Reserved for Miklós Nemesszeghy

Introduction

Rechargeable Al-ion based batteries are considered promising future electrochemical energy storage devices. As a high valence ion (+III specifically), Al-ion utilizing batteries have a potential for high energy density and capacity as each oxidation/reduction reaction transfers multiple electrons per atom/ion

A typical Al-ion battery has aluminium metal foil as the negative electrode, a chloride based ionic liquid as electrolyte, and a positive electrode, supporting Al-ion intercalation. Al-ions are transported between electrodes in the electrolyte as AlCl₄^-- and Al₂Cl₇^--ions. During discharge, the Al-ions intercalate into the positive electrode, while during charge, the Al-ions are plated onto the Al negative electrode.

{\text{Al}+7\text{Al}\text{Cl}_4^-\left(+3\text{Cl}^-\right)\leftrightarrow4\text{Al}_2\text{Cl}_7^-+3\text{e}^-.}

There are also numerous other working mechanisms of Al-ion based batteries, emerging from different electrode and electrolyte configurations. It should also be noted that Al-ion based batteries also include the so called Al dual-ion batteries. In dual-ion batteries cation as well as anion intercalation, or deintercalation, happens simultaneously

Al-ion Battery Electrodes

Positive

A good portion of the research on Al-ion batteries was directed at finding suitable positive electrodes for Al-ion intercalation. Despite the small size of the Al³⁺-ion, there are relatively few materials that exhibit reversible Al-ion intercalation, because of the high charge density of the trivalent ion

Carbon structures such as graphite, graphene and carbon nanotubes as well as other carbon based materials are amongst the most studied positive electrodes for Al-ion batteries. Graphene and graphite materials exhibits high capacity for Al-ion intercalation, a high discharge potential with metallic Al (higher than 1.7 V; in comparison a commercial Li-ion batteries exhibit typically around 3.7 V 

) and a high discharge capacity due to fast diffusion of intercalation ions

The ion intercalating into graphitic electrodes is not Al³⁺ but instead AlCl₄^- (with chloride based liquid ionic electrolytes) with only one electron transfer per intercalated ion

The various metal compound electrodes generally exhibit two storage reactions named insertion and conversion. In insertion, either Al³⁺-ions or AlCl₄^--ions are intercalated (or inserted) into the electrode structure, while in conversion, the reaction modifies the electrode structure itself. 

Al-ion Battery Electrolytes

There are generally four possible electrolyte types for rechargeable metal ion batteries: Aqueous, ionic liquid, solid and molten salt. However, molten salt electrolytes require an elevated temperature to function and are mostly suited for industrial applications

Aqueous

Aqueous electrolytes consist of metal containing salts dissolved in water. The allure of aqueous electrolytes lies in their intrinsic safety, low cost and superior ionic conductivity

A way to make the aqueous electrolytes more practical is the use of so called "water-in-salt" electrolytes. , where the concentration of the dissolved metal containing salt is higher than 9 mol/(kg water)  

Ionic liquid

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Solid

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