Carbon Material-Based Positive Electrodes for Aluminum Secondary Battery with a Chloroaluminate Ionic Liquid
Authors: Tsuda Tetsuya , Uemura Yuya , Chen Chih-Yao , Kokubo Itsuki , Kuwabata Susumu 
Published: February 2020

Secondary batteries have already been utilized widely in various equipment, e.g., electric vehicle and cell-phone, but higher power density and capacity are required for supporting our modern life more than ever before. In order to achieve it, novel battery systems that are expected to take the place of conventional Li-ion battery (LIB) are proposed by a great number of research groups around the world. Above all, secondary battery using a multivalent-metal ion, e.g., Mg(II) and Al(III), has been expected to be a next generation battery because it can flow a larger current exceeding a conventional LIB. In a Lewis acidic haloaluminate ionic liquid (IL), it is well-known that aluminum can be electrochemically deposited with a high current efficiency. It suggests that the electrode reaction is a favorable one for constructing the Al secondary battery with the chloroaluminate IL. In fact, some remarkable researches on the Al secondary battery with haloaluminate ILs have been reported since the 1980.1-4 In this study, we have investigated the electrochemical behaviors of the various carbon positive electrodes in the Lewis acidic 60.0-40.0 mol % AlCl3–1-ethyl-3-methylimidazolium chloride (AlCl3–[C2mim]Cl) IL.
The procedures used for the preparation and purification processes on the AlCl3−[C2mim]Cl IL were identical to those described in our articles. All the electrochemical experiments were conducted using a three-electrode beaker cell. A coiled Al wire was employed for the negative electrode. Five types of carbon materials were exploited to fabricate the positive electrode for an Al secondary battery. Those were an activated carbon fiber cloth covered with or without a graphene layer, a basal or an edge plane pyrolytic graphite, and a graphite-leaf powder. The reference electrode was an Al wire immersed in a neat 60.0-40.0 mol % AlCl3–[C2mim]Cl, if needed, but was separated from the bulk IL by porosity E glass frits. All experiments were carried out in an Ar gas-filled glove box with O2 and H2O < 1 ppm.
Cyclic voltammograms recorded at different carbon positive electrodes in the 60.0 mol % AlCl3–[C2mim]Cl are shown in Fig. 1. At an activated carbon fiber cloth electrode, only an oxidation reaction appeared at the potential over ca. 2.25 V, and then electric double layer capacitance was also recognized due to the porous properties. But if the graphite (not shown) and graphene-coated electrodes were employed, several redox waves appeared. A small oxidation wave observed at ca. 1.70 V (vs. Al(III)/Al) should be the following electrochemical intercalation reaction.

n C + [AlCl4]- ⇄ C
n

+[AlCl4]- + e-

However, graphite electrodes, especially an edge plane pyrolytic one, were seriously damaged by the voltage sweep over 2.30 V. After the experiment, carbon powder precipitated in the IL electrolyte. In order to improve the behavior, graphite-leaf powder composite electrode was fabricated by using polymer binders that are stable in the IL. A typical voltammogram recorded at the electrode are shown in Fig. 1. Two clear oxidation waves appeared before increasing the oxidation current corresponding to the electrochemical oxidation of [AlCl4]-. It suggests that graphite intercalation compounds with different stages, e.g., stage 4, are produced during the anodic potential sweep.3 Based on the findings described above, we designed several Al secondary battery systems with different positive electrodes. Of these, use of the graphite-leaf powder composite electrode enabled high coulomb efficiency and discharge capacity up to 99 % and 80 mAh g-1, respectively, when the cut-off voltages were 0.80 and 2.10 V at the charge-discharge rate of the 50 mA g-1. The electrode performance strongly depended on the amount of the active material. This positive electrode will be one of the choices for designing the Al secondary battery.

Acknowledgement

Part of this research was supported by the Grant-in-Aid for Scientific Research, Grant Numbers 15H03591, 15K13287, and 15H2202 from the Japanese Ministry of Education, Culture, Sports, Science and Technology and by the ALCA-SPRING program, Japan Science and Technology Agency.

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Figure 1