Garnet-type solid electrolytes are the most promising materials available, which attract much attention, due to their high ionic conductivity, wide electrochemical window, and stability with lithium. However, some challenges such as surface stability, lithium dendrite penetration, and cost remain critical for its industrialization. This review focuses on the crystal structure and ion conduction mechanisms of garnet-type oxide solid electrolyte (Li₇La₃Zr₂O₁₂, LLZO) to investigate the formation mechanisms and removal strategies for surface passivation layers, including lithium carbonate and lithium hydroxide. For the lithium dendrite penetration of LLZO, potential solutions are explored in view of the black and surface properties electrolyte. For the industrialization, this review analyzes the cost optimization strategies for garnet-type solid electrolytes, aiming to provide valuable insights for industrial advancement.

All-solid-state sodium batteries (ASSSBs) emerge as a highly promising next-generation energy storage technology due to their inherent high safety, abundant sodium resources, and potential for cost-effective production. A central to the performance of these batteries is solid electrolyte (SSE) that has a pivotal role in determining the overall electrochemical performance. Among various SSE candidates, halide solid electrolytes (HSSEs) have attracted much attention due to their high ionic conductivity, wide electrochemical window and good deformability. These characteristics make HSSEs particularly suitable for high-energy-density applications, where conventional liquid electrolytes often fall short due to safety concerns and limited voltage windows.

The crystal structures of halide SSEs are intricately linked to their ion transport mechanisms and electrochemical performance. These structures can be broadly classified based on the metal elements incorporated and the arrangement of halide ions. For instance, sodium-based halide SSEs can be categorized into three main types, i.e., those with subgroup 3 and 4 elements (i.e., Sc, Y, La-Lu, Zr, Hf), subgroup 5 elements (i.e., Nb, Ta), and subgroup 3 main group elements (i.e., Al, Ga, In). Each category exhibits distinct structural characteristics and corresponding ionic conductivities. For instance, Na₃YCl₆, which belongs to the subgroup 3 and 4 elements category, is reported to have an ionic conductivity of 1.4×10⁻⁷ S/cm at room temperature and a monoclinic crystal structure. In contrast, NaTaCl₆ as a member of the subgroup 5 elements category has an ionic conductivity of 6.2×10⁻⁵ S/cm and a monoclinic structure. These structural differences can affect the ion transport mechanisms and overall electrochemical performance of SSEs.

The synthesis methods of halide SSEs play a crucial role in determining their structural and electrochemical properties. Common synthesis methods include mechanical milling, solid-state annealing, and wet chemical synthesis. Mechanical milling, especially high-energy ball milling, can introduce structural disorder and defects, which enhance the ionic conductivity of SSEs. For instance, ball-milled NaTaCl₆ (NTC) exhibits a higher ionic conductivity of 4×10⁻³ S/m, compared to its as-synthesized form. Solid-state annealing can improve the crystallinity and phase purity of SSEs, but it may also lead to a decrease in ionic conductivity due to the reduction of structural defects. Wet chemical synthesis offers a more scalable and energy-efficient approach, but it requires careful control of the synthesis parameters to achieve the desired crystal structure and ionic conductivity.

Interface stability between SSEs and the electrodes is another critical issue that needs to be addressed for the practical application of ASSSBs. Halide SSEs generally exhibit good chemical stability and compatibility with cathode materials due to their wide electrochemical window and high oxidation resistance. However, the interface between SSEs and sodium metal anode remains a challenge. The poor electrochemical reduction stability of halide SSEs can lead to severe interfacial reactions and degradation when in direct contact with sodium metal. To mitigate this issue, various strategies are proposed, such as alloying sodium with other metals (i.e., Sn) to form a more stable interface, using protective coatings to prevent direct contact between SSEs and sodium, and employing composite electrolytes to enhance the overall stability of the interface.

In addition, the scalability and reproducibility of halide SSEs are also critical factors for their practical application. The existing synthesis methods often involve complex procedures and high-energy inputs, which limit the large-scale production of these materials. It is essential for the commercialization of halide SSEs to develop cost-effective and scalable synthesis routes. Moreover, the long-term stability and reliability of ASSSBs incorporating halide SSEs need to be thoroughly evaluated under various operating conditions (i.e., temperature, cycling rate and environmental factor).

Solid-state batteries (SSBs) are a promising next-generation secondary battery due to their potential for high energy density and enhanced safety, offering solutions to the problems inherent in conventional lithium-ion batteries (LIBs) with organic liquid electrolytes (i.e., flammability, corrosion susceptibility, and high-voltage instability). In the construction of SSBs, the selection of the cathode materials is critical to achieving a high energy density, particularly when coupled with lithium metal anodes. However, conventional cathodes often affect the energy density of SSBs due to their constrained specific capacities. It is thus crucial for achieving substantial improvements in the energy density of SSBs to develop high specific capacity cathodes. The Li-rich Mn-based layered oxide (LRMO) is a promising cathode material for SSBs for energy densities of above 600 W·h·kg^-1 due to their high discharge specific capacities. Furthermore, LRMO cathodes offer some additional advantages, i.e., reduction of Co and Ni content, leverage of the abundance of Mn to achieve lower materials costs and improved safety. Their application in SSBs also mitigates the dissolution of TM-ions into the electrolyte, thereby enhancing the structural stability and capacity retention during long-term cycling progress. In addition, the resource-efficient composition of LRMO cathodes also align with environmentally friendly and sustainable development goals.

This review represents that the LRMO cathode materials are characterized by a composite crystal structure comprising two key components, i.e., Li₂MnO₃ phase and LiMO₂(M=Mn, Ni, Co) phase. Li₂MnO₃ phase can be considered as a superlattice-structured variant of LiMO₂, formulated as Li[Li_xMn_1−x]O₂. This superlattice-structured introduces unique unhybridized O 2p states, arising from Li—O—Li configurations. These unique oxygen states enable the participation of oxygen in charge compensation processes. Consequently, the high capacity in LRMO cathodes is attributed to the synergistic contributions of both TM cations and oxygen redox reactions.

LRMO cathodes, while exhibiting a distinctive biphasic structure, encounter significantly some challenges in SSBs. Specifically, the application of LRMO cathodes in SSBs is hindered by two primary issues. Firstly, the inherent incompatibility between Li₂MnO₃ phase and SEs interfaces results in sluggish reaction kinetics, severely restricting the activation of oxygen redox activity and consequently reducing the associated capacity contribution. Secondly, a chemical potential mismatch between SEs and LRMO cathodes drives spontaneous reactions at the composite cathode interfaces. These reactions lead to the formation of mixed ionic/electronic conductive CEI. Furthermore, irreversible oxygen escape further oxidizes the SEs interface, generating the passivation layers. These passivation layers increase interfacial impedance and imped ion transport, ultimately hindering practical advancements in SSBs technology.

LRMO cathodes hold a significant promise for SSBs, as evidenced by research progress across various SEs, including sulfides, halides, polymers, and oxides. To fully realize this potential, some strategies addressing the inherent incompatibility between LRMO cathodes and SEs are crucial. These strategies encompass bulk/interfacial structure design, nanostructured particle engineering, and the construction of stable Li⁺/e^- transport pathways. These approaches can suppress oxygen escape, enhance the high-voltage stability of solid-solid interfaces, and ultimately stabilize oxygen redox while optimizing interfacial dynamics. Consequently, the implementation of these strategies leads to a significant enhancement in the electrochemical performance of LRMO-based SSBs.

Two-dimensional (2D) ion conductors have demonstrated significant advantages in applications such as water treatment, bio-medical, catalysis, energy storage and conversion. The confinement structure of 2D ion conductors provides an ideal platform for studying ion-transport behaviors at nano/angstrom-scale. When the size of the 2D channel is less than 100 nm in at least one direction, the size confinement effect and surface charge of the 2D channel can regulate ion flow, creating nanofluidic transport. This surface-charge-controlled ion transport shows many different properties (e.g. ultrahigh ion conductivity and selectivity) from bulk solutions.

The nanofluidic ion transport has been observed in biological channels (e.g. transmembrane protein on the cell) and natural layered materials (e.g. vermiculite and mica), and gained extensive research interest. To finely control the structure, research attempts have been made to synthesize artificial 2D ion conductors, which are typically achieved by "top-down" and "bottom-up" methods. These artificial 2D ion conductors are normally restacked 2D materials with extrinsic nanofluidics or lamellar crystal materials with intrinsic nanofluidics. Artificial 2D ion conductors can be categorized into three types, including inorganic (e.g. graphene), organic (e.g. covalent organic frameworks), and organic-inorganic hybrid ion conductors (metal-organic frameworks). The nanofluidics in 2D nanochannels originate from the interactions between ions and surface charge. When the spatial size of nanochannels is comparable to those of solvated ions, the ions can be partially or fully desolvated to separate solvent molecules and ions. When ions transport in the nanofluidic channels, ions with the same charge of surface charge on the inner wall can be repeled by the electrostatic force, which leads to unipolar ion transport with high flux. By contrast, ions with the opposite charge will be attracted to suppress their transport. Such ion transport within 2D nanoconfined spaces can create unusual phenomena that are distinct from bulk solution, including ion gating, ion current rectification, selective ion conduction, and ultrahigh ion flux. This review also summarizes key factors influencing nanofluidic ion transport in 2D ion conductors and strategies for regulation, which can be achieved by adjusting internal interlayer spacing and surface chemistry of 2D materials, as well as external field regulation of the confined space.

Benefiting from the studies of nanofluidic mechanisms, the development of 2D ion conductors is rapid in applications where separation and conduction play critical roles. The application can be classified into three types. The first type is the separation of ions and water molecules, which is mainly used for seawater desalination and water purification to promote efficiency and low energy consumption. The second type is the separation of ions with different valences and sizes, which is crucial for ion separation and extraction of high-value elements (e.g. lithium). The third type is accelerating the ion transport of cations or anions, which is one of the keys to determining the rate capabilities in devices for energy conversion (e.g. osmotic energy conversion and photoelectric conversion) and energy storage (e.g. supercapacitors and batteries).

All-solid-state batteries (ASSBs) emerge as a promising next-generation energy storage technology due to their potential for high energy density and enhanced safety. Among the various types of solid electrolytes, sulfide solid electrolytes have attracted much attention due to their high ionic conductivity and excellent mechanical properties. Despite these advantages, the development and industrialization of sulfide-based ASSBs face numerous scientific and engineering challenges. This review focuses on the fundamental scientific issues and engineering difficulties associated with sulfide-based ASSBs, and proposes future directions and recommendations to advance their development and commercialization.

The rapid growth of electric vehicles, consumer electronics, and energy storage systems has driven the development of lithium-ion batteries. However, conventional lithium-ion batteries with a high energy density pose safety risks due to the use of flammable liquid electrolytes. ASSBs, which replace liquid electrolytes with solid electrolytes, offer a safer alternative with a potential for higher energy density. Sulfide solid electrolytes, in particular, exhibit ionic conductivities, compared to those of liquid electrolytes, making them a leading candidate for ASSBs. Despite significant progress, several challenges remain. Sulfide solid electrolytes face issues related to electrochemical stability, humidity sensitivity, and thermal stability. The interfaces between sulfide electrolytes and electrodes (i.e., both cathode and anode) are critical for the battery performance, but are prone to poor contact, chemical reactions, and lithium dendrite growth. These issues hinder the practical application of sulfide-based ASSBs.

The stability of sulfide solid electrolytes is crucial for the performance and safety of ASSBs. Electrochemical stability ensures that the electrolyte can operate in a wide voltage range without decomposing. However, sulfide electrolytes often exhibit narrow electrochemical windows, leading to decomposition at high voltages. Humidity stability is another concern, as sulfide electrolytes tend to react with moisture, producing toxic H₂S gas. Thermal stability is also critical, as sulfide electrolytes can undergo thermal decomposition, increasing the risk of thermal runaway. The interfaces between sulfide electrolytes and electrodes are a major bottleneck in ASSBs development. The solid-solid contact between the electrolyte and electrodes often leads to a poor ion transport and an increased interfacial resistance. In addition, chemical reactions at the interface can also degrade battery performance over time. For instance, the interface between sulfide electrolytes and high-voltage cathodes can lead to the formation of high resistance interface layers, reducing battery efficiency. The compatibility of sulfide electrolytes with different anode materials, such as graphite, silicon, and lithium metal, is another critical issue. Graphite anodes suffer from lithium plating at high currents. Silicon anodes offer a high capacity but experience significant volume changes during cycling, leading to mechanical instability. Lithium metal anodes with their high theoretical capacity are prone to dendrite growth and interfacial reactions with sulfide electrolytes.

The large-scale production of sulfide electrolytes is essential for the commercialization of ASSBs. However, the synthesis of sulfide electrolytes is complex and requires a precise control of reaction conditions. High-temperature solid-state methods and liquid-phase synthesis are the two main approaches, each with its own advantages and challenges. Cost control is also a significant factor, as the raw materials for sulfide electrolytes, particularly Li₂S, are expensive. The fabrication of thin, uniform, and mechanically robust electrolyte membranes is crucial for ASSB performance. Wet and dry processing methods are commonly used, but each has limitations. Wet processing can lead to solvent-induced degradation of the electrolyte, while dry processing faces challenges in achieving uniform mixing and thin film formation. The assembly of ASSBs involves stacking multiple layers of electrodes and electrolyte membranes. Ensuring a good contact between these layers is critical for battery performance. However, the solid nature of the components makes it challenging to achieve uniform pressure distribution during stacking, leading to increased interfacial resistance.

All-solid-state lithium batteries (ASSLBs) emerge as a pivotal direction for next-generation energy storage systems due to their high energy density, intrinsic safety, and extended cycle life. Lithium-rich manganese-based layered oxides (LLOs) with their exceptional specific capacity (i.e., >300 mA·h/g) and cost-effectiveness are regarded as a promising cathode candidate for high-energy-density ASSLBs. However, critical challenges such as poor solid-solid interfacial contact between LLOs and solid electrolytes, irreversible lattice oxygen loss, and interfacial side reactions hinder their practical implementation. This review comprehensively analyzes the structural characteristics of LLOs, the anionic oxygen redox (OAR) mechanism, and interfacial challenges in ASSLBs, while systematically summarizing modification strategies across sulfide-, halide-, oxide-, and polymer-based solid electrolyte systems.

The high capacity of LLOs primarily originates from OAR, where reversible O^2-/O^- redox contributes to extra capacity. However, oxygen release and transition metal (TM) migration lead to voltage decay and structural degradation. To address these issues, gradient doping and surface coating are developed to stabilize lattice oxygen and suppress phase separation. Compared to polycrystalline materials, single-crystal LLOs exhibit superior mechanical stability and interfacial contact, effectively mitigating crack propagation caused by volume changes.

In sulfide-based systems, the space charge layer (SCL) effect and sulfur oxidation at high voltages are the main limiting factors. Strategies such as LLOs surface sulfurization, uniform dispersion by liquid-phase mixing, and functional coatings can effectively reduce the interfacial resistance and enhance the OAR reversibility. For halide electrolytes, the introduction of carbon additives and ion-conductive coatings can establish a continuous conductive transport network. Oxide-based systems benefit from co-sintering LLO with garnet-type Li₇La₃Zr₂O₁₂ and Li₃BO₃ sintering aids to improve interfacial densification, although Mn/La interdiffusion in co-sintering requires further attention. Polymer electrolytes, especially those formed by in-situ polymerization of propane sultone-based materials, are able to form a thin and uniform cathode-electrolyte interface (CEI), thereby widening the electrochemical stability window.

A critical finding across these systems is the importance of mechanical-electrochemical coupling at interfaces. "Soft-contact" interfaces with flexible ion-conductive layers are essential to accommodate volume changes. Halide electrolytes have a unique compatibility with LLOs via minimizing SCL effects, while sulfide systems demand a precise control of oxidation-prone components to prevent degradation.

The advancement of the global new energy revolution has placed stringent demands on energy storage technologies, emphasizing the need for high-energy-density and high-power electrochemical devices to enable the grid-scale and public infrastructure energy storage. All-solid-state batteries (ASSBs) exploiting solid ionic conductors as electrolytes offer advantages of high-energy-density, enhanced safety performance and long cycle life, which are considered as a crucial direction for the development of next-generation electrochemical energy storage. Designing and synthesizing solid-state electrolytes (SSEs) with ultrafast ionic conductivity, wide electrochemical windows and excellent mechanical deformability plays a crucial role to realizing the practical application of ASSBs.

To date, various types of inorganic solid electrolytes including oxides and sulfides are extensively investigated, but none of them are successfully combined the advantages of excellent interfacial wettability and outstanding oxidation stability. Oxyhalide SSEs are among the most promising solid electrolytes for the commercialization of ASSBs due to their advantages like good compatibility with high-voltage cathodes, excellent mechanical deformability and remarkable ionic conductivity compared to that of liquid electrolytes. Despite oxyhalide electrolytes use in ASSBs, challenges remain in their practical application, including the scalable preparation, enhancement of humid tolerance and improved compatibility with electrode materials.

In this review, a critical overview of the development, synthesis, ionic conduction mechanisms and challenges of oxyhalide electrolytes is given. Different synthesis routes of oxyhalide electrolytes including the promising hydrate-assisted synthesis method are summarized in detail. Furthermore, the framework structures of various types of oxyhalide electrolytes are analyzed, and the ionic conduction mechanisms in crystalline and amorphous oxyhalide electrolytes are elucidated. The interfacial compatibility of oxyhalide electrolytes with lithium metal anodes and various cathode materials is highlighted, and the latest progress of oxyhalide electrolytes in low-temperature ASSBs is also represented. Finally, future perspectives on designing high-performance oxyhalide electrolytes and their practical applications in ASSBs are provided, aiming to offer guidance for the advancement of oxyhalide-based ASSBs in energy conversion and storage.

The global transition toward renewable energy and electrified transportation has intensified the demand for advanced energy storage systems with high energy density, intrinsic safety, and long-term durability. Conventional lithium-ion batteries (LIBs) in the market face critical limitations, i.e., organic liquid electrolytes are flammable, posing significant safety risks (i.e., thermal runaway and combustion), and their energy density is approaching theoretical limits (i.e., ~300 W·h·kg^-1). These shortcomings hinder their applicability in emerging technologies such as electric vehicles (EVs), grid-scale storage, and high-power portable electronics. All-solid-state batteries (ASSBs), which replace liquid electrolytes with solid-state electrolytes (SSEs), offer a revolutionary solution. SSEs eliminate flammability risks, enable the use of lithium metal anodes (i.e., theoretical capacity of 3860 mA·h·g^-1), and facilitate compact cell designs through bipolar stacking. Among SSE candidates, halide-based electrolytes emerge as a game-changing material class due to their unique advantages like ultrahigh ionic conductivity, high voltage compatibility, mechanical adaptability, and scalable synthesis. The urgency to develop halide SSEs is further underscored due to the growing need for safe, high-energy-density batteries in critical applications. This article provides a comprehensive and updated review of the classification, structure, synthesis methods, air stability, mechanical stability, and applications of halide electrolytes at both cathode and anode interfaces.

Structural design is pivotal in developing superionic conductors as it serves as a primary determinant of ionic conductivity. Effective strategies to enhance lithium-ion conductivity include regulating the concentration and rational occupation of lithium ions and lithium vacancies within the electrolyte. Halide-based electrolytes exhibit an exceptional structural tunability, providing a foundation for designing novel electrolytes with optimized properties. Among these strategies, elemental doping stands out as one of the most common approaches to improve ionic conductivity in halide electrolytes. In recent years, high-entropy materials, characterized by their adjustable chemical compositions and unique properties, indicate widespread applications in various fields. Introducing high-entropy strategies into SSEs can effectively boost ionic conductivity through chemical disorder and local lattice distortions, while reducing reliance on rare-earth elements. In addition, amorphous SSEs have attracted much attention due to their outstanding mechanical flexibility and rapid ion transport capabilities, which arise from the decoupling of charge carriers from the supporting matrix. This review also highlights the synthesis methods for halide-based electrolytes. Simple, cost-effective, and environmentally friendly synthetic routes are crucial for advancing the industrialization of these materials. The existing approaches primarily involve solid-state, liquid-phase, and vapor-phase synthesis techniques, each of which significantly affects the resulting electrolyte structure and ionic conductivity.

Air stability is crucial for SSEs, impacting their entire lifecycle from synthesis to application. Halide-based electrolytes, derived from hygroscopic halide salts, are prone to moisture-induced degradation during synthesis and storage. To enhance their air stability, some methods like surface coating and structural modification are employed. For instance, creating physical barriers, superhydrophobic membranes, or doping with In³⁺ can significantly improve moisture resistance. In ASSBs, inorganic SSEs must have good electrochemical properties and excellent mechanical properties. Bulk modification as a common approach to improving halide-based electrolytes performance also boosts mechanical stability. For instance, O^2- doping can disrupt the crystal structure of electrolyte, making it more ductile. In addition, adjusting the particle size of halide-based electrolytes and combining them with binders to form organic-inorganic composite membranes can also enhance their mechanical properties.

As a component in direct contact with the positive and negative electrodes, understanding the electrochemical stability and interfacial reactions of halide SSEs is crucial for achieving high-energy-density and long-cycle-life halide-based ASSBs. For high-voltage cathodes, halide SSEs are among the most promising. The oxidation potential of halide SSEs mainly depends on the halogen anion, especially for chlorine- and fluorine-based halides, whose high electronegativity prevents oxidation at battery operating voltages. To further enhance the cathode interface stability of halide SSEs, element doping can broaden the thermodynamic electrochemical window or form a stable cathode-electrolyte interphase (CEI) in-situ. Structural control, like reducing the SSE's Young's modulus and particle size or coating the SSE on the cathode material surface to improve solid-to-solid contact in composite cathodes, is also effective. The instability of halide SSEs with metal lithium mainly stems from the transition metals in the halide. The high potential difference between halide SSEs and lithium metal makes direct contact with lithium or lithium alloy anodes extremely unstable. This leads to high-resistance interfacial layers that impede Li⁺ diffusion and cause SSE degradation, triggering lithium dendrite growth. To stabilize the halide SSE/anode interface, constructing interfacial buffer layers, optimizing SSE structure, and developing lithium alloy anodes are essential strategies.

With the rapid advancement of portable electronic devices, electric vehicles, and large-scale energy storage grids, next-generation rechargeable batteries with higher energy densities and enhanced safety have attracted much attention. Solid-state lithium metal batteries with high-safety solid-state electrolytes and lithium metal at the maximum specific capacity as an anode hold a tremendous potential for the next generation of rechargeable batteries. As a critical component of solid-state lithium metal batteries, solid-state electrolytes play a pivotal role in advancing their development. The existing solid-state electrolyte materials (i.e., organic polymer electrolytes, oxide electrolytes, sulfide electrolytes, and halide electrolytes) are systematically investigated, with the oxide solid-state electrolytes standing out due to their excellent overall performance. To achieve the practical application of oxide solid-state electrolytes, substantial research efforts are directed at modifying ceramic grain boundaries to improve ionic conductivity and mechanical properties and reduce electronic conductivity. Also, research on ceramic films is equally important to achieve a high energy density.

The sintering process of ceramics is a critical factor affecting material properties, especially for the improvement of grain boundary. Relative density, grain boundary strength, and ionic/electronic conductivity are dominant factors impacting the long-term stability of the battery, with relative density as the most fundamental performance of ceramic electrolyte that enables other characteristics to be manifested effectively. The regulation of the preparation process, including powder preparation method, ceramic sintering temperature, and atmosphere, can effectively enhance the ceramic relative density. The powder preparation route, in particular, has a considerable effect on the final properties of LLZO ceramic, such as grain size and size distribution, relative density, microstructure, and ionic/electronic conductivity. In the sintering, some parameters such as sintering temperature, dwelling time, pressure, and the lithium oxide atmosphere play a crucial role in promoting the densification of LLZO ceramic.

Atmospheric sintering is still the most common, simple, and low-cost method to prepare LLZO ceramics although it has a longer sintering time and a higher sintering temperature, compared to the pressurized sintering. The prolonged high-temperature process leads to lithium loss and the formation of impurity phases like La₂Zr₂O₇ in the electrolyte, which is detrimental to reducing the densification and ionic conduction behavior of the ceramics. To promote the densification and lower the sintering temperature, various kinds of sintering aids are adopted to assist the sintering process. A variety of low melting point additives, such as Li₃BO₃, LiO₂-B₂O₃-SiO₂-CaO-Al₂O₃, etc., are used as ceramic sintering aids. Subsequently, optimization of sintering processes and sintering aids are emerging, such as embedding-sintering, atmosphere-reversible control additives, lithium source-green body separation sintering, endogenous atmosphere sintering aids, etc.. LLZO ceramics sintered in the endogenous atmosphere with sintering additive of Li₆Zr₂O₇ can realize the densities of up to 97.21% in the absence of mother powder and under atmospheric pressure.

In highly densified LLZO ceramics, dendrite growth and even piercing of the ceramic sheet are still inevitable, indicating that dendrite growth is related to ceramic relative density and ionic conductivity and is also closely related to the electronic conductivity and fracture toughness of ceramics. Multifunctional additives are reported to regulate the compositions, microstructures, as well as physical and electrochemical properties at the internal grain boundaries of LLZO ceramic in addition to improving the relative density and ionic conductivity of the ceramics. Among them, (Li₂O)_0.733(ZrO₂)_0.267, Li₆Zr₂O₇, Li₂WO₄, Li₂CuO₂, LaTiO₃, etc., are considered as effective sintering additives.

To realize the high energy density of solid-state lithium metal batteries, thin-filming of ceramic electrolyte has attracted recent attention. Since the reduction of the thickness of LLZO ceramic leads to a decrease in the mechanical properties of the ceramics, a balance between the thickness and strength of the electrolyte is of great significance. Realizing the thin-film of LLZO ceramic through rational structural design is crucial to achieving a high energy density of solid-state batteries. In this case, ceramic films with different structures, such as single-layer dense ceramic, multilayer ceramic, and single-layer porous ceramic, are reported. A lithium-metal battery based on an interface-optimized 74 μm-thick single-layer dense LLZO ceramic has the excellent long-cycle stability of more than 800 cycles. A lithium-symmetric battery based on an optimized 115 μm-thick multilayer ceramic exhibits an ultra-high critical current density of 100 mA·cm^-2. A 12 μm-thick single-layer porous ceramic composited with a polymer electrolyte is expected to achieve energy densities of greater than 350 W·h·kg^-1. In addition, the quality of the green body (i.e., uniformity, flatness, and residual stress) and the sintering regime have a significant effect on the phase purity, ionic conductivity, surface state, and film flatness of LLZO after sintering due to the high sintering temperature of LLZO as well as the volatile properties of Li, La, Zr, and O in the composition. Therefore, some novel preparation processes to prepare ceramic films are developed in recent years. In particular, the combination of tape-casting and ultrafast-sintering has a promising application in the preparation of LLZO ceramic films.

The rapid expansion of lithium-ion batteries in consumer electronics, electric vehicles, and energy storage systems escalates a demand for batteries with higher energy density and enhanced safety. However, conventional lithium-ion batteries employing liquid electrolytes are increasingly constrained due to their limited electrochemical stability, susceptibility to leakage, and lithium dendrite formation, which pose risks of short circuits and thermal runaway. Solid-state batteries (SSBs) with solid electrolytes present a promising solution to these challenges, offering a potential for higher safety and energy density.

Recent advancements in SSBs include the development of solid electrolytes with a high ionic conductivity, solid-solid interface optimization, and composite electrode design. Nonetheless, some issues such as lithium-ion transport barriers across phases, interfacial impedance, and the performance limitations of thick electrodes continue to impede the commercial viability of SSBs. To address these multifaceted challenges, a concept of "lithium-ion transport throughput" is proposed as a comprehensive descriptor for evaluating SSB performance. This descriptor quantifies the quantity of lithium ions transported across the electrode/electrolyte interface per unit area over time, considering some factors such as areal capacity and charge/discharge rates.

This review systematically examines strategies to enhance lithium-ion transport throughput from three key perspectives, i.e., bulk ionic transport in solid electrolytes, electrode/electrolyte interface design, and synergistic ionic/electronic transport networks within electrodes. we aim to offer insights into improving the overall performance of SSBs to meet the demands for high safety and energy density in next-generation energy storage systems via integrating materials design with structural optimization. In addition, we also conduct quantitative calculations to analyze lithium-ion transport throughput in recently published high-performance SSBs, providing a detailed comparison of various systems. We illustrate how material advancements and interface designs affect overall performance via compiling representative data (i.e., areal capacity, charge/discharge rates, and resulting throughput values). These calculations highlight trends and identify strategies that can achieve significant throughput improvements, offering a robust foundation for future optimization efforts.