Charge-Discharge Performance of Rechargeable Organic-Magnesium Batteries Using Glyme-Based Electrolytes

Rechargeable magnesium batteries may be an option for replacing the rechargeable lithium batteries because magnesium is more widely distributed than lithium. A volumetric energy density of Mg should be greater than that of metallic Li. In addition, metallic Mg is less reactive toward ambient water and oxygen than metallic Li. To achieve rechargeable magnesium batteries, it is important to consider the combination of positive electrode and electrolyte with Mg negative electrode. We previously investigated the potential capability of 2,5-di­methoxy-1,4-benzoquinone (DMBQ) as a positive electrode material in a Mg electrolyte solution¹. Moreover, we examined some sulfone-based electrolytes by using magnesium bis(trifluoromethane sulfonyl)amide (Mg(TFSA)₂) and reported that a two-electrode cell composed of Mg|Mg(TFSA)₂/sulfone|DMBQ endured more than 50 charge-discharge cycles at 30˚C². However, the discharge voltage plateau at the 10th cycle is located at only 0.4 V.

A new class of electrolytes based on Mg(TFSA)₂ dissolved in glyme-based solvents was reported very recently^3-5. The reversible deposition and dissolution of magnesium metal was achieved in these electrolytes at ambient temperature. In the present study, we report the charge-discharge performance of DMBQ-Mg batteries using the glyme-based electrolytes.

Mg(TFSA)₂ and four glymes (monoglyme, diglyme, triglyme, and tetraglyme) were purchased from Kishida Chemical and were used without further purification. The electrolytes were prepared by dissolving Mg(TFSA)₂ in each glyme. A DMBQ positive electrode sheet was prepared by mixing DMBQ powder (Tokyo Chamical Industry), acetylene black as a conductive additive, and poly(tetrafluoroethylene) as a binder in a weight ratio of 4:5:1 and pressed onto an Al mesh current collector. Mg ribbon was used as a negative electrode. To evaluate the Mg|Mg(TFSA)₂/glyme|DMBQ system as a rechargeable battery, we tested the charge-discharge cycle performance in a two-electrode configuration using a coin-type cell (IEC R2032) at 30˚C.

The properties of a two-electrode cell consisting of Mg|Mg(TFSA)₂/glyme|DMBQ were examined as a rechargeable Mg battery. Figure 1 shows the discharge and charge curves of the cells at 30˚C. In all four cases the first discharge curve shows two potential plateaus, which was often observed for quinone-based positive electrodes in Li, Na and Mg systems¹. The discharge capacity reaches ca. 200 mAh g(DMBQ)^‑1 in Mg(TFSA)₂/monoglyme and diglyme electrolytes. The subsequent charge curve shows a plateau at above 3.0 V and the voltage slightly increases with increasing the molecular weight of glyme. This seems to be correlated to the decrease of the specific conductivity of the electrolytes. In the case of Mg(TFSA)₂/monoglyme the discharge voltage increases drastically after a few cycle. One of the reasons for this is that the overpotential of the Mg negative electrode for the dissolution decreases. By cyclic voltammetry, the large anodic current of the metallic Mg electrode in Mg(TFSA)₂/monoglyme electrolyte was observed at room temperature in the three-electrode cell. Although the capacity substantially decreases upon cycling, the cell using Mg(TFSA)₂/monoglyme retains the voltage at the discharge even after 20 cycles.

1. H. Sano et al., Chem. Lett., 41, 1594 (2012).

2. H. Senoh et al., J. Electrochem. Soc., 161, A1 (2014).

3. S.-Y. Ha et al., ACS Appl. Mater. Interf. 6, 4063 (2014).

4. Y. Orikasa et al., Sci. Rep., 4, 5622 (2014).

5. T. Fukutsuka et al., Chem. Lett., 43, 1788 (2014).