Identification of the Active Chemical Species in the Sulfone Based Electrolyte for Rechargeable Aluminum Batteries
Authors: Chiku Masanobu , Miyake Takumi , Higuchi Eiji , Inoue Hiroshi 
Published: December 2020

Introduction
The Li-ion battery is the most dominant as practical rechargeable batteries because of its high energy density, high power density, and high cyclability. However, the energy density of the Li-ion battery is not sufficient for the electric vehicle (EV), which can realize the practical driving range as long as classical gasoline and the internal combustion engine system. Multivalent cation batteries are considered as one of the most promising next-generation power sources for mobile electric devices and EVs. The Al electrode has the highest volumetric energy density (8042 mAh cm-3), which is 4-fold higher than the Li metal electrode (2060 mAh cm-3). Therefore, the rechargeable Al battery has the possibility for the high energy density power sources for EVs. Moreover, Al metal is safer than Li metal because of its high melting point and has higher reserves on the earth than Li.
Even though there are several advantages of the rechargeable Al battery, such as high energy density, high safety, and low cost, however, it is difficult to realize rechargeable Al battery because of two factors. The first one is the positive electrode material that can store the Al trivalent cation which strongly adsorb to the positive electrode materials reducing the cyclability of the battery. The second one is the electrolyte solution, which can reversibly deposit/strip Al metal. There are several amounts of researches about rechargeable Al battery, however, almost all of those researches used the solution of AlCl3 in the ionic liquid, 1-ethyl-3-methylimidazolium chloride, as an electrolyte solution. The ionic liquid show excellent reversibility for the Al metal deposit/strip reaction and the active species in the electrolyte solution was already identified as Al2Cl7. We have used the solution of AlCl3 in dipropylsulfone (DPSO2) as the electrolyte solution of the rechargeable Al battery. Here, we tried to identify the active chemical species of the solution of AlCl3 in DPSO2 to understand the difference between our electrolyte solution for rechargeable Al battery and conventional ionic liquid-based electrolyte.
Experimental
All electrochemical experiments were measured with a glass cell. Mo plates (1.3 × 1.3 cm2) were used as working electrode and current collector. An Al plate (10 mm in diameter) and wire were used as the counter and reference electrodes for three-electrode configuration. A glass fiber filter was used as separator.
Results & discussion
Figure 1(a) shows the 27Al NMR results of the electrolyte containing different ratios of AlCl3 and DPSO2. Four different Al chemical species (AlCl4
-, Al2Cl7
-, AlCl3-DPSO2, and Al(DPSO2)3) were detected, and we analyzed the ratio of each chemical species in the electrolyte containing different ratios of AlCl3 and DPSO2 (Fig. 1b). AlCl4
- is known as inactive for the electrochemical reaction; therefore, we investigated the remaining three species.
Figure 1c shows the CV results of each electrolyte solution. We analyzed the CV result from three different points of view; over voltage for Al deposition, over voltage for Al stripping, and the electrochemical window. The electrolyte with the ratio of AlCl3: DPSO2: toluene = 1: 10 (2): 5, which was mainly consisted of Al(DPSO2)3, show small over voltage for deposition/stripping of Al and wide electrochemical window. The electrolyte with the ratio of AlCl3: DPSO2: toluene = 1: 1.1 (1): 5, which mainly consisted of AlCl3-DPSO2, show large over voltage for deposition/stripping of Al and narrow electrochemical window. The electrolyte with the ratio of AlCl3: DPSO2: toluene = 1: 0.9: 5, which mainly consisted of Al2Cl7
-, which is the same as the ionic liquid-based electrolyte solution, show small over-voltage for deposition of Al, large over voltage for the Al stripping, and narrow electrochemical potential window. These results suggested the superiority of the chemical species of Al(DPSO2)3 and our electrolyte solution of AlCl3: DPSO2: toluene = 1: 10: 5, compared with the ionic liquid-based electrolyte solution, which containing Al2Cl7
- as active species.

Figure 1