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

Abstract

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.