User:Georgelieu/sandbox/Aluminium-ion battery

Aluminium-ion batteries are a class of rechargeable batteries in which aluminium-ions provide energy by flowing from the negative end of the battery, the anode, to the positive end, the cathode. After discharge, the battery can be recharged, during which aluminium-ions return to the anode. Though aluminium-ion batteries are similar in function to lithium-ion batteries, they differ in energy output level due to their components and structure.

Design

Like all other batteries, the basic structure of aluminium-ion batteries includes two electrodes connected by an electrolyte, a conductive material acting as a medium for the flow of electrons. As a battery is discharged, electrons flow from the anode, to the cathode while ions are also flowing to keep the charges balanced. During discharge, the electrons can be harnessed by an external source to power lightbulbs or electronic devices until there are no more electrons. Some batteries, including aluminium-ion batteries, can be recharged, known as secondary batteries. A larger voltage is applied against the normal flow of electrons, effectively pushing the electrons back to the anode and recharging the battery.

The amount of energy or power that a battery can release is dependent on a number of factors like the battery cell's voltage, capacity, and chemical composition of the battery. A battery can maximize its energy output levels by:

• Having a large chemical potential difference between the two electrodes, prompting greater electron transfer
• Reducing the mass of reactants while still maximizing the amount of electrons being transferred
• Preventing the electrolyte from being consumed by the chemical reactions of the battery

Aluminium-ion batteries maximize their energy output levels and efficiency by integrating these concepts into the design of the battery. Currently, various research teams are experimenting with aluminium and other chemical compounds to produce the most efficient long lasting battery.

Oak Ridge National Laboratory

The Oak Ridge National Laboratory, or ORNL, led by Gilbert M. Brown has been developing a high energy density aluminium-ion rechargeable battery. Energy capacity levels seem promising, with a energy density value of 1060 Wh/kg versus 406 Wh/kg for lithium-ion batteries. Furthermore, ORNL has been able to eliminate common problems of metal ion batteries. Normally, aluminium batteries use aqueous electrolytes that require water during the reactions, leading to the formation of hydrogen gas. The high corrosion rate of the aluminium anode is also a common problem. Both factors damage the battery. The ORNL research team combatted the hydrogen gas formation issue by using an ionic electrolyte. Made up of 3-ethyl-1methylimidazolium chloride with excess aluminium trichloride, the reactions taking place don't require water, preventing any hydrogen gas. While ionic electrolytes solve some problems, they are also less conductive, leaving a battery that is less efficient in facilitating the flow of electrons. At the same time, shortening the distance between the anode and cathode can reduce the time electrons spend flowing to the cathode, though the quicker discharge also heats up the battery. To solve the problem of the corroding aluminium anode, ORNL devised a cathode made up of spinel manganese-oxide that has the ability to insert and remove aluminium-ions, meaning corrosion is less of an issue.

Cornell University

At Cornell University, a research team led by Professor of Chemical and Biomolecular Engineering Lyden Archer kept the same ionic electrolyte as the ORNL team, but used vanadium-oxide nanowires for the cathode as opposed to ONRL's spinel manganese-oxide. Vanadium oxide consists of an open crystal structure, allowing greater surface area and space for an aluminium structure that reduces the path between cathode and anode, maximizing energy output levels. The experimental aluminium-ion battery possessed a high voltage efficiency, having a large output voltage during operation. However, the battery didn't have a high coulombic efficiency, meaning each charge cycle reduced the energy capacity of the battery in a way that the battery wouldn't be suitable for actual use.

Electrochemistry

Taking place at the anode is the following half reaction:

$\mathrm {Al} +\mathrm {7AlCl_{4}} \leftrightarrows \mathrm {4Al_{2}Cl_{7}} +\mathrm {3e^{-}}$

Taking place at the cathode is the following half reaction:

$\mathrm {2MnO_{2}} +\mathrm {Li^{+}} +\mathrm {e^{-}} \leftrightarrows \mathrm {LiMn_{2}O_{4}}$

Combining the two half reactions yields the following reaction:

$\mathrm {Al} +\mathrm {7AlCl_{4}} +\mathrm {6MnO_{2}} +\mathrm {3Li^{+}} \leftrightarrows \mathrm {4Al_{2}Cl_{7}} +\mathrm {3LiMn_{2}O_{4}}$

Aluminium-ion vs Lithium-ion

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 for aluminium-ion batteries is 8140 Whr/kg in comparison to lithium-ion's 1462 Whr/kg. The large difference in energy density is due to the fact that aluminium-ions have three valence electrons while lithium-ions only have one. Not only does aluminium have a greater energy density, it also exists in greater abundance as a natural resource, allowing for efficient batteries that are cost-effective as well.

Uses & Drawbacks

The opportunities for aluminium-ion batteries are endless. Like lithium-ion batteries, their primary advantage lies in being able to power portable electronic devices and phones. From computers to flashlights to power tools, batteries are very commonly used and the reliance on them continues to grow. Beyond powering gadgets, they can act as energy storage systems for electricity generated through wind and solar power.

Being a fairly recent idea, aluminium-ion batteries have a number of challenges to overcome. Like other batteries, particularly lithium-ion, aluminium-ion batteries have a relatively short shelf life. The combination of heat, rate of charge, and number of cycles of the battery can dramatically decrease energy capacity. Other problems like corrosion and oxide formations can be reduced with the right cathode, but the inherent problem of metal ion batteries still looms. When metal ion batteries are reduced to a charge of zero, the battery is completely dead. Lastly, the advancement of aluminium-ion batteries is hindered due to the high cost of ionic electrolytes and low cost of gasoline. As a fuel source for vehicles, batteries are only recently becoming more common while gasoline still forms the foundation of all vehicles. In order for aluminium-ion batteries to be the preferable choice, efficiency and cost must be improved, but more importantly, the safety of using batteries in cars has to be a primary focus too. In a crash, electric vehicles can crash and catch on fire, raising major concerns and forcing car companies to take greater measures to ensure the safety of the passengers.