INTRODUCTION
The advancement of large-scale energy storage technologies is critical for achieving comprehensive decarbonization of the power grid.1–7 Electrochemical energy storage is the primary representative among these technologies and has experienced a nearly threefold increase in the past 5 years, demonstrating a significant upsurge.8,9 Diverse large-scale electrochemical energy storage technologies encompass flow batteries, lead-acid batteries, sodium–sulfur (Na–S) batteries,10 supercapacitors, and lithium-ion batteries (LIBs), with LIBs being the predominant form, constituting 90.9% of the total installed electric capacity in 2021.11 However, each existing technology harbors its drawbacks; for instance, lead-acid batteries suffer from low energy density and short lifespan, while Na–S batteries pose high safety risks operating at high temperatures.12 Furthermore, the progression of LIBs is hindered by various factors, including low Li reserves, uneven distribution of Li-ores, and escalating costs, which impede their further development.12–14 Consequently, Na-ion batteries have been recognized as among the most promising alternatives to LIBs due to their abundant Na-availability, cost-effectiveness, and similar electrochemical mechanism (resembling LIBs).2,15–19 However, most existing Na-ion batteries utilize liquid organic electrolytes, which still exhibit safety concerns akin to LIBs, including electrolyte leakage, weak thermal stability, flammability, and explosibility.20,21 Compared with organic liquid electrolytes, solid-state electrolytes (SSEs) do not have the aforementioned disadvantages, and they can also inhibit the growth of dendrites to prevent internal short circuits. Therefore, solid-state sodium batteries (SSSBs) with Na anodes exhibit high energy densities and are thus anticipated to emerge as the future key energy storage technologies, considering their high performance, safety, and stability.22–26
SSEs, the key components of SSSBs, act both as ion conductors and physical barriers that separate the cathode and anode. However, their low ionic conductivity at room temperature (RT) and poor electrolyte/electrode interfacial compatibility hinder the practical applicability of SSSBs. Therefore, with recent developments, the fabrication of SSEs, with high RT ionic conductivities, high ionic transference numbers, high chemical/electrochemical stability, favorable mechanical properties, and good interfacial compatibility, using simple methods has become feasible.27 Typically, SSEs are categorized into two types: inorganic solid electrolytes (ISEs) and solid polymer electrolytes (SPEs). The ISEs, consisting of oxides,28–32 sulfides,33 hydrides34,35 and halides,36–38 possess a high ionic conductivity, good mechanical properties, high chemical/electrochemical stability, and high thermal reliability; however, given their high mechanical rigidity and roughness, hard–hard contact with the electrode is inevitable.39–42 In contrast, the contact between SPE and electrodes is much better, and SPEs are much easier to fabricate with superior film-forming properties. Nevertheless, most of them have a low RT ionic conductivity, poor chemical/electrochemical stability, and inferior mechanical strength.43–45 In addition to the low ionic conductivity, another stumbling block for SSSBs is the change in the electrode volume during cycling, which leads to loss of contact at the SSE–electrode interface, leading to the shortening of ion-transport pathways.46–48
Moreover, for developing high-performance SSSBs, investigating the interface between the SSE and electrodes is essential. Compared with liquid electrolytes, the change in the electrode volume during the cycling of SSSBs leads to loss of contact at the SSE/electrode interface, and consequently, the number of ion-transport pathways decreases, which hinders the practical application of SSSBs. Additionally, the interface compatibility between the SSEs and electrodes should be considered. Therefore, creating an intimate and stable interface between the SSEs and electrodes is another key requirement for realizing high-performance SSSBs.
Despite the extensive efforts invested to date to improve SSEs, single-state SSEs cannot fulfill the aforementioned requirements. Therefore, the focus has shifted to composite SSEs (CSSEs) to eliminate the drawbacks of single-state SSEs and develop SSEs with enhanced properties.49–52 Although SSSBs have been reviewed extensively from different viewpoints, our classification method may provide a new perspective on the design of novel SSBs. Hence, summarizing the recent advances in CSSEs and their interfaces with electrodes is necessary to elucidating the governing mechanisms.
In this review, we classify CSSEs into three groups, namely organic–inorganic (II), polymer–polymer (PP), and inorganic–polymer types (IP) composites, and discuss the relationship between the structure and performance of these CSSE types. In addition, their performance in SSSBs is analyzed in terms of ionic conductivity, chemical/electrochemical stability, and other key parameters (Figure 1). In addition, this review enumerates the inherent shortcomings of various CSSEs, presents the characteristics of different electrolyte–electrode interfaces, and summarizes effective strategies for constructing intimate and stable interfaces. Finally, the review outlines the trends in the design of high-performance CSSEs and CSSE–electrode interfaces and briefly describes future SSSB integration technologies for mass production and commercial applications. This review has been conducted to attract the attention of the global academic and industrial community and present the practical application of SSSBs.
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FUNDAMENTALS OF SSES
Utilization of SSEs in Na batteries is a feasible approach for mitigating the safety concerns inherent in conventional liquid electrolytes, including volatility, leakage, combustion, and explosion risks. Moreover, owing to their exceptional mechanical, thermal, and chemical stability, SSEs increase battery life and stability as well as facilitate the integration of high-specific-capacity cathodes and metallic Na anodes within SSSBs. However, SSSBs still exhibit low ion conductivities at ambient temperatures, constrained electrochemical windows, inadequate compatibility with electrode interfaces, and poor interfacial contacts. Hence, improving the Na+ conductivity, enhancing interface compatibility and stability, and reducing the interfacial impedance of SSEs are essential primary steps for improving the performance and commercial viability of SSSBs. Continuous research and development efforts are required to overcome these challenges and unlock the full potential of SSSBs in advancing energy storage technologies.
Key parameters of SSEs
Understanding the key physicochemical parameters of SSEs, including their ionic conductivity, ion transference number, electrochemical stability, and critical current density (CCD), is the first step toward designing SSEs with superior performance.
Ionic conductivity
Ionic conductivity is influenced by the number of mobile charge carriers (such as Na+ ions) per unit volume and the presence of structural defects that can facilitate their mobility.53 Typically, ionic transport in ISE materials follows the Arrhenius equation (Equation 1):
Here, σ represents the ionic conductivity; σ0 denotes the Arrhenius pre-exponential factor, which is related to the number of charge carriers; T is the absolute temperature; EA indicates the Na+ diffusion activation energy; and kB signifies the Boltzmann constant. Therefore, the overall conductivity hinges upon several elements, including the diffusion activation energy, ambient temperature, and the pre-exponential factor. Several methods, such as reducing the steric effect of polymer chains, increasing the number of mobile Na ions, and constructing fast Na-ion transport channels, can be used to achieve high ionic conductivity. However, there is a fundamental difference between the ionic transport mechanisms of SPEs and crystalline ISEs. Na+-ion transport in SPEs is achieved through the hopping of ions, either intrachain or interchain, an intertwined process with the motion of polymer segments with a series of coordination sites in succession.54 Generally, the SPE ionic conductivity follows the Vogel–Tammann–Fulcher (VTF) equation (Equation 2):
Here, B is the pseudo-activation energy expressed in units of Ea/k, and T0 denotes the reference temperature, which is ~50 K below the experimental glass-transition temperature (Tg). Most SSEs have relatively higher Tg compared with RT, with VTF behavior usually observed above the Tg of the matrix. In SPE, higher ionic conductivity can be achieved by reducing the degree of polymer crystallinity and increasing the concentration of dissociated Na+ ions.
For CSSEs, the comprehensive conductivity is a product of the specific compositions and structure of the components. For instance, in binary SPEs (polymers doped with Na salt) or nanofiller–polymer electrolytes (comprising polymers, Na salts, and additives), the ionic conductivity is often modeled by VTF.
Electrochemical impedance spectroscopy (EIS) is a common technique to evaluate the ionic conductivity of SSEs. The ionic conductivity of test batteries composed of an SSE and two stainless steel ion-blocking electrodes can be calculated as follows:
Na-ion transference number
The transference number (t+) of cations—specifically, Na+, is also crucial for characterizing SSEs. The ionic transfer number is defined by the following equation:
Here, iss represents the steady-state current; i0 denotes the initial current; and R0 and Rss signify the initial and steady-state resistances, respectively; and ∆V indicates the applied constant potential, which is usually maintained below 10 mV. Specifically, an SSE with a high t+ can demonstrate more effective Na+-ion transport and fast charge–discharge capability. SPEs typically have transference numbers below 0.5, indicating significant anion mobility, which can detract from battery performance.55 In contrast to the SPEs, ISEs are usually employed as single Na+-ionic conductors, resulting in a Na+-transference number that approaches 1.58
Electrochemical stability window
The electrochemical window is one of the critical parameters that define its suitability for SSEs. It is the voltage range within which the electrolyte remains stable without decomposing due to chemical reactions. Specifically, SSEs with more expansive electrochemical windows are better in high-voltage systems and will increase the energy density of SSSBs. The conventional approach for evaluating the electrochemical window of an SSE involves cyclic voltammetry or linear sweep voltammetry. This method entails using cells comprising an SSE, Na metal as the reference and counter electrodes, and a blocking working electrode.59
CCD
The maximum available current density of an SSSB is defined as CCD, a crucial index for evaluating the ability of SSEs to inhibit dendrite formation.60 Symmetric cells with nonblocking Na-electrodes are commonly employed to determine the CCD of an SSSB by gradually increasing the current. However, with the development of SSSBs, there are an increasing number of measurement protocols for CCD, yielding different CCD results. Consequently, the lack of practical reasonableness and consistency in the CCD collection results is due to the nonstandardized evaluation criteria. In particular, the magnitude of the CCD depends not only on the nature of the solid-electrolyte interface (SEI) resulting from the reduction of the SSE but also on factors like SSE thickness, applied stacking pressure, and the efficiency and timing of alkali metal plating and stripping processes.
Accordingly, the wide distribution of CCD values reported for studies employing the same SSE makes it challenging to assess the ability of the electrolyte to inhibit dendrite growth. Recently, Wang et al.61 evaluated the ability of SSEs to inhibit Li-dendrite formation using critical interphase overpotential (CIOP). CIOP, being an intrinsic electrolyte property, excludes the effects of testing pressure, SSE thickness, SSE densification, and the Li anode. Thus, CIOP offers design guidelines for high-energy-density and RT SSLBs and provides insights for SSSB design.
Mechanical properties
Mechanical properties such as strength, toughness, brittleness, hardness, and resilience influence the mechanical strength of a material and its ability to be molded in a suitable shape.62 In SSSBs, the mechanical strength of SSEs plays a crucial role in inhibiting dendrite growth and thus achieving a long cycle life, a high energy density, and high safety.63 The mechanical strength of SSEs can be characterized using Young's modulus (E, MPa) and shear modulus (G, MPa), which are defined in Equations (7) and (8), respectively.64
Here, Vl is the longitudinal velocity, ρ is the density, and υ is Poisson's ratio. These parameters can be evaluated through instrumental tests such as nanoindentation. Currently, the mechanical properties of SSEs are mostly tested at RT, whereas SSSBs are usually operated at higher temperatures, which may greatly weaken their mechanical strengths.
Consequently, examining the alterations in the mechanical properties of SSEs at different operational temperatures is crucial. In addition to Young's and shear moduli, parameters such as maximum stress, strain at break, and toughness are adopted as indicators of the mechanical characteristics of SSEs.
Battery performance under extreme conditions
Na batteries that can exhibit a robust performance at extremely low and high temperatures as well as can withstand physical damage due to collision and other factors are desirable for practical applications, including vehicles that operate at extreme conditions. In the case of liquid electrolyte-based batteries, the liquid electrolyte undergoes operational failure and hinders the kinetics of the electrolyte reactions in cold conditions, whereas at high temperatures, electrolyte decomposition and Na metal melting may be inhibited. By contrast, some SSEs do not undergo phase transition even at low temperatures.65 For example, some polymers, such as PFSA-Na membranes, exhibit an extremely low Tg, which ensures that the corresponding cell maintains a high performance even at −35°C.66 In addition, some inorganic electrolytes doped with other elements can operate at low temperatures.
Notably, some inorganic SSBs can reportedly sustain high temperatures; for example, commercial high-temperature Na–S batteries employ oxide-based electrolytes to operate at temperatures ranging from 300°C to 350°C. Nevertheless, the highly corrosive nature of molten Na metal and Na polysulfide are major safety concerns. At high temperatures, all inorganic and organic electrolytes show fast ionic conduction, whereas most SPEs undergo mechanical failure.
An effective SSB should sustain a high ionic conductivity, exhibit a low interphase resistance, ensure optimal interfacial contact, and maintain a high ionic diffusivity within its electrodes.67 Current research on solid-state ion batteries primarily concentrates on their performance at ambient conditions. Therefore, evaluating the characteristics of these batteries under extreme conditions is essential for understanding their full potential and application scope.
Single-state electrolytes
ISEs
ISEs, often called “ceramic electrolytes,” demonstrate the capability to conduct Na+ ions. ISEs provide a wide range of benefits, including strong thermal stability at ambient temperature, broad electrochemical window (0–5 V), and reasonably high ionic conductivity (>10–4 S cm–1).68 Furthermore, the mechanical integrity of ISEs contributes to the suppression of Na dendrite growth, thereby enhancing the battery cycling stability. However, interfacial Na+ transport was severely limited due to the poor interfacial compatibility between the electrodes and rigid ISEs, resulting in elevated interfacial resistance.63,69 Additionally, the unstable electrochemical interface between the electrodes and ISEs leads to unavoidable side reactions and poor battery cyclability.51 Therefore, the interface compatibility and stability between the electrodes and ISEs emerge as critical limiting factors in developing SSSBs.
Similar to the migration mechanism of Li-ions in crystalline ISE materials, Na-ion transport relies heavily on the concentrations of mobile ions and vacancies, aligning with Schottky and Frenkel defects models.70 Specifically, there are three principal transport mechanisms in the crystal structures: (1) migration of mobile Na+-ions from one vacant site to an adjacent vacant site; (2) migration of mobile Na+-ions from one interstitial site to an adjacent interstitial site, where the interstitial ion size is typically much smaller than that of the anionic sublattice; and (3) movement of mobile interstitial Na+ entering neighboring sites via substituting adjacent lattice ions. Moreover, Na+-migration in amorphous materials mirrors that in crystalline materials. Due to the unique short-range ordered structure in amorphous materials, charge carriers can effectively move between more active sites, enhancing ionic conductivity, a characteristic commonly observed in glassy S-based ISEs. The most prevalent Na-ion ISEs for SSSBs are oxide-based (e.g., BASEs and NASICONs), sulfide-based, hydride-based, and halide-based ISEs.
Na-β-Al2O3 (BASE)
In the 1960s, BASE exhibiting high ionic conductivity found successful application in high-temperature Na–S batteries and subsequently in large-scale grid energy storage systems.28–30 BASE is classified into two crystal structures with an alternating configuration with aluminum-oxygen (Al–O) spinel block and Na+-conduction planes.71,72 β-Al2O3 has a chemical formula of Na2O (8-11) Al2O3 and features a hexagonal structure (P63/mmc; a = b = 5.58 Å; and c = 22.45 Å), containing one Na+-conduction plane and two spinel blocks. Another structure, β״-Al2O3, with the chemical formula Na2O·(5 − 7)Al2O3, exhibits a rhombohedral structure (R-3m, a = b = 5.61 Å; and c = 33.85 Å), and is composed of two Na+-conduction planes and three spinel blocks (Figure 2A).71,72 β״-Al2O3 has a higher ionic conductivity than β-Al2O3 due to a greater concentration of Na+ in the conduction plane and due to a difference in the stacking of oxygen atoms within the spinel blocks.77 At 300°C, the ionic conductivity of monocrystalline β״-Al2O3 can approach 1 S cm–1, but that of polycrystalline β״-Al2O3 can only reach 0.2–0.4 S cm–1.77–79 This is mainly because polycrystalline β״-Al2O3 has a high inherent grain-boundary resistance. However, despite its high ionic conductivity, the pure β״-Al2O3 phase is sensitive to water (or H2O) and challenging to prepare owing to its unfavorable thermodynamic stability.49,77,80 β״-Al2O3 is synthesized by solid-state reactions, co-precipitation techniques, sol-gel processes, solution combustion, spray-freeze/freeze-drying, microwave heating, and mechanochemical methods.81–85 Nonetheless, it is difficult to obtain pure β″-Al2O3 through traditional solid-state reactions or solution chemistry, as these methods often result in unwanted impurities, such as β״-Al2O3 and sodium aluminate (NaAlO2), at the grain boundaries. β-Al2O3 generated in the high-temperature reaction reduces the ionic conductivity of β״-Al2O3. The resulting NaAlO2 byproduct is sensitive to water and carbon dioxide (CO2).86 As a result, the commercially available BASE is characterized by a β/β״-Al2O3 mixed phase, with the ionic conductivity primarily influenced by the β/β״ phase ratio and microstructures of these phases. Various modification techniques, including optimization of the synthesis and processing conditions, have been proposed to enhance the performance of BASE electrolytes.
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Traditional sintering methods, which can effectively form dense and continuous conducting structures, are characterized by several drawbacks, such as Na evaporation and the formation of unwanted NaAlO2 after prolonged exposure to high temperatures. Because both high ionic mobility and a well-connected ionic transportation network are essential for achieving high ionic conductivities in β-Al2O3 electrolytes, to date, various innovative sintering techniques have been developed to overcome the shortcomings of the traditional methods. Li et al.87 employed spark plasma sintering (SPS) to create a highly oriented rod-like β״-Al2O3 structure. This method notably reduces the sintering time to just 10 min at 1300°C and the orientation of the rod-like structure aids in the migration of Na+ ions, resulting in an electrolyte with a remarkably high ionic conductivity of 18.3 mS cm−1 at 350°C. However, the sintering temperature required for this method is still relatively high. Grady et al.88 explored a different approach by utilizing a hydroxide-based transient solvent in a cold sintering process. This method enables the densification of β″-Al2O3 at just 400°C, which is remarkably lower than traditional sintering temperatures. The resultant β-aluminum oxide demonstrates a high ionic conductivity of 10 mS cm−1 at 300°C. Further development of suitable preparation methods is essential to synthesizing pure β″-Al2O3 with a controllable microstructure, reduced byproduct content, and enhanced yields to improve the comprehensive performance of BASE electrolytes.
NASICONs
The NASICON-type materials, Na1+xZr2SixP3xO12 (0 ≤ x ≤ 3), were first proposed by Hong and Goodenough et al. in 1976. These materials can be derived from a solid solution of NaZr2P3O12 with partial substitution of the P5+ sites by Si4+.31,32 In contrast to ion movement within the two-dimensional (2D) planes of layered Na-β״-Al2O3, the design of NASICONs enables rapid transport of Na ions via open 3D channels. Consequently, NASICONs offer high RT ionic conductivity, excellent thermal stability, and a broad electrochemical window. Notably, Na1+xZr2SixP3−xO12 (x = 2) exhibits the ionic conductivity of 10–4 to 10–3 S cm–1 at RT and rises to 0.2 S cm–1 at 300°C. As shown in Figure 2B,49 Na3Zr2Si2PO12 (NZSP) exhibits two different crystal structures, including rhombohedral (R-3c) and monoclinic (C2/c). Both crystal phases comprise a network formed by corner-sharing ZrO6 octahedra and SiO4/PO4 tetrahedra. The rhombohedral phase contains two sodium sites, Na1 and Na2, creating a 3D framework that permits Na-ion mobility. However, a slight lattice distortion causes a monoclinic crystal phase transition, splitting Na2 into two sites (Na2 and Na3). Figure 2B illustrates the existence of four distinct migration pathways for Na+-diffusion in the monoclinic NASICON structure: two Na1-Na2 and two Na1-Na3 channels. To realize transport, Na+ must pass via the triangular bottleneck zones in the NASICON SSEs, where Na+ diffusion is directly influenced by the bottleneck's size. This factor directly influences bulk conductivity, and its adverse effect on Na-ion diffusion can be mitigated via aliovalent doping or isovalent substitution, which is applied in CSSEs. Boundary resistance is another factor impeding the efficient conduction of Na ions in SSEs. In addition, grain boundaries represent preferential sites for dendritic growth, which can cause battery failure; thus, grain-boundary decoration is another method employed to improve the performance of NASICON SSEs.
Generally, NASICON materials can be synthesized successfully through various methods, each with pros and cons, impacting the quality, purity, and efficiency of the final product, including traditional solid-state reactions, sol-gel, hydrothermal, and spray-freeze/freeze-drying routes.31,32,89–94 Among these, solid-state synthesis of NASICONs at 1000°C often leads to the evaporation and loss of P and Si, as well as the formation of impurities such as zirconium dioxide (ZrO2). Compared with solid-state reactions, sol-gel and hydrothermal techniques generate more homogeneous products with lower ZrO2-impurity contents, producing high-density and high-purity NASICON particles.95 Additionally, sol-gel processing aids in reducing the required temperature for NASICON synthesis to some extent.
Sulfides
Sulfide-based SSEs garner attention as highly advantageous for Na-based batteries due to their exceptional ionic conductivities, manageable production conditions, reduced grain-boundary resistance, and significant flexibility. Sulfur, whose electronegativity is lower than that of analogous oxides and ionic radius is larger than that of the latter, weakens the electrostatic interactions between Na+ and the electrolyte matrix. These weak interactions facilitate the generation of large ion-transport channels, which enhance the ionic conduction at RT.96 Meanwhile, sulfide-based electrolytes exhibit notable plasticity and low grain-boundary resistance. Consequently, the sulfide-based electrolyte powder can be cold-pressed and employed as an SSE without high-temperature sintering to form ceramic platelets. More importantly, the inherent softness of the sulfide-based electrolyte powders plays a pivotal role in securing sufficient contact at the electrode–electrolyte interface, which is essential for the operation of SSSBs. As depicted in Figure 2C (top), Na3PS4 is the most common sulfide-based SSE and consists of two different crystal structures, tetragonal phase (P-421c; a = b = 6.9520 Å, c = 7.0757 Å) and cubic phase (I-43m; a = b = c = 7.0699 Å).73 Typically, Na3PS4 can change from its usual tetragonal stable form to a cubic one at high temperatures. Initially synthesized in a crystalline format by Jansen et al.33 in 1992, Na3PS4 had an ionic conductivity of just 4.17 × 10−6 S cm−1 at 50°C. However, in 2012, Hayashi et al.59 succeeded in synthesizing a glass–ceramic hybrid form of Na3PS4 with a remarkable ion conductivity of 2 × 10–4 S cm–1 at RT. The evolution of Na3PS4's conductivity and its application versatility have therefore made it a focal point of interest in Na battery technology.
Another typical sulfide-based SSE is Na11Sn2PS12, which was successfully realized by Dehnen and Roling in 2018.74 The unit cell of Na11Sn2PS12 is indexed in a tetragonal structure with a space group of I41/acd (a = 13.6148(3) Å, c = 27.2244(7) Å), as presented in the Figure 2C (bottom). In Na11Sn2PS12, the unique chessboard-like 3D framework comprises an ordered arrangement of SnS4 and PS4 tetrahedra, thus guaranteeing a continuous Na+-migration channel. In addition, the Na+ vacancies in Na11Sn2PS12 facilitate high-speed Na+-diffusion, indicating the material's low activation energy of 0.25 eV. Coupled with the high ionic conductivity of 1.4 × 10–3 S cm–1 at RT, as reported by Nazar et al.,97 Na11Sn2PS12 emerges as a promising electrolyte material for developing efficient and potentially stable Na-ion batteries.
However, sulfides are highly unstable in environments containing moisture and can generate toxic H2S; these factors decrease the ionic conductivity of the SSE. Additionally, the electrochemical window is relatively narrow, resulting in undesired side reactions at the electrolyte/electrode interface. Numerous studies have been conducted to develop composite sulfide electrolytes that can overcome these limitations.40
Hydrides
Udovic et al.98 first reported the conduction of Na+ in hydrides. At RT, the ionic conductivities of two alanate SSEs, NaAlH4 and Na3AlH6, have been demonstrated to be 2.1 × 10–10 and 6.4 × 10–7 S cm–1, respectively. Subsequently, Na-conducting hydride SSEs have emerged as promising candidates for SSBs and garnered considerable attention. For example, based on studies of borohydrides including the [BH4]–, [B10H10]2–, and [B12H12]2– systems, such structures exhibit enhanced polyanion rotation for efficient ion transfer via a paddle-wheel mechanism (Figure 2D).75 Specifically, sodium borohydride (NaBH4) exhibits distinct crystal structures under various temperature and pressure conditions, including P-421c, Fm-3m, and F-43m. Matsuo et al.99 employed AC-impedance measurements to investigate Na+-conductivity in NaBH4, revealing a low conductivity of 2 × 10–10 S cm–1 at RT and an activation energy of 0.61 eV. Duchêne et al.47 proposed another type of superconductive hydride, Na2B10H10, with a pseudo-face-centered cubic framework in a monoclinic phase (P21/c, a = 6.65 Å, b = 13.13 Å, c = 11.8 Å, with β = 120.20°). The work on disordered Na2B10H10 demonstrated how larger anions in complex hydrides (B10H10− or B12H12− compared with BH4−) can exhibit significant increases in ionic conductivity above order-disorder phase transition temperature (375 K).75 The disordered state of Na2B10H10 shows a high ionic conductivity (0.01 S cm–1) and a low activation energy (0.19 eV). Similarly, Na2B12H12 exhibits strong conductivity (>0.1 S cm–1) due to its disordered, body-centered cubic shape and plenty of cation vacancies at 573 K.
Hydride-based electrolytes exhibit poor ionic conduction at RT; however, these materials undergo a phase transition, and the ordered phase transforms into a disordered high-ionic-conductivity phase at about 200–300°C. Various studies have been conducted to enhance the stability of the disordered phase at RT via doping with other anions or by increasing the degree of disorder using mechanical methods such as ball milling. Despite these hurdles, hydride-based electrolytes remain promising candidates for Na-ion SSEs.49
Halides
In recent years, Li-ion halide SSEs with high ionic conductivity and voltage stability have attracted considerable attention, sparking interest in Na-ion halide-based SSEs. By their greater electronegativity relative to O and S, halogens contribute to better oxidative stability. Additionally, halogen ions possess larger ionic radii than oxides and chalcogenides, resulting in longer ionic bond lengths and higher polarizability of halide-based SSEs. This characteristic is anticipated to facilitate ion motion and promote greater deformability. Emerging as a potent competitor to the more conventional oxide and sulfide-based SSEs, halide-based SSEs are better in ionic conductivity, thermodynamic equilibrium, deformability, and oxidation stability. Contrary to the high environmental sensitivity of sulfide-based electrolytes, especially to moisture, halide-based SSEs exhibit stability in moist conditions. However, most of the investigated halide-based SSEs have focused on Li-ion systems, with only a limited number exploring Na-ion systems.
The typical composition of halide-based Na-ion SSEs comprises Na3−xM1−xM′xX6, wherein M can be chosen from 3d transition metals, rare-earth elements, icosogen elements, pnictogens elements, or lanthanides. M′ can be zirconium (Zr) for partial or complete replacement of the M site. X is a halogen element, such as chlorine (Cl), bromine (Br), or iodine (I). Moreover, the structural characteristics of Na-ion conductive halides are opposed to those of Li-based ones, and it is rooted in the difference in ionic radii between Na+ and Li+.37,76,100 The common crystal structures of Cl-based and Br-based SSEs include the triangular P31c, the monoclinic P21/n, and the triangular R3 phases. These structures primarily depend on the type and ionic radius of the M cation in Na3MX6 (Figure 2E).76 However, previous studies have relied on theoretical calculations without experimental verification.
Although the ionic conductivity of halide-based SSEs is relatively higher than those of other types of SSEs, their performance, particularly at RT and low temperatures, is still insufficient for practical applications. Additionally, the limited migration of Na ions within these materials, compared with that in liquid electrolytes, remains a major drawback.101 A more effective solution could involve mixing anions or modifying the lattice structure to create CSSEs, which exhibit high ionic conductivities and high performances.
Polymer solid electrolytes
Unlike ISEs, SPEs have been used extensively in SSSBs because of their mechanical flexibility, lightweight, low cost, ease of manufacturing, and ability to accommodate volume changes in electrodes during charge–discharge cycles. Typically, SPEs are made by dissolving Na salts in a polymeric matrix. Polar functional groups in polymers, such as –O–, ═O, –P–, –N–, –S–, C═O, and C═N, are crucial in promoting Na-salt dissolution and the transport of Na ions.102 It is generally accepted that Na-ion transport in SPEs mainly occurs in the amorphous regions of the polymer matrix, where the movement of long-chain segments is less constrained compared with the crystallization regions. For example, polyethylene oxide (PEO) is the most commonly employed polymer host in SPEs owing to its low cost, availability, and excellent film-forming ability.103,104 Na+ can coordinate with the polar ether O groups present in PEO,105–107 and the motion of long-chain segments induced by an electric field provides adequate free volume for Na+ migration (Figure 2F). In addition to PEO, other polymer hosts include PAN, poly(vinylidene fluoride) (PVDF), and poly(vinylidene fluoride–hexafluoropropylene) (PVDF–HFP).108,109 However, SSSBs may be at risk for safety concerns since most SPEs-based SSSBs must operate at high temperatures between 60°C and 80°C, which is considerably closer to the melting point of Na metal.
COMPOSITE TYPES OF CSSES
In recent years, researchers have redirected their focus toward CSSEs to overcome the limitations of individual ISEs and SPEs27 by leveraging their synergistic benefits and paving the way for their application in diverse fields. These studies have been predominantly centered on designing innovative superionic conductors with ionic conductivities comparable to that of liquid electrolytes. Simultaneously, these studies delve into understanding CSSE-based ion-transport mechanisms, enhancing mechanical properties, and improving electrochemical performance. Accordingly, the following sections introduce and examine the distinctive properties of different CSSEs, including II-based CSSEs, such as oxide-, sulfide-, hydride-, halide-derived CSSEs, PP-based CSSEs, and IP-based CSSEs. In preparing II-based CSSEs, the composite method primarily involves doping or substituting atoms or groups within individual ISEs. This approach has proven effective in enhancing the ion-transport capacity, stability, and mechanical properties. For PP-based CSSEs, properties such as ionic conductivity, mechanical strength, thermal stability, and electrochemical properties can be improved mainly through composite modification of the polymer segments and polymer chains to overcome the limitations of conventional single-polymer electrolytes. For IP-based CSSEs, both the inclusion of particles and the implementation of skeleton structures help advance SSEs in SSSBs. These improvements are crucial in enhancing various aspects of SSEs, such as bolstering mechanical strength, aiding Na-salt dissociation, expanding the electrochemical stability window, and providing pathways for efficient ionic transportation.
Inorganic-based CSSEs
Oxide-based II CSSEs
BASE-based II CSSEs
Increasing the ionic conductivity of BASEs can be achieved by enhancing the concentration of Na+ in the conduction plane. Several endeavors have focused on doping lower-valence cations, such as Li+ and Fe2+, into the Al3+ site of the spinel block plane. This strategy induces significant stoichiometry deviations, facilitating the incorporation of excess Na+ into the structure. For example, the properties of BASEs can be substantially improved by adding lithium oxide (Li2O),110 which increases the content of the β″-Al2O3 phase and promotes the formation of microstructure layers, and consequently, the corresponding modified BASE effectively shows the preferable ionic conductivity (Figure 3A). Doping Fe2+ into BASE effectively enhances the β″-Al2O3 fraction owing to an increase in Al3+ vacancies, which increase the grain-boundary diffusivity and improve ionic conductivity.117 The ionic conductivity of Fe-doped BASE at 350°C is 1.4 × 10–1 S cm–1.
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Numerous other dopants introduced into the Al3+ site of the spinel block plane, such as Mn4+ and Ta5+, have shown effectiveness in enhancing the ionic conductivity of BASE. Lee et al.111 doped manganese dioxide (MnO2) to BASE, noting a concurrent ascent in the β″-Al2O3 phase, sintered density, and ionic transport capacity with an increment of MnO2 from 0 to 1 wt.%. The maximum ionic conductivity of 1.0 × 10−1 S cm–1 was achieved with 1.0 wt.% MnO2-doped BASE (Figure 3B). Similarly, incorporating 0.3 wt.% tantalum pentoxide (Ta2O5) into BASE resulted in a high ionic conductivity of 1.1 × 10−1 S cm–1 at 350°C. Therefore, the selection of a suitable doping component not only improves the concentration of Na+ but also regulates the phase ratio of β- and β″-Al2O3, thus improving the Na+ ionic conductivity.
However, β″-Al2O3, despite its significance for ionic conductivity, is thermodynamically a metastable phase. It undergoes irreversible phase transformations at high temperatures and precipitates undesired byproducts, such as β-Al2O3 and NaAlO2. Some of the dopant ions, mainly Li2O, magnesium oxide (MgO), titanium dioxide (TiO2), ZrO2, yttrium oxide (Y2O3), MnO2, silicon dioxide (SiO2), and ferric oxide (Fe2O3), are referred to as stabilizers because they can stabilize the structure of β″-Al2O3, suppress β-Al2O3 formation, and prevent Na loss.110–113,117–120 Ion doping is also essential in modifying the grain size and porosity of β″-Al2O3 to achieve high ionic conductivity and good mechanical properties. For example, MgO-doped BASE composite electrolyte was prepared with γ-Al2O3 via a vapor phase process in which sodium carbonate (or Na2CO3), magnesium carbonate (or MgCO3), and lithium carbonate (or Li2CO3) were used as additives. Investigation through X-ray diffraction (XRD) and scanning electron microscopy (SEM) validated that an optimal dose of MgO increases the proportion of β″-Al2O3 phases and accelerates their densification (Figure 3C).112 The electrochemical performance and bending strength of the composite electrolyte were measured, indicating that the variant with 0.4 wt.% MgO dopant exhibited the best ionic transport and mechanical firmness. In another study, researchers increased the proportion of the β″-Al2O3 phase to 98.91% by doping BASE with 0.25 wt.% NiO. Figure 3D shows that this composition exhibited uniform microstructure and higher densification, with a relative density of 98.73% of the theoretical density. Notably, the mechanical strength can reach up to 296 MPa, with the ionic conductivity attaining 0.066 S cm–1 at 350°C.113
NASICON-based II CSSEs
Efforts to improve the ionic conductivity of NASICON-type SSEs have focused on introducing additional Na+ or doping/substituting block plane atoms. These approaches have the dual benefit of heightening the number of mobile charge carriers and enlarging the constricted pathways that impede Na+ movement. For example, doping with divalent ions (such as Mg2+,121 Zn2+,122 Co2+, and Ni2+),122 trivalent ions (such as La3+ and Sc3+),123 and tetra/pentavalent ions (such as Ti4+ and V5+)124 can effectively enhance the ionic conductivity of NASICON. Pure NASICON substituted with 10 mol% Fe2+ and 1.5 mol% Ti4+ for Zr4+ exhibited conductivities of 0.16 and 0.14 S cm–1, respectively, at 300°C, exceeding those of individual NASICONs. In particular, doping the Zr4+ site with a low-valence cation reduces the electrostatic interaction between the fixed cation and the mobile Na+, thereby reducing the activation energy and accelerating Na+ mobility. For example, Park et al.114 developed a Ge-doped NASCION with an ionic conductivity of 1.4 × 10−2 S cm–1 at 150°C, approximately twice as high as that of bare NASICON (Figure 3E). In addition, the Ge-doped NASCION composite electrolyte not only precipitates a lower temperature for phase transition, enhancing the stability and fraction of the highly conductive rhombohedral phase at elevated temperatures, but also signifies a more conducive environment for Na-ion transport due to a decrease in symmetry-related activation barriers. Recently, high-entropy metal cation mixing at the Zr4+ site has been proven to improve the RT ionic conductivity of NASICONs.115 This technique simplifies the limitations accompanied by specific dopant options and the complicated synthesis process, achieving materials with superior conductivity like the Na(Ti, Zr, Sn, Hf)2(PO4)3 reported by Zeng et al. (Figure 3F). Local perturbations introduced in high-entropy materials lead to an overlap of the position energy distributions of the alkali ions, allowing penetration with minimal activation energy.
In addition, the incorporation of an anion at the O2– site can also improve the ionic conductivity. For example, Sun et al.116 recently developed a novel NASCION composite electrolyte (Na3.2Hf2Si2.2P0.8O11.85F0.3) via adding sodium fluoride (NaF) in as-synthesized Na3.2Hf2Si2.2P0.8O12, resulting in 2.39 × 10−3 S cm–1 high ionic conductivity and excellent chemical stability. This can be attributed to that the position of O is replaced by F (Figure 3G). Furthermore, strategic doping with ions like Mg2+, La3+, and Nb5+ has shown promising results in battery performance by alleviating resistance at grain boundaries, consequently fostering better ion conduction through these interfaces. Doping Na3+xZr2−xLaxSi2PO12 with La3+ leads to the construction of a new Na3La(PO4)2 phase, which positively impacts the ion conduction at the grain boundaries and increases ceramic density; as a result, the ionic conductivity of Na3.3Zr1.7La0.3Si2PO12 at −50°C (7.6 × 10−5 S cm−1) is one magnitude higher than that of Na3Zr2Si2PO12 (Figure 3H).93
Sulfide-based II CSSEs
Na3PS4-based II CSSEs
A common approach to enhancing the ionic conductivity of Na3PS4 is the introduction of Na+-vacancies or interstitials into the lattice, for example, by doping higher-valence cations into the Na sites or lower-valence anions into the S sites. For example, the Ca2+-doped cubic Na3PS4 exhibits a maximum conductivity of ~1 mS cm–1 at 25°C (Figure 4A).125 Investigations including EIS, magnetic resonance spectroscopy (NMR), and density functional theory (DFT) indicate that Ca2+ replacing Na+ in Na3PS4 stabilizes a cubic phase rich in Na-vacancies. This transformation increases the activation barriers but significantly boosts Na-ion mobility. Based on the Ca-doped Na3PS4 composite electrolyte, the TiS2|NaSn batteries achieve a commendable 200 mA h g–1 capacity at 0.06 C, demonstrating commendable cyclic durability and superior rate capability compared with batteries using nondoped Na3PS4. Wagemaker et al.132 demonstrated by DFT-molecular dynamics simulations that doping halogen (Cl–, Br–, or I–) in cubic Na3PS4 can introduce Na-vacancies, obtaining higher Na-ion conductivity. A mere 2% of vacancies can increase the conductivity by a factor of 10, up to 0.20 S cm–1, while also lowering activation energy from 0.28 to 0.16 eV. Chu et al.126 unveiled Cl-doped tetragonal Na3PS4(t-Na3−xPS4−xClx), whose Na+ conductivity surpasses 1 mS cm–1 at RT, and it also exhibits a good cycling performance in a TiS2|Na full cell at a rate of 0.1 C (Figure 4B).
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Doping lower-valent cations (especially tetravalent ions) into P5+ sites is a recognized method for not only embedding Na+-interstitials but also propelling ion conduction in Na3PS4. For example, the ionic conductivity of Na3PS4 can be improved by doping Ge4+, Ti4+, or Sn4+, with Sn-doped variants displaying conductivities up to 2.5 × 10−4 S cm–1 at RT.127 Innovative Na2+2δFe2−δ(SO4)3|Na3+xMxP1−xS4 (M = Ge4+, Ti4+, Sn4+) (x = 0, 0.1)|Na2Ti3O7 SSSBs feature high discharge capability (2 C) at elevated temperatures of 80°C (Figure 4C).
Isovalent substitution, achieved by replacing P or S with larger atoms in Na3PS4-type SSEs, is another strategy to increase the migration capacity of Na+. Furthermore, these substituted ions fine-tune the chemical and spatial dynamics within the framework, lessening Na-ion interactions with the anionic network and expanding lattice or channel dimensions. For example, with S substituted with Se, the cubic phase Na3PSe4 has an I-43m space group and a unit cell of 7.3094 Å.58 Compared with the cell size of Na3PS4, such a large unit cell along with the peculiar electron density distribution on 12d-sites can create extra free volume for Na+-transport and provide fast Na+-transport pathways in the 3D channels (Figure 4D). Consequently, this Se-doped material can achieve conductivities of 6 mS cm–1 at RT, paired with a modest activation energy of 0.21 eV. Similarly, substituting P5+ with Sb5+ in Na3PS4 and Na3SbS4 has attracted significant attention because of its elevated ionic conductivity and remarkable air stability at RT. DFT simulations illustrate that the high symmetry in the cubic Na3SbS4 lessens Na-S(Sb) atomic interactions, shortens the Na hop distance to 2.85 Å, and leads to a higher diffusion coefficient. (Figure 4E).128
Nevertheless, sulfide-based electrolytes typically exhibit vulnerability to air and moisture due to their relatively fragile P–S bonds, which could lead to the rapid release of the toxic gas H2S when in contact with humid environments.133 To promote the environmental stability of these electrolytes, the frail P-S connections should be substituted with bonds formed by aliovalent ion doping to intensify the bonding strength between sulfur atoms and the integrated dopants. Based on the hard and soft acid (HSAB) theory, by replacing P5+ with Sb5+, Na3SbS4 is more stable in air than pristine Na3PS4 owing to the strong bonding between the soft acid (Sb5+) and the soft base (S2−), as shown in the Raman and XRD analyses (Figure 4F).129
Na11Sn2PS12-based II CSSEs
Similar to Na3PS4-type SSEs, isovalent substitution offers a means of modulating the size of the lattice, thereby facilitating Na+ mobility in Na11Sn2PS12 SSEs. Upon replacing S with Se, the Na11.1Sn2.1P0.9Se12 material exhibits a similar Na+ ion conductivity of 3.0 mS cm–1 at RT compared with Na11Sn2PS12 SSEs. However, the reduced lattice rigidity and weaker Na–S interactions in these Se-doped electrolytes contribute to a significantly lower activation energy of 0.3 eV. (Figure 4G).130 Ramos et al.134 reported a single-crystal structure of Na+-ion conductor Na11Sn2SbS12, almost structurally identical to Na11Sn2PS12. However, Na11Sn2SbS12 had a lower ionic conductivity than Na11Sn2PS12 because of the reduced population of Na(6). A new structural class of tetragonal Na4−xSn1−xSbxS4 (0.02 ≤ x ≤ 0.33) electrolyte with space group I41/acd was reported by Heo et al.131 The Na4−xSn1−xSbxS4 exhibited 3D Na-ion diffusion pathways, which allowed high ionic conductivities of 0.2–0.5 mS cm–1 at 30°C. In addition, the Na4−xSn1−xSbxS4 demonstrates excellent dry-air stability without releasing H2S upon water contact, addressing a common issue with sulfide-based electrolytes. However, in contrast to the approach of aliovalent doping to increase the charge carrier concentration, incorporating additional vacancies or interstitials has shown a limited impact on enhancing the Na+-ionic conductivity in Na11Sn2PS12 (Figure 4H). This is likely due to the abundant inherent Na+-disorder present in these materials. The Na11Sn2PS12 already contains a sufficient number of charge carriers (vacancies), making the introduction of additional point defects unnecessary.
Hydride-based II CSSEs
To expand the practical applications of novel hydrides, representative hydride electrolytes must further increase the ionic conductivity and lower the phase transition temperature, which is directly achieved using composite strategies with a second phase. For example, Ngene et al.135 improved the ionic conductivity of nanocomposites by approximately three orders of magnitude by introducing suitable metal-oxide scaffolds. Specifically, NaBH4 and γ-Al2O3 were composited, and NaNH2 and SiO2 were composited, achieving conductivities of 4.7 × 10–5 and 2.1 × 10–5 S cm–1, respectively, at 80°C (Figure 5A).
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In the quest to enhance ionic conductivity and decrease the phase transition temperature, researchers have explored substituting one boron atom in (BnHn)2− with a carbon monocarba-closo-borate. The changes instigated by carbon addition affect how cations and anions interact, their preferred orientation, and the dynamics of their rotation. Replacement of the B in (BnHn)2− of the quasi-spherical cage-like closo-polyborate with different types of nest-like nido-poly(carba)borate polyanions ((B11H14)−, (CB10H13)−, and (C2B9H12)−) can improve the ionic conductivity and promote the RT stability (Figure 5B).136
In addition to replacing the B atom in a (BnHn)2– anion, substituting an H atom with halogen elements can also lower the phase transition temperature by adjusting the relative enthalpies of the ordered and disordered state. Compounds, such as Na2B12H12−xIx, developed through selective iodine replacement of hydrogen, have shown significant ionic conductivity close to 0.1 S cm–1 at RT while exhibiting a phase transition temperature between those of NaCB11H12 and NaCB9H10. However, the full substitution of hydrogen with halogen atoms in halogenated Na-closo-dodecaboranes (Na2B12F12, Na2B12Cl12, Na2B12Br12, and Na2B12I12) (Figure 5C) resulted in a higher polymorphic transition temperature in Na2B12H12, which could be related to the increasing anion size, mass, and anisotropic electron density in the covalently bound halogens.34
Anion mixing is a promising method to eliminate the phase transition temperature and maintain the desired disordered phase over a wide temperature range in hydride SSE. Such blending, evident in composites like Na3B24H23-5Na2B12H12, results in electrolytes with remarkable ionic conductivity of 1.42 mS cm–1 and transference number of 0.97 and a wide electrochemical window (5.8 V vs. Na+/Na) (Figure 5D).137 The Na-symmetrical cell with this CSSE remains stable for 100 h at 0.2 mA cm−2 and 60°C as well as at 0.7 mA cm−2 and 25°C without short-circuiting.
Nevertheless, in addition to the increase in ionic conductivity, the chemical and electrochemical stability of the hydrides should also be evaluated. For example, Na2BH4NH2 can exhibit a high ionic conductivity of 7.56 × 10–4 S cm–1 at 90°C. However, despite its high ionic conductivity, its full-cell cycle life paired with that of the TiS2 cathode is only 15 cycles because of the instability of the electrochemical properties of the electrolyte.140 By introducing Na–B–S compounds, such as NaBS3, Na2B2S5, and Na3B3S6 into NaBH4, Zhou et al.138 prepared a thioborohydride composite (Figure 5E). At 120°C, the Na+-conductivity of the synthesized NaBH4/Na–B–S electrolyte was 1.7 × 10–4 S cm–1. Moreover, the capacity of a full cell paired with a Na3V2(PO4)3 anode reached 102 mA h g–1 after 20 cycles, demonstrating that the electrochemical stability of the composite hydride electrolyte was further improved. However, it is worth noting that the cycle life is still short at 50 cycles. Pang et al.139 recently proposed an in situ reaction with NaHF2 to enhance the hydride's electrochemical stability (Figure 5F). Na2B12H12 reacted with NaHF2 to form NaF nanoparticles, which were uniformly embedded in the Na2B12H11F matrix, leading to an 87.7% capacity retention of Na/Na3V2(PO4)3 SSSBs after 100 cycles.
Halide-based II CSSEs
Similar to other SSEs, Na+ migration in halide SSEs is predominantly facilitated by intrinsic vacancies. Therefore, a key strategy to improve ionic conductivity involves boosting vacancy concentrations along the pathways for Na+ diffusion, which can be achieved by introducing a charge-compensation mechanism. Zr4+ played a significant role in facilitating this process because of its low doping formation energy. Moreover, anion mixing is a reasonable strategy for enhancing ionic conduction within halide-based SSEs. By altering lattice structures, chemical bonds, and rearranging the positions of ions, it becomes feasible to promote Na-ion conduction. Wu et al.100 have developed a new class of Na-SSEs, Na3−xY1−xZrxCl6 (NYZClx), using data-driven methods (Figure 6A). High ionic conductivities, excellent electrochemical stabilities, and chemical stabilities of up to 3.8 V versus Na/Na+ are exhibited by these SSEs. Through DFT calculations, it was hypothesized that the inclusion of Zr4+ in Na3YCl6 could dramatically elevate Na+-conductivity by three orders of magnitude. Such high Na+ conductivity in Na3−xY1−xZrxCl6 is mainly because Zr4+-substitution increases the volume of the unit cell, which in turn causes the polyanion to rotate. Diffusion of Na+ increased significantly post-Zr4+-doping due to the synergistic impact of the polyanion's rotation and the rise in the effective mobile carrier/vacancy concentration. This doping method improves conductivity and its remarkable chemical stability and wide electrochemical window of the SSEs.
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The C2/m phase of Na3MI6 is preferred and exhibits a high ionic conductivity of approximately 10–4 S cm–1, whereas doping of Na3MI6 through anion mixing (such as Br) would further enhance Na-ion migration and results in a significantly higher ionic conductivity of around 10–3 S cm–1,37 which is two magnitudes higher than that of its reported counterparts, as shown in Figure 6B. Likewise, the disorder in the chemical bonding and the vibration of the Br and I anions inside the crystal lattice are expected to increase due to anion mixing in Na3MBr3I3. This increased disorder, and larger anion vibrations facilitated Na-ion migration through the halide-based SSEs.
PP-based CSSEs
Compared with that on SSLBs, there has been relatively limited research on polymer electrolytes in SSSBs.141 Due to the unrestricted mobility of the anions resulting from the dissociation of Na-salts within the single-polymer matrix, the Na-ion transference number in polymer electrolytes tends to be relatively low, leading to a pronounced limitation on the rate performance of SSSBs.142,143 In addition, a single-polymer electrolyte is not mechanically strong enough to suppress and withstand breakthroughs by Na dendrites. Therefore, by utilizing various strategies to construct polymer–polymer composites, the limitations of traditional polymer electrolytes can be overcome, and improved properties, such as enhanced ionic conductivity, mechanical strength, thermal stability, and electrochemical performance, can be achieved.144,145 The design and development of polymer electrolytes involves a sophisticated composite strategy that encompasses the integration of monomers, polymer segments, and polymer chains. This multilevel composite approach facilitates the realization of synergistic interactions and mutual complementarity, ultimately leading to optimal performance characteristics.
PEO-based CSSEs
Among SPEs, PEO is one of the earliest identified and most thoroughly examined polymeric backbones. The use of PEO as a polymer matrix in ion batteries began in 1973 when Wright et al.146 reported the ion-conducting properties of PEO complexed with alkali metal salts. PEO is an ether-based polymer where the ether oxygen units in its structure can complex with sodium ions, facilitating electrolyte salt dissociation and promoting sodium ion transport through segmental motion. The primary challenges associated with PEO-based SPEs are their low room-temperature ionic conductivities and poor mechanical properties. PEO-based CSSEs primarily focus on the polymer segment and polymer chain levels.
Significant methods regarding composites at the polymer segment level include block copolymerization, grafting, and hyperbranching. Inserting other polymer chains in PEO effectively reduces crystallinity, resulting in higher ionic conductivity.147 However, when PEO segments are combined with other polymer segments, they can endow the composite electrolyte with corresponding properties and significantly enhance the overall performance of the composite electrolyte. Forsyth et al.148 introduced perfluoropolyether (PFPE) segments into PEO-based electrolytes to construct a novel class of block copolymers (Figure 7A). Their research showed that adding PFPE contributed to forming self-assembled structures, thereby improving the mechanical properties (the block copolymer electrolyte formed a free-standing membrane, and the PEO-based control sample was a non-free-standing membrane). At 80°C, the storage modulus of EO10-PFPE was significantly higher than its loss modulus. The microstructure, defined by its distinct phase separation, offered combined benefits that augmented the performance of the electrolyte. Molecular dynamics simulations corroborated weakened interactions between the Na+ ions and the PFPE polymer chains, alongside an amplification in the interaction with FSI– ions. The results showed that the Na+ transfer number of the composite block copolymer electrolyte increased from 0.33 to 0.46, and the Na+ conductivity was 1.5 times higher than that of the PEO-based control sample, significantly improving the long-term cycling stability of the composite block copolymer electrolyte. A constructed Na metal battery, with Na3V2(PO4)3 as its cathode, sustained a stable electrochemical performance and displayed exceptional reversibility in charge and discharge processes (CE = 99.91%) following more than 900 cycles at an elevated thermal setting of 80°C under 2 C.
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In addition to block copolymerization, grafting and hyperbranching can effectively enhance the overall performance of PEO-based CSSEs. The grafted short chain can dissociate the salt and solvated Na ions on the main chain to form a hyperbranched structure, inhibiting the crystallization of linear PEO and enhancing the complexation ability (Figure 7B).149 For example, Deimede et al.154 reported that sulfonated polysulfone (Na-salt) grafted PEO composite electrolyte materials exhibited ionic conductivity as high as 2 × 10–2 S cm–1 (80°C) when the mass fraction of modified PEO was 70%. Similarly, building polymers with special structures through hyperbranching can effectively enhance the overall performance of CSSEs. Ma et al.150 synthesized complex hyperbranched polymers with star-shaped architectures by fusing a β-cyclodextrin (β-CD) nucleus with several oligo(methyl methacrylate)-block-oligo (ethylene glycol) methyl ether methacrylate (PMMA-b-PPEGMA) appendages. They utilized these polymers to fabricate free-standing, flexible composite membranes via the atom transfer radical polymerization (ATRP) approach (Figure 7C). The resulting PEO-based CSSEs exhibited excellent thermal stability, superior mechanical properties, high ionic conductivity, and a wide electrochemical window. When the optimized polymer with 69.3 wt.% PPEGMA was used, an ionic conductivity of 1.3 × 10−4 S cm−1 and a wide electrochemical window of 5.2 V (vs. Na+/Na) were achieved at 60°C. Additionally, when this optimized CSSE was implemented in a Na/NaNi1/3Fe1/3Mn1/3O2 battery, it was able to maintain its capacity at 87.8% (88.9 mA h g−1) even after surpassing 80 charge–discharge cycles.
While the Tg of the polymer can be lowered and the ionic conductivity can be enhanced by introducing other polymer segments into the PEO segments, the stability and mechanical properties of the polymers can be compromised in some cases. Crosslinked CSSEs can be obtained by chemically crosslinking the polymer chains to improve mechanical stability. Although the chemical crosslinking of polymers can improve their mechanical properties, an increase in the degree of crosslinking reduces the mobility of the polymer segments, reducing the ionic conductivity. Therefore, the crosslinking degree must be precisely controlled to obtain a better-performance polymer electrolyte. The molecules or segments involved in crosslinking can be rationally designed to impart unique properties to the composite electrolyte. Wang et al.151 developed a flexible flame retardant polymer electrolyte (FRPE) using cyclotriphosphazene networks established through thiol-acrylate and free radical photopolymerization, with polyethylene glycol-functionalized cyclotriphosphazene (PEG-CP) (Figure 7D). This star-shaped PEG-CP took on a dual role as an ion transporter and a fire-resistant additive, which simultaneously boosted both the flame resistance and the ionic transport conductivity of the FRPE (the ionic conductivity of FRPE-0 without PEG-CP was only 1.10 × 10–6 S cm–1 at 30°C, whereas that of FRPE-40 reached 4.72 × 10–5 S cm–1). The Tg of the FRPEs and crystallinity both sharply dropped as the PEG-CP amount increased, facilitating an enhancement in ionic conductivity. Incorporating a crosslinked network structure provides polymer electrolytes with enhanced mechanical properties. Moreover, the introduction of cyclic boronic esters enhanced Na-ion migration and interface stability owing to the interaction between the Na-salt-anions and boronic esters.
A composite of PEO at the polymer chain level is primarily achieved by blending. Blending enhances the interfacial interactions among the different phases of polymer electrolyte materials, regulates molecular chain movement, and adjusts crystallinity. These aspects have contributed to the development of SPEs with excellent mechanical and electrochemical properties. Blending methods are simple to operate and allow easy control of the physical properties of the system through changes in the components. In the study of SSSBs, the main polymers reported to be blended with PEO are polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), and polyaniline (PANi).155,156 Chandra et al.157 prepared 97[75PEO:25NaPO3]/3PVP CSSEs by hot pressing method. The melting point of the blended membrane was reduced by 5°C compared with that of pure PEO, and the RT ionic conductivity was increased by two orders of magnitude (1.07 × 10–5 S cm–1) compared with that of the electrolyte without added PVP. Kunteppa et al.158 investigated the blends of PANI and PEO in PEO/PANI/NaClO4 SPEs. The electrolyte exhibited the highest ionic conductivity (2.0 × 10–5 S cm–1) at 50°C for a blend ratio of 50/20/30 (mass ratio). Incorporating polymers into a PEO matrix can effectively enhance the electrochemical stability of CSSEs. Gerbaldi et al.159 incorporated sodium carboxymethylcellulose (Na-CMC) into a CSSE based on PEO with an optimum mass ratio of PEO:NaClO4:Na-CMC = 82:9:9. Na-CMC can serve as an electrode binder, optimizing the interface contact between the electrode and the electrolyte when added to the electrolyte. SSEs of PEO/Na-CMC demonstrated a marked decrease in impedance related to charge transfer. This suggests improved interface compatibility and ion-transport mechanisms compared with electrolytes lacking Na-CMC. The performance of such solid-state battery configurations (Na|SPE|TiO2 and Na|SPE|NaFePO4) showed commendable cycling stability throughout their usage.
New polymers with unique molecular structures have been blended with PEO and conventional homopolymers. For example, Hu et al.152 successfully designed and synthesized a well-defined multi-arm fluorinated polymer, 21-β-CD-g-PTFEMA (a postmodified β-CD core and 21 poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) arms), using the ATRP technique (Figure 7E). Blending this polymer into the PEO matrix can enhance the transference number and electrochemical window stability. Through various physical and chemical characterizations, as well as theoretical analyses, they confirmed that unique supramolecular interactions (involving C═O, C–O, C–F, O–H, and C–H bonds) and multi-arm topology are vital factors in improving the properties to surpass the performance of the two independent polymers. Xue et al.160 synthesized custom-designed star polymers featuring crosslinked cores embedded with polymeric pseudo-crown ether cavities and linear PEO arms. This novel structure was created through a cation-template-assisted cyclopolymerization technique. The presence of pseudo-crown ether cavities in the crosslinked cores enables coordination with Na+, providing additional paths for cation diffusion. Furthermore, the functional linear PEO arms improve the cation conductivity and compatibility with the PEO matrix.
Other polymer-based CSSEs
Despite numerous modifications and composites made for PEO, the overall performance of PEO-based SPEs still falls short of the requirements for developing such batteries. Consequently, various new polymer matrices have been gradually applied to SSE systems.161–163 Research on composite systems with non-PEO-based solid polymeric electrolytes is still relatively limited and is in its early stages. Similarly, composite modifications were carried out at the polymer segment and polymer chain levels through copolymerization, grafting, blending, and crosslinking. For instance, Brandell et al.164 prepared a novel polymer electrolyte based on polycaprolactone-polycarbonate (PCL-PTMC/NaFSI). They found that by enhancing the salt content within the composite electrolyte to 35%, a substantial decrease in Tg was observed, from −11°C to a much lower −64°C. This notable reduction in Tg significantly increased the thermal motion of the polymer chains even at ambient temperatures. An assembled hard carbon (HC)/Na2−xFe(Fe(CN)6) SSSBs using this electrolyte exhibited stable cycling for 120 cycles at 22°C.
Single-ion ion-conducting polymer electrolytes (SIPEs), another innovative class of SPEs, have also garnered considerable interest. Anions dissociated from the Na-salt in the electrolyte usually do not interact with the polar groups in the polymer matrix. These anions move in a direction opposite to that of Na ions, causing internal polarization and leading to the formation of a polarizing electric field within the electrolyte during the charging and discharging of the battery. This polarization increases the possibility of reactions occurring at the electrode surface and the self-discharge of the battery, ultimately affecting the stability of the battery's charge and discharge current, efficiency, and lifespan. In a previous study, the fixed attachment of anions to a polymer matrix through covalent bonds was explored to design and synthesize polymer single-ion conductors in which anions do not migrate. These ion conductors have attracted significant interest because of their ability to maintain a high safety factor and effectively resolve internal polarization issues, leading to improved capacity and cycling performance. Li et al.153 synthesized sodium copoly(4-styrenesulfonyl(trifluoromethylsulfonyl)imide)-poly(ethylacrylate)-poly(ethylene) (Na(PSTFSI-co-EA)) and obtained an SIPE (Figure 7F). Although the article did not report the Na-ion transference numbers for these SPEs, similar to the polymeric anion Li salt, the polymeric anion Na-salt also exhibited a relatively high Na-ion transference number when applied in SPEs. However, one of the main challenges for single-ion conductors is their relatively low ionic conductivity, which is a key focus in developing SIPE. Bresser et al.165 proposed a novel SIPE doped with ethylene carbonate as the electrolyte system for SSSBs. This innovative SIPE exhibited a high ionic conductivity of 2.6 mS cm–1 and an electrochemical stability window of approximately 4.1 V at 40°C, allowing for stable Na stripping and plating. It also demonstrated excellent rate capability in Na/Na3V2(PO4)3 cells, even at 2 C. Notably, these cells maintained a capacity retention of approximately 85% after 1,000 cycles at 0.2 C, owing to the exceptionally high Coulombic efficiency of 99.9%. This efficiency was achieved through the excellent interfacial stability of Na metal and the Na3V2(PO4)3 cathode.
IP-based CSSEs
Particle fillers in SPEs
In this review, the SSEs consisting of ceramic particles and polymer matrices are called IP-based CSSEs. Incorporation of inorganic components into the polymer matrix can reduce the crystallinity of the polymer and promote the motion of the segments, thus improving the ability to conduct Na ions. Second, it improves the mechanical strength to inhibit the growth of Na dendrites. These inorganic fillers are usually classified into two categories: those that cannot provide Na ions and are termed passive fillers, such as nano-TiO2, SiO2, and ZrO2, and those that are termed active fillers and have their ion-transporting ability, such as BASEs, NASICONs, and Na3PS4.
Ni'Mah et al.166 introduced nanosized TiO2 into PEO and NaClO4 systems through a one-pot hydrothermal reaction. This method effectively prevents TiO2 aggregation and produces free-standing transparent films. The ionic conductivity reaches 2.6 × 10–4 S cm–1 at 60°C. Furthermore, a full cell assembled with Na2/3Co2/3Mn1/3O2 as the cathode exhibited electrochemical performance comparable to that of a liquid system (Figure 8A). Similarly, Zhu et al.173 explored the effects of different amounts of TiO2 doping within a PEO-NaFSI system. They observed that an increased quantity of TiO2 led to a reduction in the crystallinity of the PEO. Notably, this reduction in crystallinity did not translate to a “more is better” scenario. The results demonstrated that the ionic conductivity of the PEO-NaFSI-10%TiO2 CSSEs was 2.4 × 10–4 S cm–1. In contrast, the PEO-NaFSI-1%TiO2 CSSEs exhibited a higher ionic conductivity of 4.89 × 10–4 S cm–1 at 60°C. This difference may be attributed to the agglomeration of the nano-TiO2 particles, which could hinder the motion of the Na ions. Similarly, the mechanical strength and electrochemical characteristics of the polymer were enhanced by doping ZrO2 particles into the matrix.174 When 3 wt.% ZrO2 was added to a PVA/NaClO4 polymer electrolyte, the ionic conductivity could reach 4.3 mS cm–1. BN can also be used as a passive filler; for example, Hamisu et al.175 incorporated hexagonal boron nitride (h-BN) fillers into a poly(AN-co-PEGMA) polymer electrolyte. The presence of h-BN nanoparticles not only enhanced the thermal properties of the polymer but also acted as an active site to promote the dissociation of NaClO4, and the ionic conductivity of the resulting CSSE was approximately 3.6 × 10–4 S cm–1 at 100°C. This is mainly because the molecular structure of BN contains nonbonding electrons. These electrons tend to form dative bonds with the anionic groups present in the Na-salt, thus facilitating the dissociation of the Na-salt. In addition to TiO2, ZrO2, and BN, SiO2 particles are common inert inorganic fillers used to reduce the crystallinity of polymer matrices. Moreno et al.176 incorporated nanometric SiO2 (7 nm) into a PEO-NATFSI system via high-energy ball milling and achieved an ionic conductivity of 1.1 mS cm–1 at 80°C by adding 5 wt.% of SiO2 particles with an EO:Na ratio of 20:1. While an appropriate number of nanoparticles enhances the ionic conductivity, an excessive number of nanoparticles causes particle aggregation, which reduces the surface area of the particle–polymer interface and inhibits Na-ion transport. This limitation is typically addressed using in situ synthesis methods. For example, Lin et al.177 synthesized SiO2 particles from tetraethyl orthosilicates. The resulting SiO2 particles with abundant hydroxyl groups are uniformly dispersed in poly (ethylene glycol)-co-ureidopyrimidinone (PEG-UPy). The team found that, of the different silica particle morphologies, hollow mesoporous silica had the best electrochemical performance, benefiting from its large surface areas with more negatively charged sites. The organic groups grafted onto inorganic particles can interact with the anionic groups present in Na-salts. This interaction occurs while maintaining the beneficial effects of inert particles. Villaluenga et al.167 grafted SiO2 particles with significant anionic groups, specifically RSO2N(−)SO3CF3 and PEG, to ensure that Na+ remained the only conductive ion (Figure 8B). The grafting of inorganic nanofillers with polymer chains hindered anion motion, thus enhancing the ionic conductivity and transference number. This SSE, which is stable against redox reactions and hydrolysis, maintains its mechanical strength while exhibiting considerable ionic conductivity.
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Besides incorporating polymers and inert fillers, active particles, such as NASICONs, Na3PS4, and BASEs, are commonly used as additives for compounding polymers. These fillers consistently exhibited enhanced ionic transport rates and improved mechanical properties. For example, Liu et al.178 incorporated Na-β″-Al2O3 into a PVDF matrix, effectively broadening the chemical stability window to 4.77 V. Assembled into a whole cell with NaNi1/3Mn1/3Fe1/3O2(NNFM) as the cathode, the cell delivered a capacity of 109.6 mA h g−1 at 1 C rate. Fang et al.170 integrated BASE into an ultrathin CSSE by immersing Na-β-Al2O3 (~20 μm)-containing DMF solutions in a 1 M PVDF/NaPF6 electrolyte (Figure 8E). The Na-β-Al2O3 particles exhibited excellent orientation and established short percolation paths for Na-ion transport. Consequently, these particles exhibited a high transference number (0.91 at RT).
NASICON particles have the advantages of high thermal stability, superior ionic conductivity and transference numbers, and exceptional electrochemical stability. Zhang et al.179 blended PEO-, NaFSI-, and NASICON-based electrolytes (NZSP and Na3.4Zr1.8Mg0.2Si2PO12) to establish rapid ion-transport paths through inorganic particles. This system remained thermally stable up to 150°C. Notably, the full cell assembled with a Na anode showed an overpotential of only 60 mV at 80°C and 0.1 C; this overpotential is considerably lower than that of common SPEs (~160 mV). Wang et al.180 devised a CPE that harnesses the strengths of both PEO and PAN by incorporating NZSP as an inorganic filler. The resulting electrolyte exhibited a high electrochemical window (4.2 V) and remarkable tensile strength (13.84 MPa). The whole cell could operate at 80°C, which was close to the melting temperature of Na metal (~98°C), with a discharge capacity of 134.3 mAh g−1 at 0.1 C. Kim et al.181 incorporated NZSP ceramic particles into a PVDF–HFP polymer using a simple phase inversion method augmented by adding sodium triflate/TEGDME. This hybrid CSSE displayed a fourfold increase in ionic conductivity (approximately 10–4 S cm–1 at 0°C) and a high transference number of 0.92, surpassing that achieved with common mixing approaches. The Na|CSSE|NaFePO4 cells showed a reversible capacity of 103.1 mA h g−1 at 0.5 C. Moreover, the hybrid film exhibited robust thermal stability, attributed to its ceramic inclusion. Yu et al.168 used NaClO4 as the Na-salt (Figure 8C). The polarization of Na/PEO-NaClO4-NZSP/Na batteries measured approximately 85 mV at 60°C with a current density of 1.0 mA cm–2. Additionally, the substitution of PEO with PEGDA, a type of crosslinking polymer, together with the addition of a succinonitrile plasticizer and NZSP particles, Na/PEGDA-SCN-NaClO4-NZSP/Na2MnFe(CN)6 cells resulted in 95.6% capacity retention after 50 cycles.182 Wu et al.183 introduced Ga-doped Na2Zn2TeO6 (NZTO) as an alternative filler for a more environmentally friendly production process. This choice decreased NZTO's calcination temperature to around 850°C, in contrast to NZSP's 1100–1200°C. The PEO/NZTO CSSEs exhibited a high ionic conductivity of 1 × 10–3 S cm–1 at 80°C.
In addition to oxide particles, sulfide particles, exemplified by Na3PS4 or Na3SbS4, exhibit high ionic conductivity, low melting temperature, and excellent compatibility with polymer matrices and are also used as active additives. For instance, Xu et al.184 synthesized a Na3PS4-PEO (NPS-PEO) electrolyte through a reaction involving Na2S, P2S5, PEO, and NaClO4 at a low temperature of 260°C. The PEO polymer encapsulated NPS particles with thicknesses ranging from 5 to 10 nm, thus creating conduits for Na-ion transport. With the incorporation of PEO-NaClO4, the ionic conductivity of the composite membrane reached 9.5 × 10–4 S cm–1, which is approximately an order of magnitude higher than that of the NPS ceramic electrolyte.185 Beyond the notable ionic conductivity of sulfides, the SbS43− unit within Na3SbS4 can function as a Lewis base, triggering the crosslinking of pentaerythritol tetra acrylate (PETEA) to form an in situ polymerization electrolyte within the range of 120–150°C (Figure 8D).169 This novel CSSE film possessed a low thickness of 30 μm and exhibited an RT resistance of only 65 Ω cm2.
Structural design of CSSEs
Incorporating inorganic ceramic particles is the simplest way to enhance the overall performance, including the electrochemical behavior, mechanical strength, and thermal stability. Nonetheless, an excessive number of fillers can lead to the agglomeration of these inorganic particles despite some unique synthesis methods that are not universal for all situations. Employing a structurally designed inorganic component effectively circumvents this issue while simultaneously increasing the ceramic content. As demonstrated in Li-metal batteries, these structurally designed inorganic components include nanofibers,186–188 nanotubes/microtubes,189 and 3D continuous frameworks.190,191 These structural designs can be effectively employed in SSSBs to provide additional pathways for Na ions, thereby improving ion transport.
Nanowires or nanotubes/microtubes, which have a mature manufacturing process, can improve the electrochemical performance of SSBs. Lei et al.171 developed a composite inorganic ionic conductor/gel polymer electrolyte by coating β′/β″-Al2O3 nanowires (ANs) with a layer of PVDF–HFP polymer (Figure 8F). The resulting crosslinked membranes established dense and uniform conduits for Na-ion transport via ANs, facilitating uniform Na stripping and deposition. The symmetrical cells exhibited a low polarization of 200 mV, sustained for over 300 h at 0.5 mA cm–2. Furthermore, full cells assembled with the NVP cathode exhibited an impressive capacity retention of 95.3% after 1000 cycles at a rate of 1 C. At a temperature of 60°C, the cell retained 78.8% of its original capacity after 1000 cycles, demonstrating good performance at high temperatures. NASICON microtubes also serve as interconnected pathways for Na+. NZSP microtubes with diameters of around 10 μm and lengths spanning hundreds of micrometers, adopting the shape of twisted ribbons, provided a continuous and swift conduction path for Na+. When combined with an ionic liquid, such as EmimFSI, the PEO-NZSP microtube electrolyte demonstrated a superior conductivity of 0.693 mS cm–1 and an impressive transference number of 0.882 at RT.192
Studies have indicated that 3D interconnected structures offer remarkable structural stability and excellent ion conduction pathways. Wu et al.193 incorporated electrospun SiO2 nanofibers, infiltrated with a PVDF–HFP membrane using an in situ UV curing technique, with a specially designed alloy anode, remarkably extending the material's temperature compatibility range from −30°C to 130°C. This approach yielded an excellent ionic conductivity of 0.153 mS cm–1 at −30°C. The nanostructured NASICON framework served as a rapid Na-ion diffusion pathway, contributing to an impressive ionic conductivity of 0.1 mS cm–1 at RT. The addition of the flame-retardant SiO2 nanofibers improved the high-temperature performance as well as lowered the Tg of the electrolyte. At a temperature of 130°C, the full cell delivered a discharge capacity of 218 mAh g−1 at a rate of 5 C, and at −30°C, the cell showed a discharge capacity of 64 mAh g−1 at 0.1 C. Wang et al.172 presented a porous Na3Zr2Si2PO12-PEO nanostructure achieved via a sol-gel method. The porosity level can be tailored by adjusting the quantity of chemical agents within the precursor gel. Subsequently, PEO is introduced into the pores, forming continuous pathways for Na+ transport (Figure 8G). The robust NASICON framework effectively curbed Na-dendrite growth. This composite film showed an ionic conductivity of 0.14 mS cm–1 at RT. The research team employed scanning probe microscopy (SPM) to gain insight into the diffusion of Na ions across the interface. Notably, they confirmed that diffusion at the interfaces outpaced that occurring in either individual NASICON or PEO components. Lim et al.194 employed partial sintering of a NASICON SSE to establish connected ion-transport channels. Subsequently, the internal pores were filled with epoxy resin polymer. This approach increased the mechanical strength by approximately twofold without compromising the thermal and electrochemical stabilities of the NASICON material. The conductivity reached 0.145 mS cm–1, while the electrochemical window was extended to 7 V.
Including particles and implementing structurally designed inorganic components contribute to the advancement of SSEs in SSSBs. Inorganic additives play a crucial role in enhancing various aspects of SSEs, such as improving mechanical strength, aiding in Na-salt dissociation, expanding the electrochemical stability window, and providing pathways for efficient ionic transportation.
INTERFACE DESIGN
For optimal cell performance of SSSBs, it is necessary to have SSE with high ionic conductivity and a well-developed electrode–electrolyte interface. However, interface issues have become a bottleneck for developing SSSBs, mainly involving electrode–electrolyte interfacial contact and chemical compatibility.51,195,196 Given the description of the different types of SSEs mentioned above, ISEs are characterized by a relatively high ionic conductivity, wide electrochemical window, high safety, and thermal stability. However, SSSBs still face several challenges, including high electrode–electrolyte interfacial resistance, insufficient contact area with limited ion-transport pathways, and poor electrode–electrolyte interfacial compatibility during electrode volume changes. Therefore, resolving the contact issues takes priority over chemical compatibility.197,198 The SSSBs associated with SPEs have good interfacial contact between the electrode and the electrolyte.199 However, the electrochemical performance of SSSBs is poor at RT because of the low ionic conductivity of the electrolyte, unstable electrode–electrolyte interface, and mismatch of the mechanical properties. Thus, the priority solution is to improve the chemical compatibility of the interface between the electrode and electrolyte. For CSSEs, enhancing their overall efficacy necessitates advancements in various aspects, including ionic conductivity, electrochemical window, thermal/mechanical stability, and interface compatibility between the electrolyte and electrode.51 Hence, in the case of SSSBs based on CSSEs, the critical factors for enhancing the electrochemical performance not only require improving the ionic conductivity of the SSEs but also necessitate optimizing the interface contact and combability between the electrode and electrolyte, as well as reinforcing the mechanical and structural stability of the interface. This section summarizes the latest progress in designing interfaces using diverse SSEs and electrode materials.
Cathode–SSE interface
The problems related to the cathode–electrolyte interface in different types of SSEs can be summarized as follows:
- 1)
The rigid nature of ISEs leads to point-to-point contact with the cathodes, thereby negatively affecting the performance of SSSBs. Moreover, the volume change of cathodes can lead to a loss of contact at the electrode–electrolyte interface, exacerbating polarization and interface resistance within the battery system. The chemical compatibility between cathodes and ISEs has not been thoroughly examined yet. Recent experimental results show that some ISEs are also incompatible with the cathode, mainly attributed to the inherent chemical reactivity between the high-voltage cathode and the ISE.
- 2)
The interfacial contact between the electrode and electrolyte may be significantly enhanced based on the mechanical characteristics of SPEs. However, it is worth noting that the reduction and oxidation stability of most SPEs is unsatisfactory compared with the ISEs. Consequently, the electrode and electrolyte interface is susceptible to harmful side reactions that can conspicuously deteriorate the battery's electrochemical performance.
- 3)
Despite being better than SPEs, IP-based CSSEs still have interface problems, such as insufficient interface contact at the cathode/CSSE interface, which leads to a high interfacial impedance.
Several strategies have been developed to create stable, intimate interfaces between cathodes and electrolytes. These strategies can be classified as follows.
Cathode and ISE interfaces
Composites of electrolytes and cathode materials
For ISEs, the conventional approach to enhancing the interface contact involves grinding the ISE surface or a simple cold-pressing process. Alternative approaches have been explored to improve interface contact in ISEs. For example, mixing active materials with ISEs to create a nanocomposite cathode can enhance the interface contact. This method has gained significant popularity for SSSB applications. Fan et al.200 demonstrated the formulation of Na2S-Na3PS4-CMK-3 nanocomposite cathodes via sophisticated melt-casting followed by stress-release annealing. This process aims to foster a close contact of SSE Na3PS4 with mesoporous carbon CMK-3 in the mixed ionic/electronic conductive matrix, significantly reducing the interfacial impedance and enhancing the electrochemical performance, as shown in Figure 9A. Moreover, the resultant nanocomposite cathode manifests high reversible cycling behavior at 60°C with capacity retention of 810 mA h g−1 at 50 mA g−1 for 50 cycles.
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Yamauchi et al.205 constructed a porous Na-β“-Al2O3 layer to magnify the cathode–electrolyte contact area. The Na2FeP2O7 (NFP) cathode material fills the pores of the porous SSE and forms a strong physical bond with the SSE matrix through an anchoring effect, which increases the insertion/release of ions and electrons from the active material. After adding the contact interface, the internal resistance of the battery is reduced from 331 to 120 Ω. The subsequent SSSB is characterized by its high-rate charge–discharge performance and operational flexibility over a range of temperatures and even at a low temperature of −20°C. A pouch cell can also drive a motor at 0°C owing to its extremely low resistance, suggesting the potential use of such electrolytes in high-performance energy storage applications.
Co-sintering or coating
Co-sintering is another effective approach for achieving compact interface contact between the electrode and electrolyte. Wang et al.206 enhanced the grain connection and ionic conductivity of NASICON by adding Na2B4O7 (NBO) to Na3.3La0.3Zr1.7Si2PO12 (NLZSP). According to the EIS and SEM, the NBO additions significantly decreased the grain-boundary resistance, improved the interface contact between the electrode and electrolyte, and encouraged a closer contact between the NLZSP grains. Therefore, the NaTi2(PO4)3 NTP@rGO|NLZSP(NBO3)|Na SSSBs exhibited excellent battery performance, retaining approximately 91.4% of their capacity over 200 cycles at 1 C.
However, the co-sintering method for ISEs still suffers from issues, such as point-to-point contact, void space, and high interfacial resistance between the electrode and electrolyte, necessitating the exploration of improved approaches to enhance their performance further. Coating active material films on ISEs at the atomic scale can tighten the contact between the cathode and the electrolyte and decrease the interface resistance. The deposition of NaxCoO2 onto Na3.4Sc0.4Zr1.6(SiO4)2(PO4) through pulsed laser deposition results in a compact nonvoid interface, thereby showing good long-term cycling performance over 800 cycles—with a mere 8% capacity degradation at the 600th cycle. In addition to coating active materials on ISEs to improve interface contact, coating the ISE film on the active materials is also an effective method. A thin coat of ISE on Mo6S8 surfaces improves the interface contact with the Na3PS4 electrolyte (Figure 9B),201 as confirmed by the EDS analysis. Owing to this ISE-coated Mo6S8 cathode, the corresponding SSSB exhibits a good cycling performance up to 500 cycles at 60°C, thereby outperforming even liquid electrolyte batteries with the Mo6S8 cathode.
Interphase/interlayer engineering
During SSSB cycling, expansion and contraction of the cathode active materials cause structural failure and increased resistance between the rigid ISE and the electrode, accelerating capacity degradation. A straightforward solution is to create an accommodating interphase that shows commendable electrochemical stability while fostering an intimate contact between the electrode and electrolyte. For example, Yang et al.207 utilized polydopamine, an air-stable polymer, to coat the surface of a NASCICON-type (Na3.4Zr1.9Zn0.1Si2.2P0.8O12) electrolyte through an in situ polymerization technique. The coating provided strong adhesion to the electrolyte surface, intimate interfacial contact between the electrolyte and cathode, and flexibility in accommodating volume changes. It maintained structural integrity during the charging and discharging processes. The FeS2|polydopamine-Na3.4Zr1.9Zn0.1Si2.2P0.8O12|Na cell, as a result, manifests high reversible capacities, maintaining 236.5 mA h g−1 at 0.1 C after 100 cycles and an appealing 133.1 mA h g−1 at 0.5 C following 300 cycles.
In addition, the interaction of the cathode with the ISE can lead to significant side reactions, necessitating the use of a protective interlayer to inhibit electrochemical decomposition. Some researchers have provided evidence that implementing interphases can effectively mitigate interfacial resistance and facilitate efficient ion transport between cathodes and electrolytes. Asakura et al.202 introduced a novel hydroborate SSE—Na4(CB11H12)2(B12H12)—characterized by two anionic cages of distinct oxidative stabilities (Figure 9C). It has been demonstrated that [B12H12]2- anions can effectively passivate the interface of a 4V-class cathode (vs. Na+/Na) by the formation of passivating interphases and prevent interface resistance growth during cycling, while the [CB11H11]− anions stand out by ensuring adequate ionic conduction through the interlayer. This interface design strategy enables full-cell performance (cobalt-free high-voltage Na3VO2(PO4)2F cathode and Na metal anode) with discharge capacities of 104 mA h g−1 at 0.1 C and 99 mA h g−1 at 0.2 C, respectively.
Wetting agents on the interface
Alternatively, the addition of ionic liquids (ILs), which are known for their high ionic conductivity, nonflammability, nonvolatility, high thermal stability, and electrochemical stability at high temperatures, to resolve the interface issues of SSSBs is a direct and effective method for improving the interface properties. A practical illustration of this method is the incorporation of N-methyl-N-propyl-piperidinium-bis(fluorosulfonyl)imide (PP13FSI) at the cathode–electrolyte interface (Figure 9D).93 SSSBs with PP13FSI show superior electrochemical performance, sustaining specific capacities of 90 mA h g−1 even under 10 C rate after 10,000 cycles at ambient temperature. Recently, Wang et al.208 synthesized a Na [PF6]–[DEME][PF6] IL that exhibited strong oxidative stability (above 4.5 V on a conductive carbon electrode) to suppress the decomposition of BASE, thus widening the electrochemical window in a full-cell system. Moreover, the IL helps to reduce the interfacial resistance caused by poor contact between the electrolyte and cathode. The BASE/IL configuration ensured that the whole cell was composed of a high-voltage cathode, Na3V2(PO4)2F3, and a Na anode with high energy density and superior cycling performance.
Cathode and SPE interfaces
The conventional method for the preparation of cathodes in SSSBs involves mixing all components (active materials, conductive agent, and binder) and then coating them on the current collector, which often results in inefficient contact points between active particles and the formation of pores due to solvent evaporation. This method can lead to high interfacial resistances, negatively impacting performance. Therefore, incorporating cathodes with SPEs is an effective strategy for reducing interfacial resistance. For instance, when PEO is combined with the Na2V3(PO4)3 cathode material, the corresponding SSSBs can deliver a discharge capacity of 99 mA h g−1 after 100 cycles at 0.2 C and 80°C; this delivered capacity is superior to that of batteries incorporated with conventional Na2V3(PO4)3 electrodes.183
Furthermore, Pan et al.203 introduced PC into a PVDF–HFP SPE, which facilitated the formation of an organic–inorganic hybrid flexible interfacial film with a Na0.67Mn2/3Co1/3O2 (NMCO) cathode material. Hence, it increases the interfacial contact between the cathode and the SPE, prevents concentration polarization, and ensures the rapid transport of Na ions (Figure 9E). The manufactured full cell produced around 72.8 mA h g−1 during 350 cycles with a 1 C current density.
Cathode and IP-based CSSE interface
Although CSSEs have greatly enhanced the interfacial contact between the cathode and the electrolyte, further issues must be resolved. For example, the practical application of PEO-based composite electrolytes in SSSBs faces challenges such as relatively low Na+ diffusion at RT and a tendency toward oxidation at high voltages. Wang et al.180 proposed a hetero-layered composite polymeric electrolyte that enables interfacial stability with a 4.2-V high-voltage cathode (Figure 9F). This approach combined the properties of two different polymer materials. PAN was used close to the high-voltage cathode due to its oxidation resistance, while the PEO was near the anode because of its good compatibility with Na. Na3Zr2Si2PO12 (NZSP) ceramic nanofillers, processed with sand milling to achieve uniform dispersion, were added to this CSSE. Therefore, the assembled FeHCF|PEPA@NC|Na batteries achieved a capacity retention of 93.73% after 200 cycles at RT. To relieve the volume change of the cathode particles during cycling and build better transfer channels for Na+-ions between the cathode and CSSEs, succinonitrile was introduced into the cathode materials using a simple dispersion method. The Na0.67Ni0.33Mn0.67O2/SN-NaClO4|Na3Zr2Si2PO12/PEO|Na SSSBs exhibit both excellent electrochemical performances with high discharge capacity of 73.2 mA h g−1 and high capacity retention of 98.4% after 100 cycles at 0.5 C and 55°C.
Developing anion-trapping 3D fiber network-enhanced polymer electrolytes (ATFPE) (Figure 9G)204 represents another strategic design to facilitate a tight interface between the electrode and electrolyte, enabling quick Na+ transport. The ATFPE integrates electrostatically spun fibers with an orderly arranged MOF, which acts as a structural enhancer with an interconnected 3D channel network for fast ion transport and features anion-trapping capabilities through its abundance of coordination sites. This leads to a high surface area for enhanced interfacial contact and improved electrochemical stability, as evidenced by the high discharge capacity and retention rates observed in the Na3V2(PO4)3|ATFPE|Na battery system, which delivers a high discharge capacity of 117.5 mA h g−1 at 0.1 C and high capacity retention of 78% after 1000 cycles even at 1 C. Either method can increase Na-ion flux and reduce interfacial resistance by increasing the area contacted by the cathode and electrolyte.
Anode–electrolyte interface
For SSSBs, the anode is mainly Na metal or alloy, and the highly active Na anode undergoes side reactions with most SSEs, adversely affecting its electrochemical performance. Moreover, numerous studies have shown that despite the high modulus of ISE, Na dendrites can still be observed in ISE-related batteries. Although the interfacial contacts between most SPEs and the Na anode are more stable and intimate than those of ISEs, they cannot inhibit the continuous growth of dendrites. Therefore, modification of the interface between the Na anode and the different types of SSEs is essential for reducing the interface side reactions and inhibiting dendrite growth, thus improving the overall performance of the SSSBs.
Anode–ISE interfaces
Additives
ISEs synthesized via solid-state sintering or ball milling contain numerous pores and grain boundaries that could be responsible for the growth of Na dendrites. Adding additives to ISEs can reduce the defects in ISEs and thus improve the interface contact and stability between the ISEs and the electrode. For example, the work of Chi et al.25 on oxygen-doped Na3PS4 (Na3PS4−xOx) has shown promising results. The oxygen doping enhances the CCD, making this modified ISE more resistant to Na dendrite formation and enabling higher performance in Na–S batteries at RT (Figure 10A). The pressure-induced sintering of Na3PS4−xOx ISEs at RT creates a homogeneous glass structure with improved mechanical properties and the formation of a beneficial SEI at the Na/SSE interface, which enhances stability and promotes efficient and reversible Na plating/stripping cycles. Recently, Dai et al.215 reported a class of viscoelastic inorganic glasses (VIGLAS) as SSEs that required only oxygen to replace the chlorine of tetrachloroaluminate. VIGLAS exhibits high ionic conductivity for both Li+ and Na+ (~1 mS cm–1 at 30°C) and excellent chemomechanical compatibility with 4.3 V cathodes without additional stacking pressure and therefore enables unpressurized Li- and Na-based SSBs (0.1 MPa). This characteristic allows for the practical development of unpressurized SSBs, as the VIGLAS can infiltrate electrode materials at low melting temperatures, similar to liquid electrolytes, mitigating the mechanical instability issues common with ISEs. Adding cesium to BASE (Cs2O doped BASE) can lower the binding energy between Na and BASE and effectively increase Na's wettability, thus improving the interfacial contact between the electrolyte and electrode. Another example is the Sc-doped Na3.4Sc0.4Zr1.6(SiO4)2(PO4) compound, which has extended the electrochemical stability window superior to that of pure NASICON, ranging from 0.3 to 6 V.216 Furthermore, X-ray photoelectron spectroscopy (XPS) analysis confirms that Sc-doped NASICON shows no significant chemical reaction with Na anode and SEM demonstrated that the Sc-doped NASICON displays nonporous structure, indicating high chemical stability and structural integrity of electrode–electrolyte. The SSEs showed a stable anode performance with CCD up to 0.32 mA cm−1, while the NaxCoO2|NASICON|Na full cells exhibited stable cycling for over 100 cycles.
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Interlayers
Constructing an interlayer between the Na anode and ISEs is a standard method for reducing interface resistance and improving interface compatibility. The incompatibility of most ISEs with Na metal causes a high interfacial impedance, which limits their electrochemical performance at RT and triggers severe dendrite formation on the Na anode. Studies on interlayers are extensive. For example, to improve the electrochemical performance of SSSBs, Yang et al.217 successfully inserted a SnOx/Sn layer by magnetron sputtering at the interface of the Na anode and Na3Zr2Si2PO12 (NZSP). The result is not just a dramatic drop in interfacial resistance from 581 to 3 Ω cm2 but also a significant enhancement in the cycle stability of the Na/Na symmetric battery at RT, sustaining operations well past 1500 h at 0.1 mA cm–2. In addition, the battery exhibited exceptional cyclability and maintained good performance even at current densities of 0.3 and 0.5 mA cm–2. Miao et al.218 developed a novel powder-polishing synthesis technique to produce a pyrolyzed PAN-modified NASICON electrolyte. This approach aims to alleviate the polarization of SSE and the Na anode. With such interfacial optimization, the battery achieved a remarkable CCD (1.4 mA cm–2), indicative of high resistance to the formation of dendrites and offering excellent cycling performance in the assembled whole cell. Zhang et al.209 stabilized the Na anode and Na3SbS4 electrolyte using a molecular layer deposition (MLD) alucone film. This technique has the dual benefit of limiting the degradation of sulfide-based electrolytes and suppressing the formation of Na dendrites (Figure 10B). As a result, the Na-symmetric battery maintained stable cycling over 475 h. Therefore, fabricating an interlayer between the Na anode and ISEs is an effective strategy for enhancing the Na anode–electrolyte interfacial stability.
Alloys or composites of Na anodes
Research has been conducted to improve the interfaces between SSEs and Na anodes in SSSBs through modifications or by using Na alloys. For example, the low melting point of the Na–K alloy is −15°C, and its reduction potential is comparable to that of Li (−3.04 V for Li, −2.71 V for Na, and −2.92 V for K). Notably, the liquid Na–K alloy significantly inhibits dendritic growth, resulting in enhanced battery cyclability. To form a conformal electrolyte–electrode interface with a solid electrode, Guo et al.210 developed a Na–K alloy electrode (Figure 10C). Compared with solid Na metal, the utilization of a liquid metal anode has been observed to enhance the electrode–electrolyte interface contact significantly. Symmetric cells using liquid metal electrolytes exhibit significantly lower overpotentials and superior cyclability compared to cells utilizing solid Na metal. Moreover, the full cell exhibited excellent cyclability, minimal capacity decay over 500 cycles, and good rate performance. In addition, the Na–K alloy significantly improved the interface stability at low temperatures and expanded the practical application scope of SSSBs. Another local anchoring strategy in SSSB technology involves the electrochemical migration of K+ from the cathode to the anode, which can spontaneously generate the liquid Na–K interphase.219 This method mitigates the slow kinetics and dendrite growth encountered at the interface, enabling exceptionally long battery lives at 0°C, and the corresponding batteries show a relatively high capacity retention of 86% after 530 cycles.
The fabrication of composite Na anodes is an effective strategy for constructing stable interfaces in SSSB for regulated Na deposition and safe cycling processes with minimized volume changes and dendritic growth. Xiao et al.220 successfully prepared a sandwich-structured anode consisting of F-superdoped carbon nanotubes and cellulose nanofibrils composite with Na. The corresponding Na cell based on this multifunctional composite electrode exhibited no significant dendrite formation during the plating and stripping processes. Moreover, this structure led to a notably low nucleation overpotential of 7 mV, high Coulombic efficiency through 300 cycles at 3 mA cm−2, and reliable long-term performance of the full battery cell. Lu et al.211 utilized Ti3C2Tx and Na to develop a composite anode Na-Ti3C2Tx, whose nucleation overpotential is considerably lower than that of pure Na. Specifically, at current densities of 0.5 and 2 mA cm–2, the nucleation overpotential of Na-Ti3C2Tx is reduced to 55.2% and 56%, respectively (Figure 10D). This significant reduction in the nucleation overpotential suggests that the composite anode effectively lowers the barrier for Na nucleation, making it more favorable for efficient and controlled Na deposition and preventing Na-dendrite growth. The use of this composite anode, together with a PVDF–HFP-Na3Zr2Si2PO12 modified polyimide membrane in SSSBs, delivered a battery with a high specific discharge capacity of 87.9 mA h g−1 and near-perfect capacity retention over 500 cycles.
Anode–SPE interfaces
The strong reducing properties of Na make it reactive with SPE. Therefore, designing an interface between the SPE and Na is essential to minimizing the chemical reactions. Adding inorganic additives to the polymer electrolytes can positively affect the interface. Peta et al.212 compared SSSBs based on PEO with two salts: NaClO4 and NaPF6. Battery prototypes utilizing PEO-NaClO4 as the electrolyte exhibited enhanced performance in terms of specific capacity and energy content, thus outperforming those employing PEO-NaPF6. The presence of NaPF6 in the electrolyte led to the generation of HF as a byproduct, which detrimentally affected the overall performance of the cells. The detrimental effects of HF were lessened by adding TiO2 nanotubes to the PEO-NaPF6 electrolyte because TiO2 functions as an HF scavenger and improves SSSB performance (Figure 10E).
Anode–IP-based CSSE interfaces
CSSEs benefit both SPEs and ISEs by maintaining intimate interfacial contact and inhibiting dendrite growth. However, each component has its inherent disadvantages. To improve compatibility and interfacial ion conduction between CSSEs and Na anode, a novel approach to the design of the CSSE/anode interface is the use of thin layers of ferroelectrics, which can suppress the growth of SEI (Figure 10F).213 These ferroelectric layers enhance ion diffusion at the electrolyte-ferroelectric-cathode/anode interfaces. Remarkably, implementing ferroelectric materials in SSSBs has led to substantial increases in battery performance, exhibiting an extraordinarily high discharge capacity of 160.3 mA h g–1 with a retention rate of 97.4% after 165 cycles at RT. Moreover, exceptional stability was demonstrated, as the cell maintained a high discharge capacity retention of 86% over 180 cycles, even after an aging period of 2 months. Another innovative approach is the use of heterostructure designs. Yu et al.214 presented Na-ion conductive laminated polymer/ceramic polymer SSEs to resolve the issue of interfacial compatibility. A benign PEO polymer matrix was used on the anode side, which was then integrated with SN to facilitate Na+ ion conductivity at RT (Figure 10G). On the cathode side, cathode-friendly PAN was utilized as the polymer matrix, in which Na3Zr2Si2PO12 powder was incorporated. This dual-layered structure in the SSE includes a PEO-SN-NaClO4 layer that favors the anode and a PAN-Na3Zr2Si2PO12 layer that benefits the cathode. Such a laminate presents a stable electrochemical window from 0 to 4.8 V, indicating good compatibility and ensuring the proper functioning of the SSSB.
CONCLUSIONS AND PERSPECTIVES
SSSBs offer high safety and stability and are more suitable for large-scale energy storage. To date, various types of CSSEs, including II-, PP-, and IP-CSSEs, have been developed with significantly improved ionic conductivities, chemical/electrochemical stability, mechanical properties, and interfacial compatibility. However, they still fail to meet all the requirements for realizing high-performance SSSBs.50–52,79 For example, batteries composed of BASE- and NASICON-based CSSEs, which are II-based CSSEs, exhibit high RT ionic conductivities and chemical/electrochemical stability compared with SSSBs. However, they still need to be synthesized under harsh conditions, and their interfacial compatibility with the electrodes is not optimistic. Sulfur-based CSSEs exhibit high flexibility and good interfacial contact with electrodes; however, their poor chemical stability in air is a major limitation. In addition, hydride-based CSSEs exhibit limited electrochemical compatibility, and their development is constrained by economic and environmental factors. PP- or IP-based CSSEs, despite being flexible and interface-compatible, often exhibit a low ion conductivity at RT and may not remain stable during the battery's charge/discharge cycles, thus affecting the long-term performance of batteries. Furthermore, the Na+ transport mechanisms of different types of CSSEs, including their conduction behaviors, delivery pathways, and internal interactions, have not been fully elucidated. A combination of theoretical simulations, experimental studies, and advanced characterization techniques is utilized to explore the ion-transport mechanisms further and guide the development of high-performance SSBBs.
Trends in CSSEs and interfaces
- 1)
CSSE optimization
In principle, composite structures based on II-based SSEs are designed to lower the energy barrier in the ion-transport channels and incorporate large amounts of mobile charge carriers in the Na+ diffusion pathways. Reducing the lattice symmetry can further increase the ionic conductivity. Moreover, the composite synthesis method should be improved to minimize the effect of grain boundaries on ionic conductivity. The structures of IP-based CSSEs need to be continuously modified to improve their mechanical properties while enhancing the interactions between their two phases to facilitate ion migration. The properties of PP-based CSSEs can be optimized by tuning the functional units via copolymerization, crosslinking, and composite grafting. Calculations based on fundamental principles and machine learning will accelerate the discovery of novel CSSE systems with high ionic conductivities, excellent mechanical properties, and high chemical/electrochemical stabilities.
- 2)
Interface optimization
The structure and morphology of electrode–electrolyte interfaces cannot be accurately determined by most non-in situ characterization techniques. In situ techniques such as in situ XPS, scanning transmission electron microscopy (STEM), and X-ray imaging technologies can be applied to perform dynamic analyses of electrode–electrolyte interfaces and characterize the structure and evolutionary mechanism of the interface as well as further investigate the ion-transport mechanisms at the interface. In addition, techniques such as solid-state NMR spectroscopy and neutron diffraction, combined with simulation and machine learning, can be employed to elucidate the products, reaction processes, and interfacial ion-transport mechanisms and thus guide interface optimization.
Constructing SSSBs with enhanced performances
Creating high-performance SSSBs requires the construction of a suitable SSE and ensuring a stable interface between the electrode and electrolyte. The specific requirements of SSEs and interfaces in SSSBs have been discussed in detail in this paper. Given the current issues and inherent characteristics of SSSBs, the design of SSSBs needs to be further explored (Figure 11).
- 1)
Na metal anodes react chemically with almost all SSEs, change their volume during electrochemical cycling, and lose effective contact with the SSE. In addition, the dendrite growth in Na anodes has not yet been fully solved in SSSBs. To address these drawbacks of Na anodes in SSSBs, the following strategies can be utilized: (1) fabrication of mechanically strong and chemically stable protective layers directly on the surface of Na anodes; (2) formation of 3D host structures of Na anodes to inhibit dendritic growth, lower the local current density, and reduce the volume expansion of Na metal; and (3) utilization of alloy-based Na anodes that reduce the reactivity of Na, suppress the formation of SEI, lower the migration energy barrier of Na+ on their surfaces, and improve surface compatibility with SSEs. Moreover, the development of ultrathin Na/Na alloy anodes is necessary to improving the energy density of SSSBs further.
- 2)
Cathodes require the addition of a high proportion of SSEs to ensure contact between the active material and the SSE. The introduction of an SSE increases the thickness of the cathode at the same areal capacity, which reduces the volumetric and mass energy densities of the SSSBs. Therefore, a cathode preparation process should be developed to increase the proportion of active materials. In addition, strategies such as the development of high-capacity, high-voltage, low-strain, and single-crystal cathode materials or the application of novel SPS and 3D printing technologies to design composite cathodes with microstructures are effective for improving the performance of SSSBs.
- 3)
For SSSBs, the ideal electrolyte film exhibits the possibly minimum thickness. For high-energy-density SSBs, an electrolyte layer thickness similar to that of a commercial separator is necessary to competing with conventional Li/Na-ion batteries. Currently, SSEs are mainly prepared using tape casting, melt blending, and thin-film deposition methods. The tape-casting processes yield low electrolyte layer thicknesses of ~15 μm, whereas thin-film deposition techniques can further reduce their thickness to hundreds of nanometers. In addition, in situ polymerization of electrolytes, which can minimize the effects of electrode interface defects and facilitate tight integration of batteries, offers new opportunities for advancing next-generation commercial SSBs. SSBs are now integrated using the bipolar stacking method and can be computationally analyzed to explore novel architectures and configurations of cells for improved performances and stability.221 Thermal diffusion within SSE remains ambiguous owing to the spatial and temporal resolution of the characterization methods. Simulation of the complex thermal diffusion and thermal runaway processes in batteries can help in designing SSSB configurations with uniform heat distributions and enhanced thermal stability. In addition, the volume change of electrode materials during cycling leads to complex chemomechanical changes inside the SSSBs, and these changes significantly impact the electrochemical stability of the SSSBs. Phase-field method and other modeling approaches could be applied in SSSBs to elucidate the chemomechanical coupling behaviors and thus effectively guide the design of the cell structure. Further investigations on SSSBs are required to gain insights into their fundamental mechanisms and realize their practical application in diverse fields.
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ACKNOWLEDGMENTS
This study was financially supported by National Key R&D Program of China (2022YFB2502004), the Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (23520750400), the Science and Technology Commission of Shanghai Municipality (23160714000 and 21ZR1409300), China Postdoctoral Science Foundation (2022TQ0065 and 2023M730614), Shanghai Postdoctoral Excellence Program (2022029), Thailand Science research and Innovation Fund Chulalongkorn University, and The Program Management Unit for Human Resources & Institutional Development, Research and Innovation (B41G670026).
CONFLICT OF INTEREST STATEMENT
The authors declare that there are no conflicts of interests.
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Abstract
Solid‐state sodium batteries (SSSBs) are poised to replace lithium‐ion batteries as viable alternatives for energy storage systems owing to their high safety and reliability, abundance of raw material, and low costs. However, as the core constituent of SSSBs, solid‐state electrolytes (SSEs) with low ionic conductivities at room temperature (RT) and unstable interfaces with electrodes hinder the development of SSSBs. Recently, composite SSEs (CSSEs), which inherit the desirable properties of two phases, high RT ionic conductivity, and high interfacial stability, have emerged as viable alternatives; however, their governing mechanism remains unclear. In this review, we summarize the recent research progress of CSSEs, classified into inorganic–inorganic, polymer–polymer, and inorganic–polymer types, and discuss their structure–property relationship in detail. Moreover, the CSSE–electrode interface issues and effective strategies to promote intimate and stable interfaces are summarized. Finally, the trends in the design of CSSEs and CSSE–electrode interfaces are presented, along with the future development prospects of high‐performance SSSBs.
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Details
1 Department of Materials Science, Fudan University, Shanghai, China, State Key Laboratory of Molecular Engineering of Polymers, Department of Material Science, Fudan University, Shanghai, China
2 Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok, Thailand
3 Department of Materials Science, Fudan University, Shanghai, China