# Recent Progress of Metal-Air Batteries--A Mini Review
### Working Principles  
The working principle of MABs differs from that of traditional ionic batteries. The traditional ionic batteries involve the transformation of metallic ions from the anode to the cathode. In MABs, metals or alloys transform to metallic ions at anode and oxygen transforms to hydroxide ions at the cathode. Figure 3 shows the operation of a MAB in aqueous or non-aqueous electrolyte medium. In an aqueous electrolyte system, oxygen diffuses into batteries through the gas diffusion layer and transforms into receiving electrons forming oxygen anions. In a non-aqueous electrolyte system, oxygen receives electrons and transforms into oxygen anion. Metals release electrons, transform to  
Figure 1: Applications of metal–air batteries as energy source and storage systems.  
metallic ions and dissolve into electrolytes. These processes will be reversible during a charging procedure of a rechargeable MAB.  
For MABs, oxygen and metals participate in electrochemical reactions. Specific reaction formulas are as Equations (1) and (2):  
\[\text{Anode: M}\rightleftharpoons\text{M}^{n+}+\text{ne}^{-} \tag{1}\]  
\[\text{Cathode: O}_{2}+2\text{H}_{2}\text{O}+4\text{e}^{-}\rightleftharpoons 4\text{OH}^{-} \tag{2}\]  
The reaction kinetics of FABs in the alkaline aqueous electrolyte are shown in Equations (1) and (6) [10].  
\[\text{Anode: Fe}+2\text{OH}^{-}\rightleftharpoons\text{Fe}(\text{OH})_{2}+2 \text{e}^{-} \tag{3}\]  
\[\text{3 Fe}(\text{OH})_{2}+2\text{OH}^{-}\rightleftharpoons\text{Fe}_{3}\text{ O}_{4}+4\text{H}_{2}\text{O}+2\text{e}^{-} \tag{4}\]  
Figure 3: Schematic diagrams of MABs working principles for (**a**) non-aqueous electrolyte, and (**b**) aqueous electrolyte.  
Figure 2: Theoretical specific energies, volumetric energy densities, and nominal battery voltages of various metal–air batteries (MABs) [9].  
\[\text{Cathode: O}_{2}+2\text{H}_{2}\text{O}+4\text{e}^{-}\rightleftharpoons 4\text{OH}^{-} \tag{5}\] \[\text{Overall reaction: }2\text{O}_{2}+3\text{Fe}\rightleftharpoons \text{Fe}_{3}\text{O}_{4} \tag{6}\]  
The working principle of AABs in the alkaline aqueous electrolyte is shown in (9) [11].  
\[\text{Anode: Al}+4\text{OH}^{-}\rightleftharpoons\text{Al}(\text{OH})_{4} ^{-}+3\text{e}^{-} \tag{7}\] \[\text{Cathode: O}_{2}+2\text{H}_{2}\text{O}+4\text{e}^{-} \rightleftharpoons 4\text{OH}^{-}\] (8) \[\text{Overall reaction: }3\text{O}_{2}+2\text{Al} \rightleftharpoons\text{Al}_{2}\text{O}_{3} \tag{9}\]  
The working principle of ZABs in the alkaline aqueous electrolyte is shown in (13) [12].  
\[\text{Anode: Zn}+4\text{OH}^{-}\rightleftharpoons\text{Zn}(\text{OH})_{4} ^{2-}+2\text{e} \tag{10}\] \[\text{Zn}(\text{OH})_{4}^{2-}\rightleftharpoons\text{ZnO}+2 \text{OH}^{-}+\text{H}_{2}\text{O}\] (11) \[\text{Cathode: O}_{2}+2\text{H}_{2}\text{O}+4\text{e}^{-} \rightleftharpoons 4\text{OH}^{-}\] (12) \[\text{Overall reaction: O}_{2}+\text{Zn} \rightleftharpoons\text{ZnO} \tag{13}\]  
The working principle of LABs in the non-aqueous electrolyte is shown in (18) [1].  
\[\text{Anode: Li}\rightleftharpoons\text{Li}^{+}+\text{e}^{-} \tag{14}\] \[\text{Cathode: O}_{2}+\text{e}^{-}\rightleftharpoons\text{O}_{2} ^{-}\] (15) \[\text{O}_{2}^{-}+\text{Li}^{+}\rightleftharpoons\text{LiO}_{2}\] (16) \[\text{LiO}_{2}+\text{Li}^{+}+\text{e}^{-} \rightleftharpoons\text{Li}_{2}\text{O}_{2}\] (17) \[\text{Overall reaction: O}_{2}+\text{Li} \rightleftharpoons\text{Li}_{2}\text{O}_{2} \tag{18}\]