Suppressive Effect of Fe Cations on Oxidative Electrolyte Decomposition for Magnesium Rechargeable Batteries
Authors: Yagi Shunsuke , Han Jonghyun , Ichitsubo Tetsu 
Published: December 2020

Magnesium rechargeable batteries (MRBs) have attracted considerable attention since Aurbach et al. first proposed the prototype.1 Magnesium (Mg) can be used as the active material in the negative electrode, and has a high theoretical capacity (2205 mAh g-1) and volumetric capacity (3833 mAh cm-
3), the lowest standard electrode potential (−2.36 V vs. standard hydrogen electrode) among metals useable in air, and a relatively low tendency to form dendrites.2,3 Recently, several kinds of active materials that can be used as positive electrodes have been proposed to enhance the energy density of MRBs. As described in a previous report, we have demonstrated that the Mg spinel oxide, MgCo2O4, undergoes an Mg2+ ion insertion/extraction at a relatively high potential (~2.9 V vs. Mg/Mg2+).4 In addition, spinel oxides such as MgMn2O4, MgFe2O4, MgCr2O4, and Co3O4 have also been investigated, with promising results.
Although the stability of the electrolytes has been considerably improved,5 the present electrolytes are more or less anodically decomposed on the surface of the positive electrode above ~3 V vs. Mg/Mg2+. This is insufficient for the charge (oxidation) of the positive electrode active materials with redox potentials higher than ~2.5 V vs. Mg/Mg2+. Previous studies concerning electrolytes and positive electrode active materials indicate that the variation in the active material type changes the anodic current associated with the oxidative electrolyte decomposition. Therefore, in the present study, we investigate the catalytic activity of the transition metal ions in the Mg spinel oxides for anodic electrolyte decomposition, specifically by replacing the Mn ions in MgMn2O4 and Co ions in MgCo2O4 with Fe.6

Cyclic voltammograms for Mg(Mn1
-
xFex)2O4 were measured using a three-electrode cell at a scan rate of 25 μV s-
1 in the potential range of 1.0–3.2 V vs. Mg/Mg2+, as shown in Fig. 1. In all cyclic voltammograms, the anodic current due to the oxidative decomposition of the electrolyte was observed at potentials more positive than 2.9 V vs. Mg/Mg2+. The current decreases with increasing Fe content in Mg(Mn1
-
xFex)2O4. Thus, Fe cations are catalytically less active than Mn in the oxidative decomposition of the electrolyte. Using the low catalytic activity of the Fe ions for the oxidative electrolyte decomposition, we demonstrated that the cyclability of MRBs could be significantly improved by suppressing the anodic electrolyte decomposition. In the presentation, we will demonstrate that Fe in Mg(Co1
-
xFex)2O4exhibits a similar effect on the suppression of the oxidative electrolyte decomposition.
In conclusion, although suitable electrolytes for MRBs with a sufficiently wide electrochemical potential window remain unavailable, we expect our results to constitute a new approach to improve battery performance independent of electrolyte stability.
References

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Okamoto, T. Ichitsubo, T. Kawaguchi, Y. Kumagai, F. Oba, S. Yagi, K. Shimokawa, N. Goto, T. Doi, and E. Matsubara, Adv. Sci., 1500072 (2015).

Mandai, K. Tatesaka, K. Soh, H. Masu, A. Choudhary, Y. Tateyama, R. Ise, H. Imai, T. Takeguchi, K. Kanamura, Phys. Chem. Chem. Phys., 21, 12100–12111 (2019).

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Figure 1