# Aluminium-ion battery

Aluminium-ion batteries are a class of rechargeable battery in which aluminium ions provide energy by flowing from the negative electrode of the battery, the anode, to the positive electrode, the cathode. When recharging, aluminium ions return to the negative electrode, and can exchange three electrons per ion. This means that insertion of one Al3+
is equivalent to three Li+
ions in conventional intercalation cathodes. Thus, since the ionic radii of Al3+
(0.54 A) and Li+
(0.76 A) are similar, significantly higher models of electrons and Al3+
ions can be accepted by the cathodes without much pulverization.[1][2] The trivalent charge carrier, Al3+
is both the advantage and disadvantage of this battery.[3] While transferring 3 units of charge by one ion significantly increase the energy storage capacity but the electrostatic intercalation of the host materials with a trivalent cation is too strong for well-defined electrochemical behaviour.

Rechargeable aluminium-based batteries offer the possibilities of low cost and low flammability, together with three-electron-redox properties leading to high capacity.[4] The inertness of aluminum and the ease of handling in an ambient environment is expected to offer significant safety improvements for this kind of battery. In addition, aluminum possesses a higher volumetric capacity than Li, K, Mg, Na, Ca and Zn owning to its high density (2.7 g/cm3 at 25 °C) and ability to exchange three electrons. This again means that the energy stored in aluminum-batteries on a per volume basis is higher than that in other metal-based batteries. Hence, aluminum-batteries are expected to be smaller in size. Al-ion batteries also have a larger number of charge-discharge cycles. Thus, Al-ion batteries have the potential to replace Li-ion batteries.[2]

## Design

Like all other batteries, the basic structure of aluminium-ion batteries includes two electrodes connected by an electrolyte, an ionically (but not electrically) conductive material acting as a medium for the flow of charge carriers. Unlike lithium-ion batteries, where the mobile ion is Li+
, aluminum forms a complex with chloride in most electrolytes and generates an anionic mobile charge carrier, usually AlCl
4
or Al
2
Cl
7
.[5]

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:

• Increasing chemical potential difference between the two electrodes[6]
• Reducing the mass of reactants[6]
• Preventing the electrolyte from being modified by the chemical reactions[6]

## Research

Various research teams are experimenting with aluminium and other chemical compounds to produce the most efficient, long lasting, and safe battery.

### Oak Ridge National Laboratory

Around 2010[7], Oak Ridge National Laboratory (ORNL) developed and patented a high energy density device, producing 1,060 watt-hours per kilogram (Wh/kg) versus 406 Wh/kg for lithium-ion batteries.[8] ORNL used an ionic electrolyte, instead of the typical aqueous electrolyte which can produce hydrogen gas during operation and corrode the aluminium anode. The electrolyte was made of 3-ethyl-1-methylimidazolium chloride with excess aluminium trichloride.[9] 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 further reducing corrosion.[7]

### Cornell University

In 2011 at Cornell University, a research team used the same electrolyte as ORNL, but used vanadium oxide nanowires for the cathode.[10] Vanadium oxide displays an open crystal structure, allowing greater surface area for an aluminium structure and reduces the path between cathode and anode, maximizing energy output levels. The device produced a large output voltage during operation. However, the battery had a low coulombic efficiency.[9]

### Stanford University

In April 2015 researchers at Stanford University claimed to have developed an aluminum-ion battery with a recharge time of about one minute (for an unspecified battery capacity).[4] They claimed that their battery has no possibility of catching fire, offering a video of a hole being drilled into the battery while it was generating electricity.[11] Their cell provides about 2 volts, 4 volts if connected in a series of two cells.[4][12] The prototype lasted over 7,500 charge-discharge cycles with no loss of capacity.[13][14]

The "ultrafast rechargeable aluminum-ion battery" is made of an aluminum anode, liquid electrolyte, isolation foam, and a graphite cathode. During the charging process, ${\displaystyle {\ce {AlCl4-}}}$ ions intercalates among the graphene stacked layers. While discharging, ${\displaystyle {\ce {AlCl4-}}}$ ions are rapidly de-intercalated through the graphene stacked layers. The features of Al-ion batteries include:[15]

• A fast charge discharge cycle that can be completed in one minute[15]
• High durability that allows the battery to withstand more than 10,000 cycles without a capacity decay[15]
• Improved safety, as the thin battery cell is stable, nontoxic, and bendable (i.e., it will not catch fire even if damaged by drilling)[15]
• Low cost, as the acquisition cost of raw materials are relatively cheap. This could allow the Al-ion batteries to be used in not only electronic devices, but electric bikes and motorcycles, golf carts, lift trucks, wind turbines, solar cells etc.[15]

In 2016, the lab tested these cells through collaborating with Taiwan’s Industrial Technology Research Institute (ITRI) to power a motorbike. However, that version of the battery had one major drawback in that it needed an expensive electrolyte.[16] In 2017, the newest version includes a urea-based electrolyte and is about 100 times cheaper than the 2015 model, with higher efficiency and a charging time of 45 minutes. It is the first time urea has been used in a battery.[16] The battery exhibits ∼99.7% Coulombic efficiency and a substantial rate capability of ${\displaystyle {\ce {100 mA*g^{-1}}}}$ at a cathode capacity of ${\displaystyle {\ce {73 mAh*g^{-1}}}}$ (1.4 C).[17]

### ALION Project

In June 2015, the High Specific Energy Aluminium-Ion Rechargeable Batteries for Decentralized Electricity Generation Sources (ALION) project was launched by a consortium of materials and component manufacturers and battery assemblers as a European Horizon 2020 project led by the LEITAT research institute.[18][19] The objective of the project is to develop a prototype Al-ion battery that could be used for storage of electricity from decentralized sources, such as from renewable energy sources. The project is researching various cell and battery concepts as well as electroactive materials to achieve an energy density of 400 Wh/kg, a voltage of 48 volts and a charge-discharge life of 3000 cycles.

In May 2019, the project came to an end and published its final results. After four years, the project showed that the high power and cycling performance of the Aluminium-ion battery technology made it an appealing alternative to current commercial products. For example, they found that Al-ion batteries could be a serious candidate to replace lead–acid batteries in uninterruptible power supplies. They also found uses in telecommunications and stationary applications for grid energy storage. 3D printing of the battery packs allowed for the largest Al-ion cells developed, with voltages ranging from 6 to 72 volts.[20].

### University Of Maryland

In 2016, a University of Maryland team reported a rechargeable aluminium/sulfur battery that utilizes a sulfur/carbon composite as the cathode material. The chemistry is able to provide a theoretical energy density of 1340 Wh/kg. The team made a prototype cell which demonstrated a energy density of 800 Wh/kg for over 20 cycles.[21]

### Zhejiang University Department of Polymer Science

In December 2017 a team, led by professor Gao Chao, from Department of Polymer Science and Engineering of Zhejiang University, announced the design of a battery using graphene films as cathode and metallic aluminium as anode.

The 3H3C (Trihigh Tricontinuous) design results in a graphene film cathode with excellent electrochemical properties. The arrangement of graphene liquid crystals result in a highly oriented structure. A process of high temperature annealing under gas pressure produces a high quality and high channelling graphene structure. This 3H3C design creates an aluminium-graphene battery (Al-GB) which has impressive properties:

• The battery works well after quarter-million cycles retaining 91.7 percent of its original capacity.
• The battery can be fully charged in 1.1 seconds.
• The assembled battery works well across a temperature range of minus 40 to 120 degrees Celsius.
• It offers a high current capacity (111 mAh/g, 400 A/g based on the cathode).
• It can be folded.
• It does not explode when exposed to fire and the materials used are non flammable.

However, the aluminum-ion battery cannot compete with commonly-used Li-ion batteries in terms of energy density, according to Gao. [22][23]

### Clemson University

In 2017, researchers at Clemson Nanomaterials Institute built a prototype Al-ion battery that uses a graphene electrode to intercalate tetrachloroaluminate (AlCl
4
).[5] Their new battery technology uses aluminum foil and thin sheets of graphite called few-layer graphene (FLG) as the electrode to store electrical charge from aluminum ions present in the electrolyte.[24] The team constructed batteries with aluminum anodes, pristine or modified FLG cathodes, and an ionic liquid with AlCl3 salt as the electrolyte.[5] They claimed that the battery can operate over 10,000 cycles and the energy density is 200 Wh/kg.[25] Their hope is to make aluminum batteries with higher energy to ultimately displace lithium-ion technology.[5]

### Queensland University of Technology

Researchers from QUT developed earth abundant cryptomelane based electrodes as cathode for Aluminum ion battery with aqueous electrolytes.[26]

## Electrochemistry

Anode half reaction:

${\displaystyle {\ce {{Al}+{7AlCl4^{-}}<=>{4Al2Cl7^{-}}+{3e^{-}}}}}$

Cathode half reaction:

${\displaystyle {\ce {{2MnO2}+{Li+}+{e^{-}}<=>{LiMn2O4}}}}$

Combining the two half reactions yields the following reaction:

${\displaystyle {\ce {{Al}+{7AlCl4^{-}}+{6MnO2}+{3Li+}<=>{4Al2Cl7^{-}}+{3LiMn2O4}}}}$

## Lithium-ion comparison

Aluminium-ion batteries are conceptually similar to lithium-ion batteries, but possess an aluminum anode instead of a lithium anode. 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.[7]

Today’s lithium ion batteries have high power density (fast discharge) and high energy density (hold a lot of charge). It can also develop dendrites, similar to splinters, that can short-circuit a battery and lead to a fire. Aluminum also transfers energy more efficiently. Inside a battery, atoms of the element — lithium or aluminum — give up some of their electrons, which flow through external wires to power a device. Because of their atomic structure, lithium ions can only provide one electron at a time; aluminum can give three at a time.[27] Aluminum is also more abundant than lithium, lowering material costs.[8]

## Challenges

Aluminium-ion batteries have a relatively short shelf life. The combination of heat, rate of charge, and cycling can dramatically decrease energy capacity. One of the primary reasons for this short shelf life is the fracture of the traditional graphite anode, the Al ions being far larger than the Li ions used in conventional battery systems.[28] When metal ion batteries are fully discharged, they can no longer be recharged. Ionic electrolytes, while improving safety and the long term stability of the devices by minimizing corrosion, are expensive to manufacture and purchase and may therefore be unsuited to the mass production of Al ion devices.[29] In addition, current breakthroughs are only limited in laboratory settings where a lot more work needs to be done on scaling up the production in commercial settings.[25]

## References

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2. ^ a b Das, Shyamal K.; Mahapatra, Sadhan; Lahan, Homen (2017). "Aluminum-ion batteries: developments and challenges". Journal of Materials Chemistry A. 5 (14): 6347–6367. doi:10.1039/c7ta00228a.
3. ^ Eftekhari, Ali; Corrochano, Pablo (2017). "Electrochemical Energy Storage by Aluminum As a Lightweight and Cheap Anode/Charge Carrier". Sustainable Energy & Fuels. 1 (6): 1246–1264. doi:10.1039/C7SE00050B.
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5. ^ a b c d "Team designs aluminum-ion batteries with graphene electrode". Graphene-info. Retrieved 1 March 2018.
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18. ^
19. ^ HORIZON 2020 ALION Project Kick Off Meeting @ LEITAT
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