Journal of The Electrochemical Society, 161 (4) A593-A598 (2014) A593 0013-4651/2014/161(4)/A593/6/$31.00 © The Electrochemical Society A Convenient Approach to Mo 6 S 8 Chevrel Phase Cathode for Rechargeable Magnesium Battery Partha Saha, a Prashanth H. Jampani, b Moni Kanchan Datta, a,c Chris U. Okoli, b Ayyakkannu Manivannan, d, * and Prashant N. Kumta a,b,c,d,e,f, *, z a Department of Bioengineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA b Chemical and Petroleum Engineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA c Center for Complex Engineered Multifunctional Materials, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA d U.S. Department of Energy, National Energy Technology Laboratory, Morgantown, West Virginia 26507, USA e Mechanical Engineering and Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA f School of Dental Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA A two-step solution chemistry route was used to synthesize ternary Chevrel phase (Cu 2 Mo 6 S 8 ) 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 Cu 2 Mo 6 S 8 crystals with unit cell dimensions a ∼ 0.96245 nm and c ∼ 1.01987 nm of molar volume ∼818.14 × 10 −3 nm 3 . De-cuprated Mo 6 S 8 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 Mo 6 S 8 cathode renders the solution chemistry route an alternative approach for the synthesis of cuprated Chevrel phase: a known cathode system for magnesium battery. © 2014 The Electrochemical Society. [DOI: 10.1149/2.061404jes] All rights reserved. Manuscript submitted November 11, 2013; revised manuscript received February 3, 2014. Published February 20, 2014. Electrochemical energy storage technologies based on recharge- able batteries are considered as one of the leading emerging technolo- gies for stationary power application. Current battery technologies based on lead acid, nickel metal hydride, sodium-sulfur, and vanadium flow systems used for stationary power applications suffer due to var- ious environmental and economic concerns. 1 Li-ion batteries used for mobile electronics and electric vehicles can offer high energy density, however, with lithium’s geographically constraint reserve and high cost makes it imperative to explore alternative battery technologies. 2 Recently, energy storage systems based on bivalent Mg 2+ ions is be- ing touted as a promising high energy density alternative battery sys- tem among others. 3,4 Magnesium (Mg) has several positive attributes which set it apart from the Li-ion battery system. 5 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. 6–8 Additionally, magnesium is more stable in air compared to lithium, and is theoretically capable of rendering higher volumetric capacity (3832 mAh/cm 3 for Mg vs. 2062 mAh/cm 3 for Li). Further- more, magnesium is not plagued by dendrite formation unlike lithium metal batteries which led to initial safety concerns that was thankfully obviated by the intercalation of Li into graphite. 9,10 Earlier studies shows that conventional salts such as Mg(ClO 4 ) 2 , Mg(CF 3 SO 3 ) 2 , Mg[(CF 3 SO 2 ) 2 N] 2 etc. dissolved in various non- aqueous solvents develop surface passivation on the Mg anode and ef- fectively block Mg 2+ ion transport. 11–13 On the other hand, Grignard’s reagents (RMgX, R alkyl or aryl; X = Cl, Br) dissolved in ethereal sol- vents are well-known and are capable of electrochemically depositing and dissolving magnesium. 14 However, the limited electrochemical window (∼1.5 V) of Grignard’s reagents imposes a major barrier for their use in practical cell assemblies. Aurbach et al. 15,16 first in- vented Mg organohaloaluminate salts (R 2 Mg) n (AlCl 3-n R n ) m dissolved in ethereal solvents, capable of reversibly depositing and dissolving Mg yielding a 100% Coulombic efficiency in the electrochemical po- tential window ∼2.2 V. Following Aurbach’s successful invention, the interest for developing high energy density Mg storage systems has increased steadily over the past few years. 17 ∗ Electrochemical Society Active Member. z E-mail: pkumta@pitt.edu Aurbach also developed a prototype Mg cell using the Mo 6 S 8 Chevrel Phase (CP) - a new class of cathodes, Mg anode, and the 0.25 molar Mg(AlCl 2 EtBu) 2 /tetrahydrofuran electrolyte where Mg 2+ can be (de)intercalated reversibly ∼1–1.2 V offering an energy den- sity ∼60 Whkg −1 up to 2000 cycles with little fade in capacity. 15,18 Chevrel phases, Mo 6 T 8 (T = S, Se, Te) is an unique class of com- pounds that can accommodate multivalent cations within the Mo 6 anionic framework. 19 Relatively fast and easy intercalation of Mg 2+ ions at room temperature makes CPs a preferred choice of cathode for magnesium battery. However, Mo 6 S 8 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, M x Mo 6 T 8 (M = metal, T = S, Se, Te). 20 Typically, Cu x Mo 6 S 8 (Cu x CP) are synthesized by high temperature solid state reactions of elemental blends of copper, molybdenum, and sulfur powders in an evacuated quartz ampoules (EQA) at ∼1423 K for 7 days 3,15 or by a molten salt route (MS) using Mo-MoS 2 -CuS reactants in a KCl salt, and heat treating the reaction mixtures at ∼1123 K for 60 h in an Ar atmosphere. 18 Both approaches are ex- tremely tedious requiring chemical leaching either in 6M HCl/H 2 O or 0.2 M I 2 /acetonitrile solution for several days at room temperature for complete removal of copper. 21 The MS approach for synthesiz- ing the Cu x CP phase offers much improvement in terms of synthesis time (60 h vis-` a-vis 7 days at a 300 ◦ C lower temperature of 1123 K) compared to the EQA approach however, the total time of ∼two and a half days required for the synthesis of Mo 6 S 8 will likely increase the energy consumption and associated manufacturing costs. A re- cent report on ultra-fast synthesis of Cu 1.8 Mo 6 S 8 by self-propagating high-temperature synthesis (SHS) or thermal explosion method from elemental mixtures of Cu-Mo-S inside an Argon sealed Swagelok ves- sel successfully reduces the total synthesis time to 20 min at 1273 K. 22 However, Mo 6 S 8 a well-known model cathode for magnesium battery will continue to serve as the baseline system to evaluate the suit- ability of new magnesium electrolytes. 23 Thus, alternative methods to synthesize Mo 6 S 8 Chevrel phase from its stable high temperature ternary Chevrel phase will be paramount importance in future. With this in mind, in the present work, we exploit an alternative solution chemistry route for the synthesis of Mo 6 S 8 following modification of a previous manuscript 24 which reported the synthesis of the Cu analog of the Mo 6 S 8 phase. However, the approach was never explored for the generation of the Mo 6 S 8 phase devoid of Cu which is described ecsdl.org/site/terms_use address. Redistribution subject to ECS license or copyright; see 150.212.182.195 Downloaded on 2014-02-21 to IP