The Electrochemical Behavior of Magnesium Metal Negative Electrode in Rechargeable Hybrid-Ion Cells

Mg metal secondary battery using Mg metal negative electrode has attracted much attention because Mg metal has large capacity (2205 mA h g^-1, 3833 mA h cm^-3), intrinsically safe, and, large resource abundance. Recently, it was reported that the electrolytes have electrochemical window more than 3.0 V and shows highly reversible Mg metal deposition-dissolution (1-3).

On the other hand, only a few positive electrodes for Mg secondary battery which is working as ion intercalation/de-intercalation principle like commercial Li-ion secondary battery were found to be electrochemically active. Most of them showed large over potential and irreversible capacity compared with Li⁺-ion intercalation/de-intercalation. The divalent ion is intrinsically difficult to diffuse in the host structure because of the large polarization, and structure transformation to keep localized charge neutral (4).

Instead of the intercalation/de-intercalation of Mg²⁺-ion into positive electrode material, by introducing Li salt such as LiCl, and LiBH₄ into the electrolyte, typical Li-ion battery positive electrode material are reported to work as single Li⁺-ion intercalation/de-intercalation or accompanying Mg²⁺-ion co-intercalation/de-intercalation. They showed much improved electrochemical activity compared with Mg²⁺-ion intercalation/de-intercalation. Besides, under the existence of Li⁺-ion, Mg metal negative electrode showed stable cyclablity with high coulombic efficiency (5, 6).

Considering the problem of Li resource scarcity, using other abundant alkali metal cation such as Na⁺-ion is more desirable. However, there are few reportes on Mg²⁺- and Na⁺-ion hybrid cell yet to the best of our knowledge. Besides, Na-Mg alloy phase was not observed although Li forms alloy with Mg (7).

In this contribution, the electrochemical behavior of Mg metal negative electrode in the case of Mg²⁺- and Na⁺-ion hybrid-ion cells will be discussed. To begin with, NaCl was considered as Na salt. The solubility of NaCl in Glymes was very poor in contrast to LiCl. However, it was found to dissolve in the electrolyte if Mg(TFSA)₂ was added. Fig. 1 shows a typical cyclic voltammogram in 0.2 M 2Mg(TFSA)₂-NaCl / Diglyme.  The reversible metal deposition/dissolution curve was observed. To investigate the precise Na content during the deposition, chronoamperometry was conducted at -2.8 V vs. AgCl/Ag. Deposited metal was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurement and Mg molar ratio was 99.9%. The deposited metal was confirmed to be Mg metal. We did further analysis such as XRD, SEM/EDX measurements and the results will be presented. In conclusion our work demonstrates that electrochemical Mg metal deposition/dissolution from the electrolyte containing both Na and Mg salts are feasible. This work is crucial for the development of Na-Mg hybrid ion batteries.

References 

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