Energizing the Future: An Overview of Metal-Air Batteries

Authors: Aniruddha Pawar & Faiz Khan

Introduction:?

The aluminum–air battery is considered to be an attractive candidate as a power source for electric vehicles because of its high theoretical?energy density?(8100 Wh kg?1) and theoretical voltage of 2.71 V. However, some technical and scientific problems preventing the large-scale development of Al–air batteries have not yet been resolved.?

Metal–air batteries have gained great interest due to their high energy density and capacity, low cost (depending on the metal anode), negligible dependence of their capacity on operating load and temperature, and constant discharge voltage. Up to now, several different types of metal–air batteries, such as lithium (Li)–air, sodium (Na)–air, potassium (K)–air, zinc (Zn)–air, magnesium (Mg)–air, and aluminum (Al)–air batteries have been under study and development. Metal–air batteries exhibit high theoretical energy densities ranging between 2～10 folds higher than that of state-of-the-art LIBs.?

Metal-Air Batteries:?

The table summarizes the voltage, theoretical specific capacity, and energy density of the typical metal–air batteries. Metal–air batteries are equipped with a metal anode and an air-breathing cathode through a suitable electrolyte. Due to the open battery configuration of metal–air batteries, the oxygen reagent can be directly received from the surrounding air instead of prior incorporation, thus contributing to their very high theoretical energy densities.?

Among the metal–air batteries shown, the Li–air battery shows the highest theoretical energy density (13000?Wh?kg?1), which is significantly greater than other rechargeable battery technologies. Nonetheless, rechargeable Li–air batteries still have many challenges to overcome such as the blocking of porous carbon cathodes by discharge products, instability of lithium in?humid environments, insufficient understanding of the catalytic mechanism, low electrochemical efficiency owing to high charging overpotentials, and side product (such as lithium alkyl-carbonates and Li2CO3) formation during cycling. These issues negatively affect the reversible charging and cycle life of Li–air batteries. Additionally,?nonaqueous electrolytes are generally used in Li–air batteries, which raise the cost and cause safety concerns related to the flammable organic solvents. For aqueous electrolyte metal–air batteries, there are also significant challenges.??

On the other hand, Mg–air and Al–air batteries have gained much attention due to their many practical advantages, such as high energy density and theoretical voltage, safety, and abundance of raw materials.?

Cell Construction & Reactions:

Discharge Characteristics:?

Figure 2 shows the discharge curve of the Al-air battery. The curve describes that the Al-air cell can provide constant voltage for a good amount of time if the discharge current is small (less than 6.25 mA). It can be observed from the characteristic that, the voltage drop is very fast if the discharge current exceeds the value of 12.5 mA.?

Rechargeability:?

Aluminium–air batteries are?primary cells, i.e., non-rechargeable. Once the aluminium anode is consumed by its reaction with atmospheric oxygen at a cathode immersed in a water-based electrolyte to form hydrated?aluminium oxide, the battery will no longer produce electricity. However, it is possible to mechanically recharge the battery (with some electrolyte solutions) with new aluminium anodes made from recycling the hydrated aluminium oxide. Such recycling would be essential if aluminium–air batteries were to be widely adopted.?

Environmental Impacts:?

The negative potential exists for health and environmental effects due to the operation of a fleet of AAEVs. The domestic aluminium industry, as with any industry, presents potential health problems to its workers, the population in the vicinity of the plant, and the environment. The operation and servicing of the AAEV also pose potential public and occupational hazards that must be addressed. These hazards include hydrogen generation and accidental releases of electrolytes. In addition, waste NaOH electrolyte represents a potential environmental hazard due to the caustic nature of the solution and the trace elements present. However, with proper engineering and adherence to existing safety and environmental legislation, we see no significant concerns to hinder the development of the AAEV.?

Environmentally, the majority of AAEV emissions stem from aluminium production and can be managed by controls and by the location of the plant to lessen the impact.?

Conclusion:?

According to some researchers, metal-air batteries have the potential to compete with the capability of lithium-ion battery technology. Lithium-air and zinc-air cells are examples being pursued strongly for both primary and secondary applications. Aluminium’s light weight, safety, ready availability, and high energy density make it an obvious candidate to consider in the pursuit of realizing metal-air battery systems.?

Al–air batteries possess great potential for practical application due to their large energy capacity and in this review, Al–air batteries with Al anodes, electrolytes, and air cathodes have been discussed and the possibility of creating rechargeable Al–air batteries has been presented. Overall, the focus of the development of pure Al and Al alloy anodes is the prevention of self-corrosion and by-product formation. As for electrolytes, they are critical components determining the rechargeability potential of Al–air batteries. Electrolyte additives are also an important issue because they can suppress corrosion and hydrogen evolution to improve electrochemical properties. Overall, because the theoretical capacity of Al–air batteries is much larger than that of Li-ion batteries, the creation of rechargeable Al–air batteries is of great importance and can be used in the design of smart grid energy systems. These rechargeable Al–air batteries can also potentially be applied in electricity-based vehicles to promote an environmentally friendly future.?

?References:?

[1]: https://www.cei.washington.edu/lesson-plans/aluminum-air-battery ?

[2]: https://www.frontiersin.org/articles/10.3389/fenrg.2021.599846/full ?

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