|Fig. 1: Schematic configuration of metal-air batteries. (Source: I. Abate)|
The majority of our oil consumption has been for transportation. Oil is 34% of our world energy consumption and contributes to 40% of total CO2 emission.  Therefore, electrification of road transportations should be one of our biggest goals in order to reduce the effects of global warming. In addition, the amount of energy the sun releases in an hour is sufficient to meet the world energy demand in a year. However, renewable energy sources such as the sun and wind are not always there. Moreover, the peak demand does not match the peak supply from renewable energy sources.  High energy density batteries are thus crucial to electrification of vehicles and storing energy from renewables to meet peak demands.
Metal-air batteries are the most promising high energy density batteries. They are electrochemical cells that use metal as anode and ambient air as a cathode with aqueous electrolyte, as shown in Fig. 1.  The metal (M) at anode gets oxidized to produces metal ions (M+) which move through the electrolyte to the cathode and react with O2 to form metal oxides (MO2x). The reactions are: 
|Anode Reaction:||M (S) ⇔ M+ + e-|
|Cathode Reaction:||M + + xO2 + e- ⇔ MO2x|
|Net Reaction:||M + xO2 ⇔ MO2x|
Some of the most common metal-air batteries include lithium-air, sodium-air, magnesium-air and zinc-air batteries. Lithium-air battery gives the highest energy density (about 3,458 Wh kg-1) because of its highest charge to mass ratio. This is several times higher than that of Li-ion batteries (100-200 Wh kg-1), the most commonly used battery in electric vehicles and electronics today. 
There are several challenges which make the practicality of metal-air batteries very difficult. Four of these main challenges will be mentioned here. First, the metal anodes react with the electrolyte to form a passivation layer called solid electrolyte interphase (SEI) film. This film causes an irreversible loss in battery performance.  Second, the reason behind internal short circuit in batteries which lead to explosion is the dendrite growth on anodes. The growth is as the result of uncontrollable dissolution and deposition of metal anodes. Metals do not necessarily deposit onto the site where it was consumed. Dendrite growth can also degrade the performance of batteries.  Third, it is difficult to find an electrolyte with all desired properties which include high stability, low volatility, non-toxicity and high oxygen solubility and wide electrochemical window. Last but not least, the stability of materials where cathodic reaction take places in metal-air batteries is another main challenge. Carbon materials are mainly used for this application and are unstable above 3.5 V during discharge and charge processes. This chemical instability leads to side reactions which are a major cause for performance loss and reduced cyclability of metal-air batteries. 
The high energy density of metal-air batteries makes them desirable for electrification of vehicles and storing energy from renewable sources, which are only available intermittently. However, there are several limitations which have to be resolved before replacing the currently used Li-ion batteries, which has about ten times less theoretical energy density. These limitations include SEI layer formation and dendrite growth on the anode, finding an electrolyte that meets all the desired properties and stability of the cathode materials. Currently, there is a great deal research being undertaken by scientists around the world to better understand these limitations and devise solutions.
© Iwnetim Abate. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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