Development of Novel Electrolyte for Rechargeable Aluminum Battery with a Wide Potential Window

Introduction As a post lithium ion battery, new rechargeable batteries with metal negative electrodes, which are oxidized to multivalent cations during discharge, are actively being investigated because they are expected to have high power density, high energy density and low cost. Mg is the most popular material for the multivalent cation battery because it shows higher energy density (3837 mAh cm-3) than Li (2062 mAh cm-3). On the other hand, Al has twofold larger energy density (8043 mAh cm-3) than Mg. Thus Al rechargeable battery is also a promising next generation power source. However, there are several challenges to realize the Al rechargeable battery. One of the challenges is to develop a new electrolyte solution which can electrochemically deposit/dissolve Al metal. The current electrolyte solutions including AlCl3 1,2 show narrow potential windows below 2.5 V because of the decomposition of chloride, so they are unsuitable for high voltage rechargeable battery. Therefore replacing chloride with other anions is only the way to widen the potential window or to construct high-voltage rechargeable Al battery. In this study, we synthesized aluminum(Ⅲ) bis(trifluoromethanesulfonyl)imide (Al(TFSI)3) and dissolved it into acetonitrile (AN) with high dielectric constant to prepared chloride-free electrolyte solution. And we investigated electrochemical Al deposition/dissolution behavior in the chloride-free electrolyte solution. Experimental Al(TFSI)3 was prepared as follow: AlCl3 and H-TFSI (1:3 in mole ratio) were dissolved into AN, followed by drying. The electrolyte solution was prepared by dissolving Al(TFSI)3in AN with the mole ratio of 1:38, and put in a glass cell. All electrochemical measurements were performed after aging the electrolyte solution at 80 °C in an Ar atmosphere for 24 h. A Mo plate was used as working electrode, an Al plate was used as counter electrode and an Al wire was used as reference electrode for all experiments. A glass fiber filter was used as separator. Results and Discussion Figure 1 shows the a CV of Mo plate in the chloride-free electrolyte using Al(TFSI)3and AN. Onset potentials for the reduction current due to Al deposition and the oxidation current due to Al dissolution were ca. -0.3 V and ca. 1.0 V vs. Ag wire, respectively. The current for the oxidation of AN was observed at ca. 2.8 V vs. Ag wire. This suggests that the potential window of the chloride-free electrolyte is over 3 V, which is larger than conventional chloride-containing electrolyte. Figure 2 shows the SEM image of the deposits on Mo electrode after electrochemical deposition at -0.2 V vs. Al/Al3+for 7 h. EDX analysis indicated the deposits were Al, which means the Al deposition reaction certainly proceeded in the chloride-free electrolyte. The surface of deposited Al was rough but not dendritic, suggesting this electrolyte is suitable for battery application. Figure 3 shows charge/discharge curves of the Mo negative electrode and Al positive electrode. The overpotentials for Al deposition/dissolution on the Mo negative electrode were quite small, indicating that these reactions reversibly proceeded at the Mo plate. On the other hand, the overpotentials on the Al positive electrode were larger than on Mo negative electrode. This indicates that the surface of Al electrode was covered with a native Al2O3layer, which prevented Al deposition/dissolution reaction. Acknowledgement This work was partially supported by JSPS KAKENHI Grant Number 16K21288. References M. Chiku et al., ECS Appl Mater. Interfaces, 7, 24385 (2015). M. Lin et al., Nature, 520, 324 (2015). Figure 1