A Convenient Approach to Mo₆S₈Chevrel Phase Cathode for Rechargeable Magnesium Battery

Abstract

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.Electrochemical energy storage technologies based on rechargeable batteries are considered as one of the leading emerging technologies 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 various 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 being touted as a promising high energy density alternative battery system 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). Furthermore, 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 nonaqueous solvents develop surface passivation on the Mg anode and effectively 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 solvents 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 invented Mg organohaloaluminate salts (R 2 ...

Beyond Li-ion: electrode materials for sodium- and magnesium-ion batteries

“…In both cases, the new product and the solid-state products showed approximately identical electrochemical performance. A wet chemistry route to Cu 2 Mo 6 S 8 was proposed by Saha et al [150]. The precipitate obtained from the reaction between (NH 4 ) 2 MoS 4 with CuCl 2 in DMF was later annealed at 1000°C, and the resulting material was composed of micron-sized particles.…”

Status and challenge of Mg battery cathode

“…Saha et al also tried to control the particle size through a co-precipitation method followed by a conventional calcination process. 26 The particle size of the target material decreased to ∼ 500 nm and a capacity over 60 mAh/g was observed at a rate up to 1.5 C. Cheng et al used a facile process to precipitate the precursors on graphene to increase the conductivity of the final material and control its particle size. 27 Using this method, Mo 6 S 8 showed better rate performance (2 C at a capacity of 70 mAh/g) and cycling behavior (retain the capacity over 70 mAh/g over 150 cycles).…”

Electrochemical Performance of Chemically and Solid State-Derived Chevrel Phase Mo6T8 (T = S, Se) Positive Electrodes for Sodium-Ion Batteries

“…The essence of solution chemistry approach is that it reduces the total furnace heating time down to 5h compared to 60h required for molten salt (MS) route for direct synthesis of ternary CP 48 . electrode is ~108 mAhg -1 (~93% of theoretical capacity ~116 mAhg -1 ) and ~74 mAhg -1 , respectively, with a 1 st cycle Coulombic efficiency of ~68.3%.…”

“…This indicates an initial trapping of Na-ions which subsides rapidly and allows for reversible charge storage. The presence of trapping mechanisms in Chevrel phase electrodes has previously been studied in-depth in the case of magnesium and lithium ion systems48,58 . This similarity in behavior has led to this conclusion but it must be noted that the irreversible trapping of charge is quite possibly the effect of some other irreversible process such as adverse side reactions.…”

“…According to Gocke et al47 chemical reduction of Mo 6 S 8 leads to the formation of Na 3 Mo 6 S 8 phase which upon re-oxidation leads to the formation of stable rhombohedral Na~1Mo 6 S 8 phase a R =90 ○ where the sodium atom is leaching copper from Cu x CP-1 phase were synthesized by two different established routes. In the first approach, Cu x CP-1 was synthesized by two-step solution chemistry route reported by us elsewhere48 . Cu x CP-1 was also synthesized by molten salt (MS) route according to the method steel vial (powder: ball ratio = 1:10).…”