Mo6S8 Chevrel Phase Research Articles

Electrochemical control over stoichiometry via cation intercalation into Chevrel-phase sulphides (CuxMo6S8, x = 1–3)

This work presents a systematic investigation of the electrochemical intercalation of aqueous copper cations into the Chevrel phase (CP) Mo6S8 and its effect on the host's electronic and structural characteristics as a function of stoichiometry.

Insights into Corrosion of Current Collectors for Magnesium Batteries

To enable the shift towards renewable energy, the use of low-cost energy storage systems is necessary. Magnesium (Mg) is an abundant and safer raw material than lithium for electrochemical energy storage systems such as batteries, due to its reduced sensitivity to air. Promising intercalation materials for cathodes include Chevrel phase Mo6S8 and Prussian green FeFe(CN)6. Although magnesium perchlorate-based electrolytes are practical for early-stage cathode research, they are not compatible with Mg metal due to corrosion. The organo-metallic all phenyl complex (APC) based electrolyte is a potential candidate for magnesium full cells, but both systems contain reactive chloride species that cause severe corrosion of the current collector.This study utilizes linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopy to identify suitable materials for current collectors, including carbon-coated Al and Ni. The selected materials must withstand a polarization potential of 2 V vs. Mg/Mg2+. A Graphite-based current collector is shown to be the most promising candidate due to its low areal density, which can increase the energy density of practical Mg batteries.

New Insights into Self‐Discharge and Heat Generation in Magnesium Batteries

AbstractInvited for this month's cover is the group of Dr. Ijaz Ul Mohsin. For the first time, the total heat generation and self‐discharging phenomena of magnesium batteries containing the Chevrel Phase Mo6S8 and organic 14PAQ are compared for different electrolyte solvents using sophisticated calorimetry. Noticeable differences are observed and in tetraglyme solvent stable cycling and fewer self‐discharge phenomena are detected. However, the activation process needs more cycles to achieve the required capacity in the case of the Chevrel Phase. Read the full text of the Research Article at 10.1002/batt.202300137.

New Insights into Self‐Discharge and Heat Generation in Magnesium Batteries

AbstractMo6S8 in the Chevrel Phase (CP) and 14‐polyanthraquinone (14PAQ) cathode materials‐based coin cells were assembled against Mg‐foil as an anode by using 0.3 M magnesium tetrakis (hexafluoroisopropyloxy) borate Mg[B(hfip)4]2/dimethoxyethane (DME), 0.5 M Mg[B(hfip)4]2/DME and 0.5 M Mg[B(hfip)4]2/tetraglyme (G4) electrolytes. The heat generation of those three variants was compared using a sensitive MS80 Tian‐Calvet calorimeter. The Chevrel Phase Mo6S8 was found to generate less heat than the organic 14PAQ. However, its specific capacity was also comparatively lower than for the organic cathode material. It is equally important for battery kinetics to have a well‐designed electrolyte, therefore different solvents with the same electrolyte salt were utilized. Noticeable differences were observed and in tetraglyme solvent stable cycling and fewer self‐discharge phenomena were detected. However, the activation process needs more cycles to achieve the required capacity in the case of the Chevrel Phase. The generated heat during cycling indicated the high resistances, swelling/contraction in organic cathodes leading to higher heat generation, and poor capacity retention. To overcome self‐discharging in Mg batteries, side reactions/dissolution of cathode materials, electrolyte saturation and the formed interfaces on the anode side must be considered.

Cover Feature: Modeling of Magnesium Intercalation into Chevrel Phase Mo6S8: Report on Improved Cell Design (Batteries & Supercaps 5/2023)

The Cover Feature illustrates a newly-developed continuum model for magnesium-ion batteries with a Chevrel Phase Mo6S8 cathode. The model includes the unique crystal structure of Chevrel Phase Mo6S8 with two energetically different intercalation sites, desolvation prior to intercalation and a microstructure with an inhomogeneous particle size distribution with the aim to provide a better understanding of limiting processes and strategies to optimize the cell design. More information can be found in the Research Article by J. Drews and co-workers.

Chevrel Phase Mo6 S8 Nanosheets Featuring Reversible Electrochemical Li-Ion Intercalation as Effective Dynamic-Phase Promoter for Advanced Lithium-Sulfur Batteries.

Modifying sulfur cathodes with lithium polysulfides (LiPSs) adsorptive and electrocatalytic host materials is regarded as one of the most effective approaches to address the challenging problems in lithium-sulfur (Li-S) batteries. However, because of the high operating voltage window of Li-S batteries from 1.7 to 2.8V, most of the host materials cannot participate in the sulfur redox reactions within the same potential region, which exhibit fixed or single functional property, hardly fulfilling the requirement of the complex and multiphase process. Herein, Chevrel phase Mo6 S8 nanosheets with high electronic conductivity, fast ion transport capability, and strong polysulfide affinity are introduced to sulfur cathode. Unlike most previous inactive hosts with a fixed affinity or catalytic ability toward LiPSs, the reaction involving Mo6 S8 is intercalative and the adsorbability for LiPSs as well as the ionic conductivity can be dynamically enhanced via reversible electrochemical lithiation of Mo6 S8 to Li-ion intercalated Lix Mo6 S8 , thereby suppressing the shuttling effect and accelerating the conversion kinetics. Consequently, the Mo6 S8 nanosheets act as an effective dynamic-phase promoter in Li-S batteries and exhibit superior cycling stability, high-rate capability, and low-temperature performance. This study opens a new avenue for the development of advanced hosts with dynamic regulation activity for high performance Li-S batteries.

Continuum Modelling As Tool for Optimizing the Cell Design of Magnesium Batteries

Magnesium-based next-generation batteries are of great interest since magnesium is not only very abundant, which allows economic and sustainable applications, but also less prone to dendrite formation than many other metals. Together with the bivalency of the magnesium cations the resulting possibility to safely use a metal anode enables batteries with high specific capacities.However, for a successful commercialization of magnesium batteries there are still some challenges to overcome. The high charge density of the bivalent cation causes strong coulomb interactions with anions and solvent molecules. Therefore, magnesium salts are prone to form ion pairs and bigger clusters – especially at high concentrations, which may adversely affect the transport in the electrolyte and the electrochemical reaction at the electrode.[1] Moreover, energetic barriers for desolvation and solid-state diffusion of the double-charged magnesium ion are usually very high, which can have a crucial impact on the battery performance. Former can significantly hinder the electron-transfer reaction,[2] whereas latter makes the choice of suitable cathode materials very challenging.Consequently, a good understanding of the limiting processes in rechargeable magnesium batteries is key to develop novel high-capacity / high-voltage cathode materials. For instance, it is well-known that the morphology of an intercalation material can strongly influence the battery performance and smaller particles as well as thinner electrodes are common strategies for avoiding adverse effects of transport limitations. However, high mass loadings as well as suitable separators are still essential bottlenecks for commercialization of magnesium-ion batteries.Up to date Chevrel phase (CP) Mo6S8 is considered as benchmark intercalation cathode and Mg[B(hfip)4]2 / DME is seen as most promising chloride-free magnesium electrolyte.[3,4] In our contribution we carefully study this model system of a magnesium-ion battery to get a better understanding of how to overcome undesired limitations. Therefore, we present a newly-developed continuum model, which is able to describe the complex intercalation process of magnesium cations into a CP cathode. The model considers not only the different thermodynamics and kinetics of the two intercalation sites of Mo6S8 and their interplay but also the impact of the desolvation on the electrochemical reactions and possible ion agglomeration. The parameterization and validation of the model is based on DFT calculations and experimental data. Different kind of (transport) limitations and their impact on the battery performance are studied in detail. All in all, the combination of different modelling techniques with experimental measurements provides important insights into the operation of magnesium ion batteries and enables an optimization of the cell design. Acknowledgements This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 824066 (E-MAGIC). Furthermore, this work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe) and was funded by the German Research Foundation (DFG) under Project ID 390874152 (POLiS Cluster of Excellence). The simulations were carried out at JUSTUS 2 cluster supported by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant No INST 40/575-1 FUGG. References Drews, T. Danner, P. Jankowski et al., ChemSusChem, 3 (2020), 3599-3604.Drews, P. Jankowski, J. Häcker et al., ChemSusChem, 14 (2021), 4820-4835.Aurbach, Z. Lu, A. Schlechter et al., Nature, 407 (2000), 724-727.Zhao-Karger, R. Liu, W. Dai et al., ACS Energy Lett. 3 (2018), 2005-2013.

The evolution of local structure of Mo6S8 during Li+ electrochemical storage studied by in-situ tender X-ray absorption spectroscopy

Chevrel phase Mo6S8 has attracted great interest because of its capability to accommodate different types of small cations through reversible topotactic redox reactions. However, the intercalation mechanism of Mo6S8 is still far from being fully understood owing to the complexity of the crystal structure and limitation of the probe tools. In our study, the reaction mechanism of Li ​+ ​intercalation into Mo6S8 was studied as a model by in-situ tender X-ray absorption spectroscopy (itXAS) in the viewpoint of local structure. According to the isobestic points of the 1st derivative, the discharge process is divided into five regions while the charge process is divided into four regions, where most of the regions are assigned to the two-phase transition reactions. The intensity change of the two spectroscopic features of the 1st derivative is found to be correlated, implying that the intercluster bond length directly affects the localization of the unoccupied states of the intracluster bond. The asymmetric evolution of the intensity of a’ and b’ as well as the asymmetric partition of reaction regions demonstrate the asymmetry of the reaction path in discharge and charge process. The results help to clarify the remaining debates on the phase transitions. Besides, itXAS is demonstrated to be a powerful tool to study the reaction mechanism of Mo6S8, which is complementary to NMR, XRD and so on.

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

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

Effect of the Crystalline Structure of CuxMo6S8 on the Capacity of Mg-Based Secondary Batteries

Mo6S8 Chevrel-phase (CP) cathode materials, for Mg secondary batteries, have been reported to be better suited and superior to other cathode materials. We synthesized CuxMo6S8 (Cu-CP) by varying the temperature and time and found different crystalline structures of Cu-CP, depending on the time and temperature of the growth process. Using these structures as precursors for preparing cathodes was shown to influence the performance of the electrochemical cell. After etching Cu from Cu-CP, the structure was stabilized as the rhombohedral phase of Mo6S8 (CP). The morphologies of Cu-CP and CP were examined by scanning electron microscopy, and the structures were confirmed using X-ray diffraction. The coin-cell structure was fabricated, and the electrochemical properties of CP were evaluated and compared for different crystalline structures of Cu-CP. The discharge capacities were improved from 84.2 to 122.9 mAh·g–1 as the structure of Cu-CP changed from rhombohedral to triclinic.

Mechanisms of electrochemical magnesium (de)alloying of Mg-Sn and Mg-Pb polymorphs

Different polymorphs of Mg-Sn and Mg-Pb intermetallic compounds were prepared by high-energy mechanical alloying and then investigated as active material in magnesium batteries. Beside thermodynamically stable Mg2Sn and Mg2Pb crystallizing in the anti-fluorite structure, other polymorphs Mg∼2Sn and Mg∼2Pb were prepared by increasing the ball-milling time. The first dealloying process is almost complete only for the cubic polymorphs, then similar capacities are observed during the subsequent alloying and dealloying sequences. Thanks to operando X-ray diffraction, the electrochemical mechanism is revealed and shows that the cubic polymorphs Mg2Sn and Mg2Pb tend to preferentially form during the alloying whatever the pristine intermetallic. Weak traces of Mg∼2Sn and Mg∼2Pb are observed during the alloying, suggesting that these polymorphs act as a by-product and/or an intermediate phases of the electrochemical process. Finally, the compatibility of cubic Mg2Sn and Mg2Pb with Mg(TFSI)2-based electrolyte is confirmed in full cell vs. a positive electrode based on the Chevrel phase Mo6S8, although limited performance is achieved. This fundamental work provides new insights in the behavior of alloy-type negative electrodes for magnesium-ion batteries.

Revisiting the Chevrel Phase: Impact of Dispersion Corrections on the Properties of Mo6S8 for Cathode Applications**

AbstractWhile the Mo6S8 chevrel phase is frequently used as cathode material in Mg‐ion batteries, theoretical studies on this material are comparatively scarce. The particular structure of the Mo6S8 phase, with rather loosely connected cluster entities, points to the important role of dispersion forces in this material. However, so far this aspect has been completely neglected in the discussion of Mo6S8 as cathode material for mono‐ and multivalent‐ion batteries. In this work we therefore have studied the impact of dispersion forces on stability and kinetics of Mo6S8 intercalation compounds. For this purpose, a series of charge carriers (Li, Na, K, Mg, Ca, Zn, Al) has been investigated. Interestingly, dispersion forces are observed to only slightly affect the lattice spacing of the chevrel phase, nevertheless having a significant impact on insertion voltage and in particular on the charge carrier mobility in the material. Moreover, upon varying the charge carriers in the chevrel phase, their diffusion barriers are observed to scale linearly with the ion size, almost independent of the charge of the considered ions. This indicates a rather unique and geometry dominated diffusion mechanism in the chevrel phase. The consequences of these findings for the ion mobility in the chevrel phase will be carefully discussed.

A design concept for halogen-free Mg2+/Li+-dual salt-containing gel-polymer-electrolytes for rechargeable magnesium batteries

Polymer-based electrolytes can greatly promote the development of rechargeable metal-sulfur batteries owing to the improved safety and high flexibility; however, liquid electrolytes, which are mostly investigated so far, often hinder practical application due to the severe shuttling effect and possible leakage of the electrolytes. Herein, a new cell design is presented, that bridges the gap between lab cells and application by a novel halogen-free gel-polymer-electrolyte (GPE) with outstanding electrochemical performance. The GPE was prepared via an in-situ crosslinking reaction between lithium/magnesium borohydrides and poly(tetrahydrofuran). This GPE displays outstanding ionic conductivities in a wide temperature range, superior polarization behavior, remarkable reversibility and more strikingly, excellent compatibility with different sulfur containing and intercalation cathodes: S8@activated carbon cloth (ACC/S), sulfur poly(acrylonitrile) (SPAN) composite, titanium disulfide, Chevrel phase Mo6S8 and lithium titanate. The effective suppression of the ‘polysulfide shuttle’ by the GPE allows for a stable cycling of Mg||SPAN and Mg||ACC/S cells at room temperature with high discharge capacities (600 and 420 mAh∙gs−1 after 140 and 50 cycles, respectively). Remarkably, the Mg||GPE||SPAN system features low self-discharge, excellent flexibility and safety characteristics, which significantly improve the possibility for practical applications.

Nanocubes of Mo6S8 Chevrel phase as active electrode material for aqueous lithium-ion batteries.

The development of intrinsically safe and environmentally sustainable energy storage devices is a significant challenge. Recent advances in aqueous rechargeable lithium-ion batteries (ARLIBs) have made considerable steps in this direction. In parallel to the ongoing progress in the design of aqueous electrolytes that expand the electrochemically stable potential window, the design of negative electrode materials exhibiting large capacity and low intercalation potential attracts great research interest. Herein, we report the synthesis of high purity nanoscale Chevrel Phase (CP) Mo6S8via a simple, efficient and controllable molecular precursor approach with significantly decreased energy consumption compared to the conventional approaches. Physical characterization of the obtained product confirms the successful formation of CP-Mo6S8 and reveals that it is crystalline nanostructured in nature. Due to their unique structural characteristics, the Mo6S8 nanocubes exhibit fast kinetics in a 21 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte as a result of the shorter Li+ ion diffusion distance. Full battery cells comprised of Mo6S8 and LiMn2O4 as negative and positive electrode materials, respectively, operate at 2.23 V delivering a high energy density of 85 W h kg-1 (calculated on the total mass of active materials) under 0.2 C-rate. At 4 C, the coulombic efficiency (CE) is determined to be 99% increasing to near 100% at certain cycles. Post-mortem physical characterization demonstrates that the Mo6S8 anode maintained its crystallinity, thereby exhibiting outstanding cycling stability. The cell outperforms the commonly used vanadium-based (VO2 (B), V2O5) or (NASICON)-type LiTi2(PO4)3 anodes, highlighting the promising character of the nanoscale CP-Mo6S8 as a highly efficient anode material. In summary, the proposed synthetic strategy is expected to stimulate novel research towards the widespread application of CP-based materials in various aqueous and non-aqueous energy storage systems.

Nanoscale Chevrel-Phase Mo6S8 Prepared by a Molecular Precursor Approach for Highly Efficient Electrocatalysis of the Hydrogen Evolution Reaction in Acidic Media

Nanoscale Chevrel-Phase Mo₆S₈ Prepared by a Molecular Precursor Approach for Highly Efficient Electrocatalysis of the Hydrogen Evolution Reaction in Acidic Media

Establishing a Stable Anode-Electrolyte Interface in Mg Batteries by Electrolyte Additive.

Simple magnesium salts with high electrochemical and chemical stability and adequate ionic conductivity represent a new-generation electrolyte for magnesium (Mg) batteries. Similar to other Mg electrolytes, the simple-salt electrolyte also suffers from high charge-transfer resistance on the Mg surface due to the adsorbed species in the solution. In the current study, we built a model Mg cell system with the Mg[B(hfip)4]2/DME electrolyte and Chevrel phase Mo6S8 cathode, to demonstrate the effect of such anode-electrolyte interfacial properties on the full-cell performance. It was found that the cell required additional activation cycles to achieve its maximal capacity. The activation process is mainly attributed to the conditioning of the anode-electrolyte interface, which could be boosted by introducing an additive amount of Mg(BH4)2 to the Mg[B(hfip)4]2/DME electrolyte. Electrochemical and spectroscopic analyses revealed that the Mg(BH4)2 additive helps to remove the native oxide layer and promotes the formation of a solid electrolyte interphase layer on Mg. As a result, the full cell with the additive-containing electrolyte delivered a stable capacity from the second cycle onward. Further battery tests showed a reversible cycling for 600 cycles and an excellent rate capability, indicating good compatibility of the Mg(BH4)2 additive. The current study not only provides fundamental insights into the interfacial phenomena in Mg batteries but also highlights the facile tunability of the simple-salt Mg electrolytes.

Cation-synergy stabilizing anion redox of Chevrel phase Mo6S8 in aluminum ion battery

Anion redox chemistry is an essential component of many high energy density electrode materials, while accompanying with voltage hysteresis and fade, which currently hinders its widespread use. Here we demonstrate cation-synergy stabilizing anion redox of Chevrel phase Mo6S8 in aluminum ion battery. EELS and XAS reveal that S is fully reduced, and Mo6 cluster is firstly oxidized and then reduced with Al3+ ions insertion, which originates from the contraction and elongation of Mo-Mo bond in Mo6 cluster verified by atomic-resolved imaging. DFT calculations uncover that the energy level of [Mo-Mo]* antibonding orbitals is lifted and fell by Mo-Mo bond evolution, resulting in the synergetic cationic and anionic redox, which contributes to the stable structure upon cycling. Our work figures out the electrochemical redox mechanism of Mo6S8 in aluminum ion batteries and provides implications generally for the design of materials employing anion redox chemistry.

Unravelling the role of interfacial chemistry evolution in the kinetics of zinc insertion into chevrel phase

The insertion kinetics of the multivalent cations into an intercalation host is conventionally understood primarily focused on the ion migration and charge redistribution properties through the bulk solid framework. Here we show that the intercalation kinetics of zinc ions into a Mo6S8 Chevrel phase is critically dependent on the interfacial chemistry evolution associated with changes in the electrolyte pH and the formation of zinc hydroxide sulfate (ZHS) during the early cycles. The sluggish initial insertion process with a hydroxyl surface groups on Mo6S8 causes a concurrent intercalation of zinc ions into the inner and outer ring vacancy sites, leading to the temporary coexistence of three reaction phases. On the contrary, characteristic two-step, two-phase reactions associated with the sequential occupation of each vacancy site are observed after the electrolyte pH change induces ZHS deposit in the interface, which eliminates hydroxyl adsorption layer on the surface, and promotes the intercalation kinetics of zinc ions. This study provides valuable insight into insertion of multivalent cations into Chevrel phase, which often exhibits a cryptic kinetic dormancy during the initial cation intercalation stages.

Improved Non-Grignard Electrolyte Based on Magnesium Borate Trichloride for Rechargeable Magnesium Batteries

The high anodic stability of electrolytes for rechargeable magnesium batteries enables the use of new positive electrodes, which can contribute to an increase in energy density. In this study, novel Ph3COMgCl-, Ph3SiOMgCl-, and B(OMgCl)3-based electrolytes were prepared with AlCl3 in triglyme. The Ph3COMgCl-based electrolyte showed anodic stability over 3.0 V vs. Mg but was chemically unstable, whereas the Ph3SiOMgCl-based electrolyte was chemically stable but featured lower anodic stability than the Ph3COMgCl-based electrolyte. Advantageously, the B(OMgCl)3-based electrolyte showed both anodic stability over 3.0 V vs. Mg (possibly due to the Lewis acidic nature of B in B(OMgCl)3) and chemical stability (possibly due to the hard acid character of B(OMgCl)3). B(OMgCl)3, which was prepared by reacting boric acid with a Grignard reagent, was characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and X-ray absorption spectroscopy (XAS). The above analyses showed that B(OMgCl)3 has a complex structure featuring coordinated tetrahydrofuran molecules. 27Al NMR spectroscopy and Al K-edge XAS showed that when B(OMgCl)3 was present in the electrolyte, AlCl3 and AlCl2+ species were converted to AlCl4−. Mg K-edge XAS showed that the Mg species in B(OMgCl)3-based electrolytes are electrochemically positive. As a rechargeable magnesium battery, the full cell using the B(OMgCl)3-based electrolyte and a Mo6S8 Chevrel phase cathode showed stable charge-discharge cycles. Thus, B(OMgCl)3-based electrolytes, the anodic stability of which can be increased to ~3 V by the use of appropriate battery materials, are well suited for the development of practical Mg battery cathodes.

How solution chemistry affects the electrochemical behavior of cathodes for Mg batteries, a classical electroanalytical study

Mo6S8 Chevrel phase (CP) is still the most effective magnesium ions insertion material that was examined in cathodes for rechargeable Mg battery model systems. Its synthesis includes CuMo6S8 as an intermediate which undergoes Cu ions leaching. Thereby, CP materials may contain small amounts of copper. CuxMo6S8 undergoes complex magnesium insertion processes that include reversible displacement of copper. We report herein on a study of Cu0.09Mo6S8 electrodes in two electrolyte solutions which are important for secondary Mg batteries: C6H6MgCl/AlCl3/THF (all phenyl complex solutions, APC) and MgTFSI2/MgCl2/DME (Mg imide/chloride/DME solutions, ICD). There systems are ideal playgrounds for studying the nature of complex magnesium insertion processes into inorganic hosts. The analysis reported herein shows how the electroanalytical behavior of CP electrodes is affected by the electrolyte solution identity. For instance, the main Mg ions insertion process from the APC solutions occurs at 160 mV lower than from the ICD ones; the heterogeneous rate constant for charge transfer across the interface are significantly higher in APC solutions; in turn, the conversion processes related to the Cu residue in the CP electrodes are kinetically and thermodynamically favorable in the ICD solutions. As expected, the magnesium solid-state diffusion coefficient was found to be solutions independent. The study demonstrates the sensitivity of electroanalytical tools such as slow scanning rate voltammetry for reflecting fine structural changes during intercalation processes and can be generalized to other systems.