The Effect of Chloride Ion in the Electrolyte of Al Rechargeable Battery
Authors: Chiku Masanobu , Takeda Hiroki , Higuchi Eiji , Inoue Hiroshi 
Published: February 2020

Introduction

Mg metal is considered as a promising negative electrode material for the post Li-ion battery. However, we focused onto Al rechargeable battery because it shows highest energy density per volume (8043 mAh cm-3) and Al metal is abundant on the earth. Moreover, Al is stable in the atmosphere, therefore, Al rechargeable battery is essentially safe in the case of accidents.
There are limited variety of materials as a candidate for the electrolyte of Al rechargeable battery because the number of the electrolyte which enable reversible electrochemical deposition and dissolution of Al metal is limited. Ionic liquid is one of the most popular electrolyte for Al deposition and it also used as the electrolyte for Al rechargeable battery. Jayaprakash et al. reported that they successfully constructed the rechargeable aluminum-ion battery using ionic liquid electrolyte, Al metal negative electrode and V2O5 positive electrode1). In contrast, Reed et al. suggested the V2O5 was not electrochemically active material and the iron and chromium in the stainless steel current collectors acted as positive electrode materials2). We used a mixture of aluminum chloride and dipropylsulfone (DPSO2) as electrolyte and constructed Al rechargeable battery using Al metal negative electrode and V2O5 positive electrode3). Our battery could be operated with the specific capacity about 150 mAh g-1 for 30 cycles. However, the electrochemical window of the electrolyte was limited by the oxidation of contained chloride ion and it constricted the cell voltage of Al rechargeable battery.
In this study, we report the effect of chloride ion in the mixture of aluminum chloride and DPSO2 as the electrolyte for Al rechargeable battery.

Experimental

DPSO2/AlCl3 electrolytes were prepared by mixing DPSO2, AlCl3 and toluene with mole ratios of X:1:5 (0.5≦X≦10). All electrochemical measurements were examined with a two-electrode glass cell. A Mo plate was employed as the working electrode while an Al plate was employed as the counter and reference electrodes.
The chloride ion free DPSO2/AlCl3 = 10 electrolyte was prepared using AgBF4. AgBF4 was added into the DPSO2/AlCl3 = 10 electrolyte and chloride ion was precipitated as AgCl. The supernatant liquid was used as chlorine free DPSO2/AlCl3 = 10 electrolyte.

Results and discussions

Figure 1a shows the CV of Mo electrode in the DPSO2 + AlCl3 + toluene solutions with different DPSO2/AlCl3 ratios. CV with the DPSO2/AlCl3 = 10 electrolyte showed reversible deposition and dissolution of Al metal. On the other hands, the DPSO2/AlCl3 = 1 electrolyte was electrochemically inactive. Figure 1b shows the NMR spectrum of these electrolytes. [Al(DPSO2)3]3+ was the main product in the DPSO2/AlCl3 = 10 electrolyte, whereas AlCl3-DPSO2 was the main product in the DPSO2/AlCl3 = 1 electrolyte. We suspected that almost all of chlorine was combined with Al ion in the DPSO2/AlCl3 = 1 electrolyte. To investigate the effect of chloride ion, we measured XPS of Al metal sheet following impregnation into these electrolytes. It clearly showed the chlorine content inside the Al oxides layer on the Al plate impregnated into the DPSO2/AlCl3 = 1 electrolyte was decreased.
We also prepared the chloride ion free DPSO2/AlCl3 = 10 electrolyte. NMR and CV measurement results showed that almost all of Al was existed as [Al(DPSO2)3]3+ in the chloride ion free DPSO2/AlCl3 = 10 electrolyte and this electrolyte was electrochemically inactive. These features suggests the free chlorine content is essential for reversible Al deposition/dissolution reaction.

Reference

N. Jayaprakash, S. K. Das and L. A. Archer, Chem. Commun.,
47, 12610 (2011)

L. D. Reed and E. Menke, J. Electrochem. Soc., 160, A915 (2013)

M. Chiku, H. Takeda, T. Kunisawa, E. Higuchi and H. Inoue, 224th ECS Meeting # 521