Electrolyte Additives for Mg Stripping Reaction in Lithium-Magnesium Hybrid Batteries

Abstract

Rechargeable magnesium batteries have the advantages of low cost and high theoretical capacity without dendrite formation. However, the Mg batteries have critical problem of sluggish kinetic by the slow solid-state diffusion of highly polarizing divalent Mg-ions in most intercalation hosts. This problem can be overcome by hybrid batteries combining a Mg-metal negative electrode and fast lithium intercalation hosts when the Li+ ions are used with Mg2+ ions. Although two kinds of cations exist in the electrolyte, the Li ions can be easily intercalated into the host positive electrode materials and the Mg ion can be electroplated and stripped at the negative electrode because of its relatively higher standard reduction potential. However, the Mg metal electrode still have problem of the formation of the blocking layer between the magnesium metal surface and electrolyte. In case of using the Mg(TFSI)2 and glyme-based electrolyte, the huge overpotential occurs in the Mg stripping reaction during discharging process. We found that the electrolyte additives containing the halogen anion such as Cl- and Br- reduced the overpotential of the Mg stripping reaction. The Li4Ti5O12 (LTO) and mechanically polished Mg disk were used the electrode materials and the 1.50 M LiTFSI and 0.15 M Mg(TFSI)2 in glyme:diglyme=1:1 solution (standard electrolyte) was used as dual salt electrolyte. This cell showed a negative discharging voltage (about -1.0 V) due to the huge Mg stripping overpotential while the charging reaction occurs at 0.8 V. After the addition of 0.1 M LiCl, 0.1 M LiBr, or 0.5 M MgCl2, the discharging voltage increased to +0.5 V from the 2nd cycle after activation process during 1st cycle while the charging voltage was almost close to the halogen-free electrolyte. In spite of the increase of the halogen anion concentration, the further improvement was not detected. When the HMDS (hexamethyldisilazane) was added to the electrolyte with the LiCl, the overpotential of Mg stripping reaction was sharply decreased from the first cycle without activation process. But the overpotential did not decrease in case of the HMDS addition to the standard electrolyte composition (halogen-free). Therefore, the Mg/LTO hybrid cell with dual salt electrolyte containing halogen anion and HMDS showed good electrochemical performances with low polarization from the initial cycle.