Sulfur-Carbon Composite Cathodes for Aluminum-Anion Rechargeable Battery with an Inorganic AlCl3–NaCl–KCl Ionic Liquid
Authors: Tsuda Tetsuya , Sasaki Junya , Uemura Yuya , Kojima Toshikatsu , Senoh Hiroshi , Kuwabata Susumu 
Published: December 2020

Aluminum-anion rechargeable battery (AARB), in which chloroaluminate anions control the cathodic and anodic reactions, has been getting a lot of attention recently, because Al metal has several advantages, such as very high theoretical capacity (2980 mAh g-1, 8046 mAh cm-3), less expensive and mass consumable material, and moderate reactivity in air. It is known that Al metal deposition/stripping reversibly proceeds in Lewis acidic chloroaluminate melts.1

4[Al2Cl7]- + 3e- ⇌ Al + 7[AlCl4]-

In general, AARB uses layered sp2 carbon materials as cathode active materials that can cause the following electrochemical reaction.1

nC + [Al anion]- ⇌ C
n
[Al anion] + e- (Al anion: [AlCl4]- and [Al2Cl7]-)
These materials show a favorable coulomb efficiency over 99 % and are available for rapid charge-discharge. But, the charge-discharge capacities are only 60 ~ 90 mAh g-1, if the electrolyte is Lewis acidic organic chloroaluminate ionic liquids (ILs). Although, in the inorganic ones, e.g., AlCl3–NaCl–KCl, the capacities are over ca. 130 mAh g-1,2 it is still disportionate to high capacity of Al metal anode. Therefore, novel high capacity cathode active materials are required to design appealing AARBs. In this study, we have applied the sulfur-carbon composite material (SPEG) synthesized from the mixture of sulfur and polyethylene glycol3 to the cathode active material for AARB. Electrochemical behavior of the SPEG electrodes was examined in an inorganic 61.0-26.0-13.0 mol% AlCl3–NaCl–KCl IL (eutectic point: 366 K).
The procedure used for the preparation of SPEG was identical with that described in previous articles.2 SPEG composite electrodes were prepared by pressing the mixtures of x wt% SPEG, 100-x-5 wt% conductive additive (ketjen black (KB) or multi-walled carbon nanotube (MWCNT), and 5 wt% polytetrafluoroethylene (PTFE) onto molybdenum (Mo) plate current collectors. Al metal plates were employed as anodes. After purification of AlCl3, NaCl, and KCl, the inorganic AlCl3–NaCl–KCl IL was obtained by heating the mixture consisting of 61.0 mol% AlCl3, 26.0 mol% NaCl, and 13.0 mol% KCl at 403 K. The final product was clear and colorless. Electrochemical experiments were carried out using a two-electrode type sealed cell set up in an electric furnace with a temperature-control device. The experimental temperature was 393 K. All the procedures are conducted in an Ar gas-filled glove box with O2 and H2O < 1 ppm.
Figure 1 shows cyclic voltammograms recorded at Mo and SPEG composite electrodes in the two-electrode type sealed cell with a 61.0-26.0-13.0 mol% AlCl3–NaCl–KCl electrolyte at 393 K. There is no change in current density in the voltammograms at the Mo electrode, implying that Mo electrode is usable for the current collector in AARB. As to the SPEG composite electrodes, a pair of reduction and oxidation waves appeared at ca. 1.05 V and 1.25 V, respectively. However, the shapes of voltammograms depended on the conductive additive species and their weight ratio. In the case of the KB conductive additive, redox waves with the influence of electric double layer capacitance were observed, when the weight ratio was over 45 wt%. It was difficult to see the waves below 45 wt%. Meanwhile, the use of MWCNT was very effective to improve the cathode performance, and clear redox waves appeared at the additive amount of 10 wt%. Given the fact that electrochemical behavior of the sulfur in an organic AlCl3–[C2mim]Cl (1-ethyl-3-methylimidazolium chloride) IL,4 the redox waves observed in Fig. 1 would originate in the following reaction.
8[Al2Cl7]- + 6e- + 3S ⇌ Al2S3 + 14[AlCl4]-

These results suggest that the SPEG composite electrodes can work as the cathode in AARB. Charge-discharge tests were conducted under various conditions. Favorable rate capability comparable to the layered sp2 carbon materials was recognized.1 Besides, the SPEG electrodes had very high cathode capacities. The SPEG cathode with KB (45 wt%) showed 323 mAh (g-SPEG)-1 at 5000 mA (g-SPEG)-1. If the MWCNT was employed as a conductive additive, the capacity was decreased to 266 mAh (g-SPEG)-1 at 5000 mA (g-SPEG)-1, but the cyclability was stable up to 600 cycles. From these findings, we concluded that SPEG composite electrodes prepared in this research are promising cathodes for AARB.
This research was supported by the MIRAI program (grant number JPMJMI17E9), JST.

References

T. Tsuda, G. R. Stafford, and C. L. Hussey, J. Electrochem. Soc., 164, H5007 (2017) and references therein.

C.-Y. Chen, T. Tsuda, S. Kuwabata, and C. L. Hussey, Chem. Commun., 54, 4164 (2018).

T. Kojima, H. Ando, N. Takeichi, and H. Senoh, ECS Trans., 75, 201 (2017).

T. Gao, X. Li, X. Wang, A. J. Pearse, K. J. Gaskell, et al., Angew. Chem. Int. Ed., 55, 9898 (2016).

Figure 1