below is a quoted extract from the URL referenced above.
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...In order to exploit the high theoretical energy densities of an aluminum-ion battery (13.36 Wh/cm3, which is 1.6 times higher than gasoline of 8.6 Wh/cm3), a metallic negative electrode made of pure aluminum needs to be utilized. For this purpose, a stable electrolyte in regard to the electrochemical stability window is also demanded. A solid electrolyte could solve most of the issues connected to the disadvantages of highly corrosive or unstable liquid electrolytes. Finally, a positive electrode needs to be identified, which enables high capacities, high voltages, and thus high energy densities. For both the positive electrode and the (solid) electrolyte, ion conduction is of main importance. When a novel material exhibits a sufficient ion-conductivity the electronic conduction decides, whether such a material is used as a solid electrolyte (electronic insulator) or as a positive electrode (electronic conductor). It should be noted that an electronically insulating material could be transformed to a positive electrode, if, e.g., mixed with graphite or black carbon, and a redox reaction can take place at a constituting element and an electrical conductor could be doped to decrease electrical conductivity.
Therefore, one of the main tasks in this still early state of research should first be to identify materials with a sufficient ion conductivity comprising of non-critical elements. In the following section, the emphasis is set on these materials.
Aqueous or Primary Aluminum Battery
Despite its low cost, simple operation, and reduced environmental impact, aluminum batteries based on aqueous or protic systems exhibit fatal drawbacks, such as the passivating oxide film formation decreasing the battery voltage and efficiency, hydrogen side reactions, and material corrosion. The comparably low standard electrode potential of aluminum (−1.662 V vs. SHE) causes intrinsic hydrogen generation before aluminum could be plated in the process of reduction. This fact ... ultimately hinders the large-scale application of such systems .
The protective layer can be removed chemically by changing the pH value of the electrolyte from neutral, by adding potent corrosive agents such as concentrated alkaline or acidic solutions. The electrode potential then restores to its thermodynamically allowed value. Additionally, this leads to an accelerated rate of wasteful corrosion and limits the battery shelf life (Muldoon et al., 2014). Thus, corrosion inhibitors have to be added in order to prevent loss of the electrolyte (Liu et al., 2017). The negative reduction potential of non-passivated aluminum-metal causes constant (parasitic) hydrogen evolution when exposed to an aqueous/protic electrolyte solution.
A secondary aluminum-ion battery based on pure aluminum-metal as negative electrode and an aqueous electrolyte is unfeasible (Liu et al., 2017), because aluminum deposition only occurs at potentials far outside the stability region of water (see Figure 3). The electrolyte would decompose, and the ion transport gets disrupted. Primary (aqueous) aluminum batteries are summarized in Li and Bjerrum (2002). Theoretical specific energies of up to 1,090 Wh/kg are calculated, whereas real systems are reported to reach values of up to 200 Wh/kg. Both values are far below the theoretical specific energy of pure aluminum. Such batteries are applied in the marine sector utilizing a complex (active) electrolyte supply and mixing system (Shuster, 1990; Licht and Peramunage, 1993; Li and Bjerrum, 2002).
Due to the inherent hydrogen generation of the aluminum electrode in aqueous electrolytes, a different battery design is needed, in which no metallic aluminum is used. Holland et al. (2018) proposes a design in which TiO2 is used as the negative electrode, CuHCF (copper-hexacyanoferrate) as the positive electrode, and an aqueous electrolyte consisting of AlCl3 and KCl dissolved in water. The authors concluded that Al3+ is the mobile species. The discharge voltage was reported to be 1.5 V, whereas the specific energy is 15 Wh/kg at a specific power of 300 W/kg with energy efficiency remaining above 70% for over 1,750 cycles. Since such a cell design utilizes a negative electrode comprising of other materials than aluminum-metal, the overall reachable energy density is limited.
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Results in this report are probably out of date, but I haven't yet found any new materials science showing progress that I can quote here. Stand by for more later... one hopes.
Phinergy – Aluminium-Air
The aluminum-air battery technology is based on the reaction of oxygen in the air with aluminum. Because of their massive energy density, these batteries are perfectly suitable for electric vehicles, as they allow for significant weight reduction.
Italian Phinergy develops light, non-flammable and non-explosive, fully recyclable aluminum-air and zinc-air batteries with high energy density.