All-Solid-State Lithium–Oxygen Batteries: History
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Contributor:
Sara Pakseresht
,
Mustafa Celik
,
Aslihan Guler
,
Ahmed Waleed Majeed Al-Ogaili
,
Tanja Kallio

All-solid-state lithium–oxygen batteries (ASSLOBs) are emerging as a promising next-generation energy storage technology with potential energy densities up to ten times higher than those of current LIBs. ASSLOBs utilize non-flammable solid-state electrolytes (SSEs) and offer superior safety and mechanical stability. However, ASSLOBs face challenges, including high solid-state interface resistances and unstable lithium-metal anodes.

  • solid electrolytes
  • lithium–oxygen batteries
  • interfaces

1. Lithium Metal Anode for Solid-State Lithium–Oxygen Batteries

Li metal is an excellent anode material owing to its high theoretical energy density (3860 mAh g−1) and low potential (0.304 V vs. standard hydrogen electrode (SHE)). However, in conventional LOBs, significant issues arise, including safety hazards associated with organic electrolytes, the formation of harmful SEIs, electrolyte decomposition and evaporation, and the formation of dendrites due to the Li metal anode corrosion [1]. Dendrites formed and volumetric changes on the Li anode during Li coating/stripping cause a low cycle number [2]. The morphology and components of SEI formed at the electrolyte/Li interface are affected by the reaction of the Li anode with H2O, O2, and intermediates (LiOH, Li2O, Li2CO3, etc.) during the electrochemical process [3]. Another difficulty with LOBs is the instability of the electrolyte due to the decomposition at high voltage and forming parasite products [4]. Additionally, a small amount of liquid electrolyte is generally used to decrease the interface resistance between the Li metal anode and the electrolyte, and the continuous consumption of liquid electrolytes during the cycle cannot be prevented [5]. In order to inhibit the contact of H2O and O2 at the Li metal anode, some methods such as coating the Li surface with an artificial protection film, modifying the electrolyte/separator, and alloying the anode have been studied in the literature [6].
Recent studies proceed to investigate the physical and chemical stability of the contact between the Li metal anode and SE. In earlier reports, a slight amount of liquid electrolyte is typically injected to minimize the interface resistance between the Li metal anode and the electrolyte. Yet, the constant use of liquid electrolytes during cycling is not entirely feasible. It is hypothesized that even at higher current densities, the surface coating with lithiophilic compounds can effectively inhibit the Li penetration. The application of a coating material, such as metals, metal oxides, or sulfides, can facilitate the formation of a uniform interface by reacting with the metal anode. This results in the creation of a high Li-ion transport pathway, which facilitates an efficient charge transfer while preventing undesirable side reactions. Another prominent method for inhibiting the formation of Li dendrites is polymer interlayers. The SEI layer is mechanically unsteady because of the massive ion flow at the interface and the massive volume variation in the Li anode. Lithium alloys prevent dendritic formations in LOBs, effectively reducing the excessive nucleation potential of Li and lowering the interfacial resistance [7]. Deng et al. [8] obtained a Li21Si5 alloy by alloying the Li anode with Si and obtained a capacity of approximately 1000 mAh g−1 after approximately 80 cycles. While these alternative alloy anodes reduce certain issues associated with the Li metal, they generally have a limited Li source. The use of a heavier element apart from Li in the anode part will cause a decrease in the energy density of the LOBs after a while [9]. On the other hand, the three-dimensional (3D) freestanding anodes prevent volume expansion in the Li metal, thanks to their large surface areas and the voids [10]. Thus, a composite Li metal that exhibits improved cycling stability and dendrite-free characteristics is formed for LOBs. To illustrate the role of the lithium anode in the formation and decomposition of Li2O2 at the cathode, Jeong et al. [11] designed a 3D host–lithium composite anode for LOBs using Cu. In comparison with LOBs made of Li metal, this composite anode demonstrated lower polarization, improved rate capability, and cycle stability. Ma et al. [12] produced a graphene aerogel/Li freestanding anode by utilizing the porous and conductive skeleton of graphene aerogel and obtained improved cycling stability over 700 cycles. However, while 3D anodes reduce the energy density for LOBs, the high surface area elevated the possibility of side reactions [4]. Luo et al. [13] provided stable SEI formation on the surface of anode by coating the Li metal anode surface with SiO2/GO. As a result, they obtained a capacity of approximately 1000 mAh g−1 after approximately 300 cycles, preventing chemical corrosion of the anode. As a different approach, in situ passivation or SEI film formation on the Li metal anode is seen as one of the potential methods to solve the problem of dendrite formation and volume change on the Li anode. Furthermore, the use of inorganic materials that interact with Li-metal anodes while forming alloys effectively decreases the interface resistance [14].
In conclusion, improving the anode–electrolyte interface in ASSLOBs is crucial for enhancing their electrochemical performance. Following are some perspectives to improve this issue:
Developing advanced interfacial materials. Advanced interfacial materials, such as solid-state electrolytes and interlayers, can enhance the contact of the anode with electrolyte by preventing the formation of harmful SEIs, reducing polarization, and promoting ion transport.
Surface modification of the anode. Surface modification techniques, such as surface coating, can improve the wettability of the anode and enhance the contact between the anode and the electrolyte. For example, the use of hydrophilic coatings can promote the adsorption of electrolyte species and enhance their diffusion.
Designing porous anodes. Porous anodes with high surface area can enhance the contact of the anode with the electrolyte, facilitating the diffusion of ions and oxygen. This can be achieved by utilizing materials with high porosity, including carbon-based materials or metal oxides, and by controlling the morphology of the anode.
Using 3D printing technology. Three-dimensional printing technology can be used to fabricate anodes with precise geometry and control the distribution of pores and active materials, which can provide a good contact of the anode with the electrolyte.
Conducting in situ characterization. In situ characterization techniques, such as electrochemical impedance spectroscopy and scanning electron microscopy, can be employed to monitor the anode–electrolyte interface and identify the factors that affect their contact. This information can be used to optimize the design and composition of the anode and electrolyte to improve their contact.
Recently, researchers have proposed anodeless Li batteries to increase the energy densities of Li batteries [15]. In lithium-free batteries, Li ions from the cathode during the charging process form a thin Li film on the negative current collector. This unique lithium battery design can deliver ultra-high energy density of approximately 400 Wh/kg or 1200 Wh/L [16]. Anodeless Li batteries are seen as a solution to the abnormal heating, explosion, and combustion problem caused by liquid electrolytes, as well as the formation of dendrites and unstable SEI. In anodeless Li batteries in which organic liquid electrolyte is used, carbonate-weighted SEI is formed, which causes a capacity loss because of the decomposition of the organic electrolyte. In recent years, there have been reports on the integration of an anodeless configuration within all-solid-state battery systems as a means to enhance safety. By utilizing non-flammable SEs, the issue of thermal runaway can be significantly reduced compared with batteries employing flammable organic liquid electrolytes. Additionally, the robust mechanical strength of the SE serves to mitigate the Li dendrite growth towards the cathode, thereby minimizing the potential for short-circuiting. Nikodimos et al. [17] produced an anodeless solid state Li battery by filling PVDF-HFP gel electrolyte with LAMGP (Li1.6Al0.5Mg0.1Ge1.5(PO4)3) filler. The anodeless cell prepared with a polymer matrix composite electrolyte showed high mechanical strength, high ionic conductivity, and electrochemical stability at room temperature. Electrostatic interaction between the gel polymer membrane and the current collector formed a good adhesion. In the anodeless Li battery, a safe interface chemistry was created, and the dendrite growth was suppressed thanks to the composite gel polymer electrolyte.

2. Solid Electrolyte for Lithium–Oxygen Batteries

Many efforts have been made to discover an optimal electrolyte configuration. Solid electrolytes emerge as one of the most explored strategies for enhancing the safety of rechargeable lithium batteries, as they inhibit leakage, volatilization, and ignition. Typically, ceramics and polymers are utilized as SEs in solid-state battery (SSBs) systems.
The solid electrolyte is considered as the fundamental element of the SSBs. The main function of SEs is to separate the anode and cathode from each other and create a transition zone for the transportation of Li ions. The function of the SEs necessitates meeting certain requirements. First, the ionic conductivity of the SE at room temperature should be more than 10−4 S cm−1. At the same time, the electronic conductivity of the SE is expected to be negligible, and its electrochemical stability is expected to be high. Solid electrolytes could be broadly categorized into two major groups: inorganic electrolytes and organic electrolytes.
Inorganic electrolytes: NASICON-, garnet-, perovskite-, LISICON-, LIPON-, and sulfur-based electrolytes are notable examples of inorganic electrolytes that fulfill these requirements. Inorganic sulfide-based solid electrolytes, among the mentioned options, are not suitable for SSLOBs due to concerns related to their high sensitivity to humidity, which can lead to the risky leakage of toxic H2S. In the context of SSLOBs, it is preferable to use inorganic oxides and solid electrolytes such as NASICON, garnet, perovskite, anti-perovskite, and zeolite, as they offer more favorable characteristics for the operating environment.
NASICON (sodium superionic conductor) is a family of SSEs with the general formula Na1+xZr2SixP3-xO12 (where x is typically between 0.5 and 2) and has been investigated for use in various energy storage devices [18]. Typically, these materials adopt the formula AM2(PO4)3, with the A site accommodating elements such as Li, Na, or K, while the M region is commonly occupied by Ge, Zr, or Ti. In particular, the Li1+xMxTi2−x(PO4)3 (M = Al, Cr, Ga, Fe, Sc, In, Lu, Y, or La) system has been extensively investigated [19][20]. LATP ceramic, which is commonly used in SSLOBs, has high ionic conductivity at ambient temperatures above 10−4 S cm−1. Similarly, Li1.5Al0.5Ge1.5(PO4)3 has been extensively investigated due to its high ionicity (2.4 × 10−4  S cm−1) and relatively broad electrochemical durability window. However, when LATP or LAGP comes into contact with a Li anode, the presence of Al3+ and Ge4+ in the electrolyte tends to undergo reduction by the Li metal, resulting in the formation of a Li-Al alloy or Li-Ge alloy. This limits the use of these SEs with a Li metal anode. Various strategies such as ion exchange or coating the surface with polymer electrolyte have been developed to stabilize the electrolyte/Li anode interface in recent studies [21][22]. Recent research has shown promising results for NASICON-type solid electrolytes in ASSLOBs, with improvements in cycling stability, capacity retention, and rate capability. However, issues still remain in optimizing the properties of the NASICON-type solid electrolytes for practical application, such as reducing the interfacial resistance, improving the mechanical strength, and enhancing the compatibility with cathode materials. NASICON-type SEs exhibit significant potential for implementation in ASSLOBs, and current research endeavors are dedicated to tackling the remaining obstacles while advancing the progression of robust and high performing solid-state energy storage systems.
The garnet structure is defined by a space group described by the general formula A3B2(XO4)3, where A can represent elements such as Ca, Mg, Y, or La, and B can represent elements such as Al, Fe, Ge, or Mn. The garnet material stands out as a highly promising solid-state electrolyte due to its exceptional attributes, including a broad temperature range and broad electrochemical window. When the X sites are occupied by Li atoms, the garnet-type Li+ conductor can form as Li3A2B2O12 [23][24]. Murugan et al. [25] investigated a plate-like Li7-type garnet, namely Li7La3Zr2O12 (LLZO, 7.74 × 10−4 S cm−1). With subsequent studies, such as Li6.4La3Zr1.4Ta0.6O12 (1.0 × 10−3 S cm−1) [20] and Li6.55Ga0.15La3Zr2.5O12 (2.06 × 10−3 S cm−1) [26], Li ion conductivity reached high levels. Despite reaching an acceptable ionic conductivity, garnet-type SEs increase sensitivity to moisture and CO2 and increase the unexpected Li dendrite formation due to irregular interfacial contact [27]. A metal-doping strategy was attempted to improve the air stability of garnet-type SEs. According to Kobi et al. [28], the co-doping of Al and Mg in lithium lanthanum zirconate (Li7La3Zr2O12: LLZO) demonstrated enhanced air stability. Recent investigations have revealed the effectiveness of elemental doping (e.g., Ga, Nb, Ta, and La) in garnet-type solid electrolytes for improving air stability [29][30][31][32]. Furthermore, the application of a protective coating was shown to be a viable approach to enhance air stability [33][34].
SSEs with a perovskite structure typically follow a general formula of ABX3, where A represents a rare earth or alkaline earth metal, B represents a transition metal, and X denotes an anion, commonly oxygen. The crystal structure of perovskites allows the incorporation of a wide variety of cations and anions, leading to tunable properties including ionic conductivity and stability. Perovskite-type SSEs are seen as promising candidates for SSBs with their tunable properties. As an example, Li3/8Sr7/16Ta3/4Zr1/4O3 (LSTZ, 2.7 × 10−4 S cm−1), Li3xLa2/3−xTiO3 (LLTO, 3.7–1.4 × 10−5 S cm−1), and perovskite-type SSEs have been reported acceptable ionic conductivity values of 10−4 S cm−1 [35][36]. The LLTO solid electrolyte has had applications in solid state LABs but has been reported to have unstable interfaces with the lithium anode, which limits these applications. Doping the LLTO solid electrolyte is quite difficult due to the extremely small width of the unit cell. The LSTZ solid electrolyte suffers from air stability problems. Goodenough et al. have shown that ion exchange and polymer mixing are effective in solving this issue [37][38]. Goodenough et al. [38] reported that by combining the LSTZ SE with the PEO polymer electrolyte, they successfully resolved the issue of the LSTZ SE’s contact with the lithium anode. Simultaneously, this combination enhanced the stability of the polymer electrolyte. 
The anti-perovskite-type SE is a distinct class of solid electrolyte material characterized by its unique crystal structure, which is the inverse of the perovskite structure. In this structure, the anion sublattice of a perovskite is replaced by a cation sublattice, and the cation sublattice is replaced by an anion sublattice. This results in the formation of a new crystal structure with different properties. These electrolytes have a chemical formula of Li3OA, Li(3−x)Mx/2OA, where A is an anion and M is a metal cation [39]. Anti-perovskites (Li2OHX, where X = Cl, Br, F) are a specific type of anti-perovskite solid electrolyte that have high ionic conductivities, low-temperature processability, and a high electrochemical stability window [40]. Recent studies have shown that Li-rich anti-perovskite Li2OHBr-based PEs can be utilized as a flexible SSE to promote the performance of batteries [41]. Anti-perovskite electrolytes are excellent options for use in SSBs due to their structural flexibility and tunability [42]. The remarkable ionic conductivity, stability against air and moisture, and compatibility with lithium metal anodes exhibited by anti-perovskite-type solid electrolytes has attracted significant attention. The structural flexibility and tunability nature of anti-perovskite SEs make them great candidates for use in SSBs [43]. Anti-perovskite solid electrolytes are stable to low-potential anodes and have a very large electrochemical stability window, positioning them as a great option for use in AASSLOBs. Additionally, optimizing the interfaces in anti-perovskite electrolyte-based SSBs can further improve their performance. Yu et al. [44] developed an in situ “welding” technique to solve the interface problem of the Li2OHCl anti-perovskite-type solid electrolyte. A flexible and stable interface was obtained by forming an organic–inorganic compound buffer layer with a one-microliter liquid electrolyte.
Boosting the ionic conductivity of anti-perovskite SEs is also crucial for the advancement of ASSLOBs. Studies have shown that anti-perovskite-type SEs can achieve high ionic conductivity at ambient temperature, with values up to 10−3 S cm−1 [45][46]. Moreover, these electrolytes have demonstrated favorable compatibility with both Li metal anodes and air cathodes, positioning them as promising contenders for ASSLOBs. Nevertheless, additional research is required to enhance the performance and stability of these electrolytes for practical application.
Zeolite-based SEs have received great attention as potential candidates for ASSLOBs owing to their excellent ionic conductivity, thermal stability, and chemical compatibility with the electrodes [47]. Zeolites are porous aluminosilicate materials with a crystalline structure composed of a network of interconnected channels and cavities. These channels and cavities provide pathways for lithium ions to migrate through the solid electrolyte [48][49]. Zeolites provide improved characteristics, such as increased wetting, durability at high temperatures, ion conductivity, strength, and electrochemical stability, compared with conventional solid electrolytes [50].
Recently published studies have presented an integrated ASSLOB design incorporating an ultrathin, high-ion-conductive membrane made of lithium-ion-exchanged zeolite X (LiX) as the electrolyte [1][48]. An encouraging strategy for solid-state Li-Air batteries (SSLABs) entails the in situ combination of LiXZM (Li+ exchanged zeolite membrane) with cast lithium and carbon nanotubes (CNTs) [1]. This integration resulted in a remarkable ultrahigh capacity of 12,020 mAh g−1 and an extended lifespan of 149 cycles at a discharge rate of 500 mAh g−1, with a limited capacity of 1000 mAh g−1 in ambient air. Notably, this performance significantly surpassed that of LABs based on LAGP (Li1+xAlyGe2−y(PO4)3, lithium aluminum germanium phosphate), which achieved only 13 cycles. The SSLAB with the integrated cathode and LiXZM (referred to as C-LiXZM) demonstrated favorable flexibility and excellent electrochemical performance, offering promising prospects for practical energy storage devices. Zeolites, despite their potential, have certain drawbacks, including a lack of comprehensive studies, challenges in achieving proper interface contact with electrodes, an increased chance of Li dendrite growth, and issues in large-scale production [51]. Microporous materials, including metal–organic frameworks (MOFs) and zeolites, are intriguing candidates for utilization in composite SEs due to their potential for higher energy density and enhanced safety compared with conventional systems [52]. However, more research is required to comprehensively comprehend the full potential and limitations of zeolite-based solid electrolytes for ASSLOBs.
In summary, SSLOBs are highly regarded as an excellent option for high-performance energy storage. They offer superior safety features by utilizing non-flammable and non-volatile electrolytes. Moreover, these batteries exhibit high specific energy due to the utilization of lithium metal and oxygen gas as active materials. Inorganic solid-state electrolytes for SSLOBs have shown great potential for enabling high energy density, extended cycle life, improved safety, and great chemical stability. Garnet-, perovskite-, anti-perovskite-, and NASICON-type electrolytes have garnered significant attention among the different types of inorganic SSEs. It has been observed that corrosion of the anode, which is one of the difficulties of SSLOBs, can be prevented with inorganic solid electrolytes. Le et al. [53] reported that a perovskite-structured Al-doped Li-La-Ti-O (A-LLTO) SE can significantly improve the stability of LOBs for long-term operation by protecting the lithium anode from O2, CO2, and humidity in the air.
Moreover, inorganic SSEs can play a crucial role in effectively preventing short circuits resulting from lithium dendrite formation during long-term operation [34][54]. Hybrid quasi-solid-state electrolytes, which merge the benefits of inorganic and organic electrolytes, have also been proposed as a solution for LOBs with increased lifespan and safety [55]. Inorganic solid-state electrolytes are not without drawbacks, including issues including poor ionic conductivity, high interfacial resistance, and limited compatibility with other components of the battery. These limitations highlight the need for continued exploration and innovation in order to overcome these obstacles and boost the overall performance of SSBs.
A hybrid quasi-solid-state electrolyte was proposed as a unique solution for LOBs with extended lifespan and safety [56]. The selection of an inorganic solid-state electrolyte holds substantial influence over the stability, safety, and performance of LOBs. Further research is crucial for the development of novel materials that exhibit enhanced properties in order to address these aspects effectively.
Organic electrolytes. Polymer electrolytes have become potential candidates for ASSLOBs owing to their unique properties and advantages. These electrolytes, composed of polymer matrices and lithium salts, offer several benefits, such as better contact with electrodes and excellent mechanical flexibility, compared with inorganic electrolytes. Polymer electrolytes for Li batteries have seen notable advancements across three primary categories: dry solid polymer electrolytes, gel polymer electrolytes (GPEs), and composite polymer electrolytes (CPEs). These developments have contributed to enhanced performance and expanded possibilities for lithium battery technology. However, the researchers will focus on dry solid and CPEs, as gel polymers are not considered in this research. Dry solid polymer electrolytes consist of a polymer host and a Li salt, serving as a solid solvent with no liquid phase. Nonetheless, dry polymer systems often exhibit relatively poor ionic conductivity at ambient temperature, limiting their performance. To address this, composite polymer electrolytes have been shown as a bright strategy. These electrolytes involve incorporating ceramic fillers into the organic polymer host, which effectively lowers the glass transition temperatures and improves the ionic conductivity. This approach enables enhanced performance and conductivity in polymer electrolyte systems.
The fabrication of solid polymer electrolytes (SPEs) typically involves dissolving lithium salts, including Li bisfluorosulfonimide (LiFSI), Li bis(trifluoromethanesulfonyl)imide (LiTFSI), or Li trifluoropotassium sulfonate (LiCF3SO3), into various polymer hosts. Common polymer hosts employed in SPEs include poly(acrylonitrile) (PAN), poly(ethylene oxide) (PEO), poly(vinylidene fluoride), polyvinylidene fluoride–hexafluoropropylene (PVDF-HFP), poly(methyl methacrylate), poly(vinyl chloride) (PVC), poly(vinylpyrrolidone) (PVP), and poly(tetrafluoroethylene) (PTFE).
Polyacrylonitrile (PAN)-based polymer electrolytes were among the earliest developed polymer electrolytes. PAN polymers offer many benefits such as chemical stability, non-flammability, thermal durability, and cost-effectiveness [57][58][59]. However, PAN alone is not typically utilized as a substrate for PEs due to its fragility. To overcome this limitation, PAN can be modified by grafting, copolymerization, or mixing with other polymer monomers that possess high mechanical strength. By incorporating PAN into these ideal polymer electrolyte systems, both the ionic conductivity and mechanical integrity could be enhanced, resulting in enhanced overall performance for various battery applications. The inclusion of -CN groups in PAN facilitates interactions between Li ions and the -CO groups of ethylene carbonate (EC) and propylene carbonate (PC), rendering PAN multifunctional for different applications [60][61].
Tran et al. [62] reported PVA/PAN/LiTFSI/LATP/SN composite polymer electrolytes for ASSLOBs. In this study, ionic conductivity (1.13 × 10−4 S cm−1) and mechanical integrity improvability was demonstrated by modifying the PVA polymer electrolyte. The inclusion of -CN groups in PAN facilitates interactions between Li ions and the -CO groups of ethylene carbonate (EC) and propylene carbonate (PC), rendering PAN versatile for various applications. The existence of the -C≡N group in PAN provides strong electronegativity, allowing it to attract Li+ ions from lithium salts and transition metal ions from cathode materials. Moreover, PAN possesses desirable properties, such as viscosity and oxidation resistance, making it an important polymer for use as a coating layer material. In a study by Chen et al. [63], a solvothermal reaction process was employed to fabricate a thin film coating of PAN on LLZTO particles surface. There is a schematic graphic demonstrating the interparticle Li+ transition within the bulk of the PAN-coated LLZTO electrolyte. A TEM (transmission electron microscopy) analysis was performed to examine and compare the microstructures of PAN-coated LLZTO particles and pristine LLZTO particles. The TEM image revealed that LLZTO particles, with an average size of approximately 100 nm, were enveloped and interconnected by a uniform polymer coating that formed on the particle surfaces. This resulted in a uniform nanocoating of PAN, which significantly improved the Li+ transference number to 0.66. Therefore, the thin-film solid electrolyte exhibited a satisfactory ionic conductivity of 1.1 × 10−4 S cm−1.
PEO (polyethylene oxide) has been the subject of significant research in the past two decades as a polymer host for PEs. The investigation of ionic conductivity in PEO electrolyte systems was initiated by Wright in 1975 [64], and Feuillade and Perche explored a polymer plasticization with an aprotic solution composed of alkali metal salts in the same year. PEO is a popular polymer host because of its favorable characteristics, including structural stability, good capacity for salt complexation, high ionic conductivity in the amorphous phase, high corrosion resistance, reasonable cost, flexibility, and chemical durability [65][66]. However, these polymers suffer from limitations such as poor mechanical strength and low ionic conductivity at ambient temperatures (typically in the range of 10−8 to 10−7 S cm−1), which are attributed to the restricted chain motion of PEO [67]. The high crystalline phase of PEO at ambient temperatures hampers ion conduction, resulting in lower ionic conductivity [68]. Above its melting point, PEO-based polymer electrolytes display enhanced conductivity owing to the transition from crystalline to amorphous phases. However, the molten state of PEO results in a loss of dimensional durability and the formation of an extremely viscous liquid, which adversely affects the mechanical resilience of the PEO-based polymer electrolyte matrix. Various strategies have been explored to enhance the ionic conductivity of PEO-based electrolyte systems [69]. The introduction of organic plasticizers and fillers is mainly employed to achieve this. The incorporation of fillers decreases the polymeric crystallinity, leading to improved ionic conductivity. To improve the mechanical stability of PEO-based CPEs, inorganic nanomaterial fillers such as TiO2, Al2O3, and fumed silica have been employed [70][71]. The addition of cyclodextrin into both the polymer matrix and Li salt has demonstrated promising outcomes by enhancing the Li+ transport in PEO-based polymer electrolytes. This is achieved by weakening the interaction between the ether groups in PEO, facilitating faster Li+ mobility within the polymer matrix. Additionally, it creates channels for accelerated Li+ diffusion from the PEO matrix to LLZTO fillers, enabling faster ion transport. Furthermore, a new method was devised by He et al. [72], which utilized the intermolecular interaction between ethylene carbonate (EC) and Ta-doped LLZO Li7La3Zr2O12 (LLZTO) in P(EO)15LiTFSI. The LLZTO-induced ring opening reaction of EC leads to the formation of oligomers with ether oxygen chains, providing an additional pathway for efficient Li+ conduction. Furthermore, EC acts as a disruptor to the PEO chain, expanding the amorphous phase region and facilitating Li+ migration. Consequently, the PEO-based electrolyte achieves a room temperature conductivity of 1.43 × 10−3 S cm−1.
Another strategy involves blending PEO with other polymers, such as PMMA, to enhance the ionic conduction of PEO-based systems. The flexible backbone and amorphous nature of PMMA contribute to a reduction in the mechanical strength of PEO, allowing for improved ionic conduction [73]. Another effective additive for solid-state batteries is succinonitrile (SN), which also contains cyano groups. The -C≡N groups in SN have the ability to incorporate with a Li salt and other polymers. In the case of PEO-SN systems, they can establish a fast pathway for Li+ movement, even in the absence of a lithium salt. An analysis of NMR spectra conducted by Xu et al. [67] revealed that the incorporation of a high content of SN (SN:EO = 1:4) can promote the formation of fast ion channels, resulting in the development of homogeneous SSEs.
Extensive research has been dedicated to the synthetic development of PEO-based solid polymer electrolytes for ASSLOBs, including copolymerization, crosslinking, and hyperbranching [74][75]. In a study carried out by Wang et al. [76], a solution-casting technique was utilized to crosslink a hydrolyzed polymaleic anhydride (HPMA) low-molecular polymer plasticizer with PEO and Li salt. The physical entanglement between HPMA and PEO, as well as the presence of the -COOH group on HPMA, played a critical role in decreasing the crystallization and promoting the amorphous phase of PEO. This structural modification resulted in an enhancement in the ionic conductivity of the PEs.
Overall, PEO-based PEs are a potential avenue for the advancement of ASSLOBs, and research efforts are ongoing to improve their performance and stability.
PVDF has gained significant consideration in the improvement of lithium batteries owing to its excellent properties, such as a strong affinity to electrolyte solutions, excellent electrochemical durability, and high dielectric constant [77]. The existence of CF groups in PVDF chains contributes to the wide electrochemical stability range of the polymeric solid electrolyte, which can be up to 4.5 V or higher [78]. However, despite these favorable characteristics, the mechanical strength of PVDF-based electrolytes remains insufficient for practical applications [79][80]. Several strategies can be utilized to increase the mechanical features and suitability of the electrolyte for real-world applications. These approaches include incorporating reinforcing fillers or blending PVDF with other polymers, which effectively improve the mechanical performance of the electrolyte [81].
PVDF polymers indicate a high degree of crystallinity, which can affect their performance in Li-based systems. The inclusion of fluorine (-F) in the PVDF chain makes it susceptible to reactions with lithium metal, particularly during repetitive charge and discharge cycles. This reaction results in a reduction in the hydrophobicity of the membrane, allowing moisture from the air to penetrate and potentially damage the Li negative interface. To address these challenges, researchers have explored the use of PVDF-HFP membranes with oxygen selectivity. A copolymer of PVDF-HFP consists of two distinct monomers: symmetrical vinylidene fluoride (VdF) and asymmetrical hexafluoropropylene (HFP) [82]. This unique combination of crystalline PVDF and amorphous HFP in the copolymer results in a high ionic conductivity and good mechanical resilience [83]. PVDF, with its high degree of crystallinity, exhibits relatively lower conductivity. However, when combined with the PVDF-HFP host, which comprises two randomly mixed monomers, the resulting film allows for improved mobility of free lithium ions and higher amorphicity, resulting in improved conductivity [80][84]. PVDF-HFP has emerged as a highly successful matrix for polymer electrolytes in LOBs, serving as one of the key materials in this application [85]. PVDF-HFP-based polymer electrolytes with high Li+ transference numbers have been studied to enhance the cycling stability and rate capability of lithium metal batteries (LMBs) [86][87]. The detailed discussion of hybrid polymer electrolytes for LOBs encompassed various compositions, focusing on crucial aspects such as electrolyte conductivity and stability.
PMMA-based polymer electrolytes are derived from methyl methacrylate (MMA) or methyl propionate (MA). These PEs present numerous advantages, including a broad electrochemical stability range exceeding 4.5 V, excellent room temperature ionic conductivity (10−3 S cm−1), and favorable compatibility with both positive and negative electrodes [88]. However, PMMA-based PEs often undergo low mechanical integrity and high brittleness [89][90]. To address this issue, PMMA is often used in combination with other substrate materials to harness its desirable properties while improving mechanical strength. By incorporating PMMA into CEs or utilizing it as a component in polymer blends, the overall performance and mechanical strength of the electrolyte system can be improved. Ramesh and Liew [88] revealed an enhancement in ionic conductivity by blending PMMA with PVC and doping it with the LiTFSI lithium salt. Numerous studies have been dedicated to optimizing the properties and performance of PEs based on PMMA using different additives and mixing techniques [87]. Overall, PMMA-based PEs are a potential avenue for the improvement of ASSLOBs, and research efforts are ongoing to improve their performance and stability. However, additional studies are required to ascertain the specific advantages of PMMA-based polymer electrolytes for ASSLOBs [55][91][92][93].
Poly(vinyl alcohol) (PVA) has been studied as an SPE for Li batteries [62][94]. It possesses a range of desirable characteristics, including good elasticity strength, mechanical resilience, eco-friendliness, low cost, good optical characteristics, high temperature stability, and a high level of hydrophilicity [95][96]. The hydrophilicity of PVA is attributed to the abundance of polar hydroxyl groups within its structure. In addition to these properties, PVA offers advantages such as ease of production, high erosion resistance, good elasticity, biocompatibility, and high chemical and thermal durability [97][98]. PVA holds promise as a versatile and functional host polymer for various electrochemical applications.
The advancement of PEs for ASSLOBs is still an active research area. Despite the presence of certain limitations in SPEs, including poor lithium ionic conductivity due to crystallizing and the potential decomposition of the polymer matrix in open operating atmospheres, their high tolerance to battery volume changes and excellent processability make them highly promising for the development of flexible SSLOBs. Researchers are actively working on developing polymer electrolytes with enhanced lithium-ion conductivity through various strategies such as optimizing polymer composition, incorporating additives, and improving polymer morphology. Another challenge is the stability of the PE in the presence of Li and oxygen. The highly reactive nature of lithium and oxygen can cause degradation and decomposition of the polymer electrolyte, leading to a decrease in battery performance over time. Research efforts are focused on designing polymer electrolytes with improved stability and compatibility with lithium and oxygen to ensure long-term operation of ASSLOBs. Furthermore, the interface of PE and the electrode materials is vital for efficient charge and ion transport. To enhance the overall performance of ASSLOBs, it is crucial to focus on the improvement of interfacial engineering approaches and a deeper understanding of interfacial phenomena. While notable progress has been achieved in the development of polymer electrolytes for ASSLOBs, there are still technical challenges that must be overcome before they can be effectively commercialized. Further research and development efforts are required to enhance the Li ion conductivity, stability, and interface properties of PEs for ASSLOBs.
Composite Electrolytes. In LOBs, ceramic electrolytes show ideal Young’s modulus, good ionic conductivity (>10−4 S cm−1) and thermal durability, and superior conductivity, while polymer electrolytes provide good wetting, lightness, and improved flexibility [99]. However, the high temperature treatment used to produce ceramic electrolytes results in inadequate contact of the ceramic electrolyte with the electrode, which increases the interfacial resistance [100]. In polymer solid electrolytes, there are problems of poor interface contact and low ionic conductivity entailing the crystallinity of the polymer matrix and low chain movement at room temperature [101]. There is an increasing interest in composite electrolytes that merge the benefits of these two electrolytes and eliminate their disadvantages. Inorganic ceramic electrolytes are utilized primarily as fillers in composite solid electrolytes (CSEs) to enhance both the mechanical strength and ionic conductivity. Polymer matrices used in CSEs reduce the electrode–electrolyte interface resistance, increase the flexibility, and provide an easy production process [102]. Some features should be considered when designing composite electrolyte for LOBs. The composite solid electrolyte used in LOBs should be stable after contact with O2. It should also consist of hard inorganic components with 3D ion transport channels that evenly cover the Li anode surface to ensure an even distribution of Li+ ions and high mechanical strength. Composite solid electrolytes should possess a smooth surface, while maintaining sufficient flexibility to establish close contact with the electrodes [103].
GPEs, which are produced by adsorbing liquid electrolytes in the polymer matrix, find applications in LOBs because they have the properties of both solid and organic liquid electrolytes. The flexible structure of GPEs can suppress the volumetric change of the electrode during the electrochemical process, and its gel property facilitates Li+ diffusion [104]. Ceramic fillers are added to GPEs to increase their mechanical strength and ionic conductivity. Cations in the structures of ceramic fillers act as Lewis acids. These cations take the place of Li+ by reacting with O2 or other functional groups in the polymer [105]. Thus, the recrystallization of the polymer is prevented, and the Li salts are easily separated. Oxygen in ceramic fillers acts as a Lewis base. The oxygen in the structure interacts with Li+ and forms a Li+-rich state at the filler/polymer interface, acting as a new pathway for lithium-ion transition. As a result, there is an increase in the number of Li+ transported [106]. Liu et al. [93] designed composite GPE using poly(methyl methacrylate)(PMMA) and SiO2 and using LTFSI as a salt for ASSLOBs. SiO2 added to GPE prevents polymer recrystallization, and the interaction between Li+ ions and OH− groups on the SiO2 surface favors the formation of a lithium-ion migration pathway.
The composite electrolytes offer a solution to the issues found in ceramic and polymer electrolytes for ASSLOBs. Composite electrolytes can improve ionic conductivity and prevent dendrite formation at the anode by creating porous structures, reducing grain size, and incorporating conductive fillers. Additionally, appropriate fillers can reduce polarization on the cathode side and improve overall electrochemical performance. Furthermore, the flexibility of composite electrolytes can protect both the anode and cathode, thereby minimizing volumetric expansion problems.

3. Cathode Architecture for Solid-State Lithium–Oxygen Batteries

As an alternative to LOBs that use volatile and explosive organic liquid electrolytes, the development of ASSLOBs shows significant promise. However, because of the weak ionic conductivity of SEs, the considerable interfacial resistance, and the restricted reaction sites of cathodes, the implementation of high-performance SSLOBs is a challenge. The development of an ideal cathode for ASSLOBs has been impeded practically by a limited capacity and short cycle life. For the air cathode to effectively accommodate the deposition and decomposition of the discharge products, it necessitates a high porosity structure, excellent electronic conductivity, and high catalytic activity for both oxygen reduction and oxygen evolution reactions [107]. Unfortunately, the generation and dissolution of the discharge residue (Li2O2) in two-electron processes have slow reaction rates. The ASSLOBs typically lead to large discharge overpotentials and poor Coulombic efficiencies due to the low electrochemical reversibility of Li2O2. Furthermore, the high interfacial resistance between the cathode and the SE hindered the reaction rates. As a result, developing a highly effective catalyst is a “significant issue” for ASSLOBs. Carbon-based materials are frequently chosen as catalysts, as is well known owing to their remarkable catalytic activity, light weight, superior conductivity, and enriched porous structure. It appears that carbon-based materials with nanostructures can efficiently improve the electrochemical kinetic reaction.
In contrast, the organic electrolytes in current aprotic LOBs are easily decomposed by the strong oxidative radicals generated at the cathode, resulting in poor cyclability. Recently, solid-state cathodes composed of stable ceramic electrolytes rather than organic electrolytes have been designed to address this issue. These cathodes offer a capacity by formation of Li2O2 via an electrochemical reaction among the lithium ions, e−, and oxygen, although, due to the weak electron/ion transport in Li2O2 and the lack of a liquid medium for Li2O2 development, the formation of Li2O2 particles within the solid-state cathode is constrained. Consequently, the capacity of LOBs with a solid-state cathode is inherently restricted. To circumvent the restriction caused by the low growth of the Li2O2 particles within the solid-state cathode, it is possible to develop an environment that promotes the expansion of the discharge product. According to a study by Kim et al. [108], the discharge product can be converted from growth-restricted Li2O2 to easily produced LiOH by adding water steam to the oxygen used in the SSBs, and a ruthenium-based composite that conducts both electronically and ionically was developed as a solid-state cathode. The pouched cathode film was attached to an LATP plate to assemble an ASSLOB cell, and the cells demonstrated a high stability over 665 cycles. This strategy offers new perspectives on designing effective ceramic-based solid-state cathodes for feasible LOB systems.
Additionally, the integrated structure design for ASSLOBs has drawn more interest because it has proven to have high potential, even at a broad range of room temperatures (up to 120 °C). In the previous work, the researchers developed an integrated structure cathode by coating GPE on a MnO2/Ru nanowire hybrid framework [109], which established a compact interfacial contact between the GPEs and the cathode. The air cathode, utilizing the advantages of its 3D porous nanowire structure, can provide a sufficient number of active sites for catalytic reactions, such as oxygen reduction and oxygen evolution reactions. Furthermore, this structure helps to minimize resistance related to charge transfer and mass diffusion, thereby enhancing overall performance. The ASSLOB cells also provides a high specific capacity of 14,384 mAh g−1 at 200 mA g−1. In a recent study, a flexible integrated cathode–electrolyte structure was developed with the aim of establishing a robust interaction between the cathode and electrolyte. This was achieved by anchoring them onto a three-dimensional SiO2 nanofiber structure.
The development of a carbon-coated porous Li1.5Al0.5Ge1.5P3O12 (LAGP) layer through straightforward one-step annealing was reported by Zhou et al. [110]. An effective integrated design can potentially lower interface resistance and promote the electrochemical performance of the cell. Li et al. [111] proposed using a 3D SiO2 nanofiber (NF) membrane to develop a flexible integrated cathode–electrolyte structure (ICES) for ASSLOBs. The SiO2 NFs framework acts as a bridge connecting the composite solid electrolyte (CSE) and cathode, improving ionic conductivity and reaction sites. The ICES exhibits a high discharge capacity (9220 mAh g−1), rate capability, and cycle lifetime (145 cycles). The CSE also inhibits the dendrite growth and increases the battery safety. The ICES-based SSLOBs demonstrate lower resistance and improved performance compared with carbon-based ASSLOBs.
The field of ASSLOBs is continually advancing, and researchers are actively investigating different cathode materials and configurations to tackle the challenges related to stability, reversibility, and efficiency. Ongoing research and development endeavors are crucial to overcome these obstacles and unlock the complete potential of ASSLOBs.

This entry is adapted from the peer-reviewed paper 10.3390/batteries9070380

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