Breaking the Polarization Bottleneck: Innovative Pathways to High-Performance Metal–Air Batteries

Abstract

Metal–air batteries have excellent theoretical energy density and economic advantages through abundant anode materials and open cathode structures. However, the actual energy efficiency of metal–air batteries is much lower than the theoretical value due to the effect of polarization voltage during battery operation, limiting the power output and thus hindering their practical application. This review systematically dissects the origins of polarization: slow oxygen reduction/evolution reaction (ORR/OER) kinetics, interfacial resistance, and mass transfer bottlenecks. We highlight cutting-edge strategies to mitigate polarization, including atomic-level engineering of air cathodes (e.g., single-atom catalysts, low Pt catalysts), biomass-derived 3D porous electrodes, and electrolyte innovations (additives to inhibit corrosion, solid-state electrolytes to improve stability). In addition, breakthroughs in metal–H₂O₂ battery design using concentrated liquid oxygen sources are discussed. Together, these advances alleviate the battery polarization bottleneck and pave the way for practical applications of metal–air batteries in electric vehicles, drones, and deep-sea devices.

1. Introduction

Driven by the continuous rise in global energy consumption and carbon neutrality targets, the development of new electrochemical energy storage technologies has become a strategic key to solving the energy–environment dilemma. As the most mature energy storage carrier in commercialization, Li-ion batteries rely on the intercalation reaction mechanism between layered oxide cathode (e.g., NMC811) and graphite anode to successfully achieve a mass energy density of 150–250 Wh kg⁻¹ [1,2], and their liquid system based on the LiPF₆ electrolyte derives a cycle life of more than 5000 cycles [3], which has directly contributed to the high-speed development of electric vehicles globally. However, due to the theoretical capacity limit of graphite anode [1] (372 mA h g⁻¹) and the resource constraints of strategic metals such as cobalt and nickel [4], the existing system faces a fundamental challenge to achieve the 400 Wh kg⁻¹ target set by the Energy Saving and New Energy Vehicle Technology Roadmap 2.0.

As a third-generation high-energy chemical power source, metal–air batteries have attracted much attention for their breakthrough energy storage mechanism [5,6]. The remarkable theoretical energy densities of this system are metal-dependent: lithium–air (Li–air) batteries can reach theoretical values of up to 5900 Wh kg⁻¹, zinc–air (Zn–air) batteries of up to 1220 Wh kg⁻¹ (Figure 1a), and aluminium–air (Al–air) batteries of up to 8100 Wh kg⁻¹ [7]. This allows metal–air batteries to show a unique advantage in extreme application scenarios, such as UAV endurance and deep space probes. In terms of system economics, the advantages are high abundance of anode materials (e.g., Al accounts for about 8.23% of the earth’s crust, which is much higher than the 20 ppm of Li resources, Figure 1b), which significantly reduces the cost, and the open cathode structure eliminates the need for expensive cathode materials (e.g., Li cobaltate) for conventional Li-ion batteries [8]. As the most technologically mature and commercially viable metal–air battery, Zn–air batteries have been commercialized in hearing aids and navigation lights due to their reversible deposition and closed-cycle design (ZnO electrolytic regeneration) [9]. Moreover, due to their high safety and stable voltage platform, Zn–air batteries have broad application prospects in the field of high-energy density micro-batteries and grid energy storage [10,11]. Zhang et al. used PVA–KOH–K₂CO₃ hydrogel electrolyte to reduce the electrolyte volume, achieving a volumetric energy density of approximately 1400 Wh L⁻¹ and an areal peak power density of 139 mW cm⁻² for micro Zn–air batteries. Through further structural optimization, including the use of multi-layer gel electrolytes and thinning of the air cathode, the volumetric and gravimetric energy densities reached 1576 Wh L⁻¹ and 420 Wh kg⁻¹, respectively [12]. Therefore, Zn–air batteries are expected to become the most promising alternative to micro Li-ion batteries. As a primary battery, Al–air batteries exhibit an exceptionally prominent actual energy density. Additionally, Al is abundant in resources and low in cost. Currently, mature primary Al–air battery systems have been applied in special scenarios that require ultra-high energy density, long storage life (in dry state), and have no requirement for charging, such as underwater equipment and emergency backup power supplies [13,14]. The core appeal of Li–air batteries lies in their unparalleled theoretical energy density, making them a highly promising energy candidate for future electric vehicles and long-endurance drones. However, due to issues with cycle life and safety, Li–air batteries are still in the early stages of development. Currently, solid-state electrolyte modulation technology has helped Li–air batteries achieve a small polarization gap and high-rate operation [15]. In summary, we selected these three highly representative metal–air batteries as the research objects for polarization voltage. In Figure 1c, we searched the number of papers published on metal–air batteries from January 2015 to May 2025. The publication of metal–air batteries, especially Zn–air batteries, has steadily increased in the last decade, and these advances are collectively driving the critical turnaround of metal–air batteries from the laboratory to industrialization.

The loss of real energy efficiency due to polarization voltage has formed a significant techno-economic barrier [8,17]. The polarization voltage is the difference between the actual operating voltage and the theoretical equilibrium voltage of metal–air batteries, which is essentially caused by the combination of activation polarization, concentration polarization, and ohmic polarization. Polarization voltage causes the actual output voltage of the battery to be lower than the theoretical value, which results in lower efficiency [18], e.g., the actual energy density of Zn–air batteries is less than 60% of the theoretical value [19]. Moreover, the side reactions induced by the polarization voltage (e.g., hydrogen evolution reaction [20], dendritic crystal growth [21,22]) limit the power output, which affects the acceleration performance of the battery in high-power scenarios, such as the acceleration performance of an electric vehicle. In Figure 2, to further delve into the polarization voltage, this review summarizes the core causes of the polarization voltage in metal–air batteries: activation polarization due to the kinetic hysteresis of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), concentration polarization due to oxygen diffusion and ionic diffusion, and ohmic polarization influenced by the interfacial resistance and electrolyte conduction. The corresponding solution strategies are also discussed in terms of optimal design of the air cathode and electrolyte engineering. Finally, this review highlights the significance of reducing the polarization voltage to achieve the commercial application of metal–air batteries and looks forward to its broad prospects in the future energy storage field.

2. Working Mechanism and Battery Polarization Challenge of Metal–Air Batteries

2.1. Working Mechanism

The metal–air battery is a kind of high-energy density electrochemical energy storage device that uses metal as the anode and air (oxygen) as the cathode active substance, with its core advantage being the use of oxygen in the air as the cathode reactant, which significantly reduces the mass of the battery and improves the energy density [29]. The basic structure of the metal–air battery consists of the metal anode, the air cathode, the electrolyte, and the diaphragm. The metal anode is usually made of active metals such as Zn, Li, Al, etc., and exists in the form of foil, powder, or porous structure, releasing electrons through oxidation reaction; the air cathode is a multi-layered design containing a gas diffusion layer (GDL), a catalytic layer, and a collector, in which the GDL composed of porous carbon-based materials allows oxygen to penetrate and prevents the leakage of the electrolyte, and the catalytic layer is loaded with precious metals or transition metal oxides to accelerate ORR and OER. The catalytic layer is composed of a catalyst, a binder, and a conductive agent. Both the binder and the conductive agent play indispensable auxiliary roles in the air cathode. The binder acts as the “framework” of the electrode structure, ensuring mechanical strength and porous structure and reducing concentration polarization to a certain extent; the conductive agent serves as the “wire network” inside the electrode, ensuring efficient electron transmission and reducing the internal resistance of the electrode. They work together to provide a stable, efficient, and durable reaction site and transmission path for oxygen reduction and oxygen evolution [30,31]. Therefore, while improving the catalyst activity, it is also necessary to select and study the most suitable binder/conductive agent combination and its optimization process. The collector is responsible for electron conduction. Depending on the system, the electrolyte is classified as aqueous (e.g., aqueous KOH solution [32]) or organic (e.g., Li salt dissolved in an ether solvent [33]), which provides an ion transport channel and participates in the electrochemical reaction; the diaphragm, usually made of porous polymer, prevents short-circuiting of the positive and negative electrodes while allowing free ion migration.

When the battery works, the anode of the metal is oxidized to release electrons, oxygen is reduced at the cathode and combined with ions in the electrolyte to form oxides, and the electrons pass through the external circuit to form an electric current, thus realizing the conversion of chemical energy into electrical energy. Due to the difference in electrolyte systems, the working principle of the aqueous metal–air battery and the organic metal–air battery is slightly different. For aqueous metal–air batteries (Zn–air battery, for example), the principle of operation revolves around the oxidation of Zn at the anode and the reduction of oxygen at the cathode [19]. During discharge, the Zn anode undergoes an oxidation reaction in an alkaline environment, generating Zn²⁺ ions (Equation (1)), which are further decomposed into Zn oxide and water (Equation (2)), and at the same time releasing electrons that flow through external circuits towards the cathode electrode [34]. On the cathode side of the air, oxygen flows through the porous carbon substrate material, forming a current through the porous carbon material, which is then converted into electrical energy. Oxygen diffuses through the porous carbon-based material to the catalytic layer, where it combines with water and electrons to undergo a reduction reaction in the presence of a catalyst, forming hydroxide ions to complete the charge transfer (Equation (3)). The total reaction throughout the discharge process is the formation of Zn oxide from Zn and oxygen at a theoretical voltage of approximately 1.65 V (Equation (4)) [34]. During charging, a voltage applied by an external power source forces the reaction to proceed in reverse: Zn oxide is reconverted to Zn in the alkaline electrolyte, while oxygen is released from the positive electrode (Equations (5)–(7)) [16,34]. However, passivation of the Zn negative electrode (where the Zn oxide layer hinders the reaction), clogging of the electrode pores with carbonates generated by the electrolyte due to absorption of carbon dioxide from the air, and the growth of Zn dendrites during charging limit their cycle life. Despite the challenges, Zn–air batteries have been commercialized in low-power devices, such as hearing aids and sensors, due to their high theoretical energy density [9,21,35], low cost, and environmentally friendly properties, and continue to be optimized as a potential renewable energy storage solution [9,36].

Discharge process

Anode:

$$\begin{array}{c}\mathrm{Zn}+{4\mathrm{OH}}^{-}\to {\mathrm{Zn}\left(\mathrm{OH}\right)}_4^{2-}+2{\mathrm{e}}^{-} \mathrm{E}=-1.25 \mathrm{V} \mathrm{vs}. \mathrm{SHE}\end{array}$$

(1)

$${\mathrm{Z}\mathrm{n}\left(\mathrm{O}\mathrm{H}\right)}_4^{2-}\to \mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+2\mathrm{O}{\mathrm{H}}^{-}$$

(2)

Air cathode:

$${\mathrm{O}}_2+{2\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4\mathrm{O}{\mathrm{H}}^{-} \mathrm{E}=0.4 \mathrm{V} \mathrm{v}\mathrm{s}.\mathrm{SHE}$$

(3)

Overall reaction:

$$2\mathrm{Z}\mathrm{n}+{\mathrm{O}}_2\to 2\mathrm{Z}\mathrm{n}\mathrm{O} \mathrm{E}=1.65 \mathrm{V} \mathrm{v}\mathrm{s}. \mathrm{S}\mathrm{H}\mathrm{E}$$

(4)

Charging process

Anode:

$$\mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+2\mathrm{O}{\mathrm{H}}^{-}+2{\mathrm{e}}^{-}\to \mathrm{Z}\mathrm{n}+4\mathrm{O}{\mathrm{H}}^{-}$$

(5)

Air cathode:

$$4\mathrm{O}{\mathrm{H}}^{-}\to {\mathrm{O}}_2\uparrow +2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$$

(6)

Overall charge reaction:

$$2\mathrm{Z}\mathrm{n}\mathrm{O}\to 2\mathrm{Z}\mathrm{n}+{\mathrm{O}}_2\uparrow$$

(7)

Al–air batteries, however, are mostly disposable batteries that need to be “mechanically recharged” by replacing the metal anode electrode [37]. In the discharge process of organic-based metal–air batteries represented by Li–air batteries, the Li anode loses electrons to be oxidized to Li ions, and the electrons are transferred to the anode through the external circuit (Equation (8)); oxygen diffuses through the porous structure of the anode to the catalytic layer, where it combines with the Li ions and the electrons to generate Li peroxide (Equation (9)) at a theoretical voltage of 2.96 V (Equation (10)). However, the electrolyte is susceptible to oxidative decomposition by superoxide radicals (O²⁻), generating irreversible byproducts (e.g., Li₂CO₃ or LiOH) that lead to capacity decay [17,34,38]. During charging, an external power supply is required to apply a high voltage (typically >4 V) to force the decomposition of Li peroxide (Equation (11)), but this process is accompanied by intense oxidation of the electrolyte and growth of Li dendrites, which can lead to safety hazards [17,39]. In addition, dendrites generated on the surface of the Li anode may puncture the diaphragm and cause a short circuit, while Li₂O₂ at the anode further reduces the reaction kinetics due to electrode passivation caused by the insulating properties [33,34,40]. To improve the performance, researchers have used bifunctional catalysts (e.g., bioenzymes [41]) to accelerate the oxygen reduction and precipitation reactions or have introduced solid-state electrolytes (e.g., ceramic oxides [42]) to inhibit the dendrites and widen the electrochemical window. Despite the remaining barriers to commercialization, Li–air batteries show transformative potential in long-range areas such as electric vehicles and drones, making them an important area to explore for the next generation of high-energy batteries.

Discharge process

Anode:

$$\mathrm{L}\mathrm{i}\to {\mathrm{L}\mathrm{i}}^{+}+{\mathrm{e}}^{-} \mathrm{E}=-3.05 \mathrm{V} \mathrm{v}\mathrm{s}. \mathrm{S}\mathrm{H}\mathrm{E}$$

(8)

Air cathode:

$${\mathrm{O}}_2+2\mathrm{L}{\mathrm{i}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{L}\mathrm{i}}_2{\mathrm{O}}_2 \mathrm{E}=-0.09 \mathrm{V} \mathrm{v}\mathrm{s}. \mathrm{S}\mathrm{H}\mathrm{E}$$

(9)

Overall reaction:

$$2\mathrm{L}\mathrm{i}+{\mathrm{O}}_2\to {\mathrm{L}\mathrm{i}}_2{\mathrm{O}}_2 \mathrm{E}=2.96 \mathrm{V} \mathrm{v}\mathrm{s}. \mathrm{S}\mathrm{H}\mathrm{E}$$

(10)

Charging process

$${\mathrm{L}\mathrm{i}}_2{\mathrm{O}}_2\to 2\mathrm{L}\mathrm{i}+{\mathrm{O}}_2\uparrow$$

(11)

2.2. Battery Polarization Challenge

The performance of metal–air batteries is governed by a variety of polarization effects, mainly activation polarization, ohmic polarization, and concentration polarization, which together lead to the deviation of the actual battery voltage from the theoretical value, thus affecting the energy efficiency and power output [43]. Activation polarization arises from the retardation of the electrochemical reaction kinetics, in particular, the multi-step electron transfer processes of ORR and OER [44,45,46]. At very low current densities, the electrochemical reaction rate is slow, resulting in a very low consumption rate of oxygen on the electrode surface and a small concentration gradient. Meanwhile, due to the small current, the ohmic loss is also minimal. Under such circumstances, the main factor determining the drop in battery voltage is the overpotential required to overcome the activation energy barrier of the electrochemical reaction [44]. For rechargeable metal–air batteries, activation polarization will increase the charge–discharge voltage difference, leading to more energy loss [43]. Ohmic polarization, on the other hand, is caused by the internal resistance of the battery, including the low ionic conductivity of the electrolyte, the poor conductivity of the electrode materials, and the interfacial contact impedance [47]. Its effect grows linearly with the current density and manifests itself as a drop in the operating voltage [48], an accumulation of Joule heat [33], and a limitation of the power density [49,50]. Such polarization is exacerbated by a high-temperature or low-temperature environment. Concentration polarization is associated with insufficient substance transport rates, such as impeded cathode oxygen diffusion [51,52] and formation of local concentration gradients of metal ions at the anode or electrolyte mass transfer bottlenecks [53,54]. At high-rate discharges, the electrochemical reaction rate is extremely fast. The diffusion rate of reactants (especially oxygen at the cathode) to the active sites of the electrode cannot keep up with the consumption rate of the reaction, resulting in a sharp drop in the concentration of reactants on the electrode surface to nearly zero (the anode may also have problems with metal ion diffusion or product accumulation). At this point, in order to maintain a high reaction rate, a huge driving force is required to overcome the mass transfer limitation, and the concentration polarization overpotential increases sharply, becoming the most important factor for the voltage drop [21,55,56].

To mitigate the battery polarization effect, a multi-dimensional strategy is required. In order to address the activation polarization caused by the slow ORR and OER kinetics of air cathodes, the design of catalysts with bifunctional ORR/OER activity is an effective solution. Single-atom catalysts (SACs) and low platinum catalysts have been shown to exhibit excellent bifunctional activity and high utilisation of active sites. This has been demonstrated to be an effective means of reducing the high overpotential caused by activation polarization, thus improving the cycle life of batteries. For metal–air batteries operating at high power, it is necessary to reduce the concentration polarization caused by insufficient oxygen diffusion under high current densities to maintain battery performance. Biomass-derived 3D cathode materials, with their high specific surface area and abundant pore structures, provide more three-phase reaction interfaces for ORR/OER and significantly promote oxygen diffusion; in addition, optimizing battery structures such as metal–H₂O₂ batteries can increase the oxygen concentration on the cathode side and accelerate oxygen diffusion, thereby indirectly suppressing concentration polarization. Regarding ohmic polarization, in addition to the rational design of the cathode catalytic layer, it is currently more important to protect the metal anode to inhibit its self-corrosion and passivation, thereby reducing the internal resistance of the electrode. The development of electrolyte additives to form protective coatings on metal anodes and the design of solid electrolytes can effectively inhibit anode self-corrosion and passivation caused by the accumulation of reaction products, reducing the additional power loss caused by ohmic polarization. The synergistic improvement of materials, structures, and processes can effectively suppress the polarization phenomenon and promote the practical application of these batteries in the field of high-energy storage and power.

3. Air Cathode Optimized Design

3.1. Single-Atom Catalysts (SACs)

Single-atom catalysts (SACs) achieve atomic-level dispersion of active sites by anchoring isolated metal atoms to the carrier surface, thus breaking through the limitations of traditional nanoparticle catalysts in terms of atom utilization and interfacial synergistic effects [57,58,59,60] since the first report of Pt₁/FeO_x catalysts and the introduction of the concept of “single-atom catalysis” by Tao Zhang’s team in 2011 [61]. In

Table 1. Previously reported electrocatalytic and Zn–air battery performance of SACs.

Catalyst	E1/2 (V vs. RHE)	E10 (V vs. RHE)	OCV (V)	Pmax (mW cm−2)	Cycling (h/mA cm−2)	Ref.
VMoON@NC	0.861	1.417	1.392	376.4	>630/10	[57]
Fe-Nx-C	0.91	1.83	1.51	96.4	300/5	[63]
FeN4/NGO	0.91	-	-	217	100/10	[64]
Fe-Z8NC&NaCl	0.896	-	1.52	104	>100/20	[65]
Fe-NNCv	0.924	1.64	1.57	99.1	>330/10	[66]
g-Cu-SACs	0.92	-	1.48	112	>650/5	[67]
Fe-NX@NSCST-ZL	0.94	1.71	1.47	196.21	>750/5	[68]
FeSA/N,S-PHLC	0.91	-	1.47	217	>882/5	[69]
FeN-SC	0.92	-	1.46	251	140/10	[70]
Fe1/NCP	0.95	1.6	1.50	263.8	350/10	[71]
Fe3C@NCNTs	0.84	-	1.61	194	>100/10	[72]
FeS/Fe3C@Fe-N-C	0.91	1.506	1.432	113	>24/50	[73]
FAS-NSC@950	0.871	1.544	1.48	181.8	>72/10	[74]
Fe3Co7-NC	0.893	1.573	-	133	>400/2	[75]
FeCu SACs@NSC	0.89	1.68	1.47	152	>300/5	[76]
FeMn-DSAC	0.922	1.635	1.45	184	>80/2	[77]
FeNi-DSAs-PNCH	0.89	-	1.48	99.20	-	[78]
FeZn-NC-800	0.89	-	1.508	218.6	>200/10	[79]
FeCoCuZn-SACs	0.89 V	1.51	1.51	252	>225/5 and 10	[80]

, we summarize some of the SACs published in recent years; these catalysts have been characterized by their unique electronic structure (e.g., unsaturated coordination environments, quantum size effect) and nearly 100% metal atom utilization, with highly dispersed 3D transition metal–nitrogen ligand fraction (often considered to be the active center for adsorption of O₂ molecules and catalysis of ORR/OER) [5,62].

Han et al. reported an efficient single-atom Fe-N_x-C electrocatalyst [63]. The Fe atoms of the catalyst were uniformly dispersed on the nitrogen-doped carbon skeleton in the form of single atoms to form stable Fe-N_x active sites, and the rigid nanocage structure of ZIF-8 effectively inhibited the agglomeration of Fe atoms during the pyrolysis process. The application of Fe-N_x-C in Zn–air batteries resulted in an open circuit voltage (OCV) as high as 1.51 V and a power density of 96.4 mW cm⁻². These performance advantages stem from the high active site utilization of the single-atom structure and the stability of the carbon skeleton, which effectively reduces the ORR/OER overpotential while suppressing Zn anode side reactions. The additive-assisted pyrolysis strategy demonstrated significant advantages in the preparation of SACs [81]. Cao et al. disrupted the chemical bonding of bulk Fe₂O₃ by the strong polar effect of molten NaCl at high temperatures, which induced the volatilization of the Fe species and their anchoring to the ZIF-8-derived nitrogen-doped porous carbon carriers, resulting in the formation of atomically dispersed Fe-N₄-O₂ active sites (Figure 3a) [65]. XANES and EXAFS analysis confirmed the local coordination environment of Fe-N₄-O₂ (Figure 3b,c). The Fe atoms were coordinated to nitrogen in the Fe²⁺/Fe³⁺ valence state, which was shown to form a stable single-atomic configuration by HAADF-STEM (Figure 3d). Thanks to the unique single-atom active site and optimized electronic structure, Fe-Z8NC&NaCl exhibited excellent ORR performance in 0.1 M KOH, with a half-wave potential (0.896 V vs. RHE) and kinetic current density (36.1 mA cm⁻²) superior to that of commercial Pt/C (0.85 V, 6.4 mA cm⁻²) (Figure 3e). The Fe-Z8NC&NaCl cathode assembled Zn–air battery exhibited a high OCV of 1.52 V and a peak power density (P_max) of 104 mW cm⁻² (Figure 3f,g). Sun prepared Fe SACs (Fe-NNCv) by a “post-adsorption–secondary pyrolysis” strategy combined with melamine (MA) etching [66]. By introducing MA molecules into the precursor ZIF-8, the particle size was significantly reduced to about 46 nm, and the decomposition and volatilization of MA during high-temperature pyrolysis were utilized to introduce abundant carbon vacancies in the carbon matrix. Due to the abundance of vacancies generated by the high-temperature volatile etching of MA, Fe-NNC_V exhibits an excellent half-wave potential (0.924 V vs. RHE) in 0.1 M KOH and no performance degradation after 5000 cycles. The Fe-NNCv Zn–air battery exhibits a high OCV of 1.57 V and a peak power density of 99.1 mW cm⁻² with a cycle stability of more than 330 h.

Compared with the conventional gradual warming pyrolysis method, a very short thermal treatment time seems to be effective for the synthesis of nanocomposites containing small molecules [82]. Fei et al. successfully synthesised SACs by the ultrafast heating effect of microwaves [82,83]. This rapid heating process prevents metal agglomeration and thus reduces the amount of catalyst used. The transient heat treatment-based Fe single atomic catalyst FeN₄/NGO utilizes CO generated from the decomposition of DMF to repair carbon vacancy defects in graphene, which promotes the anchoring of Fe atoms on the surface of nitrogen-doped graphene and the formation of a stable Fe-N₄ coordination configuration via oxygen bridges [64]. The FeN₄/NGO cathode exhibited a peak power density of 217 mW cm⁻², 40% improvement over Pt/C, and achieved a high specific capacity of 750 mA h g⁻¹. This performance advantage stems from the synergistic optimization of active site density, electronic structure, and carrier stability by the transient thermal treatment strategy.

However, the symmetric M-N coordination structure of conventional SACs generally suffers from the intrinsic defect of excessively high intermediate adsorption energy barriers during oxygen electrocatalysis [84,85]. To address this kinetic bottleneck, recent studies have shown that the electronic structure properties of the active center can be precisely regulated by introducing second heteroatoms such as S [23,86,87], B [88], P [71], Cl [89], or nanoparticles (e.g., metal particles [90,91,92], FeS [74], Fe₃C [72]) for coordination microenvironmental modulation. On the one hand, the electron absorbing/supplying capacity of the metal center can be altered to optimize the strength of intermediate adsorption; on the other hand, an asymmetric charge distribution is induced to enhance the orbital hybridization between the catalytic site and the oxygen intermediate (*OOH/*O), which significantly improves the catalytic reaction kinetics of SACs [71]. Yang et al. prepared ultrathin mesoporous N/S co-doped graphene substrates via KSCN-assisted pyrolysis of ZIF-8 and used a volatilization-trapping strategy to anchor Cu single atomic sites [67]. The resulting Zn–air batteries assembled with g-Cu-SACs exhibited a peak power density of 112 mW cm⁻² and maintained a stable charge/discharge voltage difference (ΔV < 0.85 V) over 650 h of cycling. Zhang et al. [70] prepared FeSN-C with sulphur atoms located in the first coordination shell layer and FeN-SC with sulfur atoms located in the second coordination shell layer (Figure 4a). The sulfur atoms in FeN-SC are distributed in the secondary coordination layer of the Fe active center by indirect coordination to form a unique Fe-N-S configuration, while sulfur atoms in FeSN-C are directly involved in the formation of Fe-S bonds in the first coordination layer of Fe (Figure 4b,c). The sulfur in the second coordination shell layer modulates the charge density of the Fe center through the long-range electronic effect to optimize the Gibbs free energy of the *OH desorption step (ΔG = 0.37 eV), breaking through the activity bottleneck of the conventional Fe-N₄ symmetric structure. The Zn–air battery assembled based on FeN-SC exhibited a peak power density of 251 mW cm⁻² (Figure 4d).

Li et al. reported a Fe SAC (Fe-N_x@NSCST-ZL) prepared based on a two-dimensional blade-like ZIF-L template, in which Fe single atoms were anchored to N/S co-doped carbon nanosheets/carbon nanotubes [68]. Using sulfate as the sulfur source, Fe-N_x@NSCST-ZL formed thiophene sulfur (C-S-C) and induced carbon skeleton defects during pyrolysis while retaining the two-dimensional layered structure of ZIF-L. Fe-N_x@NSCST-ZL constructed a multi-stage pore system with a high specific surface area, where the in situ generated carbon nanotubes significantly enhanced the electron conduction and mass transfer efficiency. Yuan et al. further constructed active centers with FeN₃SOH five-coordination structures on nitrogen–sulfur co-doped porous hollow blade carbon substrates by a host-dual-guest sulfide-assisted strategy [69]. The two-dimensional ZIF-L was used as the host template in the synthesis process. Fe_SA/N,S-PHLC with a double-shell layer hollow blade structure realized the modulation of the iron coordination environment by sulfur atoms (Figure 4e,f). The unique double-shell layer hollow structure not only retains the high specific surface area property of ZIF-L but also its rich mesoporous structure (with a specific surface area of 848.94 m² g⁻¹), and sulfur doping-induced defective sites significantly enhance the exposure degree of the active sites and the material transport efficiency. XAFS analysis confirms that Fe exists in an atomically dispersed FeN₃SOH configuration, in which axial hydroxyl coordination and planar asymmetric N/S coordination synergistically optimize the 3D-orbital electronic structure of Fe (Figure 4i), and Figure 4h showed such a unique coordination environment significantly reduces the energy barrier of *O→*OOH by modulating the adsorption energy barrier of the oxygen intermediate (theoretical calculations show an overpotential of 0.329 eV). Fe_SA/N,S-PHLC exhibited a half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH (Figure 4g) and also reached 0.75 V (vs. RHE) under acidic conditions. When applied to Zn–air batteries, the structure–performance synergistic effect of Fe_SA/N,S-PHLC enables the Zn–air batteries to obtain an OCV of 1.47 V and a peak power density of 217 mW cm⁻². The stable coordination structure of the FeN₃SOH active center and the good mechanical stability of the carbon carrier together supported a cycle life of up to 882 h. Sun et al. reported a carbon-based Fe SAC (FeS/Fe₃C@Fe-N-C) prepared by a one-step pyrolysis method [73]. Its half-wave potential in 0.1 M KOH electrolyte reached 0.91 V (vs. RHE). Meanwhile, the overpotential (E₁₀) of the OER in 1 M KOH is 1.506 V (vs. RHE), which is comparable to that of RuO₂ (1.518 V). Attributed to the modulation of the electron distribution at the Fe-N₄ site by FeS/Fe₃C nanoparticles and the charge transport facilitated by the highly graphitized carbon layer, the liquid Zn–air battery assembled with FeS/Fe₃C@Fe-N-C exhibited a peak power density of 113 mW cm⁻², which was superior to that of the Pt/C+RuO₂ system.

Recently, it has been shown that the catalytic performance of ORR/OER can be significantly enhanced by introducing a second metal atom into SACs to construct a dual-atom catalyst system [77,93]. The enhancement of the catalytic performance is attributed to three key mechanisms: the effective regulation of the electronic structure of the active sites, the optimal modulation of the adsorption behaviour of oxygen intermediates by heterogeneous metal atoms, and the synergistic effect between the active sites of different metals [76,94,95]. In particular, the electronic interactions between heterogeneous atoms not only improve the binding energy of the reaction intermediates but also form more favorable catalytically active centers through geometrical restructuring [76]. Fe-Co dual single atomic catalysts (FeCo-NC) were prepared by a synergistic hydrothermal–pyrolysis strategy by Gu et al. [75] FeCo-NC retained the dodecahedral structure of ZIF-8 (Figure 5a,b) and was based on atomically dispersed Fe-N₄ and Co-N₄ sites as active centers, and the coexistence of adjacent Fe/Co double sites (spacing ~3.4 Å) was observed by AC-HAADF-STEM (Figure 5c,d). XANES and EXAFS analyses confirmed that both Fe/Co are in the +1~+2 valence states, forming a tetra-coordinated structure that is strongly coupled to the carbon matrix. This two-site synergistic effect enabled Fe₃Co₇-NC to exhibit a half-wave potential up to 0.893 V (vs RHE) in 0.1 M KOH, and the tafel slope was as low as 42 mV dec⁻¹, indicating faster reaction kinetics; the overpotential at 10 mA cm⁻² was only 343 mV, lower than that of RuO₂ (402 mV). Flexible solid-state Zn–air batteries assembled based on FeCo-NC have higher discharge plateaus and smaller charge/discharge voltage gaps at high current densities and have a long cycling stability of more than 400 h (Figure 5e).

Wang et al. synthesized FeNi bimetallic SACs (FeNi-DSAs-PNCH) [78]. Fe and Ni were anchored to the carbon skeleton in the form of single atoms to form Fe-N_x and Ni-N_x active sites. The adjacent Ni-N_x sites optimized the adsorption-desorption equilibrium of oxygen intermediates by modulating the d-band center of the Fe-N_x sites, which significantly enhanced the catalytic activity. In a 0.1 M KOH electrolyte, FeNi-DSAs-PNCH exhibits a half-wave potential of 0.89 V (vs. RHE), and the current density is maintained at 96% after 26,000 s of continuous operation. The enhancement of ORR and OER catalytic performance of SACs by doping with polymetallic elements has been reported [96]. Li et al. prepared trimetallic (e.g., FeCoZn, FeCuZn) and quatermetallic (FeCoCuZn) single-atom catalysts (Figure 5f) [80]. The catalysts were based on ultrathin nitrogen-doped carbon, and the metals were uniformly dispersed in the form of isolated single atoms. The absence of Fe, Co, Cu, and Zn metal characteristic peaks was confirmed by XRD and XPS analysis (Figure 5g–i). The XPS analyses showed that the metals formed a coordination structure with nitrogen (M-N_x), and there were significant electronic interactions among the multimetals (e.g., Fe 2p binding energy positively shifted and Cu 2p negatively shifted) that synergistically regulate the electronic structure of the active site. The quatermetallic FeCoCuZn-SACs exhibited a half-wave potential of up to 0.89 V (vs RHE) in 0.1 M KOH, a kinetic current density (14.20 mA cm⁻² @0.85 V), and an overpotential of 280 mV at 10 mA cm⁻². The FeCoCuZn-SACs assembled Zn–air battery exhibited an OCV of 1.51 V and a specific capacity of 817 mA h g⁻¹ (close to the theoretical value of 820 mA h g⁻¹). In the charge-discharge cycling test, the catalyst demonstrated excellent cycling durability with a voltage gap increase of only 0.78 V after 145 h of operation at 5 mA cm⁻² and another 80 h of stable operation at 10 mA cm⁻² (Figure 5j).

3.2. Low Platinum Catalysts

Platinum (Pt) is still the material of choice for ORR due to its excellent catalytic activity and stability, but high Pt loading (10–20%) leads to a high cost of the electrocatalyst [97,98]. In addition, Pt nanoparticles are prone to agglomeration and detachment during long-term operation, with a significant decrease in catalytic activity and lifetime [24]. Therefore, to reduce the Pt content as well as the cost and improve the catalytic efficiency of electrocatalysts, researchers have developed a variety of Pt-based alloy compounds or bimetallic nanostructures for ORR [99,100]. Li et al. prepared nitrogen-doped carbon-loaded PtSn alloy nanocatalysts (PtSn/NC) by a synergistic strategy of electroless deposition and carbothermal reduction [24]. The Zn–air battery assembled based on the PtSn/NC catalyst exhibited excellent performance with a peak power density of 150.1 mW cm⁻² at a low Pt loading (20 μg·cm⁻²). Yang et al. reported a low platinum-loaded electrocatalyst (PtCoCN(CDs-X)) based on carbon dots (CDs) doped ZIF-67 derivatives (Figure 6a) [101]. CDs were first introduced into the ZIF-67 framework to form a cobalt nanoparticle-coated nitrogen-doped carbon carrier (CoCN) (CDs-X), which was subsequently anchored with Pt nanoparticles on the surface by chemical reduction (Pt loading <5 wt.%). The CDs optimised the Pt-carrier interactions through the formation of Co-O bonds, promoting the formation of PtCo alloys. The optimized PtCoCN (CDs-0.10) assembled Zn–air battery reached a peak power density of 194.2 mW cm⁻².

Zhong et al. successfully prepared PtFeNC by stepwise introduction of Fe and Pt species into the ZIF-8 framework [100]. Attributed to the Pt-Fe dual-site synergistic promotion of O₂ dissociation and the optimization of intermediate product adsorption–desorption energy barriers, the Zn–air batteries assembled based on PtFeNC exhibit an OCV of 1.492 V and a peak power density as high as 148 mW cm⁻². Among all metallic elements, the ΔH_mix values for atomic combinations of platinum with other elements are below the upper safety limit, which means that platinum can be more easily alloyed with other elements without phase separation (Figure 6b) [103]. Taking advantage of this property of the platinum element, Luo et al. predicted the catalytic performance of the five most promising six-element platinum-based high entropy alloys by DFT calculations (Figure 6c) and finally chose PtFeCoNiMnGa (Figure 6d) [102]. The high entropy phase structure was fixed by ultrafast quenching in liquid nitrogen, and the PtFeCoNiMnGa nanoparticles were finally obtained as homogeneously loaded on the surface of the CNTs (Figure 6e). The PtFeCoNiMnGa/CNT has a low overpotential of 243 mV in 1 M KOH and a mass activity of 1.12 A mg_Pt⁻¹ (0.9 V vs. RHE) in the ORR test (0.1 M KOH), which is 5.3 times higher than that of Pt/C. Due to the high entropy phase inhibition of elemental segregation and enhanced structural stability of the CNT carrier, the OER/ORR activities were maintained at 87% and 95% respectively, after 50 h of constant-current testing. The Zn–air battery assembled based on this catalyst exhibits an OCV of 1.52 V and a peak power density of 130.6 mW cm⁻².

3.3. Biomass-Derived 3D Porous Cathodes

The ORR/OER catalytic process in metal–air batteries occurs at a complex three-phase interface (gaseous feedstock/liquid electrolyte/solid cathode) [104,105]. Therefore, a well-designed 3D porous cathode catalyst structure can effectively promote the oxygen diffusion and ORR/OER process [106,107,108]. Our group achieved the synergistic construction of in situ-grown 1D nitrogen-doped carbon nanotubes and 2D porous carbon nanosheets [109]. The unique 3D structure combines the high electrical conductivity and ion diffusion channel advantages of 1D carbon nanotubes with the high specific surface area and loading capacity of 2D nanosheets, which effectively promotes oxygen diffusion and inhibits the agglomeration of active sites. Excellent ORR performance was demonstrated under 0.1 M KOH alkaline conditions (E_1/2 = 0.87 V). The peak power density of the assembled Zn–air battery reached 294 mW cm⁻².

Recently, some biomass materials have been reported as high-performance ORR/OER catalysts due to their natural porous structure and low-cost advantage, such as wood [110,111], corn [112], chitosan [113], soybean [114,115], and algae [116,117]. In

Table 2. Previously reported ORR/OER and Zn–air battery performance of biomass-derived catalysts.

Catalyst	Specific Surface Area (m2 g−1)	E1/2 (V vs. RHE)	E10 (V vs. RHE)	Pmax (mW cm−2)	Ref.
Fe SA/NCZ	912	0.87	1.55	101	[118]
RN350-Z(1-2)-1000	1835	0.868		204	[114]
SV-900	464.94	0.822	1.56	732.77	[119]
SA-FeCNS-800	1192	0.85	1.615	209	[111]
MPWC-FeSA	454	0.85		152	[110]
N/biochar-800-7	1334	0.9		125	[117]
ZIF-67/nori-800	1454.3	0.85	1.46	476	[25]
CoN-BC-0.3	273.6	0.83	1.55	226	[120]
ZnCo-NC-S-900	279.5	0.90	1.57	510	[121]

, biomass-derived catalysts exhibit excellent bifunctional catalytic activity for both ORR and OER. Moreover, its high specific surface area and porous characteristics effectively promote oxygen diffusion, thereby enabling high-power-density zinc–air batteries. Jiao et al. reported a high-density Fe SAC (Fe SA/NCZ) prepared from corn silk as a biomass precursor, where 4.3 wt.% Fe single atom loading and 10 wt.% ultrahigh nitrogen doping are successfully achieved. Fe SA/NCZ exhibits a unique hierarchical porous tubular structure, with micropores providing single atom anchoring sites, and meso- and macropores facilitating oxygen diffusion and electron transport to form a highly efficient three-phase reactive interface [118]. Jiang et al. successfully constructed Co₃Fe₇-Fe₃C heterostructures by using soybean roots as a carbon source [115]. The Co₃Fe₇-Fe₃C heterostructure has a unique 3D honeycomb porous carbon skeleton inherited from the layered channel structure of soybean roots, while a heterojunction with strong electronic interactions is formed through the interfacial coupling of Co₃Fe₇ alloy with Fe₃C. The assembled Al–air battery achieved a peak power density of 210 mW cm⁻². By mechanically replacing the Al anode, the battery has been continuously discharged at 100 mA cm⁻² for 70 h, demonstrating excellent cycling stability. Algae can be enriched with large amounts of nutrients (nitrogen and phosphorus) from the environment and have a rich pore structure, making it a favorable and viable method of converting algae into an air cathode for metal–air batteries [116]. Our group uses a nori single-cell layer as a macroporous template and utilizes its natural microcellular array structure to adsorb Co²⁺ ions and generate ZIF-67 precursor in situ via 2-methylimidazole [25]. ZIF-67/nori-800 was activated by KOH to form a hierarchical porous structure with macropores (0.2–2 μm), mesopores (2–12 nm), and micropores (0.5–2 nm) with a hierarchical porous structure and high specific surface area (1454.3 m² g⁻¹) (Figure 7a–d). ZIF-67/nori-800 demonstrated outstanding bifunctional catalytic activity. Its ORR half-wave potential in 0.1 M KOH reached 0.85 V (vs. RHE), surpassing that of commercial Pt/C. Furthermore, for OER in 1 M KOH, ZIF-67/nori-800 achieved a low overpotential of 230 mV, significantly outperforming RuO₂ (363 mV).

Chen et al. prepared a helical carbon nanotube-encapsulated cobalt nanoparticle catalyst (CoN-BC-0.3) (Figure 7e) using natural bamboo as a carbon source and successfully constructed a structure of helical carbon nanotubes (HCNTs) by mixing bamboo powder with melamine and cobalt chloride and pyrolyzing it under argon atmosphere at 900 °C (Figure 7f,g) [120]. The liquid Zn–air battery based on this catalyst showed excellent performance: an OCV of 1.465 V (Figure 7h,i) and a peak power density of 226 mW cm⁻² (Figure 7j), which highlights the potential of biomass resources for energy storage applications through the synergistic effect of the multi-stage pores of bamboo and helical carbon nanotubes. Along similar lines, our group reported ultrafine pod-like ZnCo/N-CNTs [121]. Using single-celled spirulina as a template, carbon nanotubes with a pod-like structure were obtained (Figure 7k,l). ZnCo/N-CNTs have an average diameter of ~20 nm, and the surface fold interface provides abundant edge active sites where Co coexists in the form of ultrafine nanoparticles (~13.7 nm) and Co-N_X single atoms. In 1 M KOH electrolyte, ZnCo/N-CNTs exhibited excellent bifunctional catalytic performance: the half-wave potential (0.90 V vs. RHE) (Figure 7m); the OER overpotential was 340 mV. The natural pore structure of biomass combined with transition metal nitrogen-doped carbon-based catalysts can effectively promote oxygen diffusion and ORR, thus effectively reducing the concentration polarization and activation polarization of metal–air batteries.

3.4. Metal–H₂O₂ Batteries

Conventional metal–air batteries rely heavily on the presence of oxygen in the air, fail in low and no-oxygen environments, and face problems such as electrolyte volatilization and the formation of carbonates from CO₂ in the air to clog the electrodes [34,122]. As an oxygen source, hydrogen peroxide (H₂O₂) has an ultra-high oxygen content (about 13.5 mol L⁻¹ in a 70% H₂O₂ solution), which is 1600 times higher than the concentration of O₂ in air and about three times that of a high-pressure oxygen cylinder (12 MPa) [123,124]. And H₂O₂ can be conveniently stored in plastic drums, without the need for high-pressure devices. The high-purity oxygen produced by the decomposition of H₂O₂ can provide an additional driving force for the concentration polarization caused by insufficient oxygen diffusion under high current density and effectively inhibit the corrosion of the air cathode caused by carbonate formation. Consequently, at the structural level, metal–H₂O₂ batteries indirectly suppress the impact of concentration polarization on traditional metal–air batteries. H₂O₂ was first applied to an aluminium-hydrogen peroxide (Al-H₂O₂) battery in 1969 [124]. Swift Enterprises Ltd. et al. optimized the design and performance of an Al-H₂O₂ semi-fuel battery through the innovative design of a high-purity Al sheet anode and a gold-plated mesh cathode combined with a flow-through system [125]. Sun and his co-workers developed a Mg-H₂O₂ battery with a dual-electrolyte that completed 3000 deep-sea tests [126]. Unlike the above mentioned dual-electrolyte Al-H₂O₂ and Mg-H₂O₂ structures (Figure 8a,b), our group reported, for the first time, a novel single electrolyte Zn-H₂O₂ battery based on an H₂O₂ solution (Figure 8c) and confirmed that it can be successfully used in underwater environments and that its high safety and power can be demonstrated [26,127]. The composition of a Zn-H₂O₂ battery is as follows: KOH solution as the electrolyte, H₂O₂ as the oxygen source, activated carbon membrane as the GDL, carbon-based catalyst as the catalyst, and pure Zn as the anode. Unlike Zn–air batteries, Zn-H₂O₂ batteries have a new sealed compartment to contain H₂O₂ and the catalyst that decomposes it. The oxygen production from H₂O₂ decomposition (Equation (12)) is adsorbed and transferred in the GDL, and then ORR occurs on the catalyst layer. The generated OH^- reacts with metallic zinc on the anode to produce soluble Zn(OH)₄^2-, then it is converted to insoluble ZnO deposited on the surface of the zinc anode. The total reaction of the discharge process is Equation (13).

The discharging process:

$$2{\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\uparrow$$

(12)

$$\mathrm{Z}\mathrm{n}+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(13)

With the high concentration of oxygen from the decomposition of H₂O₂ at the air cathode, the ORR and OER performance of the cathode catalyst is challenged. Our group reports an efficient bifunctional electrocatalyst (ZnCo-NC-DMEA-900) with a multi-strategy design for enhancing the power density and stability of underwater Zn-H₂O₂ batteries [26]. ZnCo-NC-DMEA-900 was synthesized by the strategies of DMEA solvent-induced crystallization, Zn ion activation, and magnetic field-controlled carbon nanotube growth (Figure 8d), exhibiting excellent ORR and OER activities. The peak power density of the Zn-H₂O₂ battery reached 442 mW cm⁻² (Figure 8e,f), an 85% improvement over comparable Zn–air batteries (238 mW cm⁻²). The energy density is 966 Wh kg⁻¹ (50 mA cm⁻²) and has been successfully applied to high-power underwater Zn-H₂O₂ batteries.

4. Anode Protection via Electrolyte Engineering

4.1. Electrolyte Additives

The electrolyte is a key component of a metal–air battery, providing a channel for charge transfer between the metal anode and the air cathode. The ionic conductivity of the electrolyte directly affects the internal resistance and power output efficiency of the battery [122]. Traditional aqueous electrolytes face problems such as evaporation [130], carbonation (reaction with CO₂) [122], and the side reaction of some metals (e.g., Al) with water strongly aggravates their battery polarization [131]; organic electrolytes for Li–air batteries also face problems such as low oxygen solubility, leading to cathodic reaction rate limitation [33]. Zhang et al. effectively solved the carbonization problem of Zn–air batteries by simply introducing a Ca⁺ additive, which increased the cycle life of the battery by 22.2% [132]. Wan et al. used a fluorocarbon additive to significantly increase the oxygen solubility in the electrolyte, which resulted in a Li–air battery with a mass power density of 10⁴ Wh kg⁻¹ [133].

In Al–air batteries, the severe self-corrosion of Al anodes in alkaline electrolytes leads directly to low energy efficiency and significant capacity loss. Therefore, effectively inhibiting the corrosion reaction of Al anodes is crucial to improving the performance of Al–air batteries as a whole [134]. Meng et al. investigated the performance of the Al-0.1Sn-0.1In-0.05Ga alloy when used as the anode in Al–air batteries [135]. This alloy contains nanoscale second-phase particles and a uniform microstructure, which effectively inhibits local crystal corrosion. An Al–air battery with an Al-0.1Sn-0.1In-0.05Ga anode exhibits a discharge voltage of 1.523 V, a discharge capacity of 2214 Ah kg⁻¹, and an anode efficiency of 74.31%. However, the effect of a single alloyed anode on inhibiting corrosion is minimal. Recent studies show that electrolyte additives form a protective film on the Al anode, effectively inhibiting corrosion. Therefore, investigating these additives is crucial to suppress the Al anode’s self-corrosion and enhance battery performance, as summarized in Table 3 [13,136]. Corrosion inhibitors for Al air batteries can be divided into inorganic corrosion inhibitors, organic corrosion inhibitors, and composite corrosion inhibitors. Inorganic corrosion inhibitors (e.g., ZnO) reduce active site corrosion by forming a passivation layer through electrochemical deposition of metal ions on the Al surface [13,137]. Wei et al. investigated the inhibition effect of different Zn²⁺ concentrations (0.1–0.5 M) on Al anode self-corrosion by adding ZnO, ZnCO₃, and ZnCl₂, respectively, to a 6 M KOH electrolyte (Figure 9a) [138]. SEM showed (Figure 9b) that the Zn layer on the surface of the Al anode in the ZnCl₂ group was dense and smooth, whereas there were cracks in the ZnO group, uneven deposition in the ZnCO₃ group, and the strong Lewis acid groups generated by the ionization of ZnCl₂ adsorbed on the surface of the Al anode, forming a dense and uniform Zn protective layer and significantly reducing the active hydrogen precipitation sites.

Figure 9. (a) Schematic diagram of using inorganic Zn-containing additives to modify the Al anode of AAB; (b) SEM diagrams of AA1060 alloy surface after 60 min immersion in 6 M KOH electrolytes containing blank and 0.5 M various additives (Adapted with permission from Ref. [138], Elsevier 2022.); (c) Proposed corrosion mechanism of the Al anodes with/without the CA in 4 M KOH; (d) top-view and (e) side-view images of CA adsorption on Al (111) surface; (f) tafel polarization curves for Al anode in 4 M KOH with CA.; (g) top-view and (h) side-view images of BTCA adsorption on Al (111) surface; (i) galvanostatic discharge curves of Al anode in the Al–air battery with/without additives at 20 and 50 mA cm^–2 (Adapted with permission from Ref. [136], American Chemical Society 2025.).

Figure 9. (a) Schematic diagram of using inorganic Zn-containing additives to modify the Al anode of AAB; (b) SEM diagrams of AA1060 alloy surface after 60 min immersion in 6 M KOH electrolytes containing blank and 0.5 M various additives (Adapted with permission from Ref. [138], Elsevier 2022.); (c) Proposed corrosion mechanism of the Al anodes with/without the CA in 4 M KOH; (d) top-view and (e) side-view images of CA adsorption on Al (111) surface; (f) tafel polarization curves for Al anode in 4 M KOH with CA.; (g) top-view and (h) side-view images of BTCA adsorption on Al (111) surface; (i) galvanostatic discharge curves of Al anode in the Al–air battery with/without additives at 20 and 50 mA cm^–2 (Adapted with permission from Ref. [136], American Chemical Society 2025.).

Organic corrosion inhibitors reduce the rate of self-corrosion by adsorbing on the Al surface to form a protective film and inhibiting hydrogen precipitation side reactions [136]. Our group proposes an innovative tripodal molecular armoring strategy using cyanuric acid (CA) as an ultrashort-chain corrosion inhibitor to solve the self-corrosion problem of Al anode in Al–air batteries [136], where the planar triacid groups of CA are tightly adsorbed on the Al surface through the Al-O bond (bond length of 2.38 Å) in a “face-to-face” configuration to form a single-molecule protective layer (Figure 9c). It was shown that the addition of 4 M KOH electrolyte with 80 mM CA reduced the Al corrosion rate by 53.5% (to 0.112 mg cm⁻²·min⁻¹), and the corrosion potential (E_corr) of the Al anode was negatively shifted from −1.519 V in the blank group to −1.534 V (Figure 9f). The Al–air battery achieved 2680 mAh g⁻¹ at a high current density of 50 mA cm⁻² (Figure 9i) and an anode utilization of 89.9%. Compared with the structurally similar BTCA corrosion inhibitor, CA has a denser surface coverage and 52.87% corrosion inhibition efficiency (BTCA only 25.13%) due to its unique tripod configuration (adsorption energy −33.99 kcal mol⁻¹) (Figure 9d,e,g,h), and the molecular simulation further reveals that its parallel adsorption structure can effectively shield water/OH^- erosion. Organic–inorganic hybrid corrosion inhibitors combine the advantages of organic and inorganic corrosion inhibitors and can provide stronger inhibition effects at small doses to further enhance the protection of Al anodes [13]. Jiang et al. used the three-dimensional porous Al foam as the anode and systematically explored the composite system of ZnO and organic acid (acetic acid, citric acid, EDTA) on the Al self-corrosion [139]. As a phase-forming corrosion inhibitor, ZnO deposits a metallic Zn film on the Al surface to cover the hydrogen precipitation active sites, but the Zn film formed when it is used alone is loose in structure and prone to oxidative flaking. The introduction of organic acids significantly optimized this process, as the carbonyl oxygen atoms in the organic acid molecules bind to the Al surface or the Zn film by chemisorption, forming a stable RCOO-Al and RCOO-Zn composite protective layer. This synergistic effect effectively inhibits the oxidation of the Zn film and enhances the densification and bonding of the film layer, thus significantly reducing the corrosion rate of Al.

Table 3. Inhibition of Al corrosion and Al–air battery performance of previously reported electrolyte additives.

Material	Additive	Inhibition Efficiency (%)	Anode Utilization Rate (%)	Capacity Density (m Ah g−1)	Current Density (mA cm−2)	Ref.
Al alloy 1060	CA	52.87	89.9	2680	50	[136]
Al-5052 alloy	βD + DDBAB + 8-HQ	87.2	84.5	2517	20	[131]
Pure Al (99.99%)	Nonoxynol-9	92.8	81.2	2320	20	[140]
Al-5052 alloy	L-tryptophan	67.66	90.76	2702.7	20	[141]
Al alloy	APG + K2SnO3	94.14	73.87	2180	100	[13]
Al-5052 alloy	8-HQ + DG	83.7	92.3	2748	20	[142]
Al-5052 alloy	8-HQ + ZnO	71.59	70.3	2094	20	[143]
Al-0.5Mg-0.1Sn-0.1Ga alloy	Cerium Acetate + L-Glu	77.85	-	2985.075	20	[144]
Al alloy	NBLT	73.9	82.9	2469.1	20	[145]
Al alloy	AMIB	60	86.1	2564	20	[146]

Table 3. Inhibition of Al corrosion and Al–air battery performance of previously reported electrolyte additives.

Material	Additive	Inhibition Efficiency (%)	Anode Utilization Rate (%)	Capacity Density (m Ah g−1)	Current Density (mA cm−2)	Ref.
Al alloy 1060	CA	52.87	89.9	2680	50	[136]
Al-5052 alloy	βD + DDBAB + 8-HQ	87.2	84.5	2517	20	[131]
Pure Al (99.99%)	Nonoxynol-9	92.8	81.2	2320	20	[140]
Al-5052 alloy	L-tryptophan	67.66	90.76	2702.7	20	[141]
Al alloy	APG + K2SnO3	94.14	73.87	2180	100	[13]
Al-5052 alloy	8-HQ + DG	83.7	92.3	2748	20	[142]
Al-5052 alloy	8-HQ + ZnO	71.59	70.3	2094	20	[143]
Al-0.5Mg-0.1Sn-0.1Ga alloy	Cerium Acetate + L-Glu	77.85	-	2985.075	20	[144]
Al alloy	NBLT	73.9	82.9	2469.1	20	[145]
Al alloy	AMIB	60	86.1	2564	20	[146]

4.2. Solid-State Electrolytes (SSEs)

Metal–air batteries face problems such as metal dendrite formation [21,55], anode self-corrosion [35,147], electrolyte volatilization [148], and high overpotential during long-term operation [122]. To fundamentally solve the above bottlenecks in safety, lifetime, and efficiency brought about by liquid electrolytes, solid-state electrolytes are regarded as a promising and transformative strategy [149]. Solid-state electrolytes utilize solid-state ionic conductors instead of liquid systems and are characterized by high mechanical strength, good thermal stability, and a wide operating temperature range [29,50,150]. Currently, researchers have applied SSEs to metal–air batteries and have made some progress [119,151,152,153]. Sun et al. developed a high-performance composite gel electrolyte (PPM) based on PVA, which was successfully applied to a long-life, fully flexible Al–air battery system [151]. For Li–air battery, the low solubility of the discharge products (e.g., Li₂O₂) in electrolytes, which tend to be easily deposited on the surface of the electrodes to form passivation layers. SSEs have been shown to inhibit anode passivation caused by lithium dendrites and the shuttle effect of reaction products, thereby providing excellent protection for the anode [49,50]. Cui et al. proposed a polymer-constrained gel electrolyte (PCG) that uses hydrogen bonding to disrupt the low-temperature crystalline structure of DMSO by confining the high donor number solvent DMSO in a BSDE polymer matrix [152]. The electrolyte has an ionic conductivity of 0.15 mS cm⁻¹ at −20°C (5.5 times that of liquid DMSO), enabling Li–air batteries to be cycled for 450 h at −20°C and 760 h at 25°C, which is significantly better than the liquid electrolyte system. Shang et al. used acrylamide–acrylic acid copolymer (PAMA) as a substrate and introduced AMPNa and Fe³⁺, forming a physical–chemical double cross-linked hydrogel (PAMA-ANa/Fe³⁺) [154]. Strong interfacial adhesion was achieved by utilizing the multiple non-covalent bonds (hydrogen bonding, π-π stacking) of AMPNa. Chen et al. used a dynamic hydrogen bonding network (ATAC-EG -PAA) to lock the water and confer self-healing properties [155].

Inspired by the absorption of water from the air by desert plants (e.g., cacti), Zhang et al. designed a novel electrolyte system based on the coordination of Zn(OTF)₂ and acetamide (C₂H₅NO) (Figure 10a) [156]. The electrolyte was prepared by one-step UV-initiated radical polymerization to form a dual network structure (WIS-PAA/CNF) with PAA as the main body and cellulose nanofibers (CNFs) as the reinforcement (Figure 10b–d). Its dynamic adsorption–desorption equilibrium enabled the electrolyte to gain about 4% weight instead of losing it after one week at 60% relative humidity (Figure 10g). The introduction of CNF significantly enhanced the tensile strength (22.3 kPa) and ductility (770% elongation at break) of the electrolyte through hydrogen bonding and physical entanglement (Figure 10e). Meanwhile, the free-state −NH₂ group released by coordination gives the electrolyte excellent adhesion (1.3–2.0 kPa), which enables it to be tightly adhered to Zn anode, carbon cloth, or nickel foam electrode. (Figure 10f). When applied to Zn–air batteries, WIS-PAA/CNF electrolytes exhibit 1300 h of ultra-long cycle stability (Figure 10h) and can drive LED lights, an electronic watch, and a temperature and humidity sensor (Figure 10i).

5. Conclusions and Perspectives

In this review, we systematically summarize the main factors contributing to the voltage polarization of metal–air batteries and the corresponding solution strategies, including both the optimal design of air cathodes and electrolyte engineering. In terms of the optimal design of the air cathode, the oxygen reactivity and electrode durability have been significantly enhanced by developing highly efficient, stable, and low-cost bifunctional catalysts (e.g., single-atom catalysts, low-platinum catalysts) and optimizing the electrode structure (e.g., constructing hierarchical porous conductive networks, precisely tuning the three-phase reactive interfaces, and enhancing the stability of the carbon carriers). However, the intrinsic activity, long-term stability, and practical performance of the catalysts in complex reaction environments still need to be further improved; the dynamic evolution and mechanical stability of the electrode structure during deep charging and discharging also need to be more deeply understood and controlled. In electrolyte engineering, to address the volatility, carbonation, and metal anode corrosion/dendritic problems of aqueous electrolytes, as well as the low oxygen solubility, poor electrical conductivity, and poor compatibility with electrode materials (especially anode) of non-aqueous electrolytes, researchers have made positive advances in the areas of improving ionic conductivity, suppressing side reactions, and enhancing interfacial stability and safety through the development of functionalized additives, as well as solid electrolytes and other strategies. In particular, the introduction of solid-state electrolytes provides a fundamental way to solve the problems of volatilization, leakage, and dendrite penetration, but their room-temperature ionic conductivity, solid–solid interfacial contact impedance with the electrodes, and stability are still great challenges. Looking ahead, the development of metal–air batteries needs to be sustained and deepened in the following directions:

• Combine in situ characterization and multi-physics field modeling to reveal the correlation between atomic-scale reaction pathways and macroscopic properties during polarization and guide the rational design of materials. In-depth exploration of novel bifunctional catalysts with atomic-scale active sites, high intrinsic activity, and excellent stability. As shown in Table 1, single atom catalysts have high ORR and OER activities, combined with machine learning to screen the optimal coordination environment, combines the large specific surface area of biomass with the high activity of single atoms and designs bifunctional high-efficiency catalysts with high activity and high oxygen diffusion; precious metals, such as Pt and Ir, are introduced into the catalyst structure in single-atom form, making full use of the active sites of the precious metals to improve the catalytic efficiency. Develop a smarter and more stable electrode structure design to achieve efficient transport and rapid conversion of reactants and products while effectively inhibiting carbon corrosion and structure collapse. Flexible and wearable batteries also place higher demands on the anode structure. Explore the coupling of metal–H₂O₂ batteries with renewable energy sources (e.g., seawater electrolysis to make H₂O₂) to build a closed-loop system.
• Continue to explore new types of liquid electrolytes (e.g. highly concentrated electrolytes, new solvent systems) with high oxygen solubility/diffusion coefficient, wide electrochemical window, excellent chemical/electrochemical stability, and good interfacial compatibility; design smart additives with “one dose, multiple effects” to synergistically solve the problems of anode protection, oxygen reaction promotion, electrolyte stability, and inhibition of side reactions. Design of intelligent additives to synergistically solve the problems of metal anode protection, oxygen reaction promotion, electrolyte stability, and inhibition of side reactions. Optimize the interfacial compatibility of solid electrolyte and solve the contact impedance problem at the electrode/electrolyte interface; optimize the interfacial compatibility of solid electrolyte and solve the contact impedance problem at the electrode/electrolyte interface.

By continuing to conduct innovative research in the two core areas of air cathode optimization and electrolyte engineering and strengthening the synergistic design and system integration of the two, metal–air batteries are expected to overcome the existing bottlenecks and achieve a wider range of applications in areas such as portable electronic devices, electric vehicles, and large-scale energy storage.