The Effect of Chloride Ion in the Electrolyte of Al Rechargeable Battery

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

1. N. Jayaprakash, S. K. Das and L. A. Archer, Chem. Commun., 47, 12610 (2011)
2. L. D. Reed and E. Menke, J. Electrochem. Soc., 160, A915 (2013)
3. M. Chiku, H. Takeda, T. Kunisawa, E. Higuchi and  H. Inoue, 224^th ECS Meeting # 521