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 [1]. In addition, Al is the most abundant metal in Earth's crust [2], is not harmful to the environment and the relatively small ionic radii of Al3+-ion (53 pm) is promising for intercalation chemistries [1]. Some studies on Al-ion batteries were published already back in the 1900s, but large interest in the subject has emerged only recently (around 2015) to replace Li-ion batteries [2].
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. [3] Figure 1 shows the working mechanism of such a Al-ion battery. With the difference of intercalation versus plating, the oxidization/reduction reactions at the electrodes are [3]: \( {\text{Al}+7\text{Al}\text{Cl}_4^-\left(+3\text{Cl}^-\right)\leftrightarrow4\text{Al}_2\text{Cl}_7^-+3\text{e}^-.} \)
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 [4]. Al dual-ion batteries are meant to increase the capacity, operating voltage and reduce the cost of Al-ion based batteries [5].
Figure 1. Illustration of the working principle of an ionic liquid electrolyte Al-ion battery. (License: CC BY-NC 4.0) [3]
Al-ion Battery Electrodes
Positive electrode
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 [2][6]. Successful reversible intercalation was achieved by numerous types of carbon electrodes as well as some metal compounds [2], including Mo6S8 [7], VO2 and TiO2 [8], amongst numerous others (but few relative to other battery chemistries).
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 [9, p.12]) and a high discharge capacity due to fast diffusion of intercalation ions [10][11]. Graphene is considered the superior one of the two [10], however, the reason is seldom addressed in literature. In addition, it is somewhat ambiguous what certain authors consider graphene and what graphite.
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 [12]. Pairing graphite electrode with an Al electrode and ionic liquid electrolyte results in a dual-ion battery, where the electrolyte is also participating in the charge storage reaction [11]. Figure 2 illustrates the working mechanism of such a dual-ion battery.
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. [2] Positive electrode materials exhibiting Al3+-ion intercalation include, for example, TiO2 [13], and MoS2 [14]. Se doped Cu exhibits AlCl4--ion intercalation [15], while a VOCl based material exhibits both Al3+-ion or AlCl4--ion intercalation simultaneously [16]. Figure 3 shows the crystal structure of TiO2 with the intercalation sites for Al.
Figure 2. Illustration of the working mechanism of a Al-dual-ion battery. (License: CC BY-NC 4.0) [3]
Figure 3. (a) The crystal structure of anatase TiO2 with intercalated Al atoms. (b) The immediate surrounding of an intercalated Al atom in TiO2. Light blue represents Ti, red O and green Al. (Figure: Miklós Nemesszeghy. Made with VESTA and MS Paint; based on publication [13])
Co-Se compounds were found to exhibit Al storage through conversion reactions [2]. In CoSe2 based electrodes, the Al-ions partially substitute Co in the crystal structure, meaning that Co2+-ions would in turn dissolve into the electrolyte. In bulk CoSe2, there is little control over this process which leads to degradation of the electrode structure and thus capacity loss over time. [17]
In addition to the examples provided here, there are numerous other positive electrode materials under research for Al-ion batteries, including some exotic ones that can't be categorized in the above manner. Also, the examples provided are not fully investigated and may not be the best preforming solutions for an Al-ion battery positive electrode. Al-ion batteries are still in active development.
Negative electrode
The majority of research on Al-ion batteries has been focused on exploring electrolytes and positive electrodes and far less the possible negative electrodes. However, the widely used Al foil negative electrode has some considerable drawbacks. A metallic Al foil electrode is susceptible to dendrite growth and uneven corrosion which results in its eventual pulverization after repeated charge discharge cycles [18], limiting the lifetime of Al-ion battery devices.
One alternative is using an intercalation mechanism, instead of plating, at the negative electrode as well. An example would be MnO3 as negative electrode [1]. Another approach includes using a highly corrosion resistant material, such as Cu foil, as the basis for Al plating and stripping, eliminating the pulverization danger [19]. However, this does not address the issue caused by dendrite growth. There are more examples, like TiO2 nanopowder and carbon black mixture [20], and N-doped carbon nanorod array [21] as negative electrodes, but none of them are extensively studied yet.
It is also possible to protect the Al foil from pulverization and suppress dendrite growth by creating a protective oxide layer on the foil surface. The Al-ions have to deposit under the oxide layer which suppresses dendrite growth and promotes a more even stripping of the Al foil. While the oxide layer is separating the electrode and electrolyte, the electrolyte can still penetrate through defects in the oxide layer. This enables a longer operating time for the Al foil electrode. [22] However, it is also reported that these protective oxide layers are corroded over time in ionic liquid electrolytes [18].
A novel idea for a negative electrode is using liquid Ga. As a liquid, Ga provides a uniform dendrite free surface. The Al-ions go through alloying and dealloying at the Ga electrode during charge and discharge respectively. While this material can successfully eliminate dendrite growth and other electrode degradation, a battery using this electrode material would mostly be suited for stationary applications only due to the liquid nature of the electrode. [23] In addition, the battery would effectively be unusable below 30οC.
Al-ion Battery Electrolytes
There are generally four possible electrolyte types for rechargeable metal ion batteries: Aqueous, ionic liquid, solid and molten salt. Molten salt electrolytes require an elevated temperature to function and are mostly suited for industrial applications [24].
Aqueous electrolytes
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 [25]. However, the electrochemical stability window of water is only around 1.23 V [26] which means that water decomposition occurs when the battery is operated or charged above that voltage. Thus simple aqueous electrolytes are not suited for batteries with high operating voltages. In addition, water is reactive with Al negative electrode, quickly forming a dense passivating oxide layer on the Al surface [27]. Reports on aqueous Al-ion battery systems are scares, and the incompatibility of water and Al negative electrode is most likely the reason why.
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) [25], although is not a definitive limit. In water-in-salt electrolytes the number of free water molecules is very limited; generally all water molecules are bound to the ions originating from the dissolved salts. This widens the electrochemical stability window of the electrolyte and hinders direct reactions between water and the electrodes. [26] However, with the benefits of water-in-salt electrolytes also comes a somewhat reduced ionic conductivity [25]. Reported aqueous electrolytes for Al-ion batteries include AlCl3 and MnCl2 (dissolved in water) [28] and aluminium trifluoromethanesulfonate or Al(OTf)3 [27][29][30]. Figure 4 illustrates the different conditions in water-in-salt and "normal" aqueous electrolytes with a AlCl3 salt.
Figure 4. Illustration of interactions in (a) "salt-in-water" and (b) "water-in-salt" AlCl3 solution. The brown circled balls are Al3+, the green ones are Cl-, and the "V" shaped molecules are water. (Figure: Miklós Nemesszeghy. Made with Marvin JS and MS Paint.net.)
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