The global transition from fossil fuels to cleaner and more sustainable energy sources is an imperative response to the growing environmental concerns associated with the use of conventional hydrocarbon-based fuels [1]. The detrimental impacts of greenhouse gas emissions, climate change, and resource depletion have accelerated the need for innovative solutions to reduce our reliance on fossil fuels and mitigate their environmental footprint ^[1][2][1,2]. In this era of sustainable energy, high-performance energy storage systems play an important role in ensuring the reliability and efficiency of renewable energy sources ^[3][4][3,4]. These systems bridge the gap between energy generation and consumption, enabling the effective utilization of intermittent sources like wind and solar power while enhancing grid stability and resilience ^[3][4][3,4].

In the landscape of energy storage, solid-state batteries (SSBs) are increasingly recognized as a transformative alternative to traditional liquid electrolyte-based lithium-ion batteries, promising unprecedented advancements in energy density, safety, and longevity ^[5][6][7][5,6,7]. These benefits stem from the incorporation of advanced electrode materials and solid-state electrolytes, which together enable heightened energy storage capacities. Notably, the absence of flammable liquid electrolytes in SSBs mitigates the risk of thermal runaway, a paramount safety concern, especially in applications like electric vehicles (EVs) and portable electronics ^{[8][9][10][11]}[8,9,10,11]. Beyond safety, SSBs, with their augmented energy densities, champion the development of more compact, energy-efficient devices ^{[11][12][13][14][15][16]}[11,12,13,14,15,16]. Their resilience against dendrite formation and extended cycle life further accentuates their suitability for applications demanding sustained reliability and long-term energy storage ^{[15][16][17][18]}[15,16,17,18].

2. Traditional Cathode Materials

Lithium cobalt oxide (LiCoO₂) has been a foundation in the development of lithium-ion batteries. It offers high energy density but comes with challenges such as safety risks and limited thermal stability ^[19][21]. The original ideas for LiCoO₂ as a cathode material were inspired by interdisciplinary research in solid-state physics and chemical structure bonding ^[19][21]. LiCoO₂ has a theoretical capacity of 274 mAh g⁻¹ but often fails to deliver more than half of this due to structural deformation ^[20][23]. The material’s high energy density makes it suitable for applications requiring compact and lightweight batteries, such as mobile devices and certain types of electric vehicles. A recent study by Cherkashinin et al. ^[21][24] investigated into the intrinsic stability of LiCoO₂ and found that the material exhibits fully reversible electronic properties after the first electrochemical cycle, providing insights into the development of doping strategies to enhance its electronic conductivity. Lithium manganese oxide (LiMn₂O₄) is considered an environmentally friendly and cost-effective alternative to LiCoO₂. It has a theoretical capacity of 148 mAh g⁻¹ and can deliver more than 95% of its theoretical capacity ^[20][23]. However, it suffers from low conductivity, manganese dissolution in the electrolyte, and structural distortion at elevated temperatures ^[20][23]. Various strategies have been employed to improve LiMn₂O₄’s performance, including structure doping with single or multiple cations and anions, and surface modification by coating with materials like carbonaceous compounds, oxides, and phosphates ^[22][25]. Doping stabilizes the LiMn₂O₄ spinel structure and reduces the amount of electrochemically active Mn³⁺, which is responsible for manganese dissolution into the electrolyte ^[23][26]. Lithium iron phosphate (LiFePO₄) is another alternative cathode material known for its robustness and safety ^[24][28]. It has a theoretical capacity of 170 mAh g⁻¹ and is particularly stable during charge and discharge cycles ^[24][28]. LiFePO₄ is often used in applications where safety and long cycle life are more critical than energy density, such as in large-scale energy storage systems and certain electric vehicles. In a study focusing on the temperature’s effect on different cathode materials, LiFePO₄ was found to have optimal performance in a temperature range of 20–50 °C ^[25][29]. The study also highlighted that the state of charge (SOC) has a significant impact on the internal resistance of the battery, affecting its overall performance ^[26][30].  The interaction between cathode materials, solid electrolytes, particle size, and binder type plays a pivotal role in dictating the performance, safety, and longevity of solid-state batteries. Understanding and optimizing these correlations for LiCoO₂, LiMn₂O₄, and LiFePO₄ cathodes are essential for the advancement of solid-state battery technologies. The correlation between these properties is presented below: LiCoO₂ (lithium cobalt oxide): LiCoO₂ is known for its high energy density, but its performance is closely tied to the choice of solid electrolyte and the microstructural characteristics of the cathode. The optimal electrolyte for LiCoO₂ in solid-state batteries should have high ionic conductivity and compatibility with the cathode material to prevent interface degradation. LiMn₂O₄ (lithium manganese oxide): The performance of LiMn₂O₄ is significantly influenced by its interaction with the electrolyte, particularly concerning manganese dissolution issues. Solid electrolytes that provide a stable interface and minimize the dissolution of manganese, such as lithium phosphorus oxynitride (LiPON), are preferred for enhancing longevity and safety. Particle size plays a crucial role in LiMn₂O₄ cathodes, with nano-sized particles offering an improved rate capability and a reduced path length for lithium-ion diffusion. However, nano-sizing can exacerbate manganese dissolution, necessitating careful optimization. The choice of binder in LiMn₂O₄ cathodes, such as carboxymethyl cellulose (CMC), is vital for maintaining electrode integrity, especially considering the material’s susceptibility to volume changes during cycling. LiFePO₄ (lithium iron phosphate): LiFePO₄’s stability and safety are enhanced when paired with solid electrolytes that exhibit thermal stability and robustness, such as garnet-type electrolytes like LLZO. The particle size of LiFePO₄ directly impacts its rate performance; smaller particles facilitate faster lithium-ion diffusion but can lead to increased side reactions at the electrode–electrolyte interface. Binders that can accommodate the volume expansion of LiFePO₄ while maintaining good electrode integrity are crucial. Binders like poly(acrylic acid) (PAA) have been found to be effective, offering strong mechanical adhesion and contributing to the overall structural stability of the cathode.

Importance of Solid Electrolytes in Solid-State Batteries

The crucial role of SSBs extends far beyond the mere replacement of liquid counterparts. Solid electrolytes stand at the forefront of revolutionizing energy storage systems, primarily due to their intrinsic safety features and ability to enable high energy densities [12]. The non-flammable nature of solid electrolytes, typically composed of inorganic ceramics or polymers, eliminates the risk of leakage and thermal runaway, a significant concern in conventional lithium-ion batteries with organic liquid electrolytes ^[12][21][27][12,20,24]. This critical safety enhancement makes SSBs particularly suitable for high-energy-demand applications, including electric vehicles and portable electronics, where thermal stability is paramount ^[23][25][26,29]. Solid electrolytes also facilitate the integration of lithium metal anodes, as opposed to the traditional carbonaceous anodes, thus unlocking the potential for higher energy densities. Lithium metal anodes, in synergy with solid electrolytes, could theoretically provide an energy density nearly double that of current lithium-ion batteries [16]. This is chiefly due to the higher specific capacity of lithium metal (3860 mAh g⁻¹) compared to graphite (372 mAh g⁻¹). However, the compatibility between the solid electrolyte and the lithium metal anode remains a challenging area, particularly in terms of maintaining a stable interface and suppressing lithium dendrite growth during electrochemical cycling ^[23][25][27][20,26,29]. Despite these advantages, the development of solid electrolytes with high ionic conductivity comparable to liquid electrolytes remains a substantial challenge. The pursuit of solid electrolytes with high lithium-ion transference numbers, minimal electronic conductivity. Recent advancements have highlighted the promise of garnet-type electrolytes, like Li₇La₃Zr₂O₁₂ (LLZO), which exhibit high ionic conductivity and stability against lithium metal ^[24][27][20,28]. 

3. Emerging Cathode Materials

Sulfide-based cathode materials have gained significant attention in the realm of all-solid-state lithium batteries (ASSLBs) ^{[28][29][30][31][32]}[32,33,34,35,36]. These materials offer promising attributes, especially when paired with solid-state sulfide electrolytes (SSSEs). A recent study ^[30][34] introduced a novel Li_2.96P_0.98S_3.92O_0.06-Li₃N glass–ceramic electrolyte (GCE) where O and N substitution produced unique functional units that enabled superior ionic conductivity at room temperature. Emerging oxide cathodes in the context of ASSLBs refer to newer compositions or structural modifications of oxide materials that potentially offer improved energy densities, safety profiles, and compatibility with solid electrolytes. These include novel formulations or structurally engineered oxides that are different from the traditional materials in their electrochemical behavior, safety profiles, or other key characteristics. Oxide-based cathode materials, when paired with solid electrolytes, can significantly enhance the energy density of ASSLBs. The transition from liquid-based Li-ion batteries (LIBs) to ASSLBs has been driven by the potential advantages of oxide- and sulfide-based solid electrolytes ^[31][35]. A strategic approach to this transition involves analyzing the pairing of solid electrolytes with oxide cathode materials and the Li metal anode ^[31][35]. The chemical, electrochemical, and mechanical properties of these solid electrolytes play a crucial role in determining the performance of the resultant ASSLBs ^[31][35]. Emerging cathode materials present a myriad of advantages and challenges that influence their potential in next-generation energy storage systems. While they offer benefits such as fuel flexibility and environmental friendliness, they also come with inherent limitations that need to be addressed for optimal performance. The compatibility of cathode materials with solid electrolytes is a critical factor in the performance of ASSLBs. For instance, lithium aluminum germanium phosphate (LAGP) is a solid electrolyte that has gained significant attention due to its stability in air and good ionic conductivity ^[33][37]. However, challenges like poor interface compatibility with Li anodes and slow Li-ion conduction in thick pellets have been identified ^[33][37]. Advanced interface engineering strategies, such as introducing a functional interlayer, have been proposed to address these challenges ^[33][37]. As a summary of this section, emerging cathode materials in ASSLBs include sulfide-based and oxide-based types. Sulfide-based materials, especially when used with solid-state sulfide electrolytes (SSSEs), offer promising attributes like enhanced ionic conductivity and structural stability against moisture, as shown in the Li_2.96P_0.98S_3.92O_0.06-Li₃N GCE study. Oxide cathodes represent a shift in composition and structure from traditional materials, aiming for higher energy densities and safety. Their pairing with solid electrolytes like poly(ethylene oxide) (PEO) and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) in CSE membranes results in batteries with impressive performance.

4. Structural Optimization of Cathode Materials

4.1. Nanostructuring

The structural optimization of cathode materials in SSBs plays a crucial role in enhancing their electrochemical performance. Nanostructuring is a promising approach that involves reducing the dimensions of cathode materials to the nanoscale ^[34][38]. This strategy offers several advantages in terms of improved conductivity and enhanced surface area for electrochemical reactions ^[34][35][36][38,39,40]. The nanostructuring of cathode materials has been demonstrated in various studies. For instance, Sun et al. ^[34][38] demonstrated the advantages of utilizing a Li₂WO₄-coated LiCoO₂ cathode in combination with a sulfide Li₆PS₅Cl solid electrolyte, enhancing the electrochemical performance of all-solid-state batteries. In a similar vein, Liu et al. ^[36][40] enhanced the interface between the superionic conductor and the polymer electrolyte (polyvinylidene fluoride) through the in situ creation of a pyrochlore-type La₂Sn₂O₇ (LSO) ceramic layer on Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). They noted that the LSO synthesis process depletes La in LLZTO, leading to an increased concentration of Li ions in LLZTO and substantially boosting the conductivity of the composite electrolyte (LLZTO@LSO-CSE). The conductivity of LLZTO@0.9%LSO-CSE showed a significant improvement, reaching as high as 1.30 × 10⁻⁴¹ S cm⁻¹, compared to 3.15 × 10⁻⁵³ S cm⁻¹ for the unmodified sample.

4.2. Surface Coatings

Surface coatings represent another avenue for improving cathode materials in SSBs. Coating cathode particles with conductive and protective materials can mitigate issues related to structural instability, reactivity with electrolytes, and enhance overall electrode performance ^[37][38][41,42]. Recent research by Liang et al. ^[39][43] focused on surface coating strategies for LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) cathode materials. They demonstrated that a gradient oxy-thiophosphate coating improved the structural stability of NMC cathodes, reducing capacity fading and improving cycling stability.

4.3. Composite Approaches

Composite cathode materials are designed by integrating various components to leverage their complementary properties. This approach combines the advantages of multiple materials to address the challenges associated with low conductivity and structural instability in SSB cathodes ^[40][41][46,47]. One notable example is the development of composite cathodes using conductive polymers. Composite approaches also extend to incorporating ionic conductors within the cathode matrix. Liang et al. ^[42][44] observed that high-energy Ni-rich layered oxide cathode materials like LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) experience adverse side reactions and interfacial structural instability when paired with sulfide solid-state electrolytes in all-solid-state lithium-based batteries. To address this, their team introduced a gradient coating strategy for NMC811 particles using lithium oxy-thiophosphate (Li₃P_1+xO₄S_4x), achieved through the atomic layer deposition of Li₃PO₄ followed by an in situ development of a gradient Li₃P_1+xO₄S_4x coating. This customized surface engineering of NMC811 prevents the structural degradation commonly seen in the transition from layered to spinel phases at grain boundaries and significantly enhances the stability of the cathode/solid electrolyte interface during cycling. 

4.4. Addressing Issues Like Low Conductivity and Structural Instability

The structural optimization of SSB cathodes aims to mitigate two fundamental challenges: low electrical conductivity and structural instability. Various strategies, including those discussed above, are employed to address these issues ^{[36][37][38][39][40][41][42][43]}[40,41,42,43,44,45,46,47].

4.5. Role of Electrical Properties (Electronic and Ionic Conductivity)

The performance of SSBs is heavily influenced by the electrical properties of their cathode materials. These properties, specifically electronic and ionic conductivity, play a pivotal role in determining the efficiency of charge and ion transport within the battery. Electronic conductivity refers to the ability of a material to conduct electrons. In the context of SSBs, high electronic conductivity ensures that electrons can move freely from the cathode to the anode during the discharge process, and vice versa during charging ^[44][48]. Nanostructuring is a technique that has been employed to enhance this property. By reducing the size of the cathode material to the nanoscale, the pathways for electron diffusion become shorter, leading to improved electronic conductivity ^[45][49]. On the other hand, ionic conductivity pertains to the movement of ions (in this case, Li ions) within the battery. A high ionic conductivity ensures that Li ions can move swiftly from the anode to the cathode during charging and in the reverse direction during discharging. One way to boost ionic conductivity is by incorporating solid-state electrolytes within the cathode structure. This facilitates faster Li-ion transport, leading to a more efficient battery ^[45][49].

4.6. Role of Thermal Characteristics (Heat Dissipation and Thermal Stability) in Safety

Thermal characteristics are paramount for the safe operation of SSBs, particularly in high-demand applications such as electric vehicles and large-scale energy storage systems. The inherent risks associated with the thermal behavior of batteries, especially during charging, have led to numerous fire incidents in electric vehicles ^[46][51]. Solid-state polymer electrolytes (SPEs) have emerged as a promising solution due to their unique characteristics ^[47][52]. Unlike their liquid counterparts, SPEs are not prone to leakage and exhibit low flammability, excellent processability, good flexibility, high safety levels, and superior thermal stability ^[47][52]. However, the challenge remains in ensuring that these electrolytes can maintain their stability and performance under various thermal conditions. One of the critical aspects of ensuring thermal safety is understanding and mitigating the heat generated during the battery’s operation. For instance, during the charging process, irreversible heat is a significant contributor to temperature rise.

4.7. Characterization Techniques in the Development of Solid-State Cathodes

The development of solid-state cathodes for advanced battery technologies is a complex and multifaceted process, wherein characterization plays a key role. Characterization techniques are essential in understanding the physicochemical properties of cathode materials, which directly influence the performance, efficiency, and longevity of solid-state batteries. Advanced techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) provide critical insights into the crystalline structure, morphology, and compositional uniformity of cathode materials ^[5][13][48][5,13,19]. XRD, for instance, is instrumental in determining phase purity and crystallinity, which are vital for ensuring consistent electrochemical performance. SEM and TEM, on the other hand, offer high-resolution imaging capabilities, enabling researchers to examine the microstructural changes in cathode materials during battery operation, a factor crucial for assessing material stability and cycling behavior ^[48][19]. Electrochemical characterization methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are equally important in evaluating the performance of solid-state cathodes. These techniques provide valuable information about the redox processes, ion diffusion kinetics, and interfacial properties of the cathode materials ^[10][34][35][10,38,39]. CV helps in understanding the oxidation and reduction reactions within the cathode, critical for optimizing the energy density and rate capability of the battery. EIS, in particular, is crucial for studying the impedance changes over the battery’s lifespan, offering insights into the degradation mechanisms and thereby guiding the development of more durable cathode materials ^[35][39]. In addition to structural and electrochemical characterizations, thermal analysis techniques such as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are indispensable for evaluating the thermal stability of cathode materials. This aspect is particularly important in the context of solid-state batteries, where thermal management is a key concern ^[36][41][43][40,45,47]. DSC and TGA can identify the specific temperature ranges where structural changes or decomposition occur, providing vital data for ensuring the safety and reliability of the battery under various operational conditions ^[36][43][40,45]. Comprehensive thermal characterization not only aids in the selection of suitable cathode materials but also informs the design of battery management systems to prevent overheating and thermal runaway. These characterizations collectively form the backbone of cathode development for solid-state batteries, enabling researchers and engineers to tailor materials for specific applications, enhance performance metrics, and ensure safety and longevity. As the demand for high-performance energy storage solutions grows, the role of thorough and innovative characterization techniques becomes increasingly critical in the quest to unlock the full potential of solid-state battery technology. The following is a summary and a broader overview of the structural optimization of cathode materials in SSBs: (a) Nanostructuring, which reduces cathode materials to the nanoscale, significantly improves conductivity and the electrochemical reaction surface area. Studies, like those by Sun et al. and Liu et al., have showcased enhanced performance in SSBs using nanostructured cathodes like Li₂WO₄-coated LiCoO₂ and Li_6.4 La₃ Zr_1.4 Ta_0.6 O₁₂ with a pyrochlore-type La₂Sn₂O₇ ceramic layer, respectively. These structures have demonstrated substantial improvements in ionic conductivity and battery performance. (b) Surface coatings: Surface coating of cathode materials with conductive and protective materials addresses issues like structural instability and reactivity with electrolytes.

5. Integration of Advanced Cathode Materials into SSBs

5.1. Scalable Manufacturing Techniques

The industry is continuously seeking innovative and sustainable solutions to enhance battery performance, safety, and manufacturing efficiency. One such innovation is LI-OVIX™, a proprietary lithium metal product ^[49][54]. This unique printable formulation of lithium metal and other specialty materials can enhance lithium-ion battery performance, reduce manufacturing costs, and pave the way for next-generation battery technology. LIOVIX™ can be printed on a battery’s anode during electrode manufacturing in a process called pre-lithiation. This process employs an external source of lithium to offset the first cycle loss of lithium due to SEI (solid electrolyte interphase) formation, allowing for a more efficient use of lithium from the cathode and increasing the cell energy density. Importantly, LIOVIX™ technology is scalable, using industry-standard coating and printing equipment and can be adapted to various anode or cathode chemistries ^[49][54]. The introduction of LI-OVIX™ into the manufacturing process signifies a major shift in how electrodes are prepared and optimized. Traditionally, the first cycle inefficiency in lithium-ion batteries has been a significant hurdle, leading to a loss in capacity and overall efficiency. The pre-lithiation step facilitated by LI-OVIX™ effectively counteracts this issue by supplementing the anode with additional lithium. This not only improves the first cycle’s efficiency, but also enhances the long-term cycle stability of the battery. 

5.2. Electrode–Electrolyte Interfaces

All-solid-state batteries have the potential for high energy density and enhanced safety due to their nonflammable solid electrolytes. However, a significant challenge is the sluggish ion transmission at the cathode–electrolyte (solid/solid) interface, which results in high resistance at the contact ^[50][55]. This limits the practical implementation of these materials in real-world batteries. A study presented by Shi and group ^[50][55] marks a significant milestone in the evolution of all-solid-state lithium–sulfur (Li-S) batteries, a domain critically important for advancing energy storage technology. The innovative approach of fabricating a solid-state sulfur cathode using a composite of sulfurized polyacrylonitrile (SPAN), lithium bis(fluorosulfonyl)imide (LiFSI), and nanographene wire (NGW) addresses two of the most challenging aspects in solid-state batteries: enhancing interfacial contact and improving both ionic and electronic conduction within the solid cathode structure. This is particularly crucial in solid-state batteries where the absence of a liquid electrolyte can often lead to poor ion transport and interfacial instability. By optimizing the cathode structure, the study contributes significantly to the broader effort of making solid-state batteries a viable alternative to traditional lithium-ion batteries, especially in applications demanding higher energy density and safety. The results showed that at a lower current density of 0.167 mA/cm² (0.1C), the solid SPAN/LiFSI/NGW cathode in a bilayer garnet structure achieves an average discharge capacity of 1400 mAh/g over 40 cycles. This high capacity, particularly at a low C-rate, indicates an efficient utilization of active material and minimal capacity fade over the cycles, highlighting the success of the material composition and structural design in maintaining electrochemical stability.

5.3. Current Collectors

In the realm of SSBs, the role of current collectors is paramount for the effective integration of advanced cathode materials ^[51][56]. Current collectors serve as the conduit for electron flow, ensuring efficient charge and discharge processes within the battery. However, their significance extends beyond mere electron transport. Recent research ^[51][52][56,57] has highlighted the challenges associated with the interface of current collectors in SSBs, particularly concerning charge transfer kinetics.

5.4. Practical Considerations for Implementing Advanced Cathode Materials

The development and integration of advanced cathode materials are pivotal for the advancement of solid-state battery (SSB) technology, representing a significant leap toward higher energy densities, enhanced safety, and longer cycle life. However, the practical implementation of these materials into SSBs necessitates a comprehensive understanding of both their intrinsic properties and the complex interplay within the battery system, emphasizing the importance of material-specific challenges and system-level considerations. Central to these considerations is the interfacial stability between the cathode active material (CAM) and the solid electrolyte (SE), a critical factor in the performance of SSBs. Achieving a stable interface involves not only ensuring the chemical compatibility between the CAM and SE, but also maintaining their mechanical and thermal stability under operational conditions. Recent studies have focused on the use of interlayer materials that can act as a buffer between the CAM and SE, mitigating interfacial stress and preventing direct contact that could lead to unwanted side reactions ^[49][50][54,55]. Despite these advancements, the scalability of protective coatings and interface stabilization techniques presents a significant challenge. Transitioning from laboratory-scale synthesis to industrial-scale production is not straightforward and requires both the upscaling of material fabrication and the retention of material properties at larger scales. This upscaling process must be cost-effective to ensure the economic viability of SSBs. Developing scalable deposition techniques, such as spray pyrolysis or scalable ALD, is essential in this regard ^[52][57]. These methods must be optimized to maintain uniformity and quality of the coatings while being efficient and economical for large-scale production. In addition to material-level considerations, the integration of advanced cathode materials into the overall system design of SSBs is crucial. This includes ensuring compatibility with current collector materials, packaging, and thermal management systems. The design must also account for the mechanical stresses that occur during battery operation, which can affect the integrity of the CAM–SE interface ^[53][54][58,59]. The inclusion of advanced diagnostic tools and sensors in battery systems can provide real-time monitoring of battery health and performance, allowing for the early detection of interface degradation or other issues ^[53][54][58,59].

6. Comparative Evaluation of Cathode Materials

6.1. Key Performance Indicators

The energy density of a battery is a pivotal metric that quantifies the amount of energy a battery can store relative to its volume or weight ^[51][56]. This parameter is especially crucial for applications where the constraints of space and weight are paramount, such as in electric vehicles and portable electronics. A battery with a high energy density can store more energy in a smaller space, making it more efficient and desirable for these applications. The high voltage spinel material LiMn_1.5Ni_0.5O₄ (LMNO) has emerged as a promising candidate to enhance the energy density of lithium batteries ^[47][52]. This is attributed to its potential to operate at higher voltages, thereby storing more energy. However, like many advanced materials, LMNO is not without its challenges. Its performance can be compromised by long-term cycling and high-temperature stability. These challenges can lead to a decrease in capacity over time, especially when the battery is subjected to repeated charge and discharge cycles or operated at elevated temperatures ^[47][52].

6.1.1. Voltage Stability

Voltage stability is a critical parameter in battery performance, referring to the battery’s ability to maintain a consistent voltage during discharge ^[24][28]. A stable voltage is essential as it ensures consistent performance of the battery, preventing potential damage to devices powered by it. A battery that can maintain its voltage during discharge can deliver power more reliably, ensuring that the device it powers operates efficiently and safely. In the domain of energy storage, supercapacitors have emerged as a promising technology due to their high-power density and long-term durability ^[55][61]. A significant advancement in this field is the development of aqueous supercapacitors with extended voltage windows. One such innovation is the development of an aqueous supercapacitor with a high-voltage window of 2.0 V. Liu et al. ^[55][61] developed a 2.0 V high-voltage aqueous supercapacitor using core–shell MoO_3−x/polypyrrole (MP) nanocomposites as both cathode and anode materials.

6.1.2. Cycle Life

One of the critical performance metrics for batteries is their cycle life. Cycle life is a measure of a battery’s longevity and indicates the number of charge and discharge cycles a battery can undergo before its capacity drops to a predetermined percentage of its original value, typically 80% ^[37][41]. A battery with a longer cycle life is more desirable as it can be used for more extended periods without significant degradation in performance. In the quest for high-energy density lithium-ion batteries, high-voltage high-nickel low-cobalt lithium layered oxide cathodes have emerged as potential candidates ^[56][62]. These cathodes can store a significant amount of energy, making them suitable for applications that require long-lasting power. However, like many advanced materials, their cycle life can be compromised due to structural deterioration over time. This deterioration can lead to a decrease in capacity and overall battery performance.

7. Identification of Research Gaps and Challenges

7.1. Existing Research Gaps in SSB Cathodes

ASSLBs are increasingly being recognized for their enhanced safety features. To fully harness their capabilities, the integration of high-voltage cathodes is crucial ^{[57][58][59][60]}[63,64,65,66]. This would elevate the energy density of solid batteries, positioning them as strong contenders against their liquid counterparts. However, the incorporation of high-voltage cathodes is not without its challenges. The stability of the cathode material under high voltages is a significant concern ^[59][65]. The chemical stability at the interface between the cathode and the electrolyte is utmost. Recent studies have highlighted the potential of halide-based solid electrolytes, which demonstrate compatibility with cathodes and exhibit high ionic conductivity ^[60][66]. However, even these advanced materials face challenges when it comes to ultra-high voltage operations ^[60][66]. The mechanical integrity at the electrode–electrolyte interface is crucial for maintaining a consistent ionic flow and overall battery performance. The generation of gases within the battery can lead to swelling, internal pressure build-up, and potential rupture, compromising the safety and longevity of the battery ^[60][66]. To address these challenges, recent research endeavors have been directed toward enhancing solid-state electrolytes ^[61][67]. This includes the exploration of polymer solid electrolytes, sulfide solid electrolytes, and oxide solid electrolytes. Furthermore, strategies such as coating protection, synthesis modification, and the structural improvement of cathode materials have been proposed to bolster the electrochemical performance of these batteries ^[61][67].

7.2. Challenges in the Development and Commercialization of SSB Cathodes

Metal–Chalcogen Batteries (MCBs)

MCBs are increasingly recognized as potential successors in the realm of energy storage solutions ^[62][63][68,69]. Their cost-effectiveness, impressive theoretical capacity, and environmental compatibility make them stand out. However, the road to their commercialization is riddled with challenges. During the charge and discharge cycles, the electrodes in MCBs undergo significant volume changes. This can lead to mechanical degradation and reduced cycle life of the battery ^[63][69]. The dissolution and migration of soluble intermediates, particularly lithium polysulfides, can result in a phenomenon known as the shuttle effect. This not only reduces the battery’s efficiency, but also its overall lifespan ^[63][69]. The intermediate conversions in MCBs often suffer from sluggish reaction kinetics, which can impede the battery’s performance ^[64][70]. The growth of uncontrolled dendrites on alkali metal anodes can pose serious safety risks, including short-circuiting and potential battery failure ^[64][70]. To address these challenges, the integration of metal organic framework (MOF)-based materials into MCBs has been proposed. MOFs, with their unique properties such as high porosity, low density, expansive surface area, regular pore channels, adjustable pore size, and topological diversity, can effectively mitigate the aforementioned issues.