The Development of Ester-Based Electrolyte Solutions for Magnesium Rechargeable Batteries

Abstract

Introduction Magnesium (Mg) metal has attracted attention as an active material of a negative electrode for next-generation innovative rechargeable batteries because of its high volumetric capacity (3800 mAh cm-3) and low standard electrode potential (-2.73 V vs. NHE). High safety during charge/discharge cycles is also expected because Mg should be deposited uniformly without dendritic growth. Electrochemical deposition/dissolution reactions of Mg have been already reported using ether- and sulfolane-based electrolyte solutions such as Grignard reagents. However, the polarization of the Mg deposition/dissolution reactions was very high1). In addition, these electrolyte solutions are easily oxidized, and corrode aluminum current correctors. Therefore, the development of electrolyte solutions is necessary to realize magnesium rechargeable batteries with high working voltages above 3 V. In this study, we focused on ester-based electrolyte solution which is relatively stable against oxidation. We prepared electrolytes consisting of gamma-butyl lactone as a solvent and Mg(BF4)2 as a salt for magnesium rechargeable batteries. The electrochemical deposition/dissolution reactions of magnesium on Cu electrodes were investigated. Experimental The electrolyte solution used in this study was 0.1 mol dm-3 Mg(BF4)2 dissolved in gamma-butyl lactone, which was prepared in a glove box filled with Ar gas with a dew point below -80 oC. The water content was determined by Karl Fischer titration. The working, counter, and reference electrodes were Cu foil, Mg wire, and Ag/Ag+ electrode, respectively. Cyclic voltammetry was carried out between -1.00 and -4.00 V vs. Ag/Ag+ at a scan rate of 1 mVs-1 using three-electrode cells. Results and discussion 0.1 mol dm-3 Mg(BF4)2/GBL used in this study contained 680 ppm water. Figure 1 shows cycle voltammograms of Cu in 0.1 mol dm-3 Mg(BF4)2/GBL at the first and fifth cycles. The cathodic current was observed at potentials below -3.3 V during the initial scan and a corresponding anodic current was seen at around -2.8 V. The reaction potentials were close to the standard potential of Mg (ca. -3.2 V vs. Ag/Ag+), and therefore the couple of redox currents should be assigned to deposition and dissolution reactions of magnesium on Cu. These results show that reversible deposition/dissolution reactions of magnesium can proceed in ester-based electrolyte solution. Mg(BF4)2 is considered as a promising salt for electrolyte solutions for magnesium rechargeable batteries. However, cycle performance needs to be improved; both the cathodic and anodic current densities decreased with cycle number, as shown in Fig.1. Charge/discharge characteristics and surface morphology of the electrode will be shown in our poster presentation. Reference 1) D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, R. Turgeman, Y. Cohen, M. Moshkovich, and E. Levi, Nature, 407, 724 (2000). Figure 1