NASICON-type Na_1+xZr₂Si_xP_3−xO₁₂ electrolytes: progress and perspectives for solid-state sodium metal batteries

Abstract

Solid-state sodium metal batteries (SSMBs) are considered to be the most attractive alternative to lithium-ion batteries on account of their remarkable safety and low cost. Solid-state electrolytes, as a key component, greatly affect the performance of SSMBs. Sodium super ionic conductor (NASICON)-type Na_1+xZr₂Si_xP_3−xO₁₂ (NZSP) electrolytes with excellent mechanical stability, high thermal/chemical stability and high ionic conductivity are some of the most promising electrolytes for SSMBs. The good compatibility of NASICON-type electrolytes with Na metal anodes and high-voltage cathodes is of great significance for SSMBs. This paper reviews the structural characteristics, ion transport mechanism, strategies for enhancing conductivity, and synthesis methods of Na_1+xZr₂Si_xP_3−xO₁₂-based ceramic electrolytes. Simultaneously, it also introduces approaches for addressing the interfacial issues between Na_1+xZr₂Si_xP_3−xO₁₂ electrolytes and solid electrodes. Finally, prospective research directions are also presented to tackle the challenges for the practical application of Na_1+xZr₂Si_xP_3−xO₁₂ electrolytes.

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1 Introduction

Environmental concerns and energy scarcity are becoming major issues owing to the continuous depletion of fossil fuel reserves all over the world.^1–4 Renewable energy sources, including wind, solar, and tidal energy, exhibit characteristics of randomness, intermittency, and volatility, while electricity utilization demonstrates periodicity in both time and space. Consequently, there is increasing demand for the development of energy storage systems with good long-term stability. Lithium-ion batteries (LIBs), which possess high energy density, high coulombic efficiency, and low self-discharge, dominate the energy storage market and are widely applied in electric vehicles (EVs), aerospace, portable electronic devices, and other fields.^5–8 However, the rising costs and limited resources of lithium restrict the large-scale use of LIBs in future energy storage systems, making the development of alternative energy systems urgent.^9–12 Compared to lithium metal, sodium metal has larger reserves, lower costs, and similar chemical properties, making sodium-ion batteries (SIBs) a prospective alternative.^13–15

Sodium metal, with a low redox potential (E = −2.714 V vs. SHE) and a high theoretical specific capacity of 1165 mAh g⁻¹, has emerged as the most attractive anode material for sodium batteries.^11,16 However, the primary challenges with metal anodes are uncontrolled dendrite growth and the occurrence of intense side reactions at the anode–electrolyte interface, which have the potential to trigger internal short-circuits, thermal runaway, and even fires or explosions.^17,18 Conventional LIBs and SIBs assembled with organic liquid electrolytes present a risk of leakage and flammability.^19–23 Solid-state electrolytes (SSEs) possess excellent mechanical properties, acting as physical barriers to inhibit dendrite formation and penetration, thereby enhancing the safety and cycle life of sodium metal batteries.^24–29 Additionally, the high thermal stability and absence of leakage in SSEs facilitate the advancement of high-safety and stable solid-state sodium metal batteries (SSMBs).

In 1976, Goodenough et al.³⁰ first proposed sodium super ionic conductor (NASICON)-type electrolytes with the general formula Na_1+xZr₂Si_xP_3−xO₁₂ (0 ≤ x ≤ 3, abbreviated as NZSP). Owing to their outstanding mechanical stability, high thermal/chemical stability, excellent ionic conductivity (10⁻⁴–10⁻³ S cm⁻¹), and low thermal expansion, they have been widely explored as solid electrolytes for sodium batteries.³¹ The NASICON-type NZSP electrolytes were initially regarded as a substitute for β-alumina in high-temperature Na–S batteries due to their high ionic conductivity at 200 °C, which is comparable to that of β-alumina at 260 °C.³² The replacement of β-alumina electrolyte with NZSP in conventional ZEBRA batteries successfully reduced the operating temperature to 195 °C.³³ Recent research has identified NZSP electrolytes as promising candidates for sodium-ion rechargeable batteries in low-temperature/room-temperature applications, owing to their superior ionic conductivity. Unlike sulfide-based SSEs, the high stability of NZSP electrolytes against H₂O/CO₂ enables their production in an air environment, which is a considerable advantage for commercial industrialization.³⁴ However, the solid–solid contact occurring between the NZSP electrolyte and the solid electrodes gives rise to high interfacial resistance, which in turn severely restricts the performance of SSMBs.

This paper gives an overview of the research progress concerning NASICON-type Na_1+xZr₂Si_xP_3−xO₁₂ (0 ≤ x ≤ 3) solid-state electrolytes for SSMBs. First, we elaborate the fundamental principles of NZSP, including the crystal structure and Na⁺ conduction mechanism. Next, the factors that influence the ionic conductivity of NZSP and the corresponding improvement strategies are comprehensively explored. Optimizing the synthesis conditions, for example, reducing the reaction temperature and shortening the reaction time, effectively mitigates the formation of impurities and structural defects, thus promoting the comprehensive performance of NZSP electrolytes. Novel synthesis methods for NZSP solid electrolytes are introduced, offering new perspectives for future research and applications. Additionally, the NZSP electrolyte/solid electrode interface faces a series of formidable challenges, such as poor interfacial compatibility, high interfacial resistance, and the occurrence of side reactions, all of which significantly hinder the migration of Na⁺ across the interface. To tackle these issues efficiently, this paper systematically outlines multiple strategies for interface optimization.

2 Fundamentals of NZSP solid electrolytes

2.1 Mechanisms of Na⁺ ion transportation in NZSP

As a representative composition of NASICON-type electrolyte, Na_1+xZr₂Si_xP_3−xO₁₂ (0 ≤ x ≤ 3) is regarded as a solid solution of NaZr₂P₃O₁₂ and Na₄Zr₂Si₃O₁₂. It is formed by partially substituting P⁵⁺ with Si⁴⁺ in NaZr₂P₃O₁₂, and then adding excess Na⁺ to balance the anion framework.³⁵ Na_1+xZr₂Si_xP_3−xO₁₂ exhibits two different crystal structures depending on the element composition. The compositions falling within the range of 1.8 ≤ x ≤ 2.2 are in the monoclinic phase (C2/c), while all other compositions are in the rhombohedral phase (R3̄c).^36,37 The monoclinic phase, which can be regarded as a rotational deformation of the rhombohedral structure, undergoes a phase transition to the rhombohedral phase at approximately 400 K.³⁸ As shown in Fig. 1a and 2a, both phases are composed of SiO₄/PO₄ tetrahedra and ZrO₆ octahedra connected in a corner-sharing manner, forming a three-dimensional (3D) framework and facilitating the transport of Na⁺.³⁹ The oxygen ions are shared by the tetrahedra and octahedra, with each ZrO₆ octahedron connecting six SiO₄/PO₄ tetrahedra and each SiO₄/PO₄ tetrahedron connecting four ZrO₆ octahedra. The monoclinic Na_1+xZr₂Si_xP_3−xO₁₂ (1.8 ≤ x ≤ 2.2), particularly when x = 2, exhibits the highest ionic conductivity, reaching 6.7 × 10⁻⁴ S cm⁻¹ and 2 × 10⁻¹ S cm⁻¹ at room temperature and 300 °C, respectively. Therefore, most of the research on NASICON-type electrolytes is based on Na₃Zr₂Si₂PO₁₂.^40–42

Fig. 1 (a) Unit cell and Na⁺ migration channels of rhombohedral NASICON. Reproduced with permission from ref. 43. Copyright 2021, Wiley-VCH. (b) Crystal structure analysis of rhombohedral NaZr₂(PO₄)₃. (c) Na⁺ migration channels of rhombohedral NaZr₂(PO₄)₃ obtained from crystal structure analysis and the BVEL method. (d) Three distinct bottlenecks in Na⁺ conduction pathways in the rhombohedral NaZr₂(PO₄)₃. Reproduced with permission from ref. 44. Copyright 2020, Wiley-VCH.

Fig. 2 (a) Unit cell and Na⁺ migration channels of monoclinic NASICON. (b) Crystal structure analysis of rhombohedral NZSP. (c) Na⁺ migration channels of monoclinic NZSP. (d) The Na1–Na6–Na2–Na6–Na1 channel and the Na1–Na4–Na3–Na5–Na1 channel intersect at Na1 sites. (e) Four equivalent Na1–Na6–Na2–Na6–Na1 channels (A, B, C, and D) and eight equivalent Na1–Na4–Na3–Na5–Na1 channels (E, F, G, H, I, J, K, and L) in one monoclinic unit cell of NZSP. (f) Nine distinct bottlenecks in Na⁺ conduction pathways in the monoclinic NZSP. Reproduced with permission from ref. 43. Copyright 2021, Wiley-VCH.

In order to compensate for the evaporation of Na and P elements during high-temperature calcination, excess sodium and phosphorus salts are usually added to the precursor in the preparation of NZSP.⁴⁵ In 2016, Kang et al. demonstrated that incorporating 10 at% excess Na can overcome the bottleneck, leading to a substantial increase in the total ionic conductivity. The obtained Na_3.3Zr₂Si₂PO₁₂ exhibited ionic conductivity of approximately 8 × 10⁻⁴ S cm⁻¹ at 25 °C.⁴⁶ Liu et al.⁴⁷ systematically studied the dependence of the ionic conductivity of Na_1+xZr₂Si_xP_3−xO₁₂ on the Si/P ratio. When the Si/P ratio was raised to 2.4/0.6, the obtained Na_3.4Zr₂Si_2.4P_0.6O₁₂ showed total ionic conductivity of 5.09 × 10⁻³ S cm⁻¹ and bulk ionic conductivity of 1.5 × 10⁻² S cm⁻¹ at room temperature, which are sufficient for commercial sodium batteries.⁴⁸ The electrostatic repulsion between Si⁴⁺ and Na⁺ is lower than that between P⁵⁺ and Na⁺ during Na⁺ migration. A higher content of Si⁴⁺ reduces the content of P⁵⁺ in the crystal structure, thereby decreasing the electrostatic repulsion between Na⁺ and P⁵⁺ cations during Na⁺ migration. Meanwhile, the Si–O bond in SiO₄ is longer than the P–O bond in PO₄. Therefore, the migration unit that is rich in Si will be larger, which promotes the migration of Na⁺.⁴⁹ Thus, a local environment enriched with Si and Na elements can reduce the migration barrier for Na⁺, and when x = 2.4, the migration barrier reaches a relatively low level, leading to the fastest ionic transport rate.

It is generally believed that an open 3D framework is formed in the rhombohedral phase by two different sodium-ion sites, namely Na1 and Na2 in the rhombohedral phase, and Na⁺ migrates along the Na1–Na2 channels via Na⁺ hopping (Fig. 1b).^44,50 The Na1 site (6b) is defined as a sixfold oxygen-coordinated antiprismatic site, directly situated between two stacked ZrO₆ octahedra. The Na2 site (18e) is limited by an irregular eightfold oxygen polyhedron located between the ZrO₆ octahedra and the SiO₄/PO₄ tetrahedra bonds. Each Na1 site is linked to six surrounding Na2 sites through six equivalent Na1–Na2 channels. Boilot et al.^51,52 initially proposed the existence of an extra mid-Na site named Na3 (36f) in the rhombohedral phase, which is a fivefold oxygen-coordinated site located between the Na1 and Na2 sites in the rhombohedral phase. This site serves as a metastable site for the migration of Na⁺. Shi et al.⁴⁴ analyzed Na⁺ migration channels in the rhombohedral NASICON-type NaZr₂(PO₄)₃ by combining crystal structure analysis, bond valence energy landscape (BVEL) analysis, and ab initio molecular dynamics (AIMD) simulation. They pointed out that there are two conduction pathways for long-distance Na⁺ migration in the rhombohedral NZSP, which are called the Na1–Na3–Na2–Na3–Na1 and Na2–Na3–Na3–Na2 channels, as shown in Fig. 1c. The diffusion channels are composed of three basic parts: Na1–Na3, Na2–Na3, and Na3–Na3. The migration of sodium ions is restricted by a triangular “bottleneck” region composed of three oxygen atoms in the PO₄/SiO₄ and ZrO₆ clusters. As illustrated in Fig. 1d, there are three distinct bottleneck regions located within each of the three channels, designated as bottlenecks A, B, and C. Bottleneck B, located within the Na2–Na3 channel, has an area that is conspicuously narrower than the area of bottleneck A found in the Na1–Na3 channel.^44,53 The simulation results in the temperature range of 800 to 1400 K in the AIMD simulation show that Na⁺ exhibits a preference for diffusing through the Na1–Na3–Na2–Na3–Na1 channel, which is more beneficial for Na⁺ conduction. Therefore, the rhombohedral phase NZSP, characterized by larger Na1–Na3–Na2–Na3–Na1 channels, exhibits superior ionic conductivity.

Gaining insight into the origins of diffusion channels and bottlenecks in the monoclinic phase NZSP is a more intricate process, which can be attributed to the abundance of Na⁺ vacancies within the framework. The analysis of the space group subgroup indicates that the C2/c of group 15, which is a subgroup of the R3̄c of group 167, derives the monoclinic crystal structure (C2/c) from the rhombohedral crystal structure (R3̄c). The monoclinic phase is obtained through a crystal transformation induced by a slight lattice distortion, which splits the Na2 site in the rhombohedral phase into Na2 and Na3 sites in the monoclinic phase.⁵⁴ In the monoclinic phase, there are four different migration channels for the migration of Na⁺, including two Na1–Na2 channels and two Na1–Na3 channels. Despite a general consensus on the structure of the [Zr₂Si₂PO₁₂]³⁻ framework, significant differences have been found in the description of the Na sublattice. The proposed models exhibit diverse numbers of Na sites, accompanied by marked differences in the locations and occupancies of these Na sites.^39,42,51,55 In 2019, Shi et al. initially reported a five-Na-site model and clearly identified an additional Na site that had not been reported previously in the monoclinic phase.⁵⁶ Subsequently, through integrated crystal structure analysis using neutron powder diffraction (NPD) data, BVEL analysis, and AIMD simulation, they conducted an in-depth analysis of the Na⁺ diffusion channels in monoclinic phase NZSP (Fig. 2b–c).⁴³ Six nonequivalent crystallographic Wyckoff sites generated in the monoclinic phase NZSP were identified from the parent rhombohedral phase, that is, the Na1 (6b), Na2 (18e), and Na3 (36f) sites in the rhombohedral phase correspond to the Na1 (4a), Na2 (4e) + Na3 (8f) and Na4 (8f) + Na5 (8f) + Na6 (8f) sites in the monoclinic phase, respectively. The NPD results revealed the non-zero occupancy of the Na1, Na2, Na3, Na4, and Na5 sites in the monoclinic phase,⁵⁶ while the Na6 site was found to be unoccupied. This indicates that the Na6 site in the monoclinic phase functions as an interstitial site in the monoclinic phase through which Na⁺ merely passes during the migration process, rather than residing there in an equilibrium state. The simulation results in Fig. 2d show that there are two Na⁺ diffusion pathways intersecting at the Na1 site, which are called the Na1–Na6–Na2–Na6–Na1 and Na1–Na4–Na3–Na5–Na1 channels. As illustrated in Fig. 2e, within a monoclinic unit cell of Na₃Zr₂Si₂PO₁₂, there are four equivalent Na1–Na6–Na2–Na6–Na1 channels, denoted as A, B, C, and D, along with eight equivalent Na1–Na4–Na3–Na5–Na1 channels, namely E, F, G, H, I, J, K, and L. Moreover, local channels like Na4–Na5, Na4–Na5, and Na5–Na6 are present around the Na1 site, but exhibit insignificant effects on long-distance ionic migration. These channels form nine distinguishable bottleneck regions, and the positions of the bottlenecks are shown in detail in Fig. 2f. The size of the bottleneck in the vicinity of the Na1 site exceeds the combined radii of O²⁻ and Na⁺ ions (2.35 μm based on Shannon's effective ionic radius table⁵⁷), indicating that Na⁺ ions tend to hop between the Na1 site and the nearest Na4, Na5, and Na6 sites. The smallest bottleneck in the Na1–Na6–Na2–Na6–Na1 channel is bottleneck B (2.23 μm), while the smallest bottleneck in the Na1–Na4–Na3–Na5–Na1 channel is bottleneck D (2.38 μm). Therefore, the Na1–Na4–Na3–Na5–Na1 channel provides a wide enough migration pathway for Na⁺, making it more suitable for long-distance migration of Na⁺. By enlarging the bottleneck B to an appropriate size of 2.35 Å, the bulk conductivity of the monoclinic phase NZSP electrolyte has the potential to be further improved.

In conclusion, the conduction mechanism of Na⁺ in NZSP can be perceived as multiple atoms migrating simultaneously and correlatively through the relevant diffusion channels and bottlenecks, which is initiated by the Coulomb interactions occurring between the migrating ions and the neighboring mobile ions. The energy of the Na sites, the size of the bottlenecks, and the concentration of Na⁺/vacancies jointly affect the ionic transport in NZSP. When the size of the bottleneck is larger than 2.35 Å, the sum of the radii of Na⁺ and O²⁻, the concentration of Na⁺ plays a dominant role; otherwise the bottleneck is dominant.⁴³

2.2 Strategies to increase the ionic conductivity of NZSP

The total conductivity (σ_t) encompasses the bulk conductivity (σ_b) and the grain boundary conductivity (σ_GB). Herein, σ_b characterizes the inherent electrochemical properties of SSEs, while σ_GB mirrors processing flaws and secondary phases (SP). Thus, different strategies are required to enhance σ_b and σ_GB.⁵⁸ To enhance σ_b, efforts mainly concentrate on modifying the bottleneck dimensions, augmenting site occupancy, or elevating the cation concentration. Conversely, the enhancement of σ_GB depends on densification or the addition of SP. Elements conducive to increasing σ_GB are predominantly rare earth elements, while the augmentation of σ_b mainly consists of transition metal elements whose ionic radii approach that of Zr⁴⁺.⁵⁹

2.2.1 Element doping for increased bulk ionic conductivity of NZSP. The ionic conductivity of NZSP largely relies on the bottleneck size, which is determined by the framework. Ionic doping, an effective approach for improving ionic conductivity, is used to adjust lattice parameters with the intention of either augmenting the cation concentration or increasing the bottleneck size. In contrast, an excess of Na is required to maintain charge balance when low-valence ions are substituted for some of the Zr⁴⁺. The additional Na⁺ serve as mobile charge carriers, thereby elevating the carrier concentration and enhancing ionic conductivity. Furthermore, doping with cations with large ionic radii contributes to the enlarged bottleneck size and decreases the energy barrier for Na⁺ migration, thus increasing ionic conductivity. In NZSP, transition metals such as Sc³⁺,^60,61 Zn²⁺,^62,63 Fe³⁺,^58,64 Cu²⁺,^65,66 Ni²⁺,⁶⁷ and Y^3+ 68 are frequently doped, and some other metals, like Mg²⁺,^69–72 Ca²⁺,^73,74 and Al³⁺,^75,76 have also been extensively studied. Fig. 3 shows a map of the reported dopants and different doping sites in NZSP SSEs,⁷⁷ while Table 1 lists representative doped NZSP SSEs and their corresponding ionic conductivities.

Fig. 3 A map of the elements that have been studied for doping NZSP SSEs. The elements colored in green, orange, and yellow are doped into the Zr, P, and Na sites, respectively. Reproduced with permission from ref. 77. Copyright 2018, American Chemical Society.

Table 1 Summary of compositions and ionic conductivities of doped NZSP in previous reports

Doping element	Ion radius (Å)	σt at room temperature (mS cm^−1)	σb at room temperature (mS cm^−1)	Nominal composition	Ref.
Ni^2+	0.69	2.284	2.5	Na3.4Zr1.8Ni0.2Si2PO12	67
Ca^2+	1.00	1.59	—	Na3.3Zr1.85Ca0.15Si2PO12	74
Zn^2+	0.74	0.722	—	Na3.3Zr1.85Zn0.15Si2PO12	62
		5.27	6.53	Na3.4Zr1.9Zn0.1Si2.2P0.8O12	63
Mg^2+	0.72	1.16	—	Na3.2Zr1.9Mg0.1Si2PO12	69
		3.64	5.46	Na3.1Zr1.95Mg0.05Si2PO12	93
Cu^2+	0.73	1.94	8.33	Na3.4Zr1.8Cu0.2Si2PO12	66
Mn^3+	0.83	1.586	—	Na3.3Zr1.7Mn0.3Si2PO12	67
La^3+	1.03	1.1	1.43	Na3.1Zr1.9La0.1Si2PO12	94
Al^3+	0.535	6.0	—	Na3.4Zr1.95Al0.05Si2.35P0.65O12	79
		2.15	—	Na3.5Zr1.9Al0.1Si2.4P0.6O12	75
Ru^3+	0.68	2.1	—	Na3.48Zr1.92Ru0.08Si2.4P0.6O12	95
Sc^3+	0.745	1.77	—	Na3.4Zr1.6Sc0.4Si2PO12	60
		1.9	—	Na3.3Zr1.7Sc0.3Si2PO12	61
		2.1	—	Na3.3Zr1.7Sc0.3Si2PO12	96
Fe^3+	0.645	0.753	2.44	Na3.2Zr1.8Fe0.2Si2PO12	58
Sb^3+	0.76	0.51	—	Na3.1Zr1.9Sb0.1Si2PO12	85
Sm^3+	0.96	1.87	3.7	Na3.2Zr1.8Sm0.2Si2PO12	83
Ga^3+	0.62	1.06	—	Na3.1Zr1.9Ga0.1Si2PO12	97
Y^3+	0.9	0.922	—	Na3.1Zr1.9Y0.1Si2PO12	68
Gd^3+	0.94	0.923	—	Na3.1Zr1.9Gd0.1Si2PO12	68
Pr^3+	0.99	1.27	—	Na3.3Zr1.7Pr0.3Si2PO12	82
Eu^3+	0.947	1.08	—	Na3.3Zr1.7Eu0.3Si2PO12	82
Tb^3+	0.923	0.632	—	Na3.2Zr1.8Tb0.2Si2PO12	98
Sc^3+, Ge^4+	0.745, 0.53	4.64	5.50	Na3.125Zr1.75Sc0.125Ge0.125Si2PO12	99
Sc^3+, Ce^4+	0.745, 0.87	2.44	—	Na3.33Zr1.65Sc0.33Ce0.02Si2PO12	100
Te^4+	0.97	1.25	—	Na3Zr1.9Te0.1Si2PO12	101
Ce^4+	0.87	0.69	—	Na3Zr1.9Ce0.1Si2PO12	102
Nb^5+	0.63	5.51	8.39	Na3.3Zr1.9Nb0.1Si2.4P0.6O12	86
		1.6	6.5	Na3.36Zr1.96Nb0.04Si2.36P0.6O12	84
F^−	1.33	0.833	—	Na2.7Zr2Si2PO11.7F0.3	103
		1.41	—	Na2.3Zr2Si2PO11.3F0.7	104

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Doping with low-valence ions is a commonly used strategy to improve the ionic conductivity of NZSP. Song et al.⁷⁸ conducted a comprehensive study on the effects of doping NZSP SSEs with varying amounts of alkaline earth metal ions, including Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, on their crystal structures and ionic conductivities. The ionic conductivity of Na_3.1Zr_1.95Mg_0.05Si₂PO₁₂, which is prepared by doping 2.5% Mg²⁺ at the Zr⁴⁺ site, reaches 3.5 × 10⁻³ S cm⁻¹ at room temperature. Doping with Mg²⁺ enlarges the bottleneck size, reduces the migration resistance of Na⁺, and consequently enhances the ionic conductivity. Meanwhile, doping with Mg²⁺ also reduces the occurrence of the melt phase at the grain boundaries, promoting the formation of a dense microstructure in the material.⁷² Wang et al.⁷⁴ doped low-valence Ca²⁺ at the Zr⁴⁺ site, and the content of Na⁺ in the lattice was significantly boosted by preserving the charge balance. Due to the completely occupied Na1 site in the monoclinic phase, excess Na⁺ promotes the co-migration of anion and lowers the migration barrier. Ca²⁺ doped at the Zr site is also beneficial for the densification sintering of NZSP and promotes the migration of Na⁺ within the lattice. The ionic radius of Zn²⁺ is relatively close to that of Zr⁴⁺ (0.73 Å vs. 0.72 Å). Yao et al.⁶³ doped Zn²⁺ at the Zr⁴⁺ site and simultaneously increased the Si/P ratio; the obtained Na_3.4Zr_1.9Zn_0.1Si_2.2P_0.8O₁₂ had a high ionic conductivity of 5.27 × 10⁻³ S cm⁻¹. Doping with Zn²⁺ promotes bulk conductivity through enlarging the bottleneck size and forms a Zn-rich liquid phase during the sintering process, accelerating mass transfer and achieving liquid-phase sintering. This approach significantly reduces the internal pores of the electrolyte, leading to an enhancement of the NZSP density and a reduction of the grain boundary resistance. Consequently, sodium-ion transport at the grain boundaries becomes smoother, further boosting the overall ionic conductivity. Xun et al.⁷⁹ introduced inexpensive and abundant Al³⁺ into NZSP to prepare Na_3+x+yZr_2−yAl_ySi_2+xP_1−xO₁₂. Doping with Al increases the occupancy of the Na1 sites, elongates the Na1–Na3 distance, enlarges the ion migration space, and reduces the energy barrier (Fig. 4a). When x = 0.35 and y = 0.05, σ_t reaches a maximum value of 6.0 × 10⁻³ S cm⁻¹. Jin et al.⁸⁰ conducted a comprehensive comparison of doping with Al³⁺ and Zn²⁺ over a wide temperature range. At various temperatures, the ionic conductivity of the Zn²⁺-doped NZSP is generally higher than that of the Al³⁺-doped samples, while doping with Zn²⁺ is more effective in reducing the interfacial resistance of NZSP SSEs/Na anode. Furthermore, as shown in Fig. 4b, stable long-term Na stripping/deposition cycles are demonstrated, even at the low temperature of 0 °C, where a low and uniform overpotential of approximately 12.4 mV is maintained for 1800 h during the cycling. The full cell assembled with the Na₃V₂(PO₄)₃ (NVP) cathode demonstrates remarkable cycling stability at various temperatures, exhibiting a discharge capacity retention of 90%. Jolley et al.⁵⁸ compared the ionic transport in solid electrolytes prepared by doping with +2-valence cations (Co²⁺, Ni²⁺, Zn²⁺) and +3-valence cations (Al³⁺, Fe³⁺, Y³⁺). Compared with the NZSP doped with +3-valence cations, the lattice of NZSP doped with +2-valence cations contains more Na, and exhibits a lower resistance and a higher bulk conductivity σ_b.

Fig. 4 (a) Schematic of the effect of Al³⁺ doping on the microstructure. The occupancies of Na3 and Na1 sites, the distances between Na1–Na2 and Na1–Na3 sites, and the total Na content per unit formula for NZSP and NZSP-Al. Ionic conductivity of NZSP electrolytes with different compositions at −40 °C. Reproduced with permission from ref. 79. Copyright 2024, Wiley-VCH. (b) Comparison of the ionic conductivity, Na symmetric cell charge/discharge performance, and NVP/SSE/Na cell cycling performance for NZSP, NZAl_0.1SP, and NZZn_0.1SP. Reproduced with permission from ref. 80. Copyright 2024, Elsevier. (c) The changes of T1 and T2 size, and ion conductivity with the increase of the Sc-doping amount. Reproduced with permission from ref. 60. Copyright 2023, Elsevier.

Doping with rare earth elements, including La³⁺, Pr³⁺, Sm³⁺, Y³⁺, and Sc³⁺, also enhances the σ_t and the σ_b of NZSP. With a similar ionic radius to Zr⁴⁺ (0.72 Å), doping with Sc³⁺ at the Zr⁴⁺ site can form a wide range of solid solutions, such as Na_3+xSc_xZr_2–xSi₂PO₁₂ (0 ≤ x ≤ 0.6). With increasing Sc³⁺ doping, the crystal transforms from a monoclinic to a rhombohedral phase, while the length of the Na–O bond and the area of the triangular bottleneck change.⁶¹ In particular, when x = 0.4, the ionic conductivity of NZSP attains a peak value of 4.0 × 10⁻³ S cm⁻¹ at room temperature. In this case, the bond lengths of Na1–O and Na2–O reach a maximum value. Fig. 4c illustrates that the triangular bottlenecks T1 and T2 between Na1 and Na2, which have almost the same area, are more favorable for the migration of Na ions.^60,81 Doping elements with relatively large ionic radii, like La³⁺, Pr³⁺, Sm³⁺, Y³⁺, and Eu³⁺, has the potential to expand the bottleneck dimension, and their low-valence states contribute to an augmentation of the Na⁺ concentration. It is worth mentioning that the electronegativities of Pr (1.13), Eu (1.20), and Lu (1.27) are inferior to that of Zr (1.33). This disparity in electronegativity enables the strengthening of the ionicity in chemical bonds, consequently reducing the electronic conductivity in Pr/Eu/Lu-doped NZSP.⁸² Doping with rare earth elements also promotes the sintering densification of electrolytes, allowing them to effectively resist uncontrolled dendrite growth, and the dense microstructure facilitates Na⁺ diffusion at the grain boundaries, thereby improving the σ_GB and the σ_t.⁸³

High-valence ions with relatively small ionic radii, such as Nb⁵⁺ (0.64 Å), Ta⁵⁺ (0.64 Å), and Sb⁵⁺ (0.60 Å), can also be used for doping NZSP solid electrolytes.^84,85 After doping with Zr⁴⁺ using high-valence cations, the Na⁺ content does not increase. Instead, vacancies are generated and the electrostatic interaction is changed, which promotes the migration of Na⁺ and can also lower the phase transition temperature from the monoclinic phase to the rhombohedral phase (by approximately 30 K).⁴⁹ However, the high sodium content is essential for enhancing the carrier concentration, thus requiring a focus on adjusting the Si/P ratio. Zhou et al.⁸⁶ prepared Nb⁵⁺-doped Na_3.3Zr_1.9Nb_0.1Si_2.4P_0.6O₁₂ with a σ_b of up to 8.39 × 10⁻³ S cm⁻¹. σ_b is directly proportional to the quantity of charge carriers, which indicates that an increase in the Na⁺ occupancy rate in the crystal leads to an increase in the σ_b. However, an excessively high Na⁺ occupancy rate may inhibit the hopping of Na⁺ due to insufficient vacancies, resulting in a decrease in the σ_b. Therefore, an appropriate ratio between the Na⁺ occupancy rate and the number of vacancies plays an important role in determining σ_b. Ma et al.⁴⁸ proposed that the optimal ratio for the occupancy rate and the number of vacancies is approximately 3.4 : 0.6.

In the design, synthesis, and research of conventional SSEs, the trial-and-error approach of “empirical hypothesis-experimental verification” persists as the dominant methodology. This approach is contingent on researchers’ scientific experience, engendering protracted experimental cycles and considerable costs. Following extensive research, only a limited number of sodium-ion conductor materials have been identified as being stable at room temperature. In recent years, advanced computer-aided methods, including density functional theory (DFT) simulations, ab initio molecular dynamics (AIMD) simulations, and machine learning (ML) techniques, have been successfully employed to develop novel SSEs that exhibit enhanced ionic conductivity, excellent interfacial stability, and superior mechanical properties.^87,88 Ouyang et al.⁸⁹ utilized a synergistic methodology integrating experiments, natural language processing (NLP)-assisted text-mining of literature, and AIMD simulations to systematically quantify the correlations between compositional variables in NASICON compounds and sodium ionic conductivity: (1) the optimal Na content is approximately x = 3, though this is modulated by the polyanion chemistry; (2) prior to reaching the optimal cation size, compositions with larger cation sizes tend to exhibit higher ionic conductivity; and (3) high silicate ratios significantly enhance ionic conductivity. Furthermore, high-throughput DFT calculations predicted a compositional range map for synthesizable NASICON materials, which subsequently led to the successful synthesis of eight NASICON materials. Among them, Na_3.4Hf_0.6Sc_0.4ZrSi₂PO₁₂, which was optimized by integrating AIMD simulations and text-mining results, achieved a room-temperature conductivity of 1.2 × 10⁻³ S cm⁻¹. Recent advances in artificial intelligence have enabled ML to emerge as a transformative tool across diverse scientific disciplines. In contrast to traditional case-specific physical modelling, ML leverages algorithms to automatically derive non-linear correlations among multiple factors from large-scale datasets. Its fast and efficient calculations make it particularly suitable for high-throughput screening and trend prediction for complex systems. ML can be synergistically integrated with DFT and AIMD, using precise data to train models and pre-screen inefficient candidates, thereby improving research efficiency.^90,91 Min et al.⁹² proposed a novel ML-based classification method for the identification of NASICON SSEs with exceptional ionic conductivity. New features are derived from chemical descriptors, including Na content, elemental radius, and electronegativity. An ensemble model with gradient-boosted algorithms was employed to classify 3573 NASICON structures, yielding an average prediction accuracy of 84.2%. Thermodynamic stability and ionic conductivity values of the classified superionic materials were validated via DFT calculations and AIMD simulations, thereby identifying four potential NASICON structures that satisfy the essential performance criteria for SSEs.

2.2.2 Secondary phase for higher grain boundary conductivity of NZSP. In many materials for practical applications, the contribution of σ_GB to the σ_t cannot be ignored, and in certain cases σ_GB even plays a dominant role. In some ceramic materials, the existence of impurities and defects in the grain boundary regions may lead to the grain boundary resistance (R_gb) being much higher than the bulk resistance (R_bulk), making it the dominant component in the total resistance (R_total) and thus limiting the overall electrical conductivity of the material.⁴⁸ The high R_gb in NZSP is attributed to thermal expansion anisotropy, which is manifested as differences in thermal expansion along different lattice parameter directions. For instance, substantial differences in the thermal expansion coefficients of the lattice parameters along the a-axis and c-axis are exhibited during the sintering of rhombohedral NZSP. The anisotropic sintering shrinkage of grains instigates intergranular contact failure and the nucleation of microcracks along grain boundaries, thereby increasing R_gb.^105,106 Consequently, to further enhance σ_t, it is essential to reduce R_gb by modifying the grain boundaries.

Some doping ions have limited solubility in the NASICON-structured NZSP. Instead of substituting for Zr⁴⁺, most of these ions tend to dissolve in the secondary precipitation phases at the grain boundaries. During the charging/discharging process, they form a secondary phase (SP) that can react with the diffusing Na⁺. The SP with high ionic conductivity can fill the gaps between grains, increase the density, and thereby enhance σ_GB.¹⁰⁷ Ions such as Mg²⁺,^108,109 La³⁺,¹¹⁰ Pr³⁺,⁸² Eu³⁺,⁸² Tb³⁺⁹⁸, and Ce⁴⁺¹⁰⁰ can form new phases in NZSP, such as Na_3−2δMg_δPO₄, Na₃La(PO₄)₂, Na₃Pr(PO₄)₂, and Na₃Eu(PO₄)₂. Qu et al.⁷² prepared NZSP-0.2Mg by doping with Mg²⁺ at the Zr site, and its ionic conductivity reached 1.5 × 10⁻³ S cm⁻¹. Due to the limited solubility of Mg²⁺ in NZSP, excess Mg²⁺ enters the grain boundary phase of Na₃PO₄, forming a secondary phase of Na_3−2δMg_δPO₄. The formation of the Na_3−2δMg_δPO₄ phase is verified using dark-field scanning transmission electron microscopy (STEM) (Fig. 5a). At a current density of 0.3 mA cm⁻² and a low overpotential, the Na/NZSP-0.2Mg/Na symmetric cell maintains cycling stability for 7000 h. The corresponding NaCrO₂/NZSP-0.2Mg/Na SSMB shows a coulombic efficiency as high as 99.9%. After 1755 cycles at a rate of 1 C, its discharge capacity stands at 110 mAh g⁻¹, with the capacity loss per cycle amounting to just 0.035%. The excellent performance of NZSP-0.2Mg can be attributed to the fact that the active secondary phase of Na_3−2δMg_δPO₄ maintained a high Na⁺ transference number of NZSP-0.2Mg and inhibited the growth of Na dendrites. Mao et al.⁸² found that when Pr³⁺ was doped at the Zr⁴⁺ site, the formation of Na₃Pr(PO₄)₂ SP would extract some P elements from NZSP, making the Si/P ratio more than 2 and increasing the Na⁺ occupancy rate (Fig. 5b). Meanwhile, doping with Pr³⁺ enables NZSP to form a dense microstructure, which facilitates Na⁺ migration at the grain boundaries and leads to the σ_GB of Na_3.3Zr_1.7Pr_0.3Si₂PO₁₂ reaching a high value of 1.473 × 10⁻² S cm⁻¹. Moreover, Gu et al.¹¹⁰ reported that doping with La³⁺ contributes to the formation of SPs such as Na₃La(PO₄)₂, La₂O₃, and LaPO₄ among the NZSP particles. As shown in Fig. 5c, the formation and distribution of these SPs were verified using spherical aberration-corrected STEM and synchrotron X-ray nanoprobe analysis. SPs affect the Na content in the main crystal phase of NZSP by changing the Si/P ratio, and this change, which accelerates the densification process of NZSP, subsequently leads to a reduction in grain boundary resistance. Na_3.3Zr_1.7La_0.3Si₂PO₁₂ attains a relative density of 99.6%, and it demonstrates an impressively high σ_b of 4.5 × 10⁻³ S cm⁻¹ with a substantial σ_GB of 1.6 × 10⁻² S cm⁻¹ at room temperature. Guo et al.⁶² co-doped with Zn²⁺ and F⁻, which effectively formed a NaZnPO₄ SP and filled grain boundaries. Some of the NaZnPO₄ forms a Na⁺ transport path along the grain boundaries, thereby reducing the grain boundary resistance, which is beneficial for improving the total conductivity.

Fig. 5 (a) Dark-field STEM image of NZSP-0.2Mg and EDS linear scanning across a selected grain boundary particle. Nyquist plots of NZSP and NZSP-xMg tested at room temperature. Galvanostatic cycling profile of the Na/NZSP-0.2Mg/Na symmetric cell under the stepwise current densities of 0.05–0.3 mA cm⁻² and the cycling performance at 1 C rate of NaCrO₂/NZSP-0.2Mg/Na battery. Reproduced with permission from ref. 72. Copyright 2021, Wiley-VCH. (b) Ionic conductivities and relative density curve for Pr³⁺-doped NZSP. Reproduced with permission from ref. 82. Copyright 2022, Elsevier. (c) STEM and synchrotron X-ray nanoprobe analysis images of Na₃La(PO₄)₂ phase. Synchrotron X-ray nanoprobe analysis of La distribution and the high-resolution elemental distribution images of La, P, Zr, and P over the spatial regions indicated by the dotted red box. Reproduced with permission from ref. 110. Copyright 2016, Wiley-VCH.

Numerous studies have shown that sintering additives such as NaF, Na₂B₄O₇ (NBO), Na₃BO₃, Na₂SiO₃, and antimony tin oxide (ATO) can enhance the carrier transport behavior along the grain boundaries.^111–115 Wang et al.¹¹³ used the sintering additive NBO to engineer the grain boundaries to stabilize the Na/Na₃Zr₂Si₂PO₁₂ interface. Na₃Zr₂Si₂PO₁₂-10wt% NBO has a high ionic conductivity of 1.72 × 10⁻³ S cm⁻¹ at room temperature. SEM images revealed that the grain boundaries of NZSP-10wt%NBO are blurred and coated with a glass-like amorphous substance, similar to a binder, derived from the liquid phase formed during the sintering process. The time-of-flight secondary-ion mass spectrometry (ToF-SIMS) results, shown in Fig. 6a, also verified that the NBO layer covered the surface of the NZSP particles. Similarly, SEM images indicate the in situ generation of a “binder-like” glass phase between the grains of NaF-modified NZSP. As shown in Fig. 6b, XPS and solid-state ³¹P NMR spectra further qualitatively and quantitatively confirmed the formation of the Na–Si–P–O–F glass phase.¹¹¹ Analogously, Fu et al.¹¹⁶ constructed an extra glass phase (Na–Ga–Si–P–O phase) by doping with Ga ions at the grain boundaries. The molten glass phase fills the grain boundaries of NZSP, improves the internal connection of NZSP, and increases the ionic conductivity up to 1.65 × 10⁻³ S cm⁻¹. Oh et al.¹¹⁴ added 5 wt% Na₂SiO₃ during the NZSP sintering process to reduce the sintering temperature. Due to its relatively low melting point, Na₂SiO₃ forms a liquid phase between the precursor particles during the sintering process, which fills the voids and defects between grains (Fig. 6c). Subsequently, the liquid phase solidifies along the grain boundaries to form a Si-rich SP. It is the addition of Na₂SiO₃ that promotes the densification of the electrolyte and generates a higher σ_t by enhancing both σ_b and σ_GB, thus obtaining an ionic conductivity of 1.45 × 10⁻³ S cm⁻¹.

Fig. 6 (a) ToF-SIMS 3D illustration of NaO⁻, BO₃⁻ and PO₃⁻ along the depth and the depth profile of bulk (SiO₄⁻ and PO₄⁻) and NaO⁻ and BO₃⁻ signals collected from the by ToF-SIMS. The target sample is NZSP-10wt%NBO obtained at 1000 °C/10 h. Reproduced with permission from ref. 113. Copyright 2021, Elsevier. (b) Solid-state ³¹P NMR spectra of NZSP-xNaF electrolytes and the NMR spectra integral areas as the function of composition. Reproduced with permission from ref. 111. Copyright 2019, Elsevier. (c) Schematic diagram of liquid-phase sintering of NZSP with Na₂SiO₃. Reproduced with permission from ref. 114. Copyright 2019, American Chemical Society. (d) Backscattered electron SEM images of pristine NZSP and LaNbO₄-modified NZSP. Reproduced with permission from ref. 117. Copyright 2025, Wiley-VCH.

The anisotropic thermal expansion of NZSP causes adjacent grains to shrink to different extents in different directions during the sintering and cooling process. As a consequence, it leads to poor contact and microcracks between grains, which substantially augment the grain boundary resistance. The microstructure of the electrolyte can be improved by adding some non-sintering aids to introduce SPs, which can increase the density of the grain boundary region. Ma et al.¹⁰⁶ directly added 2.5 mol% Na₃La(PO₄)₂ to Na_3.4Zr₂Si_2.4P_0.6O₁₂ to counteract the effect of anisotropic thermal expansion and modify the grain boundaries of NZSP. After modification, the total conductivity of NZSP increased to 7.1 × 10⁻³ S cm⁻¹. Na₃La(PO₄)₂ does not change the sintering temperature and relative density of NZSP, so it cannot act as a sintering aid for NZSP. Although the addition of Na₃La(PO₄)₂ does not change the anisotropic thermal expansion of NZSP, it optimizes the microstructure of the grain boundaries by reducing the formation of microcracks and strengthening the inter-grain contact. Zhou et al.¹¹⁷ modified NZSP with monoclinic lanthanum niobate (LaNbO₄), which did not act as a sintering aid. LaNbO₄, a ferroelectric material, improves the mechanical properties of ceramics when it is present in the grain boundaries of the ceramic. Modification leads to an increase in the average elastic modulus and fracture stress of NZSP, which is attributed to the grain strengthening and microcrack reduction in the modified sample. As shown in Fig. 6d, the SPs formed by LaNbO₄ modification fill the remaining pores in the original sample. This indicates that LaNbO₄ improves intergranular contact and makes the ionic transport path smoother, thus increasing the ionic conductivity to 9.3 × 10⁻³ S cm⁻¹ at room temperature.

The results of the present research demonstrate the potential efficacy in the regulation of materials’ microstructure, grain boundary properties, and ionic conductivity by the insertion of specific SPs into NZSP SSEs. SPs play a role in optimizing carrier transport at the grain boundaries and promote ion conduction. For example, additives such as Na₂SiO₃ and Na₂B₄O₇ not only lower the sintering temperature but also improve the microstructure of the material, forming a glassy layer covering the particle surface and affecting the grain boundary properties. Meanwhile, the addition of NaF also triggers a phase transition in the crystal structure, further influencing the material's performance. These findings provide important references for optimizing NZSP SSEs, which is helpful for developing solid electrolytes with better performance and promoting the development of SSMBs.

In summary, the following strategies can be used to increase the ionic conductivity of NZSP: (1) Increase the Na⁺ concentration in the matrix lattice. As mobile charge carriers, increasing the Na⁺ concentration means an increase in the charge carrier concentration, which is conductive to enhancing σ_t. In parallel, it is equally crucial to regulate the proper Na⁺ occupancy/vacancy ratio for ionic conductivity. (2) Design a reasonable bottleneck size. By increasing the bottleneck dimensions, the introduction of appropriate substituents enables the activation energy and Na⁺ migration energy barrier to be reduced. (3) Reducing grain boundary resistance. The ion transportation at the grain boundaries is far more intricate compared to that within the grains. Due to the disruption of the conductive pathways at the grain boundaries, a higher activation energy is required for the ion transport between grain boundaries than that in the bulk phase, which explains the poor ionic conductivity of most oxide-based inorganic SSEs at room temperature. Consequently, decreasing the grain boundary resistance and increasing the electrical conductivity of Na⁺ at the grain boundary is a practical way to improve the ionic conductivity of SSEs.

3 Synthesis methods for NZSP solid electrolytes

Dense Na_1+xZr₂Si_xP_3−xO₁₂ (0 ≤ x ≤ 3) can be synthesized using various methods. The common methods include solid-state reaction sintering,¹¹⁸ solution-assisted solid-state reaction method,⁴⁸ and sol–gel synthesis.¹¹⁹ Solid-state reaction sintering, characterized by high temperatures (generally above 1000 °C) and extended holding durations (exceeding 10 h), is considered the most straightforward and prevalent method as high-temperature processing is simple to perform and controllable. The sintering temperature and holding time exert a substantial influence on the microstructure and electric performance of NZSP SSEs. A high sintering temperature and a long holding time can ensure that a dense and uniform NZSP with high ionic conductivity is obtained. However, during long-term high-temperature sintering, the Na and P elements are prone to evaporation, which may lead to problems with impurities, defect formation, and abnormal grain growth.^120,121

Typically, optimizing synthesis conditions can reduce the generation of impurities and structural defects, thereby improving the σ_GB. One appropriate approach is reducing the sintering temperature and holding time. A range of sintering technologies have been developed and optimized, including spark plasma sintering (SPS),^122–124 spray flame synthesis,^102,125 hot-pressing sintering,^126,127 coprecipitation,¹²⁸ and tape-casting.¹²⁹

Liquid-phase sintering is a method for adding a low-melting-point sintering additive or intergranular phase to a matrix material that is difficult to sinter to promote sintering. At a specific temperature, the liquid phase generated by the sintering additives fills the grain boundaries and defects of the precursor particles by capillary forces, thus promoting mass transfer. Consequently, highly dense SSEs can be obtained using lower sintering temperatures and shorter holding times.^112,130,131 Common sintering additives include oxides,^{115,132–136} fluorides,^111,137 silicates,^114,138 borates,^113,139,140 and glass.^71,141–143 Wang et al.¹³⁴ employed (ZnO)₂-(B₂O₃)₃ (ZBO) as a sintering aid to prepare NZSP. The liquid-phase environment formed by ZBO during sintering accelerates the mass transfer process, significantly reducing the sintering temperature from 1250 °C to 1000 °C. 0.2ZBO-NZSP exhibits an ionic conductivity of 1.58 × 10⁻³ S cm⁻¹ at room temperature. ZBO-NZSP, with its dense microstructure and ZBO-strengthened grain boundaries, exhibits a remarkably low interfacial resistance of 23.3 Ω cm² to the Na anode, thereby ensuring a stable Na stripping/deposition cycle of 1400 h with a current density of 0.3 mA cm⁻² at room temperature. By introducing the intergranular phase CuO, Wu et al.¹³⁵ effectively promoted the densification of NZSP and reduced the densification temperature to 1150 °C (Fig. 7a). NZSP-0.75%CuO has a high ionic conductivity of 1.74 × 10⁻³ S cm⁻¹ at room temperature. Its critical current density (CCD) reaches 0.6 mA cm⁻² as the introduction of CuO enhances the anode/electrolyte contact. Screening for suitable sintering additives demands continuous trial-and-error experimentation, making it inefficient and time-consuming. Zhao et al.¹⁴⁴ proposed a method for rapidly screening sintering additives with the potential for application to a broad spectrum of oxide ceramic systems. As shown in Fig. 7b, the screening is conducted by assessing the wettability of the sintering additives during the co-sintering with the NZSP ceramic substrate. The sintering additives that spread completely over the ceramic piece are considered to be able to effectively enhance the grain boundary contact and promote the densification of the ceramic precursor.

Fig. 7 (a) Schematic of sintering behavior of NZSP with/without CuO sintering additive. Reproduced with permission from ref. 135. Copyright 2022, American Chemical Society. (b) Schematic diagram of the sintering aid screening process and wettability study of some oxides and fluoride as sintering aids for NZSP. Reproduced with permission from ref. 144. Copyright 2023, Wiley-VCH. (c) Schematic of the cold sintering process. Reproduced with permission from ref. 145. Copyright 2021, Elsevier. (d) Temperature versus time curve and schematics of the ultrafast synthesis process. Reproduced with permission from ref. 146. Copyright 2023, Wiley-VCH. (e) Practical operation of the UHS process to produce dense NZSPO SSEs, where the NZSPO pellet was sandwiched between two carbon belts. Reproduced with permission from ref. 147. Copyright 2023, Elsevier. (f) Schematic representation of the impurities present in the NZSP crystal after the use of the ZrO₂ precursor and the formation of a pure NZSP phase after the use of the Zr(OH)₄ precursor. Reproduced with permission from ref. 148. Copyright 2024, Elsevier.

Microwave-assisted sintering is expected to supplant solid-state sintering in the future because of its mild sintering temperature, short sintering time, and high yield.¹⁴⁹ It stimulates the quick and uniform heating of ceramic particles through the interactions between microwaves and the microscopic atoms, ions, and dipoles present in the material. Chen et al.¹⁵⁰ employed rapid, low-temperature microwave sintering to synthesize the NZSP solid electrolyte. Excessive grain growth is significantly suppressed by microwave sintering through rapid and uniform heating, yielding fine-grained NZSP with a relative density of over 96%. The NZSP fabricated via microwave sintering at 850 °C for 30 min exhibits excellent ionic conductivity of 2.5 × 10⁻⁴ S cm⁻¹, comparable to that of the NZSP obtained by solid-state sintering at 1200 °C.

To further reduce the sintering temperature, Randall et al.^151,152 developed a cold sintering process (CSP) that can densify ceramics at extremely low temperatures. Specifically, CSP is a low-temperature liquid-phase sintering that uses water vapor or an aqueous solution as a transitional solvent. At lower temperatures (<350 °C), uniaxial pressure of several hundred MPa is applied to the wetted ceramic powder mixed with the solvent, thereby causing densification (Fig. 7c). However, in some cases, annealing is required after CSP to fully crystallize the material and reach the desired densification.¹⁴⁵ Luo et al.¹⁵³ applied CSP to Mg²⁺-doped NZSP (Na_3.256Mg_0.128Zr_1.872Si₂PO₁₂). After cold sintering at 140 °C and 780 MPa for 1 h, the obtained NZSP electrolyte pellets successfully reached about 83% of the theoretical density. The ionic conductivity of Mg²⁺-doped NZSP is enhanced to exceed 0.5 × 10⁻³ S cm⁻¹ primarily by increasing σ_GB through the subsequent low-temperature annealing treatment at 800 °C. The authors estimated and compared the total energy consumption per specimen prepared by CPS, SPS, and conventional sintering, with CPS sintering having the lowest total energy consumption per specimen.¹⁵⁴ CPS not only significantly reduces the temperature and time for NZSP sintering, but also saves energy and reduces costs, providing a new perspective for NZSP sintering.

Flash sintering can achieve ultrafast sintering within a few seconds to obtain high-purity NZSP with a relative density of approximately 91% under an alternating current electric field at a temperature as low as 700 °C. However, this method usually requires expensive Pt electrodes.¹⁵⁵ Wang et al.¹⁵⁶ from the University of Maryland invented a ceramic synthesis method named ultrafast high-temperature sintering (UHS). As shown in Fig. 7d and e, it only requires directly sandwiching a pressed green pellet of ceramic precursor powder between two Joule-heating carbon strips, and thus thermal radiation rapidly provides the high-temperature environment required for sintering.^146,147 This method has a high heating rate of ∼10³ to 10⁴ °C min⁻¹ and a high sintering temperature of up to 3000 °C, and the densification of ceramics can be achieved in about 10 s. Shi et al.¹⁴⁶ applied UHS to the sintering of NZSP. It only takes 8 s to synthesize Na_3.3Zr_1.7Lu_0.3Si₂PO₁₂ (NZLSP) with an impressive ionic conductivity of 7.7 × 10⁻⁴ S cm⁻¹ at room temperature. The symmetric cell demonstrates excellent stability, with good cycling stability for over 4800 h at a current density of 0.1 mA cm⁻².

Furthermore, enhanced mass transfer and increased reaction driving force can be achieved through mechanical milling, which affords increased surface free energy and surface area for precursor powders. In the synthesis of NZSP, the ball-milling time is shortened to 3 h by employing the high-energy milling (HEM) mechano-chemical synthesis method. After sintering at 1100 °C, dense NZSP particles were obtained with an ionic conductivity of 1.8 × 10⁻³ S cm⁻¹.¹⁵⁷ The efficient optimization of the NZSP synthesis process was attributed to the initiation of the reaction by the HEM during the ball-milling stage, which also significantly improves the efficiency of NZSP preparation.

Considerable advances have been achieved in the optimization of the synthesis process to obtain dense NZSP with superior ionic conductivity. However, the synthesis process is plagued by the presence of grain boundaries and, particularly, by the segregation of monoclinic zirconia impurity phases (m-ZrO₂, P2/c).¹⁵⁸ The formation of m-ZrO₂ impurities is closely related to the evaporation of Na and P elements at high sintering temperatures (>1100 °C) and the high thermodynamic stability of m-ZrO₂.¹⁵⁹ The presence of m-ZrO₂ impurities at the grain boundaries limits the densification of NZSP by exerting a drag force during the sintering process. In addition, the ion-insulating property of m-ZrO₂ also increases the grain boundary resistance. The presence of secondary impurity phases significantly affects the performance of NZSP in terms of ionic conductivity, relative density, and CCD. Adding additional Na/P resources¹⁶⁰ or reducing the amount of Zr precursor¹⁶¹ are commonly used to control the formation of m-ZrO₂ impurities. Mitra et al.¹⁴⁸ used Zr(OH)₄ as the Zr precursor, which reduced the sintering time to 4 hours, and completely removed the m-ZrO₂impurities (Fig. 7f). Zr(OH)₄ is converted to tetragonal zirconia (t-ZrO₂) during the sintering process, which is rapidly and completely consumed in the reaction. The rapid utilization of the Zr precursor significantly reduces the reaction time, minimizing the extent of evaporation of Na or P, which leads to the generation of a pure NZSP phase.

4 Interface engineering

Up to now, most of the research has predominantly centered on enhancing the ionic conductivity of NZSP, while the practical application of NZSP electrolytes in SSMBs is still severely restricted by interfacial issues. Among them, the interfacial issues include interfacial chemical reactions and physical contact. The malleability inherent in composite hydride and sulfide-based SSEs enables an expected physical contact between the solid electrode and the solid electrolyte to be achieved through a simple laminating and pressing process. However, due to their inherent brittleness, oxide-based electrolytes, including NASICON-type NZSP and β-alumina SSEs, usually require hot pressing to achieve intimate contact with solid electrodes. The present section is concerned with the electrical performance of NASICON-type Na₃Zr₂Si₂PO₁₂ electrolytes when paired with a Na anode and cathode at high working potentials, as well as minimizing the interfacial contact resistance.

4.1 Na anode/NZSP electrolyte interface

Currently, Na metal is considered the most attractive anode material with good application prospects given its high theoretical specific capacity of 1166 mAh g⁻¹ and low redox potential of 2.7 V with respect to the standard hydrogen electrode. However, the inadequate wettability between the Na anode and the rigid NZSP is still an obstacle to its application in SSMBs. Goodenough and colleagues recorded that the NZSP surface is incompatible with metallic Na under ambient conditions, which is the reason for the relatively large interfacial resistance between Na and NZSP.¹⁶² The study concludes that the creation of a good interlayer between Na and NZSP is the key to solving the overall problem, and proposes that NASICON be pretreated with Na to establish a robust solid electrolyte interphase (SEI) layer. The Na/NZSP interface is invariably accompanied by insufficient contact, owing to the point contact between Na and NZSP. The Na anode experiences undesirable volume changes during sodium stripping/deposition. The volume changes not only aggravate the situation, but ultimately cause a marked increase in interfacial resistance. The growth of Na dendrites during battery cycling is also an important factor hindering the development of SSMBs, which can lead to penetration of the SSEs and ultimately short-circuit the battery. Furthermore, the space charge layer within the Na/NZSP interface is affected by defects such as surface roughness, impurities, and the uneven particle size distribution of the electrolyte, leading to inhomogeneous deposition of Na⁺ and eventual generation of uncontrolled Na dendrites. The fracture of the dendrite root is attributable to the rapid loss of electrons, which occurs as a consequence of the excessively high current density experienced at the dendrite root during the discharge process. The uneven separation of dendrites can easily transform into unusable dead sodium, which can impede the transport paths of Na⁺ and electrons in the anode, causing significant battery polarization.^163,164

Some advanced characterization techniques have been employed to investigate the dynamics of sodium dendrites, such as environmental transmission electron microscopy (ETEM),¹⁶⁵ magnetic resonance imaging (MRI),¹⁶⁶ and time-of-flight secondary-ion mass spectrometry (TOF-SIMS).^167,168 Jin et al.¹⁶⁹ selected Eu³⁺ ions as a fluorescent probe and used confocal laser scanning microscopy (CLSM) to develop a fluorescence tomography (FT) technique for 3D imaging of sodium dendrites in solid electrolytes. Eu³⁺ has the characteristic of strong red-light emission under ultraviolet (UV) excitation, while Na dendrites do not emit light locally, severely weakening the original fluorescence. As shown in Fig. 8a, two black fluorescence dismissing “holes” that penetrate the red fluorescence region appear in the fluorescence slice image, which was attributed to the formation of Na dendrites. Quantitative analysis shows that after several cycles, small-sized Na islands appear, and as the cycles continue, large-sized dendrites of dozens of micrometers gradually form until the critical Na dendrite volume is reached, resulting in a short-circuit or serious performance degradation. Yin et al.¹⁷⁰ carried out an in-depth investigation into the characteristics and growth mechanisms of Na dendrites in SSMBs and classified them into four types: spalling, straight, branching, and gathering. Different types of dendrites can grow along the grain boundary network in the solid electrolyte and come into contact with each other, ultimately leading to internal short-circuit failures (Fig. 8b). The uneven stress formed by the growth of Na dendrites may promote crack propagation within SSEs, and the newly formed cracks provide space for the further growth of dendrites.

Fig. 8 (a) Schematic configuration of the CLSM for optical slicing and tomography analysis. Layer-by-layer fluorescence slicing images of NZSP-5% EO particles before and after CCD test short-circuiting and the corresponding 3D in-depth models reconstructed by the internal 3D visualizing software of the CLSM. Reproduced with permission from ref. 169. Copyright 2024, American Association for the Advancement of Science. (b) Schematic illustrations of the Na nucleation and growth behaviors of the Na/NASICON interface, and cycled NASICON with different types of dendrites together. Reproduced with permission from ref. 170. Copyright 2021, Wiley-VCH. (c) Nyquist plots showing the evolution of the Na/NZSP/Na symmetric cell impedance after making direct contact with NZSP with two Na metal electrodes. Fitted overall ASR of the cell (R_Total), the NZSP (R_SE), and the SEI type interphase (R_SEI) formed between NZSP and Na metal as a function of reaction time. Reproduced with permission from ref. 171. Copyright 2020, American Chemical Society. (d) SEM images of the NZSP at the short-circuit region and the NZSP in a cross-section, and their elemental EDX mapping of Na, showing the Na dendrite and blade structure along the GBs and voids. Reproduced with permission from ref. 172. Copyright 2024, Wiley-VCH.

NZSP is thermodynamically unstable toward Na metal, but kinetically stable. As shown in Fig. 8c, the impedance increases within the first 15 minutes of contact between the NZSP and the Na metal, indicating a rapid reaction between the two substances. The constant resistance after 15 minutes suggests the formation of a stable interphase at the Na/NZSP interface. The formed interphase, composed mainly of electron-insulating products such as Na₄SiO₄ and Na₂ZrO₃, constitutes a kinetically stable SEI layer that inhibits the reaction from extending into the bulk of the electrolyte, ensuring the interfacial compatibility between NZSP and Na.¹⁷¹ Ma et al.¹⁷³ employed in situ scanning electron microscopy (SEM) to observe the dendrite formation and found that dendrites mainly form along the grain boundaries of NZSP. The three-phase boundary (TPB) of Na metal, the NZSP grain boundaries near the surface, and the gas phase is the active region for the formation of surface dendrites. Fig. 8d shows SEM images of the surface and cross-section of NZSP particles in the Na/NZSP/Na cell after short-circuit cycling, alongside the corresponding EDS surface scan results. Initially, Na metal is deposited at the grain boundaries of NZSP and extends towards the connecting pores. The SEM images show that acicular and lamellar sodium metal has penetrated the grain boundaries and voids of NZSP particles. This reveals that Na dendrites have a tendency to nucleate and grow along the grain boundaries and the voids of NZSP particles.¹⁷² Guo et al.¹⁷⁴ utilized phase-field simulation to demonstrate that raising the current density both accelerated the rate of dendrite growth and promoted an enlarged number of lateral branches. Simultaneously, dendrites exhibited vertical growth at a faster rate than horizontal growth, potentially triggering internal short circuits.¹⁷⁵

During the cycling process of solid-state sodium batteries, the ionic flux (J_ion) across the Na/solid electrolyte interface may not match the flux (J_diffusion) derived from Na self-diffusion. The “front-row” Na⁺ enters the solid electrolyte at a relatively high rate (dominated by J_ion), while the replenishment rate of the “back-row” Na atoms is relatively low (dominated by J_diffusion), that is, J_diffusion < J_ion. This mismatch in rates leads to the formation of voids within the Na layer near the Na/solid electrolyte interface and reduces the contact area between the Na and the solid electrolyte. During cathodic deposition, dendrites are triggered to grow by the formation of point-contact sites after depletion of Na atoms at the interface, due to the extremely high local currents at the point contacts.¹⁷⁶ One method of inhibiting the formation of point contacts at the Na/solid electrolyte interface is to apply pressures ranging from a few MPa to several hundred MPa during cell cycling. As demonstrated in Fig. 9a, under such high pressures, the already generated voids and point contacts will undergo yield deformation due to the plastic deformation (J_creep) of the sodium metal.¹⁷³ Therefore, the potential condition for the generation of voids at the Na/SSE interface changes from J_diffusion < J_ion to J_diffusion + J_creep < J_ion.¹⁷⁷

Fig. 9 (a) Schematic of the impact of pressure on the interface between an SE and a metal anode. Reproduced with permission from ref. 173. Copyright 2022, Wiley-VCH. (b) Nyquist plot of the Na/NASICON/Na cell; schematic illustration of the uniaxial compression of the Na/NASICON; Nyquist plots for the Na/NASICON/Na cells under varied pressures; variation of the areal resistances for the NASICON total resistance and the Na/NASICON interface resistance as a function of the uniaxial compression pressure. Reproduced with permission from ref. 178. Copyright 2019, American Chemical Society. (c) SEM image of the cross-section of the UW-Na/NZSP and Na/NZSP interfaces. The electrochemical impedance spectra and galvanostatic cycling at a current density of 0.1 mA cm⁻² for the assembled Na/NZSP/Na and UW-Na/NZSP/Na-UW symmetrical cells. Reproduced with permission from ref. 179. Copyright 2021, Springer Nature. (d) Schematic illustration of the NZSP surface structure change in moisture and further heat treatment, and comparison of EIS profiles and dc cycling at a current density of 0.1 mA cm⁻² for symmetric cells of Na/NZSP/Na and Na/NZSP (HT)/Na. Reproduced with permission from ref. 180. Copyright 2020, American Chemical Society.

4.1.1 Physical approaches. The physical contact area of Na/SSE interface can be adjusted by applying external pressure. Increased pressure is associated with an enhancement in the number, density and size of microscopic contact points at the Na/NZSP interface. Consequently, the area of interface contact increases and the interfacial resistance decreases.¹⁸¹ In Fig. 9b, by applying a uniaxial pressure of 30 MPa to Na and the NZSP electrolyte, the interfacial resistance was reduced to 14 Ω cm². However, the high pressure is not applicable in practical applications.¹⁷⁸ Mao et al.^179,182 adopted a room-temperature ultrasonic solid welding strategy to construct a tight interfacial connection, which exhibit a relatively low interfacial impedance of 22.6 Ω cm² between Na/NZSP, as shown in Fig. 9c. The energy generated by high-frequency ultrasonic vibration can be transformed into deformation energy, frictional work, and a confined temperature increase, leading to rapid spreading of interfacial atoms and the development of tightly bound interfaces. The ultrasonic friction quickly broke the oxide film on the surface of the Na and the gas film on the surface of the NZSP, forming a tight interface without microscopic void defects between the Na and the NZSP.

The surface of the NZSP forms a sodium-repellent Na₂CO₃ and NaOH layer after prolonged exposure to humid air, which has been proposed to possess inferior Na wettability and low Na⁺ conductivity.¹⁸⁰ Therefore, this layer has the potential to augment the interfacial resistance over several magnitudes. Physical polishing or high-temperature annealing is usually used to remove the surface by-products.^183,184 The hydroxide on the surface of NZSP can be removed by annealing at 600 °C, and the annealed NZSP shows better Na wettability.¹⁸⁵ Aguadero et al.¹⁸⁶ found that the improvement of the electrochemical performance of NZSP by heat treatment is due to the in situ generation of a thermodynamically stable nano-Na₃PO₄ layer during thermal activation. Compared with bare NZSP, the DFT results showed that the surface energy of NZSP coated with Na₃PO₄ is considerably lower. This lower surface energy explained the decrease in the Na/NZSP interfacial resistance and the increase in the CCD. Huang et al.¹⁸⁰ proposed that after the heat treatment decomposes the surface by-product layer, the Na-ion-deficient surface interacts with Na to generate a Na₂O passivation layer, which acts as the primary factor suppressing the decomposition of NZSP by Na (Fig. 9d). At the same time, quasi-in situ X-ray photoelectron spectroscopy analysis by DFT was also used to confirm that the passivation layer, characterized by low interfacial resistance and chemical stability, effectively enhances the Na metal wettability.

4.1.2 NZSP SSEs optimization. Conventional surface cleaning methods, including physical polishing and high-temperature annealing, are incapable of entirely eradicating microscopic imperfections like pores and grain boundaries on the surface of NZSP. The microscopic defects are likely to have a high local current density and become potential hotspots for the formation of Na dendrites. Zhao et al.¹⁸⁷ adopted a two-step sintering approach to precisely control the NZSP electrolytes microstructure evolution, which enhanced the densification of NZSP and its interfacial compatibility with Na anode. As illustrated in Fig. 10a, during the two-step sintering process, the NZSP precursor was first heated to a higher temperature and maintained for a short time, ensuring the formation of NZSP crystals while limiting the grain growth time. Then it was cooled to a lower temperature and maintained for a long time, promoting the tight connection between grains but lacking the thermodynamic conditions for rapid grain growth. This approach facilitates the attainment of a dense microstructure characterized by uniform grain size. As a result, mechanical and functional properties, including room-temperature ionic conductivity and sodium dendrite resistance, are significantly improved.¹⁸⁸ The CCD of symmetric Na batteries with samples sintered in two steps can reach 0.95 mA cm⁻², indicating that the two-step sintered samples demonstrate a notable tolerance to Na dendrites. The dense and uniform microstructure of precisely regulated NZSP alleviates the space charge layer and improves interfacial compatibility, enabling the symmetric Na batteries to operate stably at current densities of 0.5, 0.1, and 0.2 mA cm⁻² with low polarization voltages. Additionally, the air stability of the two-step sintered samples is enhanced due to the increased Na–O bond strength (Fig. 10a). Huang et al.¹⁸⁹ added a certain amount of TiO₂ during the sintering of NZSP, which reduced the sintering temperature, thereby suppressing grain growth and facilitated a more homogeneous distribution of particle sizes. In addition, the SEM images in Fig. 10b demonstrate that the addition of TiO₂ facilitates tight contact between NZSP grains, significantly reducing the intergranular connectivity porosity and microscopic defects and enhancing the microstructure between grains. NZSP-TiO₂ has higher density, Young's modulus, and hardness, which inhibits the growth of dendrites, microcracks, and micropores along the grain boundaries. Jin et al.¹⁹⁰ introduced the ferroelectric phase BaTiO₃ (BTO) into NZSP. The issue of uneven Na⁺ diffusion and slow charge transfer kinetics at the Na/NZSP interface was solved by the built-in electric field generated by the ferroelectric effect. The ferroelectric phase is also able to toughen the ceramic substrate through piezoelectric and ferroelectric effects, relieving the stress caused by Na deposition to ensure the integrity of the interface (Fig. 10c). At room temperature, the ionic conductivity of NZSP-3wt%BTO reaches 0.96 × 10⁻³ S cm⁻¹, and an excellent CCD of 1.05 mA cm⁻² is obtained for the symmetric cell. Zhan et al.¹⁹¹ added the Na₃PO₄ phase to NZSP to form a new type of biphasic NZSP-Na₃PO₄ composite electrolyte. The addition of Na₃PO₄ promoted the densification process of NZSP as a sintering aid while enhancing the wettability of molten Na on the electrolyte, as shown in Fig. 10d.

Fig. 10 (a) Schematic diagram of the one-step and two-step sintering schedules. SEM images of the surface of the as-sintered, air-exposed, and immersed-water NZSP samples. Reproduced with permission from ref. 187. Copyright 2024, Wiley-VCH. (b) SEM images of the surface and cross-sectional microstructures of the NZSP and NZSP(TiO₂) pellets. Reproduced with permission from ref. 189. Copyright 2022, Wiley-VCH. (c) Ferroelectric-enhanced Na/NZSP interface as well as the densification deposition of Na metal. Reproduced with permission from ref. 190. Copyright 2022, Wiley-VCH. (d) Schematic illustration of the core concept of the biphasic NZSP-Na₃PO₄ electrolytes. Digital photographs of molten Na on NZSP-20NP and NZSP-40NP surfaces at 150 °C. Reproduced with permission from ref. 191. Copyright 2024, Elsevier.

4.1.3 Intermediate layer. In SSMBs, artificial intercalation is commonly incorporated to enhance the stability and wettability of the electrode/electrolyte interface, thus leading to lower interfacial resistance. The artificial interlayers play a crucial role in forming a uniform ionic channel and effectively impeding dendrite growth. Simultaneously, due to their low electronic conductivity, these interlayers enhance interfacial stability, thereby effectively suppressing harmful parasitic reactions.^192,193 Usually, these artificial interlayers can be classified as either organic or inorganic types.

Yin et al.¹⁹⁴ introduced an artificial AlF₃ interlayer on NZSP through a spin-coating and sintering process. As shown in Fig. 11a, the uniform AlF₃ layer can prevent the defects on the surface of NZSP from directly coming into contact with Na metal, thereby preventing Na dendrite growth at the pores. The AlF₃-coated NZSP has good wettability and interfacial resistance without affecting the ionic conductivity. The conversion reaction of AlF₃ with Na forms a buffer layer with high ionic conductivity in situ during the initial cycling process, which effectively enhanced the cycling stability of SSMBs. The buffer layer has the property of promoting current redistribution when the current is increased, which is achieved by inducing more Na⁺ to react with AlF₃ to release locally accumulated electrons. The interfacial optimization of the AlF₃ interlayer significantly improves the dendrite resistance of the battery, thus a high CCD value of 1.2 mA cm⁻² was acquired for the symmetric cell at 60 °C. Ni et al.¹⁹⁵ designed a wurtzite-type ZnO piezoelectric interlayer on the surface of NZSP by Radio Frequency Magnetron Sputtering. During the charging and discharging processes, the piezoelectric interlayer will be subjected to stress due to factors such as volume changes caused by the intercalation and deintercalation of ions, thereby generating a local stress-induced field. This electric field was able to induce a redistribution of the charge at the interface and to alleviate the phenomenon of charge aggregation, thus promoting the uniform deposition of sodium ions. The wurtzite-type ZnO offers a low Young's modulus, high work functions of adhesion, and an excellent effective longitudinal piezoelectric constant. As a consequence, it can provide a favorable interfacial phase, which assumes a pivotal role in augmenting the interfacial properties and reducing the interfacial resistance. As shown in Fig. 11b, according to the TOF-SIMS results, ZnO-NZSP exhibits uniform Na deposition, without evident agglomeration or the presence of undesirable interfacial species. Furthermore, the Zn element exhibits stability within the Na/ZnO-NZSP interface without penetrating into the NZSP bulk. The symmetric cells of ZnO-NZSP SSEs acquire a high CCD value of 1.1 mA cm⁻² and stable cycling performance for 4900 h with a current density of 0.1 mA cm⁻² at 30 °C. Apart from the aforementioned artificial interlayers, CuO,¹⁹⁶ SbF₃,¹⁹⁷ SnF₂,^198–200 SnS₂,²⁰¹ and Pb/C²⁰² have also been reported as interlayers between Na and NZSP.

Fig. 11 (a) Dendrite suppression mechanism enabled by the AlF₃ coating and the modeling plus the calculated interfacial energy between different SEI components and the number of Na metal formula units. Reproduced with permission from ref. 194. Copyright 2020, Elsevier. (b) Schematic of the Na dendrite evolution with a high electron leakage and with a piezoelectric electron-blocking interlayer at the Na/SSEs interface. Surface mapping of ZnO-NZSP collected in negative mode ToF-SIMS on charged fragments after cycling, and in-depth ToF-SIMS-derived models of the charged fragments for ZnO-NZSP after cycling. Reproduced with permission from ref. 195. Copyright 2023, Wiley-VCH. (c) SEM images and schematic illustrations of the Na anode recovered from cycled Na symmetrical cells. Reproduced with permission from ref. 203. Copyright 2021, Wiley-VCH. (d) Schematic illustration of the interface evolution of Na/NZSP and Na/BiCl₃@NZSP, and the electrochemical performance of Na/NZSP/Na and Na/BiCl₃@NZSP/Na symmetric cells. Reproduced with permission from ref. 204. Copyright 2024, Wiley-VCH.

Besides metals and their compounds, flexible polymers as interlayers can reduce interfacial impedance and side reactions; typical polymer matrices are PEO, PVDF-HFP, PAN, and PVP.²⁰⁵ Yin et al.²⁰³ utilized the powder-polishing method to fabricate a sulfurized PAN (SPAN) interlayer on the surface of NZSP. The powder-polishing method yielded a polymer layer with a thickness of approximately 1 μm, which not only achieves the purpose of improving the interfacial wettability but also reduces the negative effects of the polymer layer, such as low ionic conductivity. Na⁺ was easily bonded to SPAN via the short chain S–S segment, reducing polarization (Fig. 11c). The isotropic properties and delocalized radicals potentially confer upon the SPAN interfacial layer the capabilities to evenly redistribute the transported electrons/ions and preserve outstanding stability throughout the cycling process. Benefiting from the reasonably designed SPAN interlayer, the symmetric battery attains a high CCD value of 1.4 mA cm⁻² at room temperature. Guo et al.²⁰⁶ constructed a polymer interlayer composed of PEO, SN, and NaClO₄ between NZSP and the Na anode. The interlayer was in close contact with the Na anode, which likewise reduces the interfacial resistance. Fan et al.²⁰⁴ established a flexible composite interfacial layer of BiCl₃/polytetrafluoroethylene (PTFE) by the spin-coating method, effectively filling all the pores and defects on the surface of NZSP and providing excellent interfacial contact. The in situ electrochemical reaction between BiCl₃ and Na formed a multifunctional flexible interlayer mainly composed of Na_xBi and NaCl. Among them, Na_xBi effectively accelerated the diffusion of Na⁺ at the interface, and NaCl blocked the injection of electrons and inhibited Na dendrite growth at the interface. As demonstrated in Fig. 11d, owing to the synergistic effects of the hybrid interfacial layer of NaCl and Na_xBi, the Na/BiCl₃@NZSP/Na symmetric cell not only showed a dramatic enhancement of the CCD from 0.2 mA cm⁻² to 2 mA cm⁻², but also exhibited a remarkable cycling stability of 1100 h at 0.3 mA cm⁻².

4.1.4 Na anode optimization. Modification of the Na anode is relatively simple in comparison with the complex engineering strategies for electrolytes, owing to its low melting point and ease of processing. According to the binary phase diagrams of Na with other metals, Na is capable of forming thermally stable alloys with In, Ge, Sn, Sb, Bi, etc.^{103,207–209} The alloy interface can promote uniform and compact Na deposition by lowering the nucleation barrier of Na and alleviating continuous side reactions. Additionally, interfacial Na⁺ transport enhancement could be achieved by enhancing the Na⁺ diffusivity in the alloy and improving the Na wettability.²¹⁰ Among them, the sodium–potassium (Na–K) alloy is in a liquid state within a wide Na content range with an atomic ratio from 9.2% to 58.2% at room temperature, and there is no dendrite growth in this liquid alloy itself. Theoretically, in the range of liquid-phase compositions, Na–K metals as Na anodes can provide a theoretical specific capacity of 629 mAh g⁻¹, while demonstrating a similar mechanism of operation to the Na anode in battery systems.^211,212 Gu et al.²¹³ prepared a carbon fiber-supported liquid Na–K alloy anode to improve the interfacial contact with Zn²⁺-doped NZSP (ZNZSP). The Na–K alloy is in close contact with ZNZSP (Fig. 12a), contrary to the obvious gap between Na and ZNZSP. The Na–K symmetric cells demonstrate stable Na stripping/deposition, attaining an outstanding CCD of 40 mA cm⁻² and excellent cycling performance for 1000 h at a high current density of 4 mA cm⁻². Low-temperature high-resolution transmission electron microscopy (HRTEM) verified the generation of a SEI layer on the surface of ZNZSP, which is composed of highly chemically/electrochemically stable Na₂O and ionically conductive sodium phosphates (NaPO₃ and Na₄P₂O₇), thus significantly enhancing the battery cycling stability and CCD. Lu et al.²¹⁴ proposed a composite negative electrode composed of Na and Na₁₅Sn₄, which shows good wettability on the surface of NZSP and has a high CCD value of 2.5 mA cm⁻².

Fig. 12 (a) Cross-sectional SEM images of Na/ZNZSP and Na–K/ZNZSP interfaces after 50 cycles. Cycling performance of Na/ZNZSP/Na and Na–K/ZNZSP/Na–K symmetric cells. Reproduced with permission from ref. 213. Copyright 2022, American Chemical Society. (b) Physical contact schematic of the NZSP with Na and Na_xTiO₂. Digital photos of the wetting behavior of melted Na and Na_xTiO₂ on NZSP pellets. Cross-sectional SEM image of the Na/NZSP and Na_xTiO₂/NZSP interface. Reproduced with permission from ref. 215. Copyright 2024, Elsevier. (c) Schematic of the trilayer NZSP solid-state electrolyte and voltage profiles of the cycling in the trilayer and planar electrolyte-based Na symmetric cells with SnO₂ decoration. Reproduced with permission from ref. 216. Copyright 2024, Wiley-VCH. (d) Schematic diagram of sandwich composite electrolyte structure. Reproduced with permission from ref. 217. Copyright 2021, Elsevier. (e) Cross-sectional SEM of the NZSP trilayer architecture and CCD test and galvanostatic cycling of the Na/trilayer-NZSP/Na symmetric cell. Reproduced with permission from ref. 218. Copyright 2024, Royal Society of Chemistry.

It has also been proven that the preparation of hybrid anodes by compounding Na metal with other materials, such as oxides, sulfides, and halides, is an appropriate strategy. Hu et al.²¹⁵ obtained Na_xTiO₂ with a high ionic diffusion coefficient through the intercalation reaction of titanium dioxide (TiO₂) with molten Na. Na_xTiO₂ can serve as an ionic transport channel between the Na anode and NZSP, thus preventing the formation of gaps and maintaining stable interfacial contact. As demonstrated in Fig. 12b, the sodiophilic Na_xTiO₂ is distributed uniformly within the molten Na, reducing the surface tension of the hybrid anode and the interfacial formation energy between the anode and the electrolyte, ultimately achieving good chemical and physical interfacial contact. Benefiting from these advantages, the Na symmetric battery using the hybrid anode exhibits a low interfacial resistance of only 3.7 Ω cm² at room temperature and has a high CCD of 1.3 mA cm⁻². It shows remarkable cycling performance for 8000 hours at a current density of 0.1 mA cm⁻², with no Na dendrites observed to penetrate into the NZSP during cycling. In addition, it has been reported that preparing hybrid anodes by combining SbF₃,²¹⁹ SiO₂,²²⁰ and NZSP²²¹ with Na can improve the interfacial contact, presenting a notably decreased interfacial resistance and greatly enhanced cycling stability.

4.1.5 Structure design. The 3D microstructure arrangement at the Na/NZSP interface facilitates achieving tight interfacial contact and can expand the effective area for Na stripping/deposition. Lu et al.²¹⁶ prepared a 3D three-layer monolithic Ca²⁺-doped NZSP electrolyte structure composed of a dense layer and two porous support layers through co-pressing to decrease the interfacial resistance, as shown in Fig. 12c. To enhance the contact of the Na/NZSP interface, the SnCl₄ solution was injected into the porous layer to form a buffer layer with affinity for sodium. The unique design enables molten Na to infiltrate into the porous layer through capillary action and form a tight contact with the electrolyte when cooled. The symmetric cell, utilizing a three-layer monolithic electrolyte, exhibited stable cycling performance for more than 600 h within the current density range of 0.1–0.3 mA cm⁻² at room temperature, demonstrating a low overpotential. Knibbe et al.²¹⁷ prepared the same NZSP electrolyte structure through co-pressing. The electrochemical window of NZSP was widened by incorporating anti-reduction PEO into the porous NZSP electrolyte facing the Na anode and anti-oxidation PAN into the porous NZSP electrolyte facing the cathode, as shown in Fig. 12d. Simultaneously, the polymers formed a close and stable interface with the electrodes, thus significantly reducing the interfacial resistance. Wachsman et al.²¹⁸ used commercially scalable tape-casting to prepare a similar 3D structure and coated nanoscale ZnO on the surface of NZSP by atomic layer deposition (ALD), which substantially improved the Na wettability of the electrolyte. As illustrated in Fig. 12e, benefiting from the low interfacial resistance of 3.5 Ω cm², the symmetric cells using the three-layer electrolyte exhibited a high CCD of 40 mA cm⁻² and cycling stability for over 600 h at high current densities of 5, 10, and 15 mA cm⁻².

To comprehensively assess recent advances in SSE/Na interfacial modification approaches in SSMBs, Table 2 summarizes some of the principal parameters, including the modification techniques employed, the ionic conductivity of NZSP SSEs, and the electrochemical performances of Na/Na symmetric batteries and SSMBs. A summary and in-depth analysis of these advanced interfacial modification strategies will yield invaluable timely insights and serve as a critical reference for the commercialization of SSMB.

Table 2 Comparison of symmetric cell and full cell electrochemical performances of NZSP-based solid-state sodium metal batteries enabled by various anode interface designs

Strategies		SSEs	σ (mS cm^−1)	Na/Na symmetric batteries		SSMBs			Ref.
				CCD (mA cm^−2)	Cycling performance	Cathode	Cycling performance	Capacity retention
SSEs optimization	Two-step sintering	Na3Zr2Si2PO12	1.1	0.95	0.1 mA cm^−2/1000 h	Na3.5V0.5Mn0.5Fe0.5Ti0.5(PO4)3	1 C/400 cycles	90%	187
	TiO2	Na3Zr2Si2PO12(TiO2)	0.66	1	0.1 mA cm^−2/750 h	Na3V2(PO4)3/C	0.2 C/100 cycles	86.3%	189
	BaTiO3	Na3Zr2Si2PO12-3 BaTiO3	0.96	1.05	0.3 mA cm^−2/1000 h	Na3V1.5Cr0.5(PO4)3	100 mA g^−1/400 cycles	84.4%	190
	Na3PO4	Na3Zr2Si2PO12-40 Na3PO4	0.62	0.8	0.1 mA cm^−2/3000 h	Na3V2(PO4)3	0.5 C/550 cycles	93%	191
	Ga2O3	Na3Zr2Si2PO12-0.15 Ga	1.65	—	0.2 mA cm^−2/700 h	Na3V2(PO4)3	1 C/500 cycles	90%	116
	K2CO3	Na2.995K0.005Zr2Si2PO12	0.663	1.3	0.2 mA cm^−2/1400 h	Na3.5V0.5Mn0.5Fe0.5Ti0.5(PO4)3	1 C/200 cycles	84%	222
Intermediate layer	AlF3	Na3Zr2Si2PO12	0.23	1.2 (60 °C)	0.15 mA cm^−2/150 h and 0.25 mA cm^−2/300 h (60 °C)	Na3V2(PO4)3/C	1 C/100 cycles	80.7%	194
	SnF2	Na3Zr2Si2PO12	0.776	1.4	0.1 mA cm^−2/17 000 h	Na3V2(PO4)3	1 C/2000 cycles	89.3%	223
	SnS2	Na3.4Zn0.1Zr1.9Si2.2P0.8O12	5.27	9.4	0.2 mA cm^−2/1500 h	Na3V2(PO4)3	1 C/1000 cycles	88.1%	201
	TiO2	Na3Zr2Si2PO12	0.53	—	2 mA cm^−2/750 h	Na3V2(PO4)3/C	0.1 C/60 cycles	70.6	224
	Pb/C	Na3Zr2Si2PO12	0.61	0.7 (55 °C)	0.5 mA cm^−2/1800 h (55 °C)	Na3V2(PO4)3	0.5 C/300 cycles	96.5%	202
	α-Fe2O3−xFx	Na3Zr2Si2PO12	0.9	1.9 (80 °C)	0.2 mA cm^−2/800 h	Na3V2(PO4)3/C	1 C/120 cycles	96%	225
	Au	Na3Zr2Si2PO12	0.45	0.8	0.3 mA cm^−2/900 h	Na3V2(PO4)3	1 C/300 cycles	95.6%	182
	Sn	Na3Zr2Si2PO12	0.59	1	0.1 mA cm^−2/1500 h	Na3V2(PO4)3	1 C/100 cycles	71%	34
	Graphite	Na3.4Zr1.8Ca0.2Si2PO12	2.09	3.5	1 mA cm^−2/1000 h (60 °C)	Na3V2(PO4)3	1 C/500 cycles	97.75%	226
	ZnO	Na3Zr2Si2PO12	—	1.1 (30 °C)	1 mA cm^−2/5000 h (30 °C)	Na2MnFe(CN)6	2 C/1600 cycles (30 °C)	84%	195
	SnCl4	Na3Zr2Si2PO12	0.559	0.8	0.3 mA cm^−2/1000 h	Na3V2(PO4)3	0.5 C/1071 cycles	93%	227
	GaIn	Na3Zr2Si2PO12	0.206	0.8	1 mA cm^−2/3000 h	NaNi1/3Fe1/3Mn1/3O2	0.5 C/100 cycles	85.1%	228
	CuO	Na3Zr2Si2PO12	0.937	0.3	0.3 mA cm^−2/5000 h	Na3V1.5Al0.5(PO4)3	5 C/2250 cycles	99%	196
	SPAN	Na3Zr2Si2PO12	0.224	1.4	0.1–0.25 mA cm^−2/500 h	Na3V2(PO4)3/C	0.5 C/200 cycles	83.5%	203
	BiCl3/PTFE	Na3Zr2Si2PO12	0.47	2	0.3 mA cm^−2/1100 h	Na3V2(PO4)3	0.5 C/300 cycles	96.7%	204
	PVDF-HFP	Na2.5Zr1.95Ce0.05Si2.2P0.8O11.3F0.7	1.39	1.2	0.3 mA cm^−2/600 h	Na0.67Mn0.47Ni0.33Ti0.2O2	0.5 C/300 cycles	92.8%	229
	PTFE	Na3.4Zn0.1Zr1.9Si2.2P0.8O12	5.19	4 (60 °C)	2 mA cm^−2/1200 h (60 °C)	Na3V2(PO4)3/C	1 C/750 cycles (60 °C)	80.4%	230
Sodium alloy	Na–K	Zn-doped Na3Zr2Si2PO12	—	40	30 mA cm^−2/800 h	Na3V2(PO4)3	0.2 C/300 cycles (60 °C)	92.5%	213
	Na-Na15Sn4	Na3Zr2Si2PO12	—	2.5	0.5 mA cm^−2/500 h	—	—	—	214
Hybrid sodium anodes	Na-SbF3	Na3Zr2Si2PO12	0.509	1.9	0.5 mA cm^−2/2000 h	Na3V2(PO4)3	1 C/700 cycles	87.7%	219
	Na-Na3.4Zr2Si2.4P0.6O12	Na3.4Zr2Si2.4P0.6O12	4.1	3.1	0.5 mA cm^−2/6000 h	Na3V2(PO4)3/C	5 C/7300 cycles	84.8%	221
	Na-SiO2	Na3.2Zr1.9Mg0.1Si2PO12	—	0.5	0.5 mA cm^−2/135 h	—	—	—	220
	Na-TiO2	Na3Zr2Si2PO12	0.52	1.3	0.1 mA cm^−2/8000 h	Na3V2(PO4)3	1 C/800 cycles	74.5%	215
Structure design	Monolithic electrolyte	Na3Zr1.8Ca0.2Si2PO12	1.67	—	0.1–0.3 mA cm^−2/600 h	Na3V2(PO4)3/C	1 C/450 cycles	98%	216
	3D framework electrolyte	PEO, Sc, Ge co-doped Na3Zr2Si2PO12, PAN	0.413 (30 °C)	—	0.1 mA cm^−2/1000 h	Na3V2(PO4)2F3	0.2 C/460 cycles	81%	217
	Nanoscale ZnO porous coating	Na3.652Zr1.675Zn0.2Mg0.125Si2PO12	2.7	40	5–30 mA cm^−2/770 h	Na3V2(PO4)3	0.2 C/300 cycles	66%	218
Other strategies	Heat treatment	Na3Zr2Si2PO12	—	—	0.1 mA cm^−2/1500 h	—	—	—	180
					0.3 mA cm^−2/250 h
	Ultrasound solid welding	Na3Zr2Si2PO12	0.43	0.6	0.1 mA cm^−2/1300 h	Na3V2(PO4)3	0.1 mA cm^−1/900 cycles	89.81%	179
					0.2 mA cm^−2/400 h

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4.2 NZSP electrolyte/cathode interface

Due to their rigid properties, NASICON-type NZSP electrolytes tend to form poor contact interfaces when paired with typical electrode materials, resulting in relatively substantial interfacial resistance and deteriorating the battery performance. Compared with the numerous approaches for optimizing the contact problem at the Na/NZSP electrolyte interface, relatively less research has been concentrated on the compatibility of the cathode/NZSP electrolyte interface. Composite cathodes used in SSMBs typically consist of active substances, conductive additives, and binders. Consequently, the carrier transfer behavior on the cathode/electrolyte side is significantly more complicated than that on the anode/electrolyte side.

The cathode materials commonly employed in solid sodium batteries can mainly be categorized into four distinct classifications: transition metal oxides, polyanion compounds, Prussian blue analogs, and organic cathodes.^231,232 The poor wettability of these materials results in poor contact between the cathode and the SSEs, causing a lack of effective ion transport channels and increased interfacial resistance. To address this issue, rolling and polishing techniques are typically utilized during the assembly of solid-state batteries to improve the surface flatness of inorganic SSEs and cathodes. However, the microscale contact between the cathode and solid electrolyte remains as point-to-point physical contact, thereby causing increased interfacial impedance and sluggish sodium-ion migration kinetics.²³³ Most commonly used cathodes in solid-state batteries exhibit volume changes during charging/discharging processes, such as ∼8.26% for NVP²³⁴ and ∼23% for P2-Na_0.67Ni_0.33Mn_0.67O₂.²³⁵ Interfacial contact issues are exacerbated by such volume changes, leading to capacity degradation and poor cycling as well as rate performance. The inherent property of active materials to undergo expansion and contraction during the process of sodium-ion intercalation and deintercalation generates internal stresses within the cathode, leading to the formation of cracks, which subsequently results in particle fracture and pulverization. Bucci et al.²³⁶ investigated the mechanical failure of solid-state batteries induced by stress localization using a coupled electro–chemo–mechanical model. When the volume expansion exceeds 3%, cracks form throughout the solid electrolyte, resulting in the development of excessive tensile stresses at the corners of the particles. The density of the electrolyte displays an inverse correlation with the crack propagation rate. When the stiffness of the SSEs is comparable to that of the active material, it tends to generate higher compressive stresses and lower tensile stresses, thereby inhibiting crack nucleation. The existence of microcracks within a solid electrolyte has been demonstrated to induce electrode tortuosity, resulting in non-uniform sodium-ion deposition and stress concentration. This stress-induced mechanical degradation reduces sodium-ion diffusivity and battery performance.²³⁷ The integration of zero-strain cathode materials or flexible interlayers is anticipated to mitigate stress accumulation and the subsequent formation of cracks. The interdiffusion of cathode and electrolyte elements, along with the chemical stability issues at the cathode–electrolyte interface, also represent major limitations for solid-state batteries. To tackle the issue of inadequate physical contact at the cathode–electrolyte interface, high-temperature co-sintering is employed to integrate the two components. While high-temperature co-sintering improves interfacial contact, it induces interdiffusion of elements, leading to the formation of undesired secondary phases.²³⁸

A strategy of combining flexible organic cathodes with rigid NZSP electrolyte was used to achieve an SSMBs with low interfacial resistance, high cycling performance, and a stable cathode/electrolyte interface. Jin et al.²³⁹ reported a soft perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) as a typical organic cathode material showing favorable mechanical and electrochemical compatibility with NZSP. The assembled all-solid-state PTCDA/NZSP/Na cell showed a charge transfer resistance of 310 Ω cm² at 25 °C, lower than that of the PTCDA/Na battery using a conventional liquid electrolyte (460 Ω cm²). Furthermore, the PTCDA/NZSP/Na cell provided an initial capacity of 120.8 mAh g⁻¹ and a retention rate of 73.4% after 500 cycles at 200 mA g⁻¹, whereas the liquid battery exhibits rapid capacity fading after 50 cycles (Fig. 13a).

Fig. 13 (a) SEM images of the cycled PTCDA/NZSP interface and cycling performances of the PTCDA/NZSP/Na battery. Reproduced with permission from ref. 239. Copyright 2024, Elsevier. (b) Schematic representation of the NVP/NZSP/Na and NVP/IL/NZSP/Na solid-state batteries. Cycling performance of solid-state batteries with liquid electrolyte and ionic liquid. Reproduced with permission from ref. 110. Copyright 2024, Wiley-VCH. (c) Nano-CT images of cycled NZSP optimized by liquid electrolyte and PSSIA, and COMSOL simulation of the Na deposition process. (d) Cycling performance of Na/NZSP/NVP and Na/NZSP/NFM batteries. Reproduced with permission from ref. 240. Copyright 2024, Wiley-VCH. (e) Photo of the solid battery, battery stack composition, and design of the high-temperature cell. SEM backscattered electron image of the cross-section of the solid battery. Reproduced with permission from ref. 241. Copyright 2014, Elsevier. (f) Schematics of typical electrode microstructures obtained using different processing routes. Low-magnification HAADF-STEM image of the as-prepared NVP-NZSP composite collected from a typical micrometer-scale pore. Cycling performances of NVP/NZSP/Na batteries operating at 25 °C. Reproduced with permission from ref. 242. Copyright 2019, Elsevier.

The application of wetting substances, including ionic liquids (ILs) or commercially available liquid electrolytes, can thoroughly wet the cathode/electrolyte interface. This enables solid-state batteries to achieve outstanding performance, even rivaling that of conventional liquid batteries. Ionic liquids are receiving substantial interest on account of their distinctive properties, such as non-flammability, non-volatility, good thermal stability, and exceptional electrochemical stability. Gu et al. reported an instance of using IL as a wetting substance on the NVP/NZSP interface in 2016.¹¹⁰ As shown in Fig. 13b, by adding a small quantity of N-methyl-N-propylpiperidinium-bis(fluorosulfonyl)imide (PP13FSI) IL between NVP and NZSP, the interfacial resistance of the obtained NVP/IL/SE/Na battery approached that of the battery with a liquid electrolyte added. At a current rate of 10 C, after 10 000 cycles, the battery maintains a capacity of approximately 90 mAh g⁻¹ with no degradation, while the average coulombic efficiency remained at about 100.0%. The excellent electrochemical performance of the NVP/IL/SE/Na battery is ascribed to the fact that the IL effectively enhances the cathode/electrolyte interfacial contact and provides an ionic migration channel, which facilitates the ionic transport between the cathode active substances and the electrolyte. In addition, the IL offers a buffer region to accommodate the volume expansion of the cathode material during cycling.

The common drawbacks of the above strategies still do not radically mitigate the effects of volume expansion during repeated cycling due to the rigid properties of solid cathodes and ceramic electrolytes. Polymer electrolytes can be introduced to maintain the integrity of the interface, such as PEO, SN, and PVDF-HFP.^229,243,244 Yang et al.²⁴⁰ designed a Na⁺-conducting polymer solid-state interface adhesive (PSSIA) based on poly(ethylene glycol) diacrylate (PEGDA) polymerized crosslinked connections to improve the interfacial stability and optimize the physical contact. As shown in Fig. 13c, the 3D reconstruction of nanoscale X-ray computed tomography (nano-CT) images revealed that the number of cracks in the inorganic electrolyte with a conventional liquid electrolyte added increased significantly after multiple cycles. In contrast, the Na⁺-conducting PEGDA polymer was able to fill the voids and cracks in the electrolyte and construct a rapid Na⁺-transport channel between the inorganic electrolyte and the electrode. This action further inhibited the propagation of cracks within the electrolyte, which were induced by Na dendrites during long-term cycling. As demonstrated in Fig. 13d, the NVP/NZSP/Na battery bonded by PSSIA maintains a capacity of 81.96 mAh g⁻¹ after 200 cycles, even without any high stacking pressure being applied, exhibiting a capacity retention rate of 100%. To further demonstrate the advantages of PSSIA, the assembled solid-state battery with NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM) as the cathode has a discharge capacity of 115.3 mAh g⁻¹ at a rate of 1 C, with an average coulombic efficiency of 99.5%. After 100 cycles, the capacity retention rate was 86%.

It is also possible to combine the cathode with the electrolyte and form a composite cathode with good interfacial contact through co-sintering. For example, an NVP-NZSP composite cathode was prepared by SPS for electrochemical testing at 200 °C (Fig. 13e).²⁴¹ Notably, while high-temperature sintering is capable of significantly densifying the cathode/electrolyte interface, volume change in the unit cell of the active material during cycling leads to cracks on the rigid cathode side, with a significant reduction in ionic conductivity and high polarization. Grady et al.^245,246 prepared an NVP-NZSP composite cathode by CSP method. It was found that the NVP-NZSP two-phase system requires a relatively high temperature (>100 °C) and a substantial amount of solid electrolyte to reach the conductivity range of liquid electrolytes (10⁻⁴ to 10⁻³ S cm⁻¹). The current model system has relatively low practicality given the low loading of the active material and the operating temperature requirements.

Depositing the cathode layer directly onto the solid electrolyte allows the interfacial contact to be optimized to a greater extent. Kehne et al.²⁴⁷ achieved an all-solid-state Na_xCoO₂/Na_3.4Sc_0.4Zr_1.6Si₂PO₁₂/Na battery with a thin Na_xCoO₂ cathode film prepared by pulsed laser deposition (PLD). Within the voltage range from 2.0 to 4.2 V, the cathode/electrolyte interface is both chemically and microstructurally stable. However, this approach lacks universality when applied to diverse cathode types. Different cathodes possess distinct crystal structures and conductive principles, giving rise to substantial disparities in their ionic and electronic transport capabilities. Hayashi et al.²⁴⁸ prepared a dense and thin Na₃Ti₂(PO₄)₃ (NTP) cathode layer on the surface of NZSP by spin-coating and glass-ceramic-assisted sintering. The sample with a 0.6 μm-thick NTP cathode layer had stable charge-discharge cycling performance at 0.1 C and 25 °C, with a capacity of about 60 mAh g⁻¹ and low polarization (<0.03 V); the capacity retention rate at 0.1 C is about 80% at −20 °C.

A network of hybrid ionic-electronic conductors in close contact is established by the deposition of electrode materials in the porous framework of 3D-structured electrolytes, which also enhances the interfacial contact. The porous framework of the 3D-structured electrolyte is able to effectively limit the expansion of the electrode material during cycling, thereby improving the coulombic efficiency and cycling performance.⁷⁵ Wei et al.²⁴² proposed a method for cathode structure design by combining the chemical infiltration of the electrode material and in situ synthesis. In this process, the cathode material precursor was inserted into the side of the matrix, which is a pre-sintered porous NZSP electrolyte. Eventually, the target NVP cathode phase was obtained by post-high-temperature sintering in Ar/H₂. Even after 100 cycles, the NVP/NZSP interface maintained a tight physical contact without any interfacial cracks or phase separation, which was further verified by high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM). The structure was able to achieve high stability of battery performance at room temperature while avoiding the use of a liquid or polymer phase as a regulating medium. As shown in Fig. 13f, the NVP/NZSP/Na full cell provides 96.5 mAh g⁻¹ at 0.6 C after 100 cycles, with a capacity retention rate of 90.6%. In contrast to the conventional co-sintering, a lower temperature was employed to chemically infiltrate the electrolyte and electrode materials for bonding, enabling a wider range of materials to be combined in SSMBs.

5 Conclusions and perspectives

Solid-state sodium metal batteries (SSMBs) use solid electrolytes instead of liquid electrolytes, eliminating potential hazards such as leakage and flammability of the liquid electrolyte, and fundamentally solving the safety problems of batteries. As the core component of SSMBs, solid electrolytes play a decisive role in the safety and electrochemical performance of SSMBs. NASICON-structured Na₃Zr₂Si₂PO₁₂ (NZSP) has been widely used in SSMBs due to its significant advantages, such as high chemical stability, good safety, and excellent mechanical strength.

The composition, crystal structure, and mechanism of Na⁺ conduction in NZSP are reviewed. Meanwhile, the influencing factors and methods for improving ionic conductivity are also discussed in detail. Methods such as aliovalent ion substitution to increase the concentration of Na⁺ in the lattice, the precise regulation of the bottleneck size for Na⁺ transport, and the reduction of grain boundary resistance are all conducive to improving the ionic conductivity of NZSP. In addition, various synthesis methods for NZSP are also introduced. The poor interfacial contact between the NZSP electrolyte and the electrodes is a challenge for the practical applications of SSMBs. Electrolyte composition optimization, construction of artificial interlayers, anode alloying, and establishment of a 3D electrolyte structure are effective strategies to mitigate the interfacial contact problem between the Na anode and NZSP. In addition, it is essential to gain an in-depth understanding of the reaction mechanism, products, and interface evolution between Na and NZSP during the charging/discharging processes. To achieve this goal, advanced characterization techniques and instruments, such as cryogenic transmission electron microscopy (cryo-TEM), in situ high-resolution transmission electron microscopy (in situ HRTEM), solid-state nuclear magnetic resonance (NMR), in situ micro-X-ray absorption near edge structure (μXANES), and cryo-electron microscopy, will play an important role in revealing the interaction behavior at the Na/NZSP interface and acquiring profound comprehension of the underlying mechanism of Na⁺ transport at the interface. Moreover, approaches such as composite cathodes, interfacial wetting agents, and polymer interlayers have also been proposed to solve the interfacial problems between the cathode and NZSP electrolyte. Despite the fact that research on the modification of NZSP solid electrolytes is still in its infancy, the significant progress made in this area shows great potential and bright prospects for the further application of advanced SSMBs.

The practical application of SSMBs is envisaged to be facilitated by the following improvements to NZSP solid electrolytes: (1) Theoretical calculations are helpful for exploring more rational designs for NZSP electrolytes with a rapid Na⁺ conduction path. (2) Advanced in situ characterization techniques, such as in situ X-ray photoelectron spectroscopy (XPS), in situ scanning transmission electron microscopy (STEM), and in situ X-ray diffraction (XRD), should be preferentially adopted to effectively analyze the charge transfer behavior between interfaces and the dynamic interfacial evolution information during the cycling process, which are of great significance for achieving high-performance SSMBs. (3) It is necessary to construct interfaces that are electrically conductive, dendrite-free, and kinetically and thermodynamically stable by hybrid interfacial engineering, optimizing the Na metal anode, and modulating microstructures. (4) As a prerequisite for practical applications, it is essential to develop a straightforward, scalable, and cost-effective approach for the large-scale production of NZSP SSEs. (5) Thinning the electrolyte represents a key trend in the development of SSMBs, and this approach can promote ionic transport and improve battery performance. However, sufficient mechanical strength must be considered and thin solid electrolytes ought to have a Young's modulus value ≥5 MPa to avoid fracture or Na dendrite penetration during long-term cycling. (6) Composite solid electrolytes have the merits of both inorganic solid electrolytes and polymer electrolytes. This hybrid design has the potential to enhance ionic conductivity, mechanical flexibility and thermal stability. Furthermore, it also provides a promising strategy for addressing the interfacial problems between the electrolyte and the electrode in SSMBs.

Conflicts of interest

The authors declare that they have no competing financial interests.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

The authors acknowledge the financial support of the National Key R&D Program of China (No. 2023YFE0115800), the National Natural Science Foundation of China (No. 52472271), and the 2024 National Sustainable Development Agenda Innovation Demonstration Zone Construction Science and Technology Support Project in Ordos City (KCX2024007).

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