Effect of Supporting Electrolytes on AlCl3/Diglyme Aluminum Electrodeposition Bath
Authors: Kato Yukiya , Kitada Atsushi , Fukami Kazuhiro , Murase Kuniaki 
Published: February 2020

Aluminum (Al) is an important resource for manufacturing due to its mechanical and electrochemical properties as well as its natural abundance. With its high energy density and low negative reduction potential, Al is an attractive candidate for negative electrode materials for post lithium-ion secondary batteries. However, it is well known that electrodeposition of Al metal is not possible from aqueous solutions since its redox potential is too low (–1.68 V vs. SHE); practically only the electrolysis of water occurs. Many researches were done on non-aqueous solutions such as organic solvents and room temperature ionic liquids, in which it is known that electrodeposition of Al is possible [1]. However, these electrolytes are highly volatile or highly viscous, and/or expensive for practical use. Contrastingly, glymes have boiling points over 150 °C, which makes their volatilities quite low at room temperature. Recently, we have found out that Al metal can be electrodeposited from AlCl3/diglyme solution at room temperature [2]. Here, we considered adding supporting electrolytes in order to improve the anodic stability of the electrolyte. In Mg salt/glyme solution, related research has been done. Anodic stability of Mg salt/glyme solutions have improved by adding an ionic liquid PP13-Tf2N (N-methyl-N-propylpiperidinium bis(trifluoromethyl sulfonyl) amide) as a supporting electrolyte [3]. There also is an interest in the electrochemically active species that exists with different amounts of supporting electrolytes added.
All electrochemical experiments were performed at room temperature in an Ar-filled glove box. Al plates were used as counter and quasi reference electrodes. Cu plate was used as working electrode (WE). 1-ethyl-3-methylimidazolium chloride (EMICl) was selected as a supporting electrolyte. AlCl3/diglyme/EMICl solutions with molar ratio of AlCl3:diglyme:EMICl = 1:5:1 and 2:5:1 were used as electrolytes. The former solution was prepared by mixing EMICl into AlCl3:diglyme = 1:5 solution; the latter by mixing AlCl3:EMICl = 1:1 solution into AlCl3:diglyme = 1:5 solution.
Figure 1 shows the cyclic voltammogram (CV) of AlCl3:diglyme:EMICl = 1:5:1 and 2:5:1 solution. Range is between +0.8 V to –1.0 V vs. Al QRE with the scan rate of 20 mV s–1. In the CV of AlCl3:diglyme:EMICl = 1:5:1 solution (Figure 1(a)), two peaks were observed in both oxidation and reduction current. Peaks C1 and Al likely correspond with the redox of Al. Peaks C2 and A2 likely correspond with the redox of Cu. In the CV of AlCl3:diglyme:EMICl = 2:5:1 solution (Figure 1(b)), redox of Al was suggested, but the current was smaller than that of AlCl3:diglyme:EMICl = 1:5:1 solution.
This work was financially supported by Grant-in-Aid for Scientific Research (A) (No. 25249106 and No. 16H02411), Grant-in-Aid for challenging Exploratory Research (No.15K14193), and Grant-in-Aid for Young Scientists (B) (No. 15K18253) from the Japan Society for the Promotion of Science (JSPS). We also thank Iketani Science and Technology Foundation for their financial support (No. 0271016-A).
[1] Y. Zhao and T. J. VanderNoot, Electrochim. Acta, 42(1), 3 (1997).
[2] A. Kitada, K. Nakamura, K. Fukami, and K. Murase, Electrochemistry, 82(11), 946 (2014).
[3] A. Kitada, Y. Kang, K. Fukami, R. Hagiwara, and K. Murase, J. Electochem. Soc., 162(8) D389-D396 (2015).

Figure 1