Aluminium-ion battery

Aluminium-ion batteries are a class of rechargeable batteries 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 anode.

Design

Like all other batteries, the basic structure of aluminium-ion batteries includes two electrodes connected by an electrolyte, an ionically (but not electronically) conductive material acting as a medium for the flow of charge carriers.

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
• Reducing the mass of reactants
• Preventing the electrolyte from being modified by the chemical reactions

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 the Oak Ridge National Laboratory (ORNL) developed and patented a high energy density device, producing 1,060 Wh/kg versus 406 Wh/kg for lithium-ion batteries. 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. 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.

Cornell University

In 2011 at Cornell University, a research team used the same electrolyte as ORNL, but used vanadium oxide nanowires for the cathode. 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.

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). 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. They also claim that their battery is inexpensive. Their cell provides about 2 volts. Connecting 2 cells in a series circuit will provide 4 volts. The prototype lasted over 7,500 charge-discharge cycles with no loss of capacity.

ALION Project

In June 2015 a European Horizon 2020 research project on Aluminium-Ion batteries was launched at the technological research center LEITAT. The project aims at developing a prototype with the help of various European industries and research institutes. The project entitled High Specific Energy Aluminium-Ion Rechargeable Batteries for Decentralized Electricity Generation Sources, or ALION for short, pursues an integral approach comprising electroactive materials based on “rocking chair” mechanism, robust ionic liquid-based electrolytes as well as novel cell and battery concepts, finally resulting in a technology with much lower cost, improved performance, safety and reliability with respect to current energy storage solutions (e.g. Pumped hydro storage, Compressed air energy storage, Li-ion battery, Redox flow battery etc.). The project covers the whole value chain from materials and component manufacturers, battery assembler, until the technology validation in specific electric microgrid system including renewable energy source (i.e. mini wind turbine, photovoltaic system, etc.). Thus, the final objective of this project is to obtain an Al-ion battery module validated in a relevant environment, with a specific energy of 400Wh/kg, a voltage of 48V and a cycle life of 3000 cycles.

Electrochemistry

Anode half reaction:

$\mathrm{Al}+\mathrm{7AlCl_4^-}\leftrightarrows\mathrm{4Al_2Cl_7^-}+\mathrm{3e^-}$

Cathode half reaction:

$\mathrm{2MnO_2}+\mathrm{Li^+}+\mathrm{e^-}\leftrightarrows\mathrm{LiMn_2O_4}$

Combining the two half reactions yields the following reaction:

$\mathrm{Al}+\mathrm{7AlCl_4^-}+\mathrm{6MnO_2}+\mathrm{3Li^+}\leftrightarrows\mathrm{4Al_2Cl_7^-}+\mathrm{3LiMn_2O_4}$

Lithium-ion comparison

Aluminium-ion batteries are conceptually similar to lithium-ion batteries, but possess an aluminium anode instead of a lithium anode. While the theoretical voltage for aluminium-ion batteries is lower than lithium-ion batteries, 2.65V and 4V respectively, the theoretical energy density potential for aluminium-ion batteries is 1060 Wh/kg in comparison to lithium-ion's 406 Wh/kg limit. The large difference in energy density potential is due to the fact that aluminium ions have three valence electrons while lithium ions only have one. Aluminium is also more abundant than lithium, lowering material costs.

Challenges

Aluminium-ion batteries have a relatively short shelf life. The combination of heat, rate of charge, and cycling can dramatically decrease energy capacity. When metal ion batteries are fully discharged, they can no longer be recharged. Ionic electrolyte materials are expensive. Like most batteries, they have a far lower energy density than gasoline.

See also

• Aluminium-air battery
• Comparison of battery types
• List of battery types
• Energy density

External links

• Stanford unveils aluminum-ion battery on YouTube