# Recent Progress of Metal-Air Batteries--A Mini Review
### Configuration of MABs  
Based on packaging and practical application requirements, MABs can be classified as traditional static batteries, flow batteries, and novel flexible batteries [13]. In this part, three kinds of batteries will be discussed briefly. The latter part is based on the analysis of solid-state batteries.  
**Traditional static batteries:** As shown in Figure 3, traditional static air batteries have four main parts: cathode, separator, electrolyte, and anode. Compared to the fast kinetics of the anode reaction, oxygen reaction on cathodes is kinetically sluggish in nature. A three-phase reaction boundary of solid (catalyst)-liquid (electrolyte)-gas (oxygen) contributes to the oxygen reduction reaction (ORR). Meanwhile, a reversed oxygen evolution reaction (OER) occurs on a two-phase boundary solid (catalyst)-liquid (electrolyte) at the cathode [14]. Highly efficient bifunctional catalysts are thus required to facilitate both OER and ORR. In addition, since an active electrolyte is employed in traditional static MABs, it is challenging to completely overcome the issue of insoluble deposition of by-products on the surface of both the metal anode and air cathode during the charge-discharge cycles. These deposited by-products consequently block the electrode pores limiting the diffusion of air that eventually results in a lower battery performance [15].  
**Flow batteries:** This type of MAB consists of an electrode, separator, electrolyte, and an electrolyte bank installed as an additional part. Usually, a pump is also integrated to drive electrolyte flow, as shown in Figure 4b. Flowing-electrolyte configuration addresses some of the problems associated with the metal anode and an air cathode. For instance, in zinc-nickel batteries, a large volume of flowing electrolyte decreases the formation of dendrites and irregular shape changes of zinc and thereby, avoid passivation by improving current distribution and reducing concentration gradients [16]. However, the complicated flowing-configuration of MABs has some shortcomings, including decreasedenergy efficiency and volume density, and increased complications as additional pumps and tubes are needed to drive the flow of electrolyte during the discharge.  
**Flexible batteries**: With the increasing demand for portable electronics in recent years, light and small form-factor flexible batteries have become a hot research topic [17; 18]. The main components of a flexible battery include a cathode, anode, separator, and high conductivity electrolyte. Conventional electrolyte for a flexible battery system is a solid-state electrolyte. A thin metallic plate is used as a metal anode to reduce the battery weight. Various nano compounds and nanocomposites are being explored as a potential cathode material. Specific materials include carbon fibers, carbon nanotubes, and graphene. According to the current development trend, flexible batteries are undergoing an evolution, from polymer batteries, flexible alkaline batteries, lithium-based batteries to metal-air batteries. Currently, ZABs and AABs are the ideal flexible batteries due to low cost, safe operation, and high energy density [19].