Advancements in Lithium–Oxygen Batteries: A Comprehensive Review of Cathode and Anode Materials

Abstract

As modern society continues to advance, the depletion of non-renewable energy sources (such as natural gas and petroleum) exacerbates environmental and energy issues. The development of green, environmentally friendly energy storage and conversion systems is imperative. The energy density of commercial lithium-ion batteries is approaching its theoretical limit, and even so, it struggles to meet the rapidly growing market demand. Lithium–oxygen batteries have garnered significant attention from researchers due to their exceptionally high theoretical energy density. However, challenges such as poor electrolyte stability, short cycle life, low discharge capacity, and high overpotential arise from the sluggish kinetics of the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charging. This article elucidates the fundamental principles of lithium–oxygen batteries, analyzes the primary issues currently faced, and summarizes recent research advancements in air cathodes and anodes. Additionally, it proposes future directions and efforts for the development of lithium–air batteries.

1. Introduction

The extensive utilization of fossil fuels has played a significant role in promoting economic success and enhancing living conditions. However, it has also intensified the problems of global warming and environmental pollution. The worldwide community has shown significant interest in the long-term viability of energy provision. It is crucial to decrease the use of fossil fuels and promote the advancement of environmentally friendly, renewable, and non-polluting energy sources, including solar, wind, hydro, and tidal power [1,2,3]. However, these technologies are susceptible to natural environmental conditions and geographical locations, leading to inherent uncontrollability and intermittent supply issues. Hence, the imperative lies in the advancement of energy storage systems that are both eco-friendly, technologically advanced, and cost-effective. Presently, there exists a multitude of rechargeable batteries, including lithium-ion batteries [4], which have advanced significantly and are now widely utilized because of their inexpensive, high energy density, and favorable environmental compatibility [5,6,7]. However, the energy density of traditional lithium-ion batteries has approached its theoretical limit, necessitating the development of battery systems with higher energy density [8,9,10].

Metal–air batteries, as representatives of the new generation of green secondary batteries, have an exceptionally high theoretical energy density and are expected to play a significant role in electric transportation and energy storage [11]. Rechargeable lithium–oxygen (Li–O₂) batteries boast a satisfactory theoretical energy density (11,400 Wh kg⁻¹, based on pure lithium), nearly equivalent to gasoline (12,800 Wh kg⁻¹); the actual energy density also approaches that of gasoline, at approximately 1700 Wh kg⁻¹. Even based on Li₂O₂ calculations, Li–O₂ batteries exhibit a high specific energy density of up to 3500 Wh kg⁻¹. Given its abundance and low cost, oxygen may be readily acquired from the air and used as the electrode reactant, showing great potential in the fields of energy storage and conversion [12,13]. Through continuous efforts by researchers, significant progress has been made in the charging and discharging mechanisms, electrode/electrolyte design, and performance optimization of Li–O₂ batteries, demonstrating vast development prospects [14].

However, the development of this system is still in its early stages, with numerous key scientific issues and technological bottlenecks yet to be resolved. The actual performance of the battery, such as energy conversion efficiency, rate capability, and cycling performance, has far from reached the standards for commercial applications. Additionally, some technological bottlenecks, such as the development and preparation of inexpensive bifunctional catalysts and the corrosion prevention of active metal negative electrodes, result in difficulties in decomposing discharge products, electrolyte decomposition, short cycle life, dendrite growth, parasitic reactions, and slow reaction kinetics, becoming obstacles in the commercialization process of Li–O₂ batteries. Hence, the advancement of exceptionally efficient and enduring electrode materials is of utmost importance to facilitate the widespread implementation of Li–O₂ batteries in industries.

2. Basic Principles and Fundamental Issues of Lithium–Oxygen Batteries

Among all rechargeable metal–air batteries, Li–O₂ batteries (typically aprotic Li–O₂ batteries) have the highest theoretical energy density. Based solely on the mass of the lithium metal anode, the theoretical energy density of Li–O₂ batteries can reach 11,400 Wh kg⁻¹. When considering the masses of both Li and Li₂O₂, the theoretical energy density of Li–O₂ batteries remains impressively high at 3505 Wh kg⁻¹, which is significantly higher than that of lithium-ion batteries (LIBs) [15]. The remarkable energy density of Li–O₂ batteries is mainly due to two factors: first, the cathode material, oxygen, is obtained from the surrounding environment instead of being kept within the battery, resulting in a decrease in the weight of the completed battery. Furthermore, as the battery is being discharged, the lithium anode exhibits a remarkably high specific capacity and a comparatively low electrochemical potential (versus the standard hydrogen electrode (SHE) at −3.04 V), ensuring ideal discharge capacity and high operating voltage [16].

2.1. Basic Principles of Lithium–Oxygen Batteries

A typical non-aqueous Li–O₂ battery consists of a lithium metal anode, a non-aqueous electrolyte (comprising organic solvents and lithium salts), a separator, and a porous cathode. The porous cathode does not contribute directly to energy storage as an active material (Figure 1). Instead, it functions as a gas transport channel, an electron conduction medium, a host matrix for discharge products (non-catalytic role), and as a catalyst for the nucleation, growth, and decomposition of discharge products (catalytic role) [17]. During discharge, lithium metal loses electrons to form lithium ions (Li⁺). The Li⁺ ions react with oxygen and electrons from the external circuit to produce lithium peroxide, which is the discharge product (Li₂O₂), which deposits on the cathode material. The reaction process is as follows:

Li + O₂ → LiO₂

(1)

LiO₂ + O₂ → Li₂O₂

(2)

During the subsequent charging process, Li₂O₂ decomposes into oxygen and Li⁺, which then combine with electrons transferred from the external circuit to form metallic lithium. The reactions are as follows:

Li₂O₂ → xLi + Li_2−xO₂

(3)

Li_2−xO₂ → (2 − x)Li + O₂

(4)

2.2. Fundamental Issues in Lithium–Oxygen Batteries

The internal redox reactions in Li–O₂ batteries are notably complex due to the occurrence of oxygen reduction and oxygen evolution reactions at the gas–liquid–solid three-phase interface. Despite the significant advantages of lithium–oxygen batteries, research on this battery system is still in its infancy. The determinants impacting the charge and discharge mechanisms are inadequately comprehended, and extensive research is required to achieve commercial applications. The current challenges faced by Li–O₂ batteries include:

Reactive Lithium Metal Anode: The lithium anode readily reacts with atmospheric CO₂ and H₂O, leading to anode degradation. Additionally, the electrolyte tends to decompose during charge and discharge cycles, generating water, which further exacerbates the degradation of the lithium anode, significantly increasing safety risks. Moreover, the high reactivity of lithium metal can lead to the formation of dendrites, therefore shortening the battery’s lifespan.

Accumulation of Discharge Products: During discharge, O₂ is sequentially converted to superoxide and lithium peroxide. As these reactions proceed, the discharge products continuously accumulate, which can corrode battery components and reduce the battery’s endurance capacity.

Poor Cycling Stability: Lithium peroxide, the discharge product, has very low solubility in the electrolyte, causing it to accumulate on the electrode, which can terminate the discharge process and adversely affect the battery’s cycle life. Additionally, high overpotentials can negatively impact the cycling stability of the battery.

High Overpotential: This significantly reduces the charge–discharge efficiency compared to lithium-ion batteries. The primary cause is the high overpotential required to decompose the discharge products. An effective approach to address this problem is to design highly efficient bifunctional catalysts capable of facilitating both the ORR and OER.

To advance the commercialization of Li–O₂ batteries, it is crucial to develop highly active and stable electrode materials that can overcome these challenges.

3. Advances in Anode Material Research

Organic-based metal–air batteries have become a research hotspot in recent years due to their high theoretical energy density. Taking Li–O₂ batteries as an example, the typical structure of an organic-based Li–O₂ battery consists of a lithium anode, an organic electrolyte containing Li⁺, and a cathode material. Currently, most Li–O₂ battery research focuses on the cathode interface reactions. However, the highly reductive nature of the lithium anode leads to side reactions, complicating the reactions involved in Li–O₂ batteries. Oxygen and electrolytes diffusing from the cathode can react with the lithium anode; simultaneously, by-products generated at the lithium anode can diffuse to the cathode, interfering with cathode reactions. Additionally, the potential formation of dendrites on the anode significantly reduces the battery’s safety, hindering the practical application of the Li–O₂ battery system. Therefore, it is urgent to study and resolve the stability and safety issues within the metal lithium anode. Below is a brief overview of the latest research on the protection and modification of lithium anodes in organic-based Li–O₂ batteries.

3.1. Strategies for Lithium Anode Alternatives (Non-Pure Lithium Anodes)

Given the similarities between Li–O₂ batteries and lithium metal batteries, insights can be drawn from the development of lithium metal batteries. Using anode materials from lithium-ion batteries instead of lithium metal can address the dendrite issue, making this a viable approach for Li–O₂ batteries. Lithium-ion batteries offer a notable benefit by utilizing lithium-intercalated graphite anodes in place of lithium metal. Intercalated carbon materials can create LiC₆, which has a theoretical specific capacity of 372 mAh g⁻¹. However, this capacity seems to be reduced when used in Li–O₂ batteries. To address this issue, Hirshberg et al. introduced the concept of lithiated hard carbon as an alternative anode material for Li–O₂ batteries. This material offers a higher specific capacity compared to lithiated graphite [18]. Fully charged batteries operated for nearly 30 cycles. While the process of introducing lithium ions into hard carbon is significantly safer, it is important to note that hard carbon is not a cost-effective option. In addition to carbon materials, Si, Sn, and Al can combine with Li to create alloys that can be used as anodes containing lithium. Silicon, which has great potential as an anode material in lithium-ion batteries, has also been thoroughly investigated for its use in lithium–oxygen batteries [19,20,21,22].

Zhou’s research team has effectively created a high-performing lithium-ion oxygen (Li–O₂) battery by utilizing commercially available silicon (Si) particles as the anode [22]. A robust solid–electrolyte interface (SEI) coating was formed on the surface of the silicon (Si) anode. This enhanced solid–electrolyte interphase (SEI) film not only improves the long-term durability of the silicon (Si) electrode in cycling but also functions as a protective barrier to effectively inhibit the undesired chemical reactions induced by oxygen diffusion into the anode. The study reveals that the SEI film contains organic substances, lithium carbonate (Li₂CO₃), lithium fluoride (LiF), and specific fluorocarbon compounds. These components endow the Si electrode with protection against oxidative side reactions, thus achieving superior battery performance. The Li–O₂ battery demonstrated excellent cycling stability, capable of undergoing 100 discharge–charge cycles at low overpotentials. In addition, the energy density of this Li-ion O₂ battery has reached 678 Wh kg⁻¹, which is considerably higher than that of traditional lithium-ion batteries (384 Wh kg⁻¹), and it is anticipated to have a maximum energy density of 1764 Wh kg⁻¹ (Figure 2a–c). Experiments also confirmed the long-term cycling potential of these Li–O₂ batteries.

Researchers have also synthesized and directly utilized lithium-rich Li–Si alloys (e.g., Li₂₂Si₅ and Li₁₃Si₄) as anode materials for air batteries. X-ray diffraction (XRD) analysis revealed that the formation of charge products (Li_xSi_y phases) during discharge/charge cycles is directly influenced by the stoichiometry of the initial alloy anode. Under a fixed capacity condition (800 mAh g⁻¹), the assembled Li–O₂ battery exhibited a specific energy density of approximately 1840 Wh kg⁻¹, with an operating voltage of around 2.3 V. The use of Li–Si alloy anodes in Li–air batteries demonstrated high Coulombic efficiency (approximately 97%) and low polarization (around 1.7 V), indicating excellent electrochemical performance [23]. The Li–Si alloy anodes effectively mitigate the reactivity of metallic lithium anodes and address issues related to volume changes during the Si charge–discharge process. These alloys, which do not require electrochemical lithiation, are easy to scale up for practical applications, showcasing significant potential as anode materials in Li–O₂ batteries. Among them, the Li₂₂Si₅ alloy, due to its high lithium content, exhibited particularly satisfactory electrochemical performance (Figure 2d). This advancement underscores the viability of using Li–Si alloys to enhance the stability, efficiency, and scalability of next-generation lithium–air batteries, highlighting their potential for achieving superior energy storage solutions.

Under an Ar atmosphere, silicon and lithium powders are ball-milled into lithium–silicon alloy powder, directly used as the anode in Li–O₂ batteries. The extended lifespan of batteries containing commercial silicon can be linked to the formation of a robust solid–electrolyte interphase (SEI) film on the surface of the silicon anode (Figure 2e). This film acts as a barrier, preventing oxygen from crossing over and suppressing unwanted interactions between the anode and electrolyte. Elia et al. conducted a study on the use of Sn-based anodes in Li–O₂ batteries. They found that the inclusion of Sn nanoparticles in a carbon matrix of micron size improved the mechanical stability of the anodes [24]. Nevertheless, batteries equipped with these anodes had a limited lifespan of only nine cycles. The decrease in the discharge and charge voltage distributions was a result of the irreversible consumption of the anode. The ongoing production of LiOH and Li_xO_y on the surface of the Li_xSnC anode resulted in the permanent depletion of the available lithium source on the anode, ultimately leading to the failure of the battery. Additionally, lithiated Al–C composite electrodes were prepared electrochemically and applied to Li–O₂ batteries. Compared to batteries with a metallic Li anode, those using Li_xAlC anodes exhibited better cycling performance [25].

Besides the aforementioned anodes, special cathode materials such as LiFePO₄ are often used as anodes in Li–O₂ batteries. This material acts as a reference electrode, stabilizing at approximately 3.45 V relative to Li/Li⁺, and involves a two-phase reaction between Li_1−xFePO₄ and FePO₄. It exhibits higher resistance to oxidation caused by oxygen in the electrolyte and can minimize the presence of impurities resulting from the breakdown of the electrolyte at the lithium anode. Consequently, a cleaner electrochemical system for Li–O₂ batteries can be achieved [26,27]. However, due to the high potential of LiFePO₄, it is impractical to use it as an anode material for Li–O₂ batteries.

3.2. Electrolyte Design

The electrically insulating and ionically conducting solid–electrolyte interphase (SEI) film is rapidly formed at the interface between the anode and the electrolyte. This film effectively protects the lithium metal from corrosion. This film is typically delicate and susceptible to fracturing while being transported on a bicycle.

Introducing N,N-Dimethyltrifluoroacetamide (DMTFA) into Li–O₂ batteries helps form a stable SEI film in the presence of N,N-Dimethylacetamide (DMA), preventing sustained reactions with lithium metal [28]. They also developed DMA-based Li–O₂ batteries using LiNO₃ as the electrolyte salt [29].

A stable solid–electrolyte interface (SEI) coating effectively inhibits the reactions between metallic lithium and the electrolyte [29]. LiNO₃ can serve as a co-additive with vinylene carbonate (VC) in Li–O₂ batteries [30]. In this investigation, the electrolyte consisted of LiClO₄/dimethyl sulfoxide (DMSO). By adding VC and LiNO₃, the Coulombic efficiency increased from 25% to 82.5%. Another inorganic salt, InI₃, proposed by Zhou’s group, serves as an effective additive for Li–O₂ batteries [31]. This additive combines the redox mediator (RM) and lithium protection in an inventive way. The discharge product Li₂O₂ is oxidized by soluble I₃⁻ ions, which function as the redox mediator [32]. However, when oxidant I₃⁻ interacts with metallic lithium in the system, it may shuttle to the negative electrode, as it takes several minutes to oxidize Li₂O₂. Huang’s group has proposed the use of boric acid (BA) as an additive for forming a solid–electrolyte interphase (SEI) on lithium metal surfaces [33]. This approach facilitates the formation of a continuous and dense SEI layer composed of nanocrystalline lithium borates and amorphous borates. The resulting SEI film exhibits excellent ionic conductivity and mechanical strength, effectively inhibiting the permeation of oxygen and electrolyte solvents as well as the growth of lithium dendrites. Thanks to the inclusion of BA, the cycle life of lithium–oxygen batteries (LOBs) has been extended by more than six times. Researchers believe this method offers a promising solution to enhance the stability of lithium metal anodes, making practical, rechargeable LOBs a viable reality. This innovation underscores the potential of boric acid as an effective strategy for improving the durability and performance of next-generation energy storage systems (Figure 3a,b).

Li–O₂ batteries have utilized solid-state electrolytes such as solid polymer and ceramic electrolytes. Currently, two types with high ionic conductivity (in the range of 10⁻⁴–10⁻³ S cm⁻¹), Li–Al–Ge–PO₄ (LAGP) [11] and Li–Al–Ti–PO₄ (LATP) [36], have been used in Li–O₂ batteries. However, the performance is not satisfactory. Therefore, Zhou et al. introduced a polymer/ceramic/polymer electrolyte [37]. In this structure, the binding between LATP and cross-linked poly(ethylene glycol) methyl ether acrylate provides an effective soft interface and high mechanical strength ceramic blocks. Using a polypropylene–poly(methyl methacrylate)–polystyrene nanocomposite with SiO₂ as the electrolyte for Li–O₂ batteries, this electrolyte exhibits lower interfacial resistance with the lithium anode compared to liquid electrolytes, showing better compatibility with the lithium metal anode and improving long-term cycling performance [38]. Jang et al. studied the effect of a poly(vinylidene fluoride–co-hexafluoropropylene) (PVdF–HFP) coating on the cycling stability of Li–O₂ batteries [39]. The PVdF–HFP coating reduces interface resistance during charge and discharge and suppresses dendritic growth, therefore improving the cycling stability of Li–O₂ batteries. Subsequently, the Peng group also developed all-solid-state Li–O₂ batteries mainly composed of polymer electrolytes, primarily consisting of PVdF–HFP [34]. The researchers initially created an innovative Li–O₂ battery that consists of fibers. This battery exhibits excellent electrochemical performance and is also flexible. The battery’s discharge capacity is 12,470 milliampere-hours per gram (mAh g⁻¹), and it can consistently function for 100 cycles in an ambient environment, maintaining good electrochemical performance after bending and twisting. It can also be wearable, forming flexible power textiles for various electronic devices (Figure 3c–e).

Huang et al. have developed a dual-salt electrolyte containing trifluoroacetate ions (TFA⁻) alongside lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a G4 solvent (LiTFA/LiTFSI–G4) [35]. This electrolyte system is designed to regulate the solvation structure, therefore reducing the interaction between lithium ions and the G4 solvent. This solvation regulation strategy promotes the formation of a stable solid–electrolyte interphase (SEI) layer predominantly composed of inorganic components, which lowers the desolvation energy barrier for lithium ions and enhances interfacial kinetics. The dual-salt electrolyte strategy not only improves interfacial dynamics but also significantly enhances the cycling performance of lithium–oxygen batteries. Under conditions with a capacity-limited lithium anode (7 mAh cm⁻²), this electrolyte system extends the battery’s cycle life to 120 cycles, compared to only 10 cycles achievable with a single LiTFSI/G4 electrolyte (Figure 3f–h).

Zhou et al. developed a fibrous gel polymer electrolyte (GPE) using a polyacrylonitrile (PAN) matrix through electrospinning. The 3D structure of the GPE enhances the electrolyte’s absorption capacity, while its interconnected design promotes strong interactions between Li+ ions and the polar groups within the PAN matrix, therefore improving ion transport efficiency. In practical tests, both lithium symmetric batteries and Li–O₂ batteries demonstrated the ability to operate over long cycles at high current densities [40].

3.3. Protective Layers and Membrane Modifications

Compared to in situ generated SEI films, pre-treatment processes producing artificial protective layers are more commonly used to protect lithium anodes in lithium–oxygen batteries. A protective film for lithium anodes was developed by incorporating dextrin/β-cyclodextrin nano slurry into modified polyether ether ketone (PWC). This film features a 3D network that shields the highly reactive lithium anode from oxygen crossover [41]. A protective film composed of Al₂O₃ and PVdF–HFP demonstrated significant advantages in suppressing electrolyte decomposition and dendrite growth. After 80 cycles, batteries with protected lithium anodes exhibited discharge capacities approximately three times higher than those without any protection [42].

Park et al. fabricated ultrathin films of graphene oxide (GO) by employing a straightforward vacuum filtering technique on a porous substrate [42]. Because of their tiny dimensions, GO nanochannels preferentially permit the transit of smaller Li⁺ ions while blocking 5,10-dihydro-5,10-dimethylphenazine. Kim et al. explored phosphorene as a protective layer, which subsequently formed Li₃P through an electrochemical lithiation process [43]. The lithium plating on the lithium phosphide layer is thermodynamically unfavorable (Figure 4a–d).

Zhu et al. prepared a 30-micron-thick organic–inorganic hybrid layer by immersing Li in a mixed solution of 1-chlorodecane and cyclohexane, followed by treatment with a mixed O₂/CO₂ gas [47]. This layer serves as a barrier against Li dendrite growth, organic solvent, and dissolved oxygen corrosion. Additionally, certain special methods have been employed to protect the Li anode. Before cycling lithium–oxygen batteries in an oxygen atmosphere, electrochemical pre-treatment in pure argon gas forms an ultrathin passivation film on the lithium metal surface, further inhibiting the side reaction. Asadi et al. adopted a similar pre-treatment approach in Li–CO₂ batteries, running the battery in a pure CO₂ atmosphere to form a Li₂CO₃/C composite protective coating via the reaction between Li and CO₂ [44]. This protective layer, combined with the cathode and electrolyte, endowed the optimized lithium–oxygen battery with an ultra-long-cycle life of 700 cycles (Figure 4e–g).

Lim et al. improved the cycle stability of lithium–oxygen batteries from 65 to 130 cycles by preparing a polyethylene glycol (PEO) film on the lithium metal anode (LMA) and electrochemically precharging it in an oxygen atmosphere [45]. This process forms a polymer-supported solid–electrolyte interface (PS–SEI) layer, where inorganic components (e.g., Li₂CO₃ and LiF) are uniformly distributed within a flexible PEO matrix. This PS–SEI layer exhibits excellent mechanical and chemical properties, effectively inhibiting side reactions between the LMA and electrolyte. This method can also synergize with redox mediators (RM) to further reduce overpotential and extend the battery’s cycle life (Figure 4h,i).

Chen et al. used triethylamine trihydrofluoride (TREAT–HF) as a solution and reactant to prepare a uniform, dense, continuous LiF protective layer on Li by soaking the metal and controlling the reaction temperature. This LiF layer effectively suppresses side reactions between the Li anode and electrolyte while also inhibiting dendrite growth. It demonstrated good stability in symmetric and full lithium cells. Compared to bare Li anodes, the LiF-coated anodes significantly improved corrosion resistance to humid air. Even at high current densities (4.5 mA/cm²), lithium–oxygen batteries with LiF@Li anodes maintained stable cycling for over 150 cycles with minimal polarization voltage.

Coating the separator with charged materials can effectively suppress the redox shuttle effect, and MXenes, with their rich surface functional groups, show immense potential in lithium battery applications [48,49]. Although MXenes have proven successful in mitigating the shuttle effect in lithium–iodine, lithium–sulfur, and lithium–selenium batteries, there have been no reports of their use in lithium–oxygen batteries containing redox mediators. The shuttle effect of triiodide ions (I₃⁻) severely compromises the performance of lithium–oxygen batteries, leading to the failure of redox mediators and the anode. Researchers developed a chemically bonded membrane modification strategy based on MXene, featuring a three-dimensional porous hierarchical structure [46]. The -OH functional groups abundantly present on the MXene surface effectively suppress I₃⁻ migration, while the three-dimensional porous structure ensures rapid lithium-ion transport. Li–O₂ batteries using this MXene–BC@GF membrane did not exhibit the redox mediator shuttle phenomenon during charge and discharge processes and achieved a cycle life of up to 100 cycles, three times that of the control samples. This indicates that the strategy effectively mitigates the I₃⁻ shuttle effect, significantly enhancing the battery’s cycling performance. This work provides new insights into developing membrane modification strategies suitable for lithium–oxygen batteries containing redox mediators (Figure 4j).

Liu et al. proposed an interface polymerization (IP)-based membrane modification method, forming a selectively permeable functional layer on the surface of a polyetherimide (PEI) membrane [50]. This functional layer is designed to inhibit the shuttle effect of large ions such as EMIM⁻, TFSI⁻, and BF₄⁻ between the electrodes while enhancing lithium-ion conductivity. Li–O₂ batteries assembled with the modified P2-PEI membrane demonstrated significantly improved performance, achieving stable operation for 135 cycles at a current density of 0.05 mA cm⁻² and a fixed capacity of 1000 mAh g⁻¹, substantially outperforming batteries with unmodified membranes.

Furthermore, electrospinning can produce nanofibrous materials with high specific surface areas and porous structures, which enhance the electrochemical reactivity and ion transport efficiency of the electrodes in Li–O₂ batteries. Zhang et al. fabricated polyetherketone nanofiber membranes by electrospinning. These membranes exhibited excellent chemical and thermal stability, as well as mechanical strength. When used as a separator in Li–O₂ batteries, the cycle stability of the batteries significantly increased, achieving 194 cycles at 200 mA g⁻¹ and 500 mAh g⁻¹ [51].

4. Advances in Cathode Material Research

4.1. Progress in Cathode Material Research

The cathode is a critical component of metal–air batteries. Discharge products form on the cathode surface, and gases diffuse through the porous cathode. Consequently, cathode materials are a significant focus in metal–air battery research. Current metal–air battery cathodes typically consist of porous materials and catalysts or sometimes just one of these components. Generally, porous carbon materials serve as hosts for discharge products, while catalyst materials enhance the generation and decomposition rates of these products. Some modified carbon materials also exhibit catalytic properties, working synergistically with catalyst materials to improve the electrochemical performance of metal–air batteries.

4.1.1. Advances in Carbon-Based Cathode Materials

Porous carbon composite electrodes are predominantly used as cathodes in metal–air batteries [52]. Due to their porosity, light weight, and high conductivity, porous carbon materials (e.g., Ketjen black) and other carbon forms (e.g., graphene) are among the most promising candidates for metal–air battery cathodes [53,54]. These carbon electrodes have yielded a series of results regarding discharge products and specific capacities. Additionally, significant technological advancements in metal–air batteries have been achieved [55,56,57].

One of the most challenging issues is the insulation of discharge products that fill the pores of carbon air electrodes, leading to high charging overpotentials and the subsequent decomposition of the carbon cathode due to this high overpotential. Researchers are thus focused on understanding the decomposition mechanisms of carbon cathodes and the catalytic decomposition of products in metal–air batteries. McCloskey et al. first reported the instability of carbon-based cathodes in Li–O₂ batteries [58], indicating that cell potentials exceeding 4 V can lead to the decomposition of both the electrolyte and Li₂CO₃. Additionally, intermediates such as peroxide (O₂⁻) have been widely reported in recent studies. Spezia et al. revealed the degradation process of 1,2-dimethoxyethane (DME) through first-principles methods [59], investigating the reactivity of DME with superoxides in both oxygen-poor and oxygen-rich environments using density functional theory (DFT) calculations. Mahne et al. made a significant discovery, identifying the presence of singlet oxygen during the onset of discharge and charging, as well as during high-potential charging [60]. According to their findings, singlet oxygen is a primary impediment to both discharge and charge processes.

Following these results, the modification of carbon-based cathodes has garnered substantial interest. Porous nanostructuring of carbon materials has emerged as a promising and enduring approach. In addition to structural design, research over the past few decades has explored carbon cathodes doped with elements such as nitrogen (N), sulfur (S), oxygen (O), and phosphorus (P) [61,62,63]. DFT simulations have shown that N,O co-doped carbon materials as cathode catalysts can alleviate the disordered growth and slow decomposition of Li₂O₂. Specifically, porous carbon materials with high surface areas and N,O content were prepared through pyrolysis and air activation of guanosine 5′-monophosphate disodium salt (5′-GMP, 2Na) [64]. This material altered the morphology of Li₂O₂ from large toroids to films, improving battery performance. This research provides a new method for preparing high-performance porous carbon materials, aiding in the optimization of Li₂O₂ formation and decomposition processes in lithium–oxygen batteries.

Biomass-derived carbon materials are emerging as ideal cathode candidates for Li–O₂ batteries owing to their renewability, low cost, and environmental friendliness. These materials significantly enhance the electrical conductivity and electrochemical stability of the batteries. Zhao et al. have showcased a biomass-derived carbon material composed of bulk carbon nanosphere clusters synthesized via a low-cost, straightforward, and controllable nanoscale method [65]. The material’s open, slit-like hierarchical structure offers ample active surface sites and transport channels for the electrode. Impressively, this carbon nanomaterial exhibits a high specific capacity of 20,300 mAh g⁻¹ and a cycling life of 543 cycles in Li–O₂ batteries. The synthesis method is versatile and could be able to the three main organic components of plants and biomass waste, indicating potential for large-scale application. The prepared carbon materials demonstrate excellent conductivity and electrochemical stability, with broad application prospects in energy storage, catalysts, and adsorbents (Figure 5a).

Carbon nanotubes (CNTs), with their lightweight, high porosity, and large surface area, are also promising electrode materials. These characteristics enable CNTs to provide abundant active and catalytic sites, facilitating electrochemical reactions. Researchers have transformed the original randomly distributed three-dimensional CNT sponge into a low-curvature structure with regular, orthogonally interconnected channels, approximately 5 μm in lateral dimension [66]. This design effectively reduces path tortuosity, improving the kinetics of electrochemical reactions. The in situ growth of bifunctional transition metal hydroxide catalysts (such as NiFe(OH)_x) on the CNT surface significantly enhances battery catalytic efficiency. These catalysts address the challenges of long-cycle life and high-rate performance in Li–O₂ batteries. This CNT platform exhibits low overpotential and high areal capacity (27.5 mAh cm²), achieving a cycle life of 500 cycles at a current density of 0.5 mA cm². Even at a higher current density of 1 mA cm², this design maintains 275 cycles, outperforming existing porous electrode materials (Figure 5b–e).

In the context of the growing demand for efficient and clean energy, porous carbon materials play a crucial role in high-performance electrode materials due to their excellent conductivity and structural tunability. Addressing this, Shi et al. introduced a novel strategy for synthesizing monodisperse carbon rings through template patch droplets, creating lithium–oxygen battery electrodes with hierarchical porous structures [67]. In this method, patch-like polystyrene (PS) discs serve as templates and hollow carbon rings are produced via a secondary templating process. These carbon rings can be directly used as self-supporting, binder-free electrodes. The study found that these carbon ring electrodes provide high discharge capacity (20,658 mAh g⁻¹), significantly enhancing the performance of lithium–oxygen batteries. This strategy is not limited to carbon ring synthesis but can also be applied to the synthesis of other materials, such as silica rings. Electrodes manufactured using this method exhibit excellent ion transport and gas diffusion performance, demonstrating substantial potential in electrocatalysis and electrochemical energy storage (Figure 5f–i).

Furthermore, Yang et al. prepared metal-free oxygen-rich carbon nanotubes (CNTs) by controlling oxidation modifications, achieving an optimized combination of electrocatalytic performance and electrical conductivity. Electrochemical results indicated that oxygen functional groups facilitate O₂ adsorption, enhancing the electrolysis of O₂ on the CNT substrate during discharge. Moreover, the oxygen groups on the oxidized CNT surfaces promoted the formation of defective Li₂O₂, which exhibited poor crystallinity and could decompose at a low charging potential of approximately 3.5 V [68].

Studies have shown that modifying carbon-based materials through defect and interface engineering can significantly enhance the electrocatalytic performance and cycling stability of batteries. Researchers have developed a metal-free N,P co-doped porous activated carbon (N,P-PAC) electrode via KOH activation and phosphorus doping. This N,P-PAC cathode demonstrated a high specific discharge capacity of 3724 mAh g⁻¹ at 100 mA g⁻¹, excellent cycling stability with up to 25 cycles at a limited capacity of 1000 mAh g⁻¹, and a low charge–discharge voltage gap of 1.22 V at 1000 mA g⁻¹. The low overpotential (EOER-ORR) of 1.54 V was also noted. The outstanding electrochemical performance of the N,P-PAC electrode is primarily attributed to its large active surface area and the oxygen-containing functional groups generated during the KOH activation and phosphorus doping processes [69].

4.1.2. Noble Metals and Their Oxides

Various noble metals (e.g., Pt, Au, Pd, Ru, Ag) and their oxides have been incorporated with conductive carbon supports. Among these, palladium and platinum have been extensively studied over recent decades and continue to attract significant research interest [70,71,72].

Ye et al. reported a straightforward method to prepare Pd nanodendrite structures on graphene nanosheets, which were then applied as cathode catalysts in non-aqueous Li–O₂ batteries [72]. Xu et al. described a standalone Pd-modified hollow carbon sphere deposited on a carbon paper cathode featuring a specific porous honeycomb structure [73]. This design provided the Li–O₂ battery with high specific capacity, relatively low overpotential, and excellent rate performance. In recent years, due to their outstanding catalytic activity during the oxygen evolution reaction (OER), they have also been widely applied in Li–O₂ batteries. Zhou’s group has conducted remarkable research on materials based on Ru and RuO₂, such as CNT@RuO₂, RuO₂ nanosheets, and Ru@Ni foam [74,75].

Guo et al. introduced an advancement in Li–O₂ battery research, particularly using PtIr (platinum–iridium) multibranch structures as cathode materials to enhance battery performance [76]. Density functional theory (DFT) calculations revealed that the electron transfer from iridium (Ir) to the more electronegative platinum (Pt) reduces the Lewis acidity of Pt, therefore weakening its interaction with the LiO₂ intermediate. Batteries employing the PtIr cathode demonstrated exceptional performance: the total discharge and charge overpotential was only 0.44 V, significantly lower than most other noble metal-based cathode materials. Additionally, the battery exhibited outstanding cycling stability, maintaining performance over 180 cycles with negligible degradation. In summary, this study successfully demonstrated the immense potential of PtIr multibranch structures as cathode materials for Li–O₂ batteries. This innovative design not only significantly reduces the ORR and OER overpotentials but also provides excellent cycling stability. The findings present a novel approach to designing low Lewis acidity, high-efficiency noble metal-based catalysts to enhance battery performance (Figure 6a,b).

Research indicates that the use of graphene-based cathodes in combination with iridium nanoparticles can stabilize and purify the formation of crystalline lithium superoxide (LiO₂) in batteries [77]. Specifically, researchers achieved the stabilization of the LiO₂ structure by uniformly distributing iridium nanoparticles on a graphene-based cathode, which catalyzes the reaction process. Previously, although Li–O₂ batteries had been extensively studied, none had demonstrated stability based on pure LiO₂ (Figure 6c–e).

Single-atom catalysts have been successfully applied to metal–air battery electrode materials due to their exceptional catalytic performance, thermal and chemical stability, and superior selective catalytic properties, therefore enhancing the overall performance of these batteries. In 2019, Zhou and his team pioneered the use of g-C₃N₄ nanosheet-supported Pt single atoms as cathode catalysts in lithium–oxygen batteries (LOBs), initiating research on the application of single-atom catalysts in LOBs [78]. By early 2020, the publication of two papers on the application of Co single atoms in LOBs marked a peak in research interest in single-atom catalysts for these batteries [79,80] (Figure 6f–h).

Ruthenium (Ru), as one of the platinum group metals, is known for its excellent catalytic activity and stability. Xu and his team [81] developed nitrogen-doped porous carbon materials (NC-Ru SAs) incorporating Ru single atoms as cathode materials for LOBs and conducted electrochemical performance tests. The results demonstrated that LOBs with optimized NC-3% Ru SAs as cathode catalysts exhibited a minimal overpotential of only 0.55 V (0.02 mA cm⁻²). Furthermore, in situ Differential Electrochemical Mass Spectrometry (DEMS) revealed an e-/O₂ ratio of only 2.14 over a complete charge–discharge cycle, indicating excellent reversibility of the battery. This study confirms the catalytic effectiveness of Ru single-atom catalysts in LOBs, highlighting their potential for significantly enhancing battery performance. Through various characterization techniques, the study shows that LiO₂ can exist stably without forming Li₂O₂. This crystalline LiO₂, produced via a novel templating mechanism, allows for repeated charge and discharge cycles at low charging potentials, significantly enhancing battery performance. This breakthrough promises to advance the development of high-energy-density batteries and opens up new applications in fields such as oxygen storage.

Figure 6. (a) The XRD patterns of Pt nanocrystals and PtIr multipods. (b) The galvanostatic profiles for Pt and PtIr electrodes during the discharging and charging process with a limited capacity of 1000 mAh g⁻¹ at 0.1 A g⁻¹ [76]. Copyright 2021, Wiley–VCH. (c) Raman spectra of discharge product on Ir-rGO for first and second discharges. (d) Schematic showing lattice match between LiO₂ and Ir₃Li that may be responsible for the LiO₂ discharge product found on the Ir-rGO cathode. (e) DFT electronic band structure (left) and density of states (DOS) plot (right) of ferromagnetic bulk crystalline LiO₂ close to the Fermi level (Ef) based on a spin-polarized calculation with electronic spin-up and spin-down states shown [77]. Copyright 2016, Springer Nature. (f) HAADF–STEM images of Ru0.3 SAs-NC (Ru single atoms are marked with red circles). (g) Corresponding EDS maps reveal the homogeneous distribution of Ru and N within the carbon support of Ru0.3 SAs-NC. (h) Gibbs free energy diagrams at 2.97 V for the discharge−charge reactions on the active surface of pyrolyzed ZIF-8, Ru0.1 SAs-NC, and Ru0.3 SAs-NC [81]. Copyright 2020, American Chemical Society.

Figure 6. (a) The XRD patterns of Pt nanocrystals and PtIr multipods. (b) The galvanostatic profiles for Pt and PtIr electrodes during the discharging and charging process with a limited capacity of 1000 mAh g⁻¹ at 0.1 A g⁻¹ [76]. Copyright 2021, Wiley–VCH. (c) Raman spectra of discharge product on Ir-rGO for first and second discharges. (d) Schematic showing lattice match between LiO₂ and Ir₃Li that may be responsible for the LiO₂ discharge product found on the Ir-rGO cathode. (e) DFT electronic band structure (left) and density of states (DOS) plot (right) of ferromagnetic bulk crystalline LiO₂ close to the Fermi level (Ef) based on a spin-polarized calculation with electronic spin-up and spin-down states shown [77]. Copyright 2016, Springer Nature. (f) HAADF–STEM images of Ru0.3 SAs-NC (Ru single atoms are marked with red circles). (g) Corresponding EDS maps reveal the homogeneous distribution of Ru and N within the carbon support of Ru0.3 SAs-NC. (h) Gibbs free energy diagrams at 2.97 V for the discharge−charge reactions on the active surface of pyrolyzed ZIF-8, Ru0.1 SAs-NC, and Ru0.3 SAs-NC [81]. Copyright 2020, American Chemical Society.

Wu et al. have identified the development of highly active and durable bifunctional oxygen catalysts as a crucial factor in enhancing battery performance. The researchers have demonstrated an exceptionally active and durable bifunctional electrocatalyst (Pt/RuO₂/G) by strongly anchoring Pt and RuO₂ onto graphene. The highly exposed active sites of this catalyst significantly stabilize Li–O₂ batteries. Owing to its two-dimensional porous morphology, ultrathin thickness (approximately 2.5 nm), and the superior (110) facet formation due to spatial confinement, the Pt/RuO₂/G electrocatalyst exhibits an extremely narrow OER/ORR voltage gap of only 0.633 V [82].

4.1.3. Transition Metal Oxides/Carbides

While precious metals and their oxides exhibit excellent catalytic performance, their high material costs impede practical applications in Li–O₂ batteries. Therefore, it is essential to develop effective oxygen cathode catalysts for oxygen reduction (ORR) and oxygen evolution (OER) with lower costs [83]. Various non-precious metal materials, such as transition metal oxides and perovskites, have attracted considerable interest due to their outstanding oxygen electrocatalytic activity and low cost.

In 2007, P. Bruce’s group compared several transition metal-based oxides (La_0.8Sr_0.2MnO₃, Fe₂O₃, Co₃O₄, CuO, NiO, CoFe₂O₄), commonly used as cathode catalysts [84,85,86]. In organic lithium–air battery systems, CuO, Fe₃O₄, and CoFe₂O₄ demonstrated good capacity retention, with Co₃O₄ showing the highest initial discharge capacity and capacity retention, as well as the lowest charging voltage. Subsequently, in 2008, they studied a series of manganese oxides with different crystal structures and morphologies [87]. α-MnO₂ nanowires outperformed other manganese oxides, achieving an initial discharge capacity of up to 3000 mAh g⁻¹ and excellent cycling stability (Figure 7a,b).

Due to the favorable oxygen reduction catalytic properties of manganese oxides, extensive research has been conducted on their application in lithium–air batteries [87,88,89]. S. Suib’s group used a one-step coprecipitation method to dope titanium into octahedral γ-MnO₂ hollow spheres, achieving a capacity of 2300 mAh g⁻¹ as a cathode catalyst [90]. Zheng et al. grew α-MnO₂ nanowires in situ on a graphene substrate and compared the electrochemical performance of these in situ-grown electrodes with mechanically mixed α-MnO₂ nanowires and graphene [91]. The in situ-grown MnO₂/graphene composite electrodes exhibited significantly higher charge–discharge capacities and lower overpotentials compared to the mechanically mixed electrodes. This study highlights the importance of the interaction between the catalyst and the carbon substrate in influencing catalytic activity and battery performance. Numerous studies confirm that manganese oxides significantly enhance the overall performance of lithium–air batteries, making them a popular choice for cathode catalyst research.

Recent advancements have been made in the use of transition metals and their carbides, nitrides, phosphides, and sulfides [92,93]. Molybdenum carbide (MoC_x), due to its various electronic properties and catalytic performance related to its adjustable phase composition, emerges as a promising non-precious metal catalyst. Among these, Mo₂C exhibits the best performance due to its electronic configuration near the Fermi level (EF) [94]. Efforts have been made to develop Mo₂C nanostructures with abundant active sites and composites integrating conductive substrates, such as carbon nanotubes (CNT) and graphene [95,96,97]. However, the negative hydrogen bonding energy (DGH*) on Mo₂C indicates strong adsorption of H on its surface, which favors the reduction of H⁺ (Volmer step) but limits its desorption (Heyrovsky/Tafel step) [94]. Therefore, optimization of electronic characteristics is necessary. Element doping has been attempted, but improvements are limited due to inadequate modification and inevitable structural damage [98]. Interface engineering, such as heterojunctions, has been explored to enhance the electrochemical performance of Mo₂C catalysts [99]. For example, Mo₂N–Mo₂C heterojunctions are efficient hydrogen evolution catalysts [100], showing superior catalytic activity compared to Pt/C at high current densities (>88 mA cm⁻²) in alkaline media.

Two-dimensional transition metal carbides/nitrides, known as MXenes, exhibit significant potential as catalysts in Li–O₂ batteries due to their unique layered structures and excellent conductivity [101]. However, further enhancement of their catalytic performance remains a focus of research. Researchers prepared nitrogen-doped Ti₃C₂ MXene catalysts using hydrothermal and annealing methods, resulting in increased disorder and abundant active sites. Density functional theory (DFT) calculations indicated that nitrogen doping adjusted the occupation of Ti 3d orbitals in N-Ti3C₂(H), enhancing the electron exchange between Ti 3d orbitals and O 2p orbitals, thus accelerating oxygen electrode reactions. Li–O₂ batteries based on N-Ti₃C₂(H) exhibited a large discharge capacity of 11,679.8 mAh g⁻¹ and stable cycling performance over 372 cycles. In situ Differential Electrochemical Mass Spectrometry (DEMS) confirmed the effective accumulation and decomposition of Li₂O₂ on the N-Ti₃C₂(H) surface. This study proposed a valuable strategy to boost oxygen electrode reaction catalytic activity by tuning the 3d orbital occupation in MXenes, highlighting the potential of nitrogen-doped Ti₃C₂ MXene for high-performance Li–O₂ batteries (Figure 7c).

Figure 7. (a) Schematic representation of a rechargeable Li/O₂ battery. (b) Variation of discharge capacity with cycle number for several porous Mn-based [87]. Copyright 2008, Wiley–VCH. (c) The schematic diagram of the formation and decomposition of discharge product Li₂O₂ on the N–Ti₃C₂(H) surface during the discharge and charge process, respectively [101]. Copyright 2023, Wiley–VCH. Top (d) and side (e) views of the Ti₂C MXene monolayer. The capitals in the figure are the possible adsorption sites. Green and gray atoms are Ti and C, respectively. (f) Calculated total density of states (left) and projected density of states of Ti 3d (right) for Ti₂C, Ti₂CO₂, Ti₂CF₂, and Ti₂C(OH)₂. The vertical dashed line at E = 0 eV represents the Fermi energy [102]. Copyright 2019, American Chemical Society. (g) Schematic diagram of the reaction during the cycling process in LOBs (* represents adsorptive state). [103]. Copyright 2021, Elsevier.

Figure 7. (a) Schematic representation of a rechargeable Li/O₂ battery. (b) Variation of discharge capacity with cycle number for several porous Mn-based [87]. Copyright 2008, Wiley–VCH. (c) The schematic diagram of the formation and decomposition of discharge product Li₂O₂ on the N–Ti₃C₂(H) surface during the discharge and charge process, respectively [101]. Copyright 2023, Wiley–VCH. Top (d) and side (e) views of the Ti₂C MXene monolayer. The capitals in the figure are the possible adsorption sites. Green and gray atoms are Ti and C, respectively. (f) Calculated total density of states (left) and projected density of states of Ti 3d (right) for Ti₂C, Ti₂CO₂, Ti₂CF₂, and Ti₂C(OH)₂. The vertical dashed line at E = 0 eV represents the Fermi energy [102]. Copyright 2019, American Chemical Society. (g) Schematic diagram of the reaction during the cycling process in LOBs (* represents adsorptive state). [103]. Copyright 2021, Elsevier.

Yang et al. investigated the activity of Li–O₂ batteries by constructing adsorption models of Li_xO₂ (x = 4, 2, 1) on Ti₂C-MXene with different functional groups (-O, -F, -OH) [102]. Their computational results indicated that Ti₂CO₂ MXene exhibits superior catalytic activity, with an ORR overpotential of 0.10 V and an OER overpotential of 0.16 V. The polarization of the Ti 3d orbitals near the Fermi level determines the reducibility of Ti₂C MXene, directly influencing the oxidation reaction of O₂ (Figure 7d–f).

Recently, Li et al. obtained similar results through both experimental and theoretical calculations. They used thermally treated Ti₂C MXene nanosheets with surface -O and -F functional groups as cathode catalysts for Li–O₂ batteries. At a current density of 100 mA g⁻¹, the discharge capacity reached 15,635 mAh g⁻¹. At a current density of 200 mA g⁻¹, a fixed capacity of 600 mAh g⁻¹ was maintained over 250 cycles, demonstrating long-cycle stability. Computational results showed that Ti₂CO₂ has the most suitable adsorption strength for Li₂O₂, outperforming Ti₂CF₂ and bare Ti₂C in catalytic performance. Due to surface heterogeneity, discharge products primarily nucleate at Ti₂CO₂ sites during the discharge process and accumulate spatially to form a porous structure, effectively promoting mass transfer and cyclic stability.

Despite their optimized charge density and extended electronic properties, MXenes tend to oxidize and restack under high anodic potentials. Molybdenum oxides, with their multiple valence states and high electronegativity, show potential for ORR/OER catalysis [104]. Nitrogen-doped MoO₂ can enhance LiO₂ adsorption and promote uniform growth of Li₂O₂. Mo-based compounds can form heterojunctions, accelerating charge distribution and improving chemical stability. Sun et al. achieved strong self-assembly of MoOx and Ti₃C₂ MXene, creating a novel stable bimetallic Mo/Ti-O coupled catalyst with optimized interfacial electronic structure, high chemical stability, and enhanced ORR/OER bifunctional activity. Experimental results showed that Li–O₂ batteries with MoOx@Ti₃C₂ MXene cathodes exhibited lower voltage polarization, longer lifespan, and superior rate performance compared to other monoclinic catalysts. Experimental and theoretical analyses further revealed the crucial impact of bimetallic–oxygen coupling in promoting ORR/OER kinetics and optimizing Li₂O₂ adsorption/desorption (Figure 7g).

Studies have demonstrated that the rational design and adjustment of the structure and morphology of catalytic cathode materials can effectively optimize battery performance. Xu et al. designed a three-dimensional open-structured Co₃O₄@MnO₂ heterojunction composite material through a simple two-step hydrothermal method [105]. This composite, serving as a bifunctional catalyst and featuring a three-dimensional open structure, guides the uniform and fluffy deposition of discharge products like Li₂O₂. When used as a cathode for Li–O₂ batteries, it exhibits excellent electrochemical performance, including an initial discharge-specific capacity as high as 12,980 mAh g⁻¹.

4.1.4. Perovskite Materials

To date, perovskite-based composite oxides (ABO₃) have emerged as promising alternatives to noble metal bifunctional catalysts in lithium–oxygen (Li–O₂) batteries due to their significantly lower cost, excellent catalytic activity and high oxygen mobility. Various perovskites such as Sr_0.95Ce_0.05CoO_3−x, La_0.8Sr_0.2MnO₃, and Sr₂CrMoO_6−x have been utilized in Li–O₂ batteries [106,107,108]. However, perovskite catalyst materials synthesized by conventional methods often exhibit low intrinsic electronic conductivity and small specific surface areas, which limit their catalytic activity and practical application.

Additionally, synthesizing perovskites with porous structures that offer efficient oxygen pathways and high specific surface areas has been pursued to boost catalytic performance. For example, Zhang et al. reported the use of 3D ordered macroporous (3DOm) LaFeO₃ and La_0.75Sr_0.25MnO₃ nanotubes (PNT–LSM) [109,110] (Figure 8a,b). These porous nanotubes consist of nanoparticles with typical sizes ranging from 10 to 30 nm. When first used as electrocatalysts in Li–O₂ batteries, PNT–LSM significantly suppressed the overpotentials for oxygen reduction (ORR) and particularly oxygen evolution (OER), therefore improving round-trip efficiency. The high catalytic activity of PNT–LSM and the synergistic effects of their unique hollow channel structure endowed the Li–O₂ batteries with high specific capacities, excellent rate capabilities, and good cycling stability (Figure 8c,d).

Typical perovskite oxides like LaMO₃ (M = Mn, Fe, Co, Ni) have garnered wide attention. Oxygen defect engineering and morphology control are forward-looking strategies to address these issues. Regulating oxygen vacancies can accelerate electron transport and reduce reaction energy barriers, while oxygen vacancies also serve as efficient sites to promote the binding and decomposition of intermediate and discharge products like Li₂O₂. Morphology and surface structure significantly influence catalytic efficiency, with optimal surface structures providing more active sites to facilitate electrochemical reactions. Scientists have suggested a combined approach of flaw and surface engineering, where oxygen vacancies are regulated on the surface of LaCo_xMn_1−xO_3−σ, and cation substitution is achieved via a template-free growth method to optimize morphology, therefore enhancing electrocatalytic performance. It was found that LaCo_0.75Mn_0.25O_3−σ exhibited the best electrochemical performance among all candidate materials. Its unique porous hollow structure and rough surface nanosheets increased active site accessibility, promoted mass transfer, and enhanced catalytic performance (Figure 8e–g). This catalyst demonstrated a low overpotential (only 1.12 V), a high initial specific capacity (up to 10,301 mAh g⁻¹), and a stable cycling life [111].

Additionally, researchers have employed LaNi_0.5Co_0.5O₃ (LNCO) as a bifunctional catalyst and cathode substrate for Li–O₂ batteries, eliminating the need for carbon materials [112]. In a molten nitrate electrolyte at 160 °C, LNCO exhibited an ultra-low charge–discharge overpotential of 50 mV, an energy efficiency of up to 98.2%, and remained stable for over 100 cycles at a current density of 0.1 mA/cm². These demonstrated long-term cycling stability and efficient operation. Density functional theory (DFT) calculations confirmed the electronic structure and excellent catalytic activity of LNCO. The study highlighted LNCO’s semimetal characteristics and the coexistence of surface Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ redox pairs, effectively catalyzing both ORR and OER reactions. For the first time, porous and fluffy Li₂O was identified as the discharge product, and a detailed four-electron transfer mechanism was proposed (Figure 8h–j).

4.1.5. Emerging Cathode Materials

In the realm of lithium–oxygen (Li–O₂) battery research, traditional catalysts often face limitations due to low activity and instability. High-entropy alloys (HEAs), composed of multiple metal elements, exhibit unique properties such as lattice distortion effects, sluggish diffusion, and the “cocktail effect,” which show considerable potential in electrocatalysis and rechargeable batteries. These alloys optimize electron transfer and intermediate adsorption energies through the synergistic effects of multiple metals, therefore lowering redox barriers (Figure 9a–c).

Guo et al. have developed an innovative synthesis method using silver nanowires (Ag NWs) as templates to create ultrathin 2D high-entropy alloy subnanometer ribbons (SNRs) with up to eight metal elements and a layer thickness of just 0.8 nm, the thinnest HEA reported to date [113]. This method includes extensive metal exchange reactions, co-reduction, and the removal of the core silver. Pentanary HEA (PtPdIrRuAg) SNRs exhibited exceptional mass activity for the ORR in alkaline electrolytes, surpassing commercial Pt/C catalysts by 21 times. Hexanary HEA (PtPdIrRuAuAg) SNRs demonstrated low charge overpotentials (0.49 V) and excellent cycle life (100 cycles) in Li–O₂ batteries. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations have been used to study the structural changes and atomic arrangements of HEA SNRs, with electronic structure analyses revealing a balance between metal selection, composition, and the number of elements contributing to their electrocatalytic activity and stability.

Developing HEAs with spatial heterostructures to manipulate the d-band center of interfacial metal atoms and adjust electronic distributions is an essential challenge and fundamental research direction to enhance HEA catalyst electrocatalytic activity. By growing uniform Pt dendritic structures on a PtRuFeCoNi HEA core, a unique HEA@Pt spatial heterostructure electrocatalyst was constructed. This structure aids in accelerating ORR and OER kinetics [114]. Theoretical calculations indicated that the electron redistribution at the interface regulated the d-band center of metal atoms, optimizing oxygen species adsorption energy and reducing ORR/OER reaction barriers. This charge transfer and redistribution improved the electrocatalytic activity and cycling performance of the catalyst. Li–O₂ batteries based on HEA@Pt electrocatalysts exhibited low polarization potential (0.37 V) and long-term cycling stability (210 cycles) at a cutoff capacity of 1000 mAh g⁻¹, outperforming most previous noble metal catalysts. Additionally, HEA@Pt demonstrated excellent catalytic activity and stability in alkaline media, indicating its significant potential in a wide range of catalytic applications.

The Sabatier Principle, proposed by French chemist Paul Sabatier, is a guiding theory for designing and selecting heterogeneous catalysts. It has broad applications in various chemical reactions, including hydrogenation, ammonia synthesis, and water–gas shift reactions. In fuel cells, industrial chemical reactions, and the latest electrocatalyst research, such as Li–O₂ batteries, it guides the selection of effective catalysts. Based on the Sabatier Principle, researchers have systematically used high-entropy strategies to construct a series of catalysts with widely distributed d-band centers (i.e., a wide range of adsorption strengths), uncovering the Sabatier relationship in electrocatalysts for Li–O₂ batteries [115]. A Li–O₂ battery using FeCoNiMnPtIr (HEAPtIr) as the catalyst demonstrated over 80% energy conversion efficiency and stable operation for 2000 h at a fixed specific capacity of 4000 mAh g⁻¹. This study confirmed the applicability of the Sabatier Principle in designing advanced heterogeneous catalysts for Li–O₂ batteries, providing insightful guidance for HEA catalyst design (Figure 9d–f).

Metal–organic frameworks (MOFs) offer high specific surface areas and adjustable structures, providing numerous dispersed active sites, making them promising candidates for improving catalytic efficiency and reaction rates in Li–O₂ battery cathodes. By designing different metals and ligands, MOFs can flexibly optimize battery performance and enhance electron and ion transport efficiency. Additionally, their ordered structures and high stability contribute to improved cycling life and overall performance of Li–O₂ batteries. For instance, synthesizing a single-crystal MOF with PbO₇ nodes (naphthalene-lead-MOF, known as Na–Pb–MOF) significantly improved the kinetics of discharge and charge processes in Li–O₂ batteries [116]. Compared to traditional PbO₆ nodes, the increased Pb–O bond length in PbO₇ nodes weakens orbital coupling, optimizing the adsorption of intermediate LiO₂, reducing activation energies for LiO₂ reduction to Li₂O₂ and LiO₂ oxidation to O₂, and therefore lowering the overall reaction overpotential. Consequently, Li–O₂ batteries with Na–Pb–MOF electrocatalysts exhibited a low total discharge–charge overpotential (0.52 V) and superior cycling performance, maintaining stability for 140 cycles. Additionally, solvothermal or in situ construction methods can precisely control MOF crystal growth and metal node distribution, further optimizing catalytic performance and cycling life of Li–O₂ batteries presenting new directions for electrocatalyst design (Figure 9g,h).

Figure 9. (a) HAADF–STEM image and EDS element mapping images of one segment of an HEA–PtPdIrRuAg SNR. Pt (blue), Pd (red), Ag (magenta), Ir (orange), and Ru (green) signals. (b) Possibility of an atomic pair in different HEAs. The color bar represents the possibility of the bonding between different metals, which ranges from 0 to 1. (c) ORR performances of quinary PtPdIrRuAg SNRs [113]. Copyright 2023, American Chemical Society. (d) Schematic diagram of electron transfer in HEAPtIr. (e) Orbital interactions between LiO₂/Li₂O₂ and catalysts with different d-band centers and (f) the corresponding catalytic effects [115]. Copyright 2023, Wiley–VCH. (g) The single-crystal structure of Na–Pb–MOF. (h) Comparison of overall overpotentials of Na–Pb–MOF and the reported MOF-based electrocatalysts [116]. Copyright 2023, American Chemical Society. (i) Schematic illustration of the role of the redox mediator (RM) in a Li–O₂ battery system made using a hierarchical CNT fibril electrode [32]—copyright 2014, Wiley–VCH.

Figure 9. (a) HAADF–STEM image and EDS element mapping images of one segment of an HEA–PtPdIrRuAg SNR. Pt (blue), Pd (red), Ag (magenta), Ir (orange), and Ru (green) signals. (b) Possibility of an atomic pair in different HEAs. The color bar represents the possibility of the bonding between different metals, which ranges from 0 to 1. (c) ORR performances of quinary PtPdIrRuAg SNRs [113]. Copyright 2023, American Chemical Society. (d) Schematic diagram of electron transfer in HEAPtIr. (e) Orbital interactions between LiO₂/Li₂O₂ and catalysts with different d-band centers and (f) the corresponding catalytic effects [115]. Copyright 2023, Wiley–VCH. (g) The single-crystal structure of Na–Pb–MOF. (h) Comparison of overall overpotentials of Na–Pb–MOF and the reported MOF-based electrocatalysts [116]. Copyright 2023, American Chemical Society. (i) Schematic illustration of the role of the redox mediator (RM) in a Li–O₂ battery system made using a hierarchical CNT fibril electrode [32]—copyright 2014, Wiley–VCH.

Efficient catalysts are crucial for improving Li–O₂ battery performance [117]. Although solid-state catalysts have demonstrated excellent electrocatalytic performance, their complex preparation methods and limited active sites hinder large-scale application. In contrast, soluble catalysts (e.g., ORR and OER catalysts) uniformly distributed in the electrolyte can better contact reactants, reduce discharge/charge overpotentials, and improve battery cycling performance. Lim et al. demonstrated a novel lithium–oxygen battery that achieved high reversibility and good energy efficiency using a layered nanoporous air electrode and soluble LiI. This design delivered a reversible capacity of 1000 mAh g⁻¹ and sustained 900 cycles with reduced polarization. The future development of lithium–oxygen batteries will require the synergistic integration of multiple technological elements to achieve overall performance enhancement (Figure 9i). Zhou et al. adopted an organometallic soluble catalyst, tris(acetylacetonato)iridium [Ir(acac)₃], whose multivalent metal active centers can capture O₂⁻ to form intermediate complexes (Ir(acac)₃-O₂⁻), effectively modulating ORR and OER pathways while suppressing side reactions caused by oxygen radicals. Ir(acac)₃, by forming reversible intermediate complexes (Ir(acac)₃-O₂⁻), prolonged the diffusion time of superoxide radicals during discharge, enhancing the solution-based ORR mechanism and mitigating cathode passivation [32]. During charging, Ir(acac)₃ acted as a redox mediator, accelerating Li₂O₂ decomposition, reducing charge overpotential, and effectively inhibiting singlet oxygen (¹O₂) formation. Experimental results showed that Li–O₂ batteries containing Ir(acac)₃ exhibited low overpotential, high discharge capacity, large capacity, and good stability (130 cycles). This study demonstrated a new approach to enhancing Li–O₂ battery performance by developing multifunctional soluble catalysts, indicating the need to further explore alternative catalysts to improve overall battery efficiency.

5. Summary and Outlook

This review presented the latest advances in anode and cathode materials for lithium–oxygen batteries, emphasizing their significant potential for high-energy-density applications. Research on anode materials has explored alternatives to pure lithium metal, such as silicon and lithium-rich alloys, to improve stability and mitigate dendrite formation. The development of durable solid–electrolyte interfaces (SEI) and innovative alloy compositions has shown promise in enhancing cycling stability. On the cathode side, the focus has been on improving the ORR and OER kinetics through advanced carbon-based materials and bifunctional catalysts. These advancements address the accumulation of discharge products and high overpotentials, leading to improved electrochemical performance.

The future development of Li–O₂ batteries will concentrate on refining electrode materials, optimizing pore structures, and enhancing electrolyte stability. By addressing these challenges, Li–O₂ batteries could become a viable solution for high-energy storage, combining low costs, high specific capacity, and good durability, making them promising candidates for environmentally friendly energy devices.