Electrochemical Behavior of Magnesium Alloy in Alkali-Tfsa Ionic Liquid for Magnesium – Battery Negative Electrode

Introduction

Due to a strong demand of a high capacity battery, magnesium battery has been attracting strong attention. Among the various electrolytes developed for high capacity magnesium batteries, ionic liquids possess attractive characters such as incombustibility, refractory, chemical stability, and high ion conductivity. The alkali metal TFSA salts are used at intermediate temperature between 373 and 673 K and are a promising electrolyte owing to its high electrochemical stability [1, 2]. Magnesium batteries using Mg metal negative electrode is necessary for high capacity magnesium battery. However, as far as authors are concerned, the electrochemical behaviors of Mg metal negative electrodes in the ionic liquids operated at the elevated temperatures have not been reported. In this study, the electrochemical properties of various magnesium alloy electrode were evaluated in the alkali TFSA ionic liquid at 423 K.

Experimental

The ionic liquid electrolyte was prepared by mixing CsTFSA, LiTFSA, and Mg(TFSA)₂ in 80: 10: 10 / at% fraction. Magnesium metal, Mg-5wt%Li, and ACM522 magnesium die-casting alloy were polished before being used as the working electrode. Mg₂Sn and MgZn₂alloy was prepared by heating metal mixtures in a vacuumed quartz tube. Electrochemical measurements were conducted in the glove box using a beaker cell with a potentiostat / galvanostat (Biologic Science Instruments, VMP3). Magnesium ribbon was used as the counter electrode and lithium metal was used as the reference electrode. All the experiments were conducted on a heater pan at 423 K by controlling the temperature of the ionic liquid using a thermocouple. The morphology of deposited magnesium was observed using a scanning electron microscope (SEM; JEOL Ltd., JSM-6010LA). The Mg metal surface was observed by hard X-ray photoelectron spectroscopy (HX-PES) measurements performed at the beam line BL28XU of the synchrotron radiation facility SPring-8 (Hyogo, JAPAN). The X-ray incident energy was 8 keV.

Results and discussions

The magnesium dissolution from Mg metal electrode in an alkali TFSA salts mixture consisting of LiTFSA, CsTFSA, and Mg(TFSA)₂ proceeded via breakdown of the surface films at substantial high overpotentials to show large hysteresis behavior. The results of surface characterization of Mg surface in the alkali TFSA salts at 423 K will be presented in detail at the presentation. As to prevent formation of the surface films on Mg surfaces, various Mg alloy electrodes were evaluated. The surface modified Mg alloys composed of Li, Al, Ca, misch metals, and MgZn₂ alloy electrode resulted in a larger overpotential for magnesium dissolution due to the formation of stable surface films compared to that of the pure Mg metal electrode. Lowering of Mg activity in Mg₂Sn was found effective in reducing the passivation film formation on the Mg surface. The anodic dissolution behaviors of Mg₂Sn electrode showed small hysteresis profiles, which indicated the surface films were spontaneously repaired during the magnesium dissolution. Due to a smaller ionization tendency of Sn, Sn in Mg₂Sn alloy stayed inactive against the ionic liquid and exclusively contributed in lowing Mg activity of Mg₂Sn alloy electrode. The choice of metal species is important in design of magnesium alloy negative electrodes for magnesium batteries.

Acknowledgement:

This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project from New Energy and Industrial Technology Development Organization (NEDO) of Japan. The HX-PES experiments in this study were carried out at SPring-8 under the following proposals: 2013B7601.

Reference:

[1] A. Watarai et al, J. Power Sources, 183, 724 (2008).

[2] T. Nohira et al, J. Power Sources, 205, 506 (2012).