5th most abundant metal in the earth crust, its divalent character and its ionic radius similar to lithium ionic radius gives it a volumetric capacity higher than lithium (3833 Ah/L and 2046 Ah/L respectively). Eventually the absence of dendritic growth on metallic magnesium is a plus in term of security. Several cathode materials were proposed for Mg-ions battery, nevertheless the reference positive electrode material in Mg-Ion technology remained the Chevrel Phase with the formula Mo6S8. The Chevrel phase, synthesized by R. Chevrel et al 2 in 1974, was described as an octahedral cluster of molybdenum located in a cube of sulfur that forms a Mo6S8 pattern. Structural analysis3 (X-Ray Diffraction) showed the existence of 12 insertions sites localized inside a cube of sulfur surrounded by Mo6S8clusters as illustrated in Fig. 1. The electrochemistry highlighted 2 types of insertion sites able to host Mg2+at different potentials. Being considered as the reference cathode material for Mg-ion systems, it has been extensively studied, especially on the structural and electrochemical point of view. However, the precise red-ox processes taking place during Mg insertion-deinsertion remain unclear. In this study X-ray Photoemission Spectroscopy (XPS) has been used to get a direct observation of the redox mecanism during charge/discharge of the material. We performed ex-situ XPS core peak analyses on Mo6S8electrodes at different states of charge to probe the molybdenum and sulfur oxydation state and the environment (polarized or metallic) of inserted magnesium ions. These results were discussed based on a step-by-step XRD and XPS study before and after cycling, and correlated with the electrochemical performance of Chevrel phase. Fig 2 illustrates Mo 3d and Mg 2p orbitals in Mg1.72Mo6S8. In one side we can see the evolution of molybdenum oxidation states during discharge and on the other side the two environments of inserted magnesium is underlined. References : (1) D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich & E. Levi, Nature vol 407 (2000) 724 (2) R. Chevrel et al, Mat. Res. BuLl. Vol. 9, pp. 1487-1498, 1974M. D. Levi, E. Lancry, H. Gizbar, Z. Lu, E. (3) R. Chevrel, M. Sergent Superconductivity in Ternary Compounds I Volume 32 of the series Topics in Current Physics pp 25-86 Figure 1

Diffusion Measurements of Mg in High Capacity Thiospinel MgxTi2S4

The demand for smaller batteries capable of storing the same amount of energy as conventional Li-ion technology has led to the pursuit of several new technologies including rechargeable Mg batteries. Mg metal is attractive as a negative electrode material because it has a higher volumetric capacity density (3833 mAh/mL) than Li metal (2062 mAh/mL), is the 8th most abundant element in the earth’s crust, is safe to handle in ambient atmosphere, and can be electrodeposited (charged) without the formation of dendrites.1 The seminal work by Aurbach et al. in 20002 established an electrolyte and a positive electrode material, the Chevrel phase (Mo6S8), that was paired with Mg metal to form the first rechargeable Mg battery. Mg2+ intercalation in host materials is more difficult than that of Li+ or Na+, displaying lower ion mobility in solid oxide hosts3 and a probable higher desolvation energy penalty.4 No further positive electrode materials with both notable capacity and cycle life have been demonstrated since the Chevrel phase, until now. In this presentation, we will demonstrate that the thiospinel Ti2S4 reversibly intercalates Mg2+ with a 2ndcycle capacity of about 165 mAh/g, which drops to only 140 mAh/g after 40 cycles at C/10 as shown in Figure 1. Of crucial scientific importance is that Ti2S4 provides a second example of a material that supports facile Mg2+ diffusion, which could help elucidate why Mg2+ intercalation is so difficult in other potential cathode materials. In exploring Mg2+ diffusion, the first step is to measure the chemical diffusion coefficient, D. The galvanostatic intermittent titration technique (GITT)5 is a versatile method of carrying this out for an intercalant like Mg2+ if a reliable cell can be constructed that has a long enough diffusion length to produce a linear potential vs time response reflecting Fick’s laws of diffusion. Figure 2 shows a typical GITT experiment on thiospinel MgxTi2S4, which displays the required potential vs time response. This talk will elaborate on the results of the diffusion measurements we have performed and compare those results to theory. References J. Muldoon, C. B. Bucur, and T. Gregory. Chem. Rev. 114, 11683-11720 (2014).D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi. Nature 407, 724-727 (2000).E. Levi, Y. Gofer, and D. Aurbach. Chem. Mater. 22, 860-868 (2010).L. F. Wan, B. R. Perdue, C. A. Apblett and D. Prendergast. Chem. Mater. 27, 5932-5940 (2015).W. Weppner and R. A. Huggins. Solid-State Science and Technology 124, 1569-1578 (1977). Figure 1

Organic Cathodes for Energy Storage Systems

Redox-active organic molecules have the potential as economical and environmentally benign cathodes for lithium ion batteries. The same concept of using organic molecules as cathodes was extended to magnesium ion batteries, however, pervious work showed considerable capacity loss upon the cycling. Furthermore, no compatible electrolytes toward both anode and cathode have been demonstrated. Her, we have focused on several organic cathode materials as cathodes, mainly for magnesium ion batteries. For example, 2,5-dimethoxy-1,4-benzoquinone (DMBQ) was reinvestigated as a cathode material with magnesium electrolytes that are capable of plating/stripping magnesium for rechargeable magnesium-ion batteries. Two electrolytes, the magnesium bis(trifluoromethylsulfonyl)imide mixed with MgCl2 in dimethoxyethane (Mg(TFSI)2-2MgCl2 in DME) electrolyte, and the Mg(TFSI)2 in diglyme were selected. The Mg(TFSI)2-2MgCl2 in DME enabled Mg-DMBQ batteries with a discharge potentials above 2.0 V vs Mg/Mg2+, which is superior to the previous reported potential in Mg-DMBQ batteries with conventional magnesium salt-based electrolytes (1.1 V vs Mg/Mg2+ ), and also excels the well-known Chevrel phase Mo6S8 in magnesium-ion batteries (1.2 V vs Mg/Mg2+). The other organic cathodes explored for magnesium ion batteries are the redox active polymers. With emphasis on the structure motif, several polymers were investigated as high performance cathode candidates for magnesium storage. Excellent electrochemical reversibility was established for polymers within the range of 1.5-2.0 V (vs Mg), with a 99% capacity retention in 100 cycles (104.9 mAh g-1 at 100th cycle). The superior cycling stability was further demonstrated by 1000 discharge-charge cycles with very small amount of capacity loss, highlighting the great potential of redox-active organic polymer cathodes for advanced Mg-ion batteries. Figure 1

Express and low-cost microwave synthesis of the ternary Chevrel phase Cu2Mo6S8 for application in rechargeable magnesium batteries

The ternary Chevrel phase Cu2Mo6S8 was successfully synthetized using a simple and cost-effective solid-state microwave-assisted reaction. While solid-state routes require days of high-temperature treatment under inert atmosphere, highly pure and crystalline Cu2Mo6S8 could be obtained in only 400s from this precursor, the Chevrel binary phase Mo6S8 was then obtained by copper removal through acidic leaching, and was evaluated as a positive electrode material for Mg-battery. The electrochemical performance in half-cell configuration shows reversible capacity exceeding 80mAh/g, which is comparable to previous works carried out with materials synthesized by conventional high-temperature solid-state routes.

Layered TiS2 Positive Electrode for Mg Batteries

Magnesium batteries are a good candidate for high energy storage systems, but the limited discovery of functional positive electrode materials beyond the seminal Chevrel phase (Mo6S8) has slowed their development. Herein, we report on layered TiS2 as a promising positive electrode intercalation material, providing 115 mAh g–1 stabilized capacity in a Mg full cell. Reversible Mg2+ intercalation into the structure is proven by elemental analysis combined with X-ray diffraction studies that elucidate the phase behavior upon cycling. The voltage profiles reveal distinct Mg2+ cation ordering, unlike the solid solution behavior exhibited by Li+. Our findings not only point to the important role of “soft” lattices to facilitate divalent solid-state cation mobility but also now provide an alternative sulfide to serve as a platform for the fundamental understanding of Mg2+ intercalation in lattices.

Role of Chloride for a Simple, Non-Grignard Mg Electrolyte in Ether-Based Solvents.

Mg battery operates with Chevrel phase (Mo6S8, ∼1.1 V vs Mg) cathodes that apply Grignard-based or derived electrolytes, which allow etching of the passivating oxide coating forms at the magnesium metal anode. Majority of Mg electrolytes studied to date are focused on developing new synthetic strategies to achieve a better reversible Mg deposition. While most of these electrolytes contain chloride as a component, and there is a lack of literature which investigates the fundamental role of chloride in Mg electrolytes. Further, ease of preparation and potential safety benefits have made simple design of magnesium electrolytes an attractive alternative to traditional air sensitive Grignard reagents-based electrolytes. Work presented here describes simple, non-Grignard magnesium electrolytes composed of magnesium bis(trifluoromethane sulfonyl)imide mixed with magnesium chloride (Mg(TFSI)2-MgCl2) in tetrahydrofuran (THF) and diglyme (G2) that can reversibly plate and strip magnesium. Based on this discovery, the effect of chloride in the electrolyte complex was investigated. Electrochemical properties at different initial mixing ratios of Mg(TFSI)2 and MgCl2 showed an increase of both current density and columbic efficiency for reversible Mg deposition as the fraction content of MgCl2 increased. A decrease in overpotential was observed for rechargeable Mg batteries with electrolytes with increasing MgCl2 concentration, evidenced by the coin cell performance. In this work, the fundamental understanding of the operation mechanisms of rechargeable Mg batteries with the role of chloride content from electrolyte could potentially bring rational design of simple Mg electrolytes for practical Mg battery.

Understanding the Good Kinetics of Mo6S8 As Cathode in Mg Ion Batteries By Key Electronic States

Mg rechargeable batteries are being considered as one of the most prospective alternatives for Li rechargeable batteries because of their merits including Mg abundance, absence of Mg dendrite and high theoretical specific capacity attributed to two-electron charge transfer reaction process. [1] However，the achievable performance of Mg rechargeable batteries is still limited regarding their practical application. The key limitation to this lies in cathode. To date, Chevrel phase Mo6(SxSe1-x)8 is the only cathode material in which Mg2+ can be efficiently intercalated/deintercalated. Extensive theoretical and experimental studies have been done to screen out promising cathode materials by constructing the crystal structure similar to Chevrel phase. However, successful research has been rarely reported. It indicates that only taking into account of the crystal structure is not sufficient enough to explore the Mg storage/transport behavior in the electrode. Given the facts that the two-electron charge transfer process was involved in Mg batteries while the great success has been made in the studies on lithium ion batteries through the key electronic states [2-5], the evolution of electronic structure in Chevrel phase might provide more important information to elucidate the physical nature of Mg storage/transport in the electrode. However, the experimental study on the evolution of electronic structure in Chevrel phase cathode is rare. In this report, the study on the unoccupied states of a typical Chevrel phase Mo6S8 including Mo L-edge and S K-edge were conducted by tender X-ray absorption spectroscopy. In contrast with the conventional understanding that only Mo cation is involved in the charge transfer process, the key electronic states at the pre-edge of S spectra were also found to be evolved regularly with different states of charge/discharge. It is assigned to the hybridization between Mo 4d states and S 3p states. The evolution of this state opens a gate to reveal the nature of good kinetics in Chevrel phase. 1. Aurbach, D., et al., Prototype systems for rechargeable magnesium batteries. Nature, 2000. 407(6805): p.724-727.2. Liu, G., et al., Polymers with Tailored Electronic Structure for High Capacity Lithium Battery Electrodes. Advanced Materials, 2011. 23(40): p. 4679-4683.3. Liu, X., et al., Phase Transformation and Lithiation Effect on Electronic Structure of LixFePO4: An In-Depth Study by Soft X-ray and Simulations. Journal of the American Chemical Society, 2012. 134(33): p. 13708-13715.4. Yang, W., et al., Key electronic states in lithium battery materials probed by soft X-ray spectroscopy. Journal of Electron Spectroscopy and Related Phenomena, 2013. 190, Part A(0): p. 64-74.5. Liu, X., W. Yang, and Z. Liu, Recent Progress on Synchrotron-Based In Situ Soft X-ray Spectroscopy for Energy Materials. Advanced Materials, 2014: 7710-7729.

Advanced Electrolyte for Rechargeable Magnesium Batteries

Recently, rechargeable magnesium batteries gains more and more attention as a commercially-viable future energy storage system for mid-/large-scale applications such as ESS and electric vehicles (EV).[1-4] Magnesium is attractive as an anode material, since it is not only naturally abundant, but delivers a high gravimetric and volumetric capacity of 2,205 mAh/g and 3,833 mAh/cm3 at relatively low reduction potential of -2.372 V vs. standard hydrogen electrode. Therefore, batteries based on magnesium metal negative electrode can be potentially manufactured at very low cost and possess a market competitiveness in their energy density. However, the most intriguing problems of these systems are slow development of electrolyte systems having a reversible Mg plating/stripping character on the anode without a dendrite formation and at the same time exhibiting a wide electrochemical window, which would hire a cathode material having a high electrode potential for the high energy density of the system. It is well-known that the conventional Grignard solution (RMgX, R = alkyl or aryl, X = Cl, Br) which has a good reversibility on Mg plating/stripping reaction, functions as strong nucleophile and its anodic stability is poor so that it cannot be applied to common cathode materials. In this work, we will present a new electrolyte system which can be prepared by a simple process and can be directly applied to magnesium electrode with a high coulombic efficiency more than 99.9 % and stability up to 4.0 V vs. magnesium electrode. We will characterize this electrolyte system through many analysis tools including NMR, single-crystal XRD and other spectroscopic methods. We will also show that full cells employing Mo6S8 Chevrel phase cathode and this new electrolyte can be run for hundreds of cycles without a noticeable capacity fading.