Metal–air (oxygen) batteries (MABs) have the advantage of using the lightest cathode material available in nature: oxygen. Furthermore, O2 is not stored inside the cell, but it is continuously supplied from air or tanks outside the cell. Therefore, cell capacity is not limited by cathode active material depleting. In addition, in conventional (not-flow) MABs, the anode is a thin metal foil with an extremely high density. Combining a metal anode and an O2 cathode enables to use most of the system volume for the anode material, and this results in a battery with an extremely high energy density.
Several MAB chemistries have been proposed, including those based on alkali, transition, and multivalent metals [
17‐
19]. Table
11.2 summarizes the cell reactions, the metal and discharge product densities, the nominal cell voltage, and the theoretical specific capacity and specific energy and energy density
of different MABs.
Li, Na, K, Mg, and Al-MABs feature cell voltages higher than 2 V and therefore require the use of nonaqueous electrolytes. In the case of Li and Na, which are not stable in water, organic/inorganic hybrid electrolytes have been proposed. In these systems, the metal is in contact with an organic electrolyte, and the cathode operates with an aqueous electrolyte. Anode and cathode are alienated by a solid ionic conductor separator. Zn- and Fe-MABs featuring cell voltages lower than 2 V can operate with aqueous electrolytes, and they typically make use of alkaline solutions.
The specific capacity evaluated based on both metal and oxygen contents depends on the number of electrons exchanged for mole of reactants and mainly on the atomic mass of the metal. It spans from 0.46 Ah kg−1 of the Fe–O2 cells (four-electron process) to 1.17 Ah kg−1of the Li–O2 (two-electron process), which features the lightest metal, i.e., lithium.
All the MABs listed in Table
11.2 feature high theoretical specific energy densities
that are about two- to tenfold higher than that of today’s lithium-ion batteries
. Among the nonaqueous MABs, the Li–O
2 cells exhibit the highest value that in theory can be as high as 3.5 kW kg
−1. Among the aqueous MABs, Zn–O
2 holds the best promises with 1.1 kW kg
−1.
Table 11.2
Cell reactions, metal and discharge product densities, nominal cell voltage, theoretical specific capacity and specific energy and energy density of different MABs. The energy density values have been calculated referring to the metal (first value) and the discharge product (second value) densities
2Li + O2 ⇆ Li2O2 | 0.534 | 2.3 | 2.96 | 1.17 | 3.5 | 1.9–8.0 |
2Na + O2 ⇆ Na2O2 | 0.968 | 2.8 | 2.33 | 0.69 | 1.6 | 1.6–4.5 |
K + O2 ⇆ KO2 | 0.890 | 2.3 | 2.48 | 0.37 | 0.9 | 0.8–2.2 |
Mg + 1/2O2 + H2O ⇆ Mg(OH)2 | 1.738 | 2.3 | 3.09 | 0.92 | 2.8 | 4.9–6.6 |
4Al + 3O2 + 6H2O ⇆ 4Al(OH)3 | 2.700 | 2.4 | 2.71 | 1.03 | 2.8 | 7.5–6.8 |
Zn + ½ O2 ⇆ ZnO | 7.140 | 5.6 | 1.65 | 0.66 | 1.1 | 7.7–6.1 |
3Fe+2O2 ⇆ Fe3O4 | 7.874 | 5.2 | 1.28 | 0.46 | 0.6 | 4.6–3.1 |
In MABs, at the anode, metal stripping/deposition occurs. At the cathode, the sluggish kinetics of the oxygen reduction and evolution reactions (ORR/OER) are promoted by catalysts (or mediators) that are supported on the electrode surface, typically a porous carbon with high surface area. Like in fuel cells, in static MABs, the optimization of the cathode three-phase boundary (catalyst–electrolyte–gas) is of paramount importance to achieve a high conversion efficiency and fast cell response under high-current regimes. Today, typical discharge currents are lower than 0.5 mA·cm−2 for nonaqueous Li-MABs and 500 mA·cm−2 for aqueous Zn-MABs. In air-breathing cells, the slow natural diffusion of O2 to the cathode is one of the processes that cause not negligible cell overvoltages during discharge at high currents. Furthermore, during the discharge, insoluble by-products are deposited on the surface of the metal anode and air cathode, therefore passivating the electrodes, clogging cathode pores, and further limiting the diffusion of oxygen. These passivating products limit the discharge capacity of MABs to values that can be 50% lower than the theoretical ones. They also cause high recharge overvoltages and, consequently, low recharge energy efficiency that is typically lower than 70%.
As it concerns the metal anode, stripping/deposition inherently induces changes in the metal surface morphology, dendrite growth, and metal fragmentation into particles with subsequent loss of electric contact and of material. Here, the electrolyte can play a role by forming a suitable solid electrolyte interphase that protects the anode and controls the uniform metal deposition.
Despite such promising theoretical performance, still many challenging problems need to be solved to let MABs become a consolidated technology. Combining MAB chemistries with a flow cell design in flow metal–air batteries (FMABs) can be an answer. FMAB cell commonly consists of a metal anode (tin foil or a flowable anolyte), a separator soaked in the electrolyte, and a flowable air (oxygen) cathode. At the anode, the metal stripping/deposition occurs. At the cathode side, an electrolyte enriched with oxygen is flowed across a porous current collector where the oxygen reduction and evolution reactions (ORR/OER) occur. As in their static counterparts, to overcome the sluggish kinetics of the ORR/OER, the surface of the porous electrodes is decorated with catalysts or mediators.
The exploitation of a flowable cathode (catholyte) is a smart strategy to overcome some of MAB’s intrinsic challenges, such as the slow oxygen diffusion at the cathode and the passivation of electrodes by deposition of insoluble by-products [
1]. The convective transport of the catholyte allows for overcoming the mass transport limitations due to the oxygen diffusion [
20,
21]. To reduce the current collector passivation, driven by the deposition of the insoluble discharge products (such as metal oxides), a valuable strategy is the exploitation of slurries rather than solutions, in which the suspended particles act as nucleation centers [
1,
22]. Moreover, to alleviate the dendrite formation, while increasing the current density at the anode, a possible solution is the exploitation of anolyte slurries [
23]. The main drawback is the nontrivial design of the flow frame, which strongly depends on the cell chemistry and the rheological properties of the electrolyte.
Nowadays, zinc, aluminum, and lithium are the main metallic anodes on which the research activity in MAFBs has been focused. According to the metal reactivity, both aqueous and nonaqueous electrolyte media have been explored [
24,
25].
Zn-MAFBs are the most mature technology. Indeed, the company “Zinc 8” is currently manufacturing 100 kW, targeting 1 MW installation, forecasting a price below 100 € kW−1. Li-MAFBs, for their exceptionally high theoretical energy density, are holding great promises for energy storage.
By using the abundant, readily available seawater as catholyte, the seawater battery (SWB) arises as an attractive option for low-cost, large-scale energy storage [
26‐
28]. During its charge, at the cathode, the electrolysis (oxidation) of seawater occurs, contemporary with the reduction of Na
+ ions, extracted from seawater on the anode side. Indeed, seawater features a salinity of ≈3.5% (35 g L
−1) in which Na
+ and Cl
− ions account for most of the dissolved salts. The metallic sodium requires an anhydrous anolyte, aprotic solvent solutions with sodium-based organic salts, e.g., 1 M sodium trifluoromethanesulfonate (NaCF
3SO
3) in tetraethylene glycol dimethyl ether (TEGDME) or 0.1 M sodium bis(fluorosulfonyl)imide (NaFSI) in ionic liquid solutions [
27]. The anolyte chamber must be physically separated from the aqueous catholyte while being in ionic contact. Therefore, Na-ion conducting, solid electrolytes (e.g., NASICON) that separate the anhydrous anodic chamber and the aqueous cathodic chamber are adopted [
26‐
28]. To improve the kinetics of the ORR and OER, Pt/C- and Ir/Ru-based catalysts could be exploited [
29]. However, in SWB, the presence of Cl
− in the catholyte requires the use of a proper current collector to control its oxidation reactions during charge, and this represents an additional problem.
SWB features a theoretically high cell voltage ≈3.48 V, with reported practical voltage of 2.2 V. Although extremely promising, today, this technology is still in R&D phase, and efforts are required to decrease the cost of the components to efficiently scale up [
26].