Rapid Solution Chemistry Approach for Synthesizing Mo6S8 Chevrel Phase Cathode for Rechargeable Magnesium Battery
Authors: Kumta Prashant N , Datta Moni Kanchan , Manivannan A. , Saha Partha 
Published: February 2020

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.

References

[1] Aurbach D, Suresh G, Levi E, Mitelman A, Mizrahi O, Chusid O, et al. Progress in Rechargeable Magnesium Battery Technology. Advanced Materials. 2007;19:4260-7.
[2] Kim HS, Arthur TS, Allred GD, Zajicek J, Newman JG, Rodnyansky AE, et al. Structure and compatibility of a magnesium electrolyte with a sulphur cathode. Nat Commun. 2011;2:427.
[3] Aurbach D, Lu Z, Schechter A, Gofer Y, Gizbar H, Turgeman R, et al. Prototype systems for rechargeable magnesium batteries. Nature. 2000;407:724-7.
[4] Rabiller P, Rabiller-Baudry M, Even-Boudjada S, Burel L, Chevrel R, Sergent M, et al. Recent progress in chevrel phase syntheses: A new low temperature synthesis of the superconducting lead compound. Materials Research Bulletin. 1994;29:567-74.
[5] Lancry E, Levi E, Mitelman A, Malovany S, Aurbach D. Molten salt synthesis (MSS) Of Cu2Mo6S8- New way for large-scale production of Chevrel phases. Journal of Solid State Chemistry. 2006;179:1879-82.
[6] Nanjundaswamy KS, Vasanthacharya NY, Gopalakrishnan J, Rao CNR. Convenient synthesis of the Chevrel phases metal molybdenum sulfide, MxMo6S8(M = copper, lead, lanthanum or gadolinium). Inorganic Chemistry. 1987;26:4286-8.

Acknowledgements:

The authors gratefully acknowledge the ﬁnancial support as part of the Department of Energy’s National Energy Technology Laboratory’s program DOE-NETL) (contract number DE-FE0004000). PNK also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for partial support of this research.