Magnesium rechargeable batteries (MRBs) have attracted attentions as next-generation secondary batteries. Mg metal is a promising negative-electrode material because it has a high volumetric capacity (3830 mAh cm^-3) and a low standard potential (-2.37 V vs. NHE). In addition, MRBs ensure a higher safety compared to lithium metal rechargeable batteries, since Mg metal forms no dendrite. We previously reported that g-butyrolactone (GBL)-based electrolyte solutions allowed the electrochemical deposition/dissolution reactions of Mg and the insertion of Mg²⁺ into Chevrel phase Mo₆S₈ positive-electrodes. Mo₆S₈ is a prototypical active material that has a wide path for rapid diffusion of Mg²⁺. However, it works at extremely low potentials (1.1 V vs. Mg²⁺/Mg), and has a small theoretical capacity (129 mAh g^-1). On the other hand, MoS₂, a popular member of metal dichalcogenides, has a larger theoretical capacity (223.2 mAh g^-1) at a higher working voltage around 1.8 V ¹⁾. In this work, we synthesized MoS₂ nanoparticles with a short diffusion path of Mg²⁺, and investigated the charge and discharge properties of AZ31|MoS₂ cells using 0.3 mol dm^-3 Mg(CF₃COO)₂ in GBL as the electrolyte solution.

MoS₂ nanoparticles were synthesized by a solvothermal method ²⁾. Mo(CO)₆ powder and sulfur powder were mixed into 120 ml 2-propanol as a solvent. The solutions were stirred under heating at 80^oC for 10 h, and then the solution was cooled to room temperature naturally. The dark powders were anneled at 800^oC for 2h under Ar atmosphere. The resultant powder was characterized by an X-ray diffraction and Raman spectroscopies. The morphology was observed with a scanning electron microscope. Electrochemical measurements were conducted at a constant current of ca. 5-7 mA cm^-2 at 100^oC using a three-electrode cell. The counter and reference electrode were AZ31 foil (Mg alloy). After the initial discharge and charge, the MoS₂ electrode was analyzed by Raman spectroscopy.

The resultant MoS₂ was impurity-free, and spherical in shape with a diameter of 80-100 nm. The lattice fringe of 0.623 nm corresponds to the spacing of the MoS₂ (002) planes. The Raman bands at 378 cm^-1 and 405 cm^-1 were clearly observed, corresponding to in-plane vibrational E¹_2g mode and out-of-plane vibrational A_1g mode, respectively ³⁾. The initial discharge capacity of AZ31|MoS₂ cell was ca. 180 mAh g^-1. After the intial discharge, the Raman bands shifted to higher frequencies of 382 cm^-1 and 407 cm^-1, which is quite similar to those for the insertion of Li⁺ into MoS₂ ⁴⁾. After initial charge, each Raman band returned to its original position. These results indicated that the insertion/extraction reactions of Mg²⁺ at the MoS₂ occur in the GBL-based electrolyte solutions.

References:

1) Liang, et al., Adv. Mater., 23, 640-643 (2011).

2) Tao, et al., CrystEngComm, 14, 3027-3032 (2012).

3) Lee, et al., ACN Nano, 4 (5), 2695-2700 (2010).

4) Wang, et al., PNAS, 49, 19701-119706 (2013)