Electrochemical Properties of Nano-Sized MoS2 Positive-Electrodes in Ester-Based Electrolyte Solutions for Magnesium Rechargeable Batteries

Abstract

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 80oC for 10 h, and then the solution was cooled to room temperature naturally. The dark powders were anneled at 800oC 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 100oC 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 A1g 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. Reference s : 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)