Investigation of Discharge Products of Mg-Air Battery with Non-Aqueous Ether-Based Electrolytes
Authors: Inoue Hiroki , Akita Yasuhiro , Munakata Hirokazu , Kanamura Kiyoshi 
Published: February 2020

Introduction

Metal-air batteries are expected as one of the post lithium ion batteries. Many researchers have focused on strategies enabling the use of high energy cathode such as O2 and Li metal anode, e.g. Li-air batteries. However lithium metal leads to some problems such as low safety due to a formation of lithium metal dendrite. Recently, it has been claimed that multi valence metal anodes such as magnesium metal realize higher energy density of battery. They may offer large specific/volumetric energy density as a negative electrode owing to two electron transfer redox by Mg2+. In addition, large natural abundance of Mg resources and its chemically stable nature allows a utilization to battery. Despite such remarkable potential, cycling behavior of Mg metal anode coupled with air cathode (O2and oxide anions) has been unexplored yet, and reaction products with superoxide anions and Mg ions is still unknown. Here, we studied the reaction mechanism of Mg with superoxide anions in non-aqueous ether-based electrolytes by cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) to investigate the discharge product and electrochemical behavior during oxygen reduction reaction with superoxide anions and Mg ions.

Experimental

Triglyme (G3), Magnesium bis(trifluoromethane sulfonyl)imide (Mg(TFSI)2) and tetraethylammonium bis(trifluoromethane sulfonyl)imide (TEATFSI) were purchased and used as received. The electrolyte solutions were prepared by mixing salts and G3 in the Ar-filled glove box, and the molar ratio of salt toward G3 was fixed at 1/5. Cyclic voltammetry was performed with a typical three-electrode cell in which a glassy carbon (GC) disk (3 mm) and a platinum wire were used as a working electrode and a counter electrode, respectively. A reference electrode was fabricated by soaking Ag wire in (0.01 M AgNO3 + 0.1 M Mg(TFSI)2)/G3, confined in a glass tube with a liquid junction of a Vycol glass. Mg(TFSI)2/G3 (1:5 in mol) and TEATFSI/G3 (1:5 in mol) solutions were saturated with O2 by bubbling for 1h before measurements. The CV measurements of GC electrode at a scan rate of 50 mV s−1 were performed in two electrolytes (Mg(TFSI)2/G3 and TEATFSI/G3) solutions, to clarify the role of Mg2+ ion during charge and discharge reactions. The surface of GC electrode after cathodic polarization at a constant voltage of −1.35 V vs. Ag/Ag+was subsequently characterized by XPS.

R
esults and Discussion

Figure 1 shows the cyclic voltammograms of GC electrode in two electrolytes (Mg(TFSI)2/G3 and TEATFSI/G3) . The electrochemical response of GC electrode strongly depends on the constituent cation in the electrolytes. For TEATFSI electrolyte, redox couple of oxygen, e.g. O2 + e− → O2
− at −1.3 V vs. Ag/Ag+ and O2
− → O2 + e− at −1.2 V vs. Ag/Ag+, were clearly observed, indicating sufficient stability of TEA+ cation (and also TFSI− anion) made capable to be cycled with superoxide O2
− anion. In contrast, no anodic current was found before oxidative decomposition of the electrolyte (at +1.5 V vs. Ag/Ag+) for the Mg(TFSI)2/G3 electrolyte while the reduction of O2 to O2
− would be took place. XPS spectra suggest the formation of MgO and MgF2 on the working GC electrode during cathodic scan. As MgO and MgF2 are difficult to be decomposed electrochemically because of their high stability. Precipitates disturb favorable electrochemical reaction of oxygen with Mg, thereby poor cycling ability was observed.

Reference

[1] Catalytic Cycle Employing a TEMPO−Anion omplex to Obtain a Secondary Mg−O2 Battery

Figure 1