Mo6S8 Chevrel Phase Research Articles

Self-Formation of Mg Metal / Electrolyte Interface in Rechargeable Mg Batteries

Rechargeable magnesium batteries have recently gained more and more attention for a possible candidate of post lithium-ion batteries, because of i) potentially high energy density ((3.0V x 2e- for Mg) / (4.0V x 1e- for Li) = 1.5), ii) low cost (high Clarke number; Mg (1.93) vs. Li(0.006)) and iii) intrinsic safety (no dendrite growth for Mg, while needle-like dendrite for Li). In 2000, a great success of Mg battery was achieved by Prof. Aurbach in the prototype system comprising Mg metal anode, Chevrel phase Mo6S8 cathode and Grignard based electrolyte [1]. Guided by this discovery, intensive researches on anode, cathode and electrolyte materials have accelerated all over the world, to maximize the advantages of the Mg battery system. A major challenge of the rechargeable Mg batteries is that there is still lack of practical cathode materials. Chevrel phase Mo6T8 (T = S, Se) compounds are only well-known materials showing high reversibility over hundreds of cycles at reasonably high rates [2]. Unfortunately, the Chevrel phase showed low operation voltage (1.2 V vs. Mg) and low capacity (122 mAh/g), and therefore lots of cathode candidates have been extensively investigated to improve the capacity as well as the operating voltage, aiming a higher energy density battery system. Some candidates beyond the Chevrel phase have been found to date, however still on an exploratory level. The main issue is sluggish diffusion of Mg2+ in the host structure, retarding findings of potential cathode materials. On the other hand, we found that some cathode materials have not been monitored properly because of high impedance caused by a Mg metal anode / electrolyte interface. Figure 1 shows the Nyquist plots of a symmetrical Mg/Mg cell with PhMgCl-AlCl3 (APC) electrolyte as a function of holding time under open circuit voltage. Inset figure shows the Nyquist plot of the same symmetrical cell under a bias voltage of 150 mV after holding for 12 h. As can be seen, the interfacial resistance between Mg metal and electrolyte was getting bigger and bigger with holding time even under open circuit voltage. However, once a bias voltage was applied for the cell, the observed impedance drastically decreased [3]. As a result, such huge impedance potentially veiled any electrochemical responses for the entire cell, which may regard as no possibility of cathode materials. The interfacial issue on a Mg metal anode has been pointed out by Prof. Aurbach et al [4,5]. Here, we newly found that the huge impedance was switchable under open circuit and bias potentials in the currently-developed Mg battery electrolytes, and then strongly depended on electrochemical states [6]. Thus, mechanistic understanding of the interface between Mg metal and electrolyte is significantly important to find out future cathode materials as well as to develop highly stable electrolytes which are compatible with Mg deposition and dissolution. In this presentation, we will present our findings of the Mg metal / electrolyte interface. Then, we will discuss a new idea of self-formed Mg metal / electrolyte interface, which is one of the approaches to decrease the huge impedance on the Mg metal anode.

Development of Nonaqueous Electrolyte for Multivalent Batteries

The electrolytes play an important role in the multivalent batteries. Here we will report the development of a series of new electrolytes for magnesium batteries. The electrolyte has a high coulombic efficiency for Mg plating/dissolution and can be stable up to 2.8 V vs Mg. The preparation and characterization of the new electrolytes will be presented, including the syntheses, multinuclear NMR spectroscopy, the hypothesis of the product formation, the electrochemical characterization such as ionic conductivities and transport numbers. The capabilities of the electrolyte with Chevrel Phase Mo6S8 and other cathodes will also be discussed.

Beyond Chevrel Phase Mo6S8 : High Energy Density Mg Battery Cathodes

Rechargeable magnesium batteries have recently gained more and more attention for a possible candidate of post lithium-ion batteries, because of i) potentially high energy density ((3.0V x 2e- for Mg) / (4.0V x 1e- for Li) = 1.5), ii) low cost (high clarke number; Mg (1.93) vs. Li(0.006)) and iii) intrinsic safety (no dendrite growth for Mg, while needle-like dendrite for Li). In 2000, a great success of Mg battery was achieved by Prof. Aurbach in the system comprising Mg metal anode, Chevrel phase Mo6S8cathode and Grignard based electrolyte [1]. The prototype Mg battery can work very well over >2000 cycles. Guided by this discovery, intensive researches on anode, cathode and electrolyte materials have accelerated all over the world, to maximize the advantages of the Mg battery system.Although there have many hurdles to be overcome in the Mg batteries, the biggest hurdle is that the cathode energy density stays much lower than expected. The Chevrel phase Mo6S8 is the only reported cathode material with good cyclability, but due to low operation voltage (1.2 V vs. Mg) and low capacity (122 mAh/g), it is theoretically difficult to realize the Mg battery which is competitive with lithium-ion batteries in term of energy density. Therefore, cathode materials having high operation voltage and high capacity are strongly desired. Previously, we worked on MnO2 cathodes and tried to understand its reaction mechanisms. Although MnO2 cathodes show higher average voltage (1.5 V vs. Mg) and high initial capacity (280 mAh/g) [2], they faced certain restrictions (i.e. structural deformation, thermodynamically preferred conversion reaction and resistive shell formation (Figure 1)) during magnesium ion intercalation into the host structure, resulting in a severe capacity fading during cycling [3,4]. Based on this knowledge, we have been currently researching solutions to such issues while continuously trying to find new cathode candidates. For example, CaFe2O4 phase MgMn2O4 [5], carbon cluster fullerene (C60) [6], amorphous vanadium oxide [7], and so on.In this presentation, we will discuss about key challenges in improving the cathode performances of these materials and in further enhancing the energy density of Mg batteries.

Organo Metallic Free Electrolytes for Magnesium Rechargeable Batteries

Developing an organo-metallic free electrolytic solution is crucial step towards a practical magnesium battery system. Implementing Grignard reagents in a commercial battery would pose a great impediment owing to the material’s high cost, its hazardous nature, and high susceptibility towards oxidation. The Magnesium Aluminum Chloro Complex (MACC) solutions, developed at the Bar Ilan University in conjunction with its allies at Pellion, Cambridge, MA, consist of solely inorganic salts; MACC is the reaction products of the acid base reaction between MgCl2 and AlCl3 in the right proportion in THF. MACC solutions are endowed with high Mg deposition/stripping cycling efficiency, upwards of 99%, low over-potential for magnesium deposition and a wide electrochemical window of 3.1V. Reversible intercalation/de-intercalation of Mg ions into Chevrel-phase Mo6S8, from a MACC solution has also been demonstrated. Normally, freshly synthesized MACC solution exhibits very poor magnesium deposition/dissolution reversibility. However, it was discovered serendipitously that by an electrochemical process, coined “conditioning”, it is possible to significantly enhance the electrochemical characteristics of such electrolyte solutions. The electrochemical “conditioning” consist of slowly electrochemically cycling the solutions in a sealed cell, using Pt as WE and Mg foil as CE in the potential range of -1.2V and 2.8V. This study is an important step forward, eliminating the need for Grignard/organo-metallic based electrolytes with their intrinsic disadvantages, and towards a more “simple salt” based system.

Rapid Solution Chemistry Approach for Synthesizing Mo6S8 Chevrel Phase Cathode for Rechargeable Magnesium Battery

Recently, energy storage systems based on bivalent Mg2+ ions spurred considerable interest as a promising high energy density alternative battery system among others [1, 2]. Magnesium (Mg) has several positive attributes which set it apart from Li-ion battery system. It is environmental friendly, cost effective (~$ 2700/ton for Mg compared to $64,000/ton for Li) and is relatively more abundant in the earth’s crust (~13.9% Mg compared to ~0.0007% of Li) compared to hitherto used popular systems. Additionally, magnesium is more stable in air compared to lithium, and is theoretically capable of rendering higher volumetric capacity (3832 mAh/cc for Mg vs. 2062 mAh/cc for Li).In the year 2000, Aurbach and coworkers successfully demonstrated a prototype Mg cell using the Mo6S8 Chevrel Phase a new class of cathodes, Mg anode, and the 0.25 molar Mg(AlCl2EtBu)2/tetrahydrofuran electrolyte where Mg2+ can be (de)intercalated reversibly ~ 1-1.2V offering an energy density ~ 60 Whkg-1 up to 2000 cycles with little fade in capacity [3]. Relatively fast and easy intercalation of Mg2+ ions at room temperature makes Mo6S8 a model cathode for magnesium battery. However, Mo6S8 is a metastable phase at room temperature, and is therefore indirectly stabilized when generated via leaching of the metal from the thermodynamically stable ternary Chevrel phase compounds, MxMo6T8 (M = metal, T = S, Se, Te) [4]. Typical synthesis approach of CuxMo6S8 (CuxCP) requires high temperature reactions of elemental blends in an evacuated quartz ampoules (EQA) at ~1150○C for 7 days [3] or by a molten salt (MS) route using Mo-MoS2-CuS reactants in a KCl salt, and heat treating the reaction mixtures at ~850○C for 60h in an Ar atmosphere [5]. Both approaches are extremely tedious and require chemical leaching either in 6 molar HCl/H2O or 0.2 molar I2/acetonitrile solutions for several days at room temperature for complete removal of copper [5].Herein, we report a rapid solution chemistry route (total manufacturing time required for the synthesis of CP is only ~12h) for the synthesis of Mo6S8 following modification of a previous report [6] which only reported the synthesis of the Cu analog of the Mo6S8 phase. The structural analysis (XRD and SEM) shows the formation of phase-pure micrometer (~1-1.5 mm) size cuboidal shaped Cu2Mo6S8 and Mo6S8 crystals [See Fig. 1(a-d)]. Electrochemical performance of the resultant Mo6S8 cathode exhibits a discharge capacity ~ 76 mAhg-1 with excellent capacity retention up to ~100 cycles, when cycled at a current rate of 20mA/g (~C/6). The excellent cyclability, rate capability and high Coulombic efficiency (~99.3% at ~1.C rate) of the Mo6S8 cathode, renders the solution chemistry route a convenient approach for synthesizing the electrochemically active model Chevrel phase Mo6S8. Results of these studies will be presented and discussed.

High Lithium Ionic Mobility in Mo6S8 Chevrel Phase Cathodes

Rechargeable magnesium batteries have been recognized as an alternative solution to lithium ion batteries because of their advantages of excellent safety and cost [1]. Chevrel phase compounds, MxMo6S8(M = metal), are promising cathode materials in Mg batteries, because they display exceptionally fast cation transport for multi-valent ions compared to any other host materials. However, depending upon the composition and synthesized temperature of the Chevrel phase, the kinetics of Mg diffusion is highly affected at both room temperature and elevated temperature.In addition, there are many on-going efforts to discover improved electrolytes for Mg batteries. It has been recently demonstrated that using an electrolyte with a concentrated lithium salt could benetit the thermodynamic and kinetic in a lithium-sulfur cell [2]. Here, we report the experimental and computational calculation results to explain unusually high ionic mobility of lithium in Mo6S8 chevrel phase cathode materials. When a concentrated lithium salt of 0.5M LiCl is added to the conventional APC electrolytes, almost full theoretical capacity can be achieved at room temperature in Mo6S8 Chevrel phase cathode materials at very high C-rates up to 20, 30C.

Magnesium(II) Bis(trifluoromethane sulfonyl) Imide-Based Electrolytes with Wide Electrochemical Windows for Rechargeable Magnesium Batteries

We present a promising electrolyte candidate, Mg(TFSI)2 dissolved in glyme/diglyme, for future design of advanced magnesium (Mg) batteries. This electrolyte shows high anodic stability on an aluminum current collector and allows Mg stripping at the Mg electrode and Mg deposition on the stainless steel or the copper electrode. It is clearly shown that nondendritic and agglomerated Mg secondary particles composed of ca. 50 nm primary particles alleviating safety concern are formed in glyme/diglyme with 0.3 M Mg(TFSI)2 at a high rate of 1C. Moreover, a Mg(TFSI)2-based electrolyte presents the compatibility toward a Chevrel phase Mo6S8, a radical polymer charged up to a high voltage of 3.4 V versus Mg/Mg(2+) and a carbon-sulfur composite as cathodes.

A Convenient Approach to Mo6S8 Chevrel Phase Cathode for Rechargeable Magnesium Battery

A two-step solution chemistry route was used to synthesize ternary Chevrel phase (Cu2Mo6S8) with 5 h annealing at 1000°C under reducing atmosphere. The approach marks a synthesis route different from hitherto described conventional solid-state methods. X-ray diffraction and scanning electron micrograph shows the formation of ∼1–1.5 μm size cuboidal shape Cu2Mo6S8 crystals with unit cell dimensions a ∼ 0.96245 nm and c ∼ 1.01987 nm of molar volume ∼818.14 × 10−3 nm3. De-cuprated Mo6S8 exhibits a discharge capacity ∼76 mAhg−1 with good capacity retention up to ∼50 cycles when cycled at the current rate of 20 mA/g (∼C/6). The excellent rate capability and high Coulombic efficiency (∼99.3% at ∼1.5C rate) of the Mo6S8 cathode renders the solution chemistry route an alternative approach for the synthesis of cuprated Chevrel phase: a known cathode system for magnesium battery.

Coordination Chemistry in magnesium battery electrolytes: how ligands affect their performance

Magnesium battery is potentially a safe, cost-effective, and high energy density technology for large scale energy storage. However, the development of magnesium battery has been hindered by the limited performance and the lack of fundamental understandings of electrolytes. Here, we present a study in understanding coordination chemistry of Mg(BH4)2 in ethereal solvents. The O donor denticity, i.e. ligand strength of the ethereal solvents which act as ligands to form solvated Mg complexes, plays a significant role in enhancing coulombic efficiency of the corresponding solvated Mg complex electrolytes. A new electrolyte is developed based on Mg(BH4)2, diglyme and LiBH4. The preliminary electrochemical test results show that the new electrolyte demonstrates a close to 100% coulombic efficiency, no dendrite formation, and stable cycling performance for Mg plating/stripping and Mg insertion/de-insertion in a model cathode material Mo6S8 Chevrel phase.

New Polymorph of Mo3S4 Prepared using a High-pressure Synthesis Technique: Crystal Structure, Electronic Property, and Band Calculation

A new polymorph of Mo3S4 was synthesized at 13 GPa and 1450 °C from a stoichiometric mixture of elements. It crystallizes in a triclinic unit cell (space group P1 (No. 2)) with cell constants of a = 6.364(2) Ǻ, b = 6.608(2) Ǻ, c = 6.809(2) Ǻ, α = 103.899(3) °, β = 117.753(3) °, γ = 103.958(3) °, and V = 224.25(13) Ǻ(3). The structure of Mo3S4 is composed of edge- and face-sharing MoS6 octahedra. It was closely related to the structure of MMo2S4 type compounds (M = V, Cr, Fe, and Co). Mo3S4 can be regarded as a derivative with M = Mo. The calculated density of 6.160 g/cm(3) was much larger than 5.191 g/cm(3) of famous polymorphic Mo6S8 (Chevrel phase). Mo3S4 was metallic and did not show any superconducting transition down to 2 K. The bond-valence sums suggested that Mo3S4 can be classified in the class III-B of mixed-valence compounds; all Mo ions have a similar nonintegral valence. Electronic structure calculations revealed that the conduction band of Mo3S4 contains much contribution of the relatively narrow Mo 4d bands as well as the bands composed of hybridized Mo4d-S3d orbitals.

Mo6S8 Electrochemical Transfer Junction for Selective Extraction of Co2+-Ions from Their Mixture with Ni2+-Ions

This work unveils physico-chemical mechanisms of highly selective extraction of Co2+-ions from their mixtures with Ni2+-ions using Mo6S8 (Chevrel phase) electrochemical transfer junction (ETJ). ETJ separates the feed and recovery compartments of the cell, in which electrolysis using different current densities is carried out. By parallel cyclic voltammetry, chronopotentiometry and electrochemical impedance studies in 3 electrode cells it was found that the potential of Co2+-ions extraction from the Mo6S8 electrode is by 0.1 V more negative, and its interfacial ion-transfer kinetics is more facile as compared to that for Ni2+-ions. This allows the surface of the ETJ, facing the recovery compartment of the cell, to operate as a highly selective ion gate: Co2+-ions are able to readily cross the ETJ/recovery solution interface, while Ni2+ ions are effectively blocked by this interface since the potential of these ions extraction is lower than that for Ni2+-ions from Mo6S8 electrode. The results obtained are important for emerging new technologies for separation/recovery of hazardous metal ions from leached waste solutions of spent rechargeable batteries.

Magnesium Borohydride: From Hydrogen Storage to Magnesium Battery

Beyond hydrogen storage: The first example of reversible magnesium deposition/stripping onto/from an inorganic salt was seen for a magnesium borohydride electrolyte. High coulombic efficiency of up to 94 % was achieved in dimethoxyethane solvent. This Mg(BH(4))(2) electrolyte was utilized in a rechargeable magnesium battery.

Ultrafast anode for high voltage aqueous Li-ion batteries

Clustered Mo sulfide Mo6S8 (Chevrel phase) demonstrates outstandingly high rate capability in contact with concentrated aqueous Li2SO4 solutions. Slow galvanostatic cycling can be performed with good Faradaic efficiency at a moderate capacity of 32.1 mAhg− 1 (1e− reduction of Mo6S8), which in combination with 1e− oxidation process in Li x Mn2O4 cathode, results in equilibrium cell voltage of 1.5 V. Higher rates of charge and discharge have been also achieved within a considerably extended apparent electrochemical stability window involving second two-electron reduction of Mo6S8 which leads to apparent cell voltage of 1.85 V, specific capacity about 74.7 mAhg−1 at 90 % Faradaic efficiency, specific energy of 74 Wh kg−1 (related to both electrodes) at extremely high charge–discharge rate of 60 C. Our study highlights the generic feature of Li-ion aqueous cells, namely, a high rate capability coupled with a relatively fast self-discharge which necessitates a more profound understanding of the nature of self-discharge in Li-ion insertion hosts in contact with aqueous solutions.

The effect of milling on the performance of a Mo6S8 Chevrel phase as a cathode material for rechargeable Mg batteries

In the present study, we explored how milling Mo6S8 Chevrel phase in inert or air atmosphere affects their electrochemical behavior as a Mg-ion insertion material for rechargeable Mg batteries. Electrochemical tools such as slow scan rate cyclic voltammograms and potentiostatic intermittent titration technique have been used in conjunction with X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy. In contrast to the deterioration observed for milling Mo6S8 in air, it’s milling under Ar results in specific capacity increase due to improved Mg-ion diffusion kinetics. It was shown that in spite of the conservation of the bulk crystallographic structure, both for air and the Ar-milled materials, they differ significantly in the average particle sizes and the degree of surface oxidation state.

Distinction between Energetic Inhomogeneity and Geometric Non-Uniformity of Ion Insertion Electrodes Based on Complex Impedance and Complex Capacitance Analysis

Electrochemical insertion of Mg ions into Mo6S8 Chevrel phase is a unique, model system for studying the nature of the energetic inhomogeneity of the host sites suitable for ions accommodation. We show that the two energy state model can be successfully used for describing the very specific Mg ions insertion kinetics into the host, in particular, as relates to a drastic increase of Mg ions mobility in the vicinity of the critical potential of 1.25 V (vs Mg). This is accompanied by very pronounced changes of the impedance spectra. On the other hand, similar behavior of impedance spectra could be obtained for geometrically nonhomogeneous intercalation electrodes, comprising a distribution of thicknesses. One can frequently meet both these cases in practice for a vast variety of intercalation electrodes (e.g., for lithiated graphite, composite Li(x)MO2, M = Mn, Ni, Co, etc.). In this paper, we developed a methodology aimed at a reliable distinction between the two alternatives, based on complex impedance and complex capacitance analysis.

Phase transitions and diffusion kinetics during Mg2+- and Li+-ion insertions into the Mo6S8 chevrel phase compound studied by PITT

Careful potentiostatic intermittent titrations (PITT) were performed in order to study Li+- and Mg2+-ion insertions into the Mo6S8 chevrel phase compound. Potential dependences of equilibrium and dynamic characteristics of the insertion processes, such as the differential intercalation capacitance, Cdif, the chemical diffusion coefficient, D, and the specific ionic conductivity, κ, were measured and interpreted in terms of lattice-gas models with highly attractive interactions between the inserted ions. Li-ion conductivity of the slightly intercalated chevrel phase is much higher than that of Mg-ions, which can be attributed to a higher charge density of the latter ion and to its higher polarization effect on the S8 anions’ lattice of the host material. Some crystallographic arguments related to the low mobility of the Mg-ion on the one hand, and the extremely high mobility of most of the divalent d-metal cations in the Mo6S8 chevrel phase on the other hand, are discussed.

Microstructure of Cu(x)Mo6S8 Chevrel phase thin films on R-plane sapphire

Cu(x)Mo6S8 Chevrel phase thin films epitaxially grown on R-plane sapphire substrates have been studied using field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HREM). For samples grown in optimal conditions, the SEM images evidence oriented grains of about 100 nm average size; the HREM images of cross-section samples allow to deduce the local epitaxial relations between the substrates and films. HREM also evidences two grain families, previously deduced from X-ray and reflection high-energy electron diffraction studies. No defects have been observed in the films; a higher molybdenum content in the films results in Mo-precipitation at the film-substrate interface.