Aluminium-ion battery
Aluminium-ion batteries (AIB) are a class of rechargeable battery in which aluminium ions serve as charge carriers. Aluminium can exchange three electrons per ion. This means that insertion of one Al3+ is equivalent to three Li+ ions. Thus, since the ionic radii of Al3+ (0.54 Å) and Li+ (0.76 Å) are similar, significantly higher numbers of electrons and Al3+ ions can be accepted by cathodes with little damage. Al has 50 times (23.5 megawatt-hours m-3) the energy density of Li-ion batteries and is even higher than coal.
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Aluminum electrodes date back to the 1850s, appearing as a cathode in an 1855 Zn(Hg)/H2SO4/Al battery and an anode in an 1857 nitric acid cell (Al/HNO3/C). These early designs were non-rechargeable. First rechargeable aluminum battery appeared in 1972, when a system using molten salt was developed. The high working temperature made this cell impractical, and researchers subsequently focused on room-temperature ionic liquid electrolytes. In 2011, Jayaprakash et al. produced a working secondary cell using a vanadium pentoxide cathode and a chloride-based electrolyte. Although the prototype failed after 20 cycles, it established the electrolyte chemistry used in later work, including the stable Al/graphite battery developed by Lin et al. in 2015.
Imagem: Chris Wevers · BY-NC-SA · Openverse
Like all other batteries, aluminium-ion batteries include two electrodes connected by an electrolyte. Unlike lithium-ion batteries, where the mobile ion is Li+, aluminium forms a complex with chloride in most electrolytes and generates an anionic mobile charge carrier, usually AlCl4− or Al2Cl7−. The amount of energy or power that a battery can release is dependent on factors including the battery cell's voltage, capacity and chemical composition. A battery can maximize its energy output levels by:
Electrolyte
Since 2020, the most commonly used electrolyte for rechargeable Al batteries have been acidic room temperature non-aqueous ionic liquids (IL) made of aluminium chloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride (C6H11ClN2). This addressed the initial issue that prevented Al batteries from becoming rechargeable: Al readily reacts to form a passivating oxide coating that is chemically inert and an extremely high potential is necessary to push ions through this layer. This high potential would degrade the electrolyte during recharging. The use of the ionic liquid as an electrolyte prevents passivation and allowed Al batteries to become rechargeable. As mentioned earlier, the active species in the IL electrolyte are AlCl4− and Al2Cl7−.
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Aluminium-ion batteries are conceptually similar to lithium-ion batteries, except that aluminium is the charge carrier instead of lithium. While the theoretical voltage for aluminium-ion batteries is lower than lithium-ion batteries, 2.65 V and 4 V respectively, the theoretical energy density potential for aluminium-ion batteries is 1060 Wh/kg in comparison to lithium-ion's 406 Wh/kg limit. Today's lithium-ion batteries have high power density (fast charge/discharge) and high energy density (hold a lot of charge). They can also develop dendrites that can short-circuit and catch fire, whereas the non-volatile and nonflammable ionic liquid electrolyte[which?] in the Al battery improves its safety. The use of an Al metal anode as opposed to Li metal also provides increased safety due to the former having better air stability. Aluminium also transfers energy more efficiently because of its 3 electrons. Aluminium is more abundant and therefore costs less than lithium, lowering material costs.
Aluminium-ion batteries to date have a relatively short shelf life. The combination of heat, rate of charge, and cycling can dramatically affect energy capacity. One of the reasons is the fracture of the graphite anode. Al atoms are far larger than Li atoms. Ionic liquid electrolytes, while improving safety and the long term stability of the devices by minimizing corrosion, are expensive and may therefore be unsuitable. However, recent advances in research have introduced safer and less expensive kinds of Al-ion batteries. This includes a "high safety, high voltage, low cost" Al-ion battery introduced in 2015 that uses carbon paper as the cathode, high purity Al foil as the anode, and an ionic liquid as the electrolyte.
Various research teams are experimenting with aluminium to produce better batteries. Requirements include cost, durability, capacity, charging speed, and safety.
Anode
In 2021, researchers announced a cell that used a 3D structured anode in which layers of aluminium accumulate evenly on an interwoven carbon fiber structure via covalent bonding as the battery is charged. The thicker anode features faster kinetics, and the prototype operated for 10k cycles without signs of failure.
Electrolyte
Around 2010, Oak Ridge National Laboratory (ORNL) developed and patented a high energy density device, producing 1,060 watt-hours per kilogram (Wh/kg). ORNL used an ionic electrolyte, instead of the typical aqueous electrolyte which can produce hydrogen gas and corrode the anode. The electrolyte was made of 3-ethyl-1-methylimidazolium chloride with excess aluminium trichloride. However, ionic electrolytes are less conductive, reducing power density. Reducing anode/cathode separation can offset the limited conductivity, but causes heating. ORNL devised a cathode made up of spinel manganese oxide that further reduced corrosion.
Cathode
In 2011 a research team used the same electrolyte as ORNL, but used vanadium oxide nanowires for the cathode. Vanadium oxide has an open crystal structure with greater surface area and reduced path between cathode and anode. The device produced a large output voltage. However, the battery had a low coulombic efficiency. In April 2015 researchers at Stanford University claimed to have developed an aluminium-ion battery with a recharge time of about one minute (for an unspecified battery capacity). Their cell provides about 2 volts, 4 volts if connected in a series of two cells. The prototype lasted over 7,500 charge-discharge cycles with no loss of capacity.
Redox battery
Another approach to an aluminium battery is to use redox reactions to charge and discharge. The charging process converts aluminium oxide or aluminium hydroxide into ionic aluminium using electrolysis, typically at an aluminium smelter. This requires temperatures of 800 °C (1,470 °F). One report estimated possible efficiency at around 65%. Although ionic aluminium oxidizes in the presence of air, this costs less than 1% of the energy storage capacity. Discharging the battery involves oxidizing the aluminium, typically with water at temperatures less than 100 °C. This yields aluminium hydroxide and ionic hydrogen. The latter can produce electricity via a fuel cell. The oxidation in the fuel cell generates heat, which can support space or water heating.
