Effect of Electrolyte Composition on Positive Electrode Properties of Rechargeable Aluminum-Graphite Batteries
Authors: Yang Xue , Higuchi Eiji , Chiku Masanobu , Inoue Hiroshi 
Published: December 2024

Introduction

The use of fossil fuels and the resulting greenhouse gas emissions, particularly CO2, have caused significant environmental damage. Rechargeable batteries are attracting attention as a storage application for renewable energy, and the emergence of next-generation rechargeable batteries that exceed the energy density of lithium-ion batteries is eagerly awaited. Aluminum is a promising material for batteries due to its cost-effectiveness, chemical stability in air, safety, and ease of handling. The oxidation of Al is a 3-electron reaction, resulting in a high theoretical volumetric capacity of 8046 mAh cm-3. Therefore, rechargeable aluminum batteries (RABs), which use aluminum as the negative electrode, are considered one of next-generation rechargeable batteries.

Graphite has attracted considerable attention as the positive electrode active material in RABs due to its low cost, high stability, excellent electrical conductivity, and high redox potential. As for electrolyte, room temperature ionic liquids (RTILs) such as 1-ethyl-3-methylimidazolium chloride (EMImCl) offer several advantages, including high ionic conductivity, low vapor pressure, non-flammability, a wide potential window. It is known that AlCl3, a Lewis acid, forms a stoichiometric complex, AlCl4
-, with Cl-, a Lewis base, and AlCl4
- forms the dinuclear complex, Al2Cl7
-, with excess Cl-. So, the mixing ratio of AlCl3 to EMImCl (AlCl3/EMImCl molar ratio) will have a significant effect on the composition of RTILs, and the reaction kinetics of the graphite positive electrode. In this study, we investigated the effect of AlCl3/EMImCl ratio on the positive electrode properties.

Experimental

A slurry was prepared by mixing graphite powder and polyvinylidene fluoride (PVDF) as a binder with N-methylpyrrolidone (NMP), followed by ultrasonication. An appropriate amount of prepared slurry was applied to a Mo foil and dried under vacuum conditions to prepare a graphite positive electrode. In a glove box, AlCl3 and EMImCl were mixed in different molar ratios and stirred until a light-yellow liquid was obtained. Polished aluminum foil was used as the negative electrode.

Results and Discussion

Figure 1 shows Raman spectra of ionic liquids with AlCl3/EMImCl ratios from 1.1 to 1.9. In each spectrum, peaks assigned to AlCl4
- and Al2Cl7
- in addition to EMIm+ were observed. As the AlCl3/EMImCl ratio increased, the intensity of the AlCl4
- peak at 350 cm-1 decreased and that of the Al2Cl7
- peaks at 310 cm-1 and 430 cm-1 increased. The intensity of AlCl4
- and Al2Cl7
- linearly decreased and increased with an increase in the AlCl3/EMImCl ratio, respectively, indicating the reaction, AlCl4
- + AlCl3 → Al2Cl7
-, proceeds quantitatively.

The cyclic voltammogram of the graphite electrode suggest that some couples of redox peaks of graphite were observed in a potential range between 0.5 V and 2.5 V. Each redox peak shifted negatively as the AlCl3/EMImCl ratio increased. Galvanostatic charge-discharge tests at 500 mA g-1 between 0.5 and 2.5 V were performed using Al/graphite cells containing electrolytes with different AlCl3/EMImCl ratios. In each AlCl3/EMImCl ratio, discharge curve with two plateaus was observed. The discharge capacity at the higher plateau voltage for the electrolyte with AlCl3/EMImCl = 1.1 was lower than that for the other electrolytes, while the discharge capacity at the lower plateau voltage was similar to each other. Irrespective of AlCl3/EMImCl ratio, discharge capacity hardly decreased over 100 charge-discharge cycles, while coulombic efficiency decreased with increasing the AlCl3/EMImCl ratio. This study compared the electrochemical properties of graphite in different molar ratios of electrolytes, which may help with future research and practical applications of RABs.

Figure 1