Aluminum Secondary Battery Utilizing Graphite-Leaf Powder As an Active Material for Positive Electrode
Authors: Uemura Yuya , Chen Chih-Yao , Tsuda Tetsuya , Kuwabata Susumu 
Published: February 2020

Secondary battery has driven various technologies forward in past decades. Now we can use a laptop PC on a fright across the Pacific without inserting a plug in a socket. Still development of secondary batteries with improved energy density and reduced cost are of significance. In particular, the volumetric energy density of a battery becomes highly important when apply it in a limited space. To meet the need, several approaches are proposed so far, such as using extremely-high capacity electrodes and multivalent-ion. Some scientists are interested in the Al of which trivalent feature provides a theoretical volumetric capacity as high as 8040 mAh cm–3, nearly four times larger than that of lithium and more than twice of magnesium.1 In fact, electrodeposition of Al has a rich history. Haloaluminate ionic liquids (ILs), allowing reversible deposition/dissolution of Al with a high coulombic efficiency, is suitable for the electrolyte. However, there are only limited researches on positive electrode materials in the haloaluminate ILs. Because of this, most of Al secondary batteries show limited capacity or inferior rate performance.2-4 Herein we report that graphite-leaf powder can be utilized as a new positive electrode material for rechargeable Al batteries, which exhibits a good rate performance and a stable cyclability up to 1,000 cycles.
The procedures used for the preparation and purification processes on the AlCl3−[C2mim]Cl IL were identical to those described in our previous articles. In this study, 60.0-40.0 mol% AlCl3−[C2mim]Cl IL was used for the electrolyte. For the positive electrode, after graphite-leaf powder was thoroughly dispersed in N-methyl-2-pyrrolidone solution containing suitable amount of polysulfone binder, the resultant slurry was coated onto a molybdenum current collector and dried in vacuo at 473 K overnight. Graphite-leaf powder used in this study has an averaged thickness of 6-8 nm and thickness of 5 µm. Al coil and Al wire of high purity were used as a negative and reference electrode, respectively. Cell assembly and electrochemical measurements were conducted in an Ar-filled glovebox.
Cyclic voltammogram recorded at a graphite-leaf powder electrode in the 60.0 mol% AlCl3−[C2mim]Cl showed several redox waves during the anodic scan from the rest potential. Those waves should be relate to the electrochemical intercalation reaction of the [AlCl4]– as follows:5

n C + [AlCl4]- ⇄ C
n

+[AlCl4]- + e-

Although graphite rod electrode yields carbon powder precipitation after the electrochemical measurements over ca. 2.3 V ((vs. Al(III)/Al)), the graphite-leaf powder electrode was very stable. This favorable behavior would be caused by the flexibility of the graphite-leaf powder. Figure 1 shows the typical charge-discharge curves of graphite-leaf powder electrode. The cut-off voltage range was 0.80 to 2.40 V. Quasi voltage plateaus was observed at approximately 2.30 V, which is higher than previous reported cathodes.2-4 A reversible capacity of 70 mAh g–1 was obtained at current density of 500 mA g–1. These values were estimated from the amount of graphite-leaf powder on the positive electrode. Increasing the current density did not cause a distinct capacity decline, implying that the intercalation/deintercalation of [AlCl4]– anion readily proceeds on the electrode. The electrode performance was certainly enhanced by further improvement of the electrode fabrication method, e.g., the optimization of the type of binder and of the composition in slurry, the use of conductive additive.

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