Magnesium rechargeable batteries have much attention for next generation batteries because of the high theoretical volumetric capacity, safety, and abundance of magnesium metal anode. In order to improve the efficiency of magnesium metal anode, it is required to develop the electrolyte that enables magnesium deposition and dissolution reversibly. Although Grignard reagents have been widely known as the electrolyte for magnesium deposition/dissolution^[1], the stability of the electrolytes in high potential and their corrosive nature are major problems. In recent years, magnesium deposition/dissolution in magnesium inorganic salts with triglyme solvents have been reported^[2,3]. For the further development of electrolyte for magnesium batteries, the reaction mechanism at electrolyte/magnesium metal interface should be elucidated. In this study, we have developed operando soft X-ray measurement system and apply to the analysis of electronic and local structure of electrolyte during magnesium deposition. Furthermore, bulk structure of electrolyte is investigated by Raman spectroscopy, X-ray absorption spectroscopy (XAS) and the theoretical calculation.

  0.5 M Mg(TFSA)₂/triglyme and 0.5 M Mg(TFSA)₂/2-MeTHF electrolytes were prepared by mixing Mg(TFSA)₂ (KISHIDA CHEMICAL Co., Ltd., 99.9%>) with triglyme (KISHIDA CHEMICAL Co., Ltd. 99%>) or 2-MeTHF (Merck, 98%>) in an Ar-filled glove box. Electrochemical properties of the electrolyte were investigated by cyclic voltammetry (CV) with the three-electrode cell. Platinum plate and magnesium rod were used as working electrode and counter electrode, respectively. Operando XAS measurements at Mg K-edge were carried out at the beam line BL27SU at SPring-8 (Japan). XAS spectra were obtained at different potential during magnesium deposition. Raman spectroscopic measurements were carried out using LabRAM HR-800 (HORIBA, Ltd.) equipped with He-Ne laser (633 nm) at room temperature.

   While magnesium deposition/dissolution is observed in Mg(TFSA)₂/triglyme and Grignard system, magnesium is not deposited in Mg(TFSA)₂/2-MeTHF. From the results of Raman spectroscopy, TFSA anion exists as uncoordinated state with magnesium ion in the triglyme system, while TFSA anion coordinates with magnesium ion in the 2-MeTHF system. Fourier transform from Mg K-edge EXAFS oscillation measured at different potentials in 0.5 M Mg(TFSA)₂/triglyme and 0.5 M Mg(TFSA)2/2-MeTHF indicate that peak intensity of Mg-O bond decreases with decreasing potential.The decreases of peak intensity reflect the desolvation process of triglyme and 2-MeTHF from magnesium ion. These results suggested that reason of high overpotential in 0.5 M Mg(TFSA)₂/2-MeTHF is specific adsorption of TFSA anion. The specific adsorped TFSA anion may inhibits charge transfer raction of magnesium ion.

ACKNOWLEDGMENTS

 This study was partially supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) Project under the auspices of New Energy and Industrial Technology Development Organization (NEDO), Japan. Synchrotron radiation experiment was performed at BL27SU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI).

REFERENCES

[1]  Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich M.; Levi, E., Nature, 407, 724 (2000).

[2]  Ha, S.-Y.; Lee, Y.-W.; Woo, S.W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K.T.; Choi, N.-S., ACS Appl. Mater. Interfaces 6, 4063 (2014).

[3] Orikasa, Y.; Masese, T.; Koyama, Y.; Mori, T.; Hattori, M.; Yamamoto, K.; Okado, T.; Huang, Z. -D.; Minato, T.; Tassel, C.; Kim, J.; Kobayashi, Y.; Abe, T.; Kageyama, H.; Uchimoto, Y., Sci. Rep., 4, 5622 (2014).