Recent Advancements in Na Super Ionic Conductor-Incorporated Composite Polymer Electrolytes for Sodium-Ion Battery Application

Abstract

Sodium-ion batteries (SIBs) have garnered significant attention as a cost-effective and sustainable alternative to lithium-ion batteries (LIBs) due to the abundance and eco-friendly extraction of sodium. Despite the larger ionic radius and heavier mass of sodium ions, SIBs are ideal for large-scale applications, such as grid energy storage and electric vehicles, where cost and resource availability outweigh the constraints of size and weight. A critical component in SIBs is the electrolyte, which governs specific capacity, energy density, and battery lifespan by enabling ion transport between electrodes. Among various electrolytes, composite polymer electrolytes (CPEs) stand out for their non-leakage and non-flammable nature and tunable physicochemical properties. The incorporation of NASICON (Na Super Ionic CONductor) fillers into polymer matrices has shown transformative potential in enhancing SIB performance. NASICON fillers improve ionic conductivity by forming continuous ion conduction pathways and reduce polymer matrix crystallinity, thereby facilitating higher sodium-ion mobility. Additionally, these fillers enhance the mechanical properties and electrochemical performance of CPEs. Hence, this review focuses on the pivotal roles of NASICON fillers in optimizing the properties of CPEs, including ionic conductivity, structural integrity, and electrochemical stability. The mechanisms underlying sodium-ion transport facilitated by NASICON fillers in CPE will be explored, with emphasis on the influence of filler morphology and composition on electrochemical properties. By scrutinizing the recent findings, this review underscores the potential of NASICON-based composite polymer electrolytes as appropriate material for the development of advanced sodium-ion batteries.

1. Introduction

In recent decades, electricity production has depended on fossil fuels, though fossil fuels harm the environment and affect the economy as their prices keep increasing. Their consumption is expected to have doubled by 2050 and have tripled by 2100 [1]. Hence, there is a need to look for renewable energy sources. Energy can be stored in different ways, such as thermal, mechanical, chemical, or electrochemical forms. The global energy system is shifting towards more sustainable and environmentally friendly energy systems, such as renewable energy. This is gaining substantial attention and growing market interest due to rising concerns about the excessive use of fossil fuels and its impact on climate change [2]. Since renewable solar and wind energy sources may not cater for society’s requirements, reliable energy storage is essential. Among the various energy storage options available, such as fuel cells and capacitors, batteries play a key role in future energy needs. Recent research has been directed towards developing battery systems based on elements like lithium (Li), sodium (Na), zinc (Zn), magnesium (Mg), and potassium (K) [3]. Among these, sodium-ion batteries (SIBs) hold significant promise due to the abundance of sodium sources, their lower cost, and the more environmentally sustainable extraction of sodium compared to the lithium used in lithium-ion batteries (LIBs) [4,5].

Table 1. Theoretical specific energies of Li, Na, and K ion batteries [6,8,9,10,11].

Element	Ionic Radii (pm)	Theoretical Specific Energy Density (Wh/kg)
Lithium (Li)	71	285
Sodium (Na)	97	160
Potassium (K)	141	150

compares the theoretical specific energy densities of Li, Na, and K ion batteries, from which it can be understood that the theoretical energy density corresponding to SIBs is comparatively lesser than that of lithium-ion batteries [6]. Although sodium ions are bulkier and heavier than lithium ions (ionic radii are compared in Table 1), SIBs are primarily intended for large-scale grid energy storage systems and commercial electric vehicle applications, where the size and weight of the ions are less essential in comparison to the cost and resource availability [7].

A sodium-ion battery is composed of several essential components, namely, the cathode, anode, separator, electrolyte, and current collector. Of these, the electrolyte is considered one of the most significant components in battery systems, as it plays a significant role in defining the different properties of the battery, such as the specific capacity, energy density, and lifespan. It serves as the transporting medium for the preferred ions (Li⁺, Na⁺, etc.) between the electrodes during charge–discharge cycles. The electrochemical stability window (ESW) can be defined as the electrode potential range within which the electrolyte is stable without undergoing any kind of oxidation/reduction reactions [12]. The range of the ESW is critical for the performance and safety of electrochemical systems. An electrolyte should have a wide ESW to withstand the electrolyte interaction with highly reducing or oxidizing electrode materials. Within polymer electrolytes, the gel polymer electrolyte incorporating NASICON filler demonstrated the highest ESW, reaching up to 5.27 V [13]. There are liquid, solid, and gel electrolytes that can be used in batteries, each offering distinct advantages and limitations which are shown in Figure 1. Although liquid electrolyte-based batteries can result in higher energy density in comparison to solid electrolytes, the solutions can degrade over time, generating various degraded byproducts that cause the formation of insoluble products and form protective layers, such as the solid electrolyte interphase (SEI), on the negative electrode and the surface layer (SL) on the positive electrode. However, these layers must be maintained at an optimal level; otherwise, they can impede ion transport and result in capacity fading [14]. While liquid electrolytes are traditionally favored due to their high ionic conductivity and low cost, solid electrolytes present several benefits, such as being flame-retardant, non-volatile, safe, corrosion resistant, and able to maintain good thermal stability over a wide range of temperatures. The future of battery technology is trending towards solid electrolytes (SEs), where the cathode, anode, and electrolyte are entirely solid-state components. However, the full realization of solid electrolytes faces significant challenges, primarily due to the low ionic conductivity at room temperature (RT) and poor electrode/electrolyte interfaces [15].

Solid electrolytes are classified into three main categories:

• Inorganic solid electrolytes (ISEs),
• Solid polymer electrolytes (SPEs),
• Composite polymer electrolytes (CPEs).

ISEs, primarily made up of ceramic crystals, show better ionic conductivity ranging from approximately 10⁻³ to 10⁻² S/cm at RT, due to efficient ion conduction [16]. However, the rigidity and brittleness of ceramics usually result in high interfacial resistance at electrode–electrolyte interfaces [16]. In contrast, SPEs typically have lower ionic conductivity, around 10⁻⁶ to 10⁻⁸ S/cm [17] but offer advantages such as flexibility and ease of processing, which enhance their compatibility with electrodes. However, their lower ionic conductivity limits their commercialization. CPEs aim to combine the benefits of both ISEs and SPEs by incorporating ceramic fillers into a polymer matrix, which can enhance both ionic conductivity and mechanical properties. By optimizing the composition and processing parameters of CPEs, it is possible to achieve high ionic conductivity while maintaining the desirable flexibility and good contact with electrodes necessary for effective battery performance [16].

1.1. Inorganic Solid Electrolytes (ISEs)

Inorganic solid electrolytes, including materials like beta alumina, NASICON (Na Super Ionic CONductor), and sodium thiophosphates, exhibit higher conductivity (>10⁻⁴ S cm⁻¹ at RT), excellent thermal stability, and serve as single-ion conductors. However, limitation due to poor mechanical properties impedes their usage in battery systems. Among ISEs, sulfide-based electrolytes like Na₃PS₄ [18] exhibit higher conductivity (~2 × 10⁻⁴ S/cm at RT) but are moisture-sensitive and prone to electrochemical instability, requiring protective layers. On the other hand, they are much more expensive than liquid electrolytes [19]. Halides like NaAlCl₄ [20] offer mechanical flexibility but moderate room-temperature conductivity (~3.9 × 10⁻⁶ S/cm) [20]. Oxide-based electrolytes, such as beta-alumina (NaAl₁₁O₁₇) and NASICON (Na Super Ionic Conductor) materials, are known for their good electrochemical stability. In 1971, Whittingham [21] measured the ionic conductivity (σ) of 1.4 × 10⁻² S/cm at 25 °C for small single-crystal beta-alumina, but it required complex and high-temperature sintering for densification [22]. However, pure beta alumina is highly sensitive to moisture [23] due to its porous structure, which allows water molecules to penetrate and adsorb onto the surface. In addition, it exhibits poor mechanical properties. Na β-alumina was used in Na-S batteries by the Ford Motor Company as early as the 1970s. However, its potential applications were limited due to the requirements of high-temperature operation and its reliance on two-dimensional ion transport [15]. Beta-alumina has a layered crystal structure that facilitates ion transport primarily within two-dimensional (2D) conduction planes. This structure allows for relatively fast ion hopping within the planes perpendicular to the c-axis, while conductivity along the c-axis is effectively negligible; this limitation can hinder overall ionic conductivity when ions need to traverse through three-dimensional space [24]. Thus, in comparison to all other ISEs, NASICON is a promising solid electrolyte that results in superior ionic conductivity (with the order of 10⁻³ S/cm) at RT, along with good thermal and chemical stability, making them one of the few materials that can to achieve both high ionic conductivity and better electrochemical stability. A comparison list of the ionic conductivity of different ISEs is shown in

Table 2. List of ionic conductivity of inorganic solid electrolytes.

Chemical Formula	Ionic Conductivity [S/cm] at RT	Reference
NASICON	~10−3	[25]
Na0.7La0.7Zr0.3Cl4	2.9 × 10−4	[26]
Na3MI6	~10−4	[27]
Na3PS4	2 × 10−4	[18]
Amorphous Na2P2S6	5.7 × 10−8	[28]
Crystalline Na2P2S6	2.6 × 10−11	[28]

. It can be seen that NASICON shows better conductivity for sodium-ion conduction in comparison to other ISEs.

1.2. Solid Polymer Electrolytes (SPEs)

Solid polymer electrolytes, formed by incorporating sodium salts in a polymer matrix, offer flexibility and close electrode contact but are limited due to low ionic conductivity and limited oxidation potential. Commonly studied polymer matrices include poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), and poly(ethylene oxide) (PEO) [29]. Despite their flexibility, they generally show poor electrochemical performance compared to inorganic electrolytes such as ceramic electrolytes and sulfide-based solid electrolytes, which typically offer higher ionic conductivities and better mechanical properties. Polymer blending is a crucial method for engineering materials tailored to specific applications, capitalizing on the distinct properties of individual polymers [30]. Thus, the blending of polymers can also be utilized in the fabrication of SPEs.

1.3. Composite Polymer Electrolytes (CPEs)

Composite polymer electrolytes (CPEs) are obtained when a polymer matrix is incorporated with inorganic fillers to improve their ionic conductivity and mechanical properties. Polymer composites are not only used as polymer electrolytes but also in various applications such as nanogenerators and EMI shielding applications due to their hybrid advantages of both organic and inorganic components [31]. CPEs typically exhibit higher ionic conductivity of up to two to three orders of magnitude higher in comparison to pure SPEs due to the incorporation of inorganic fillers [32], which provide additional pathways for ion transport and show better flexibility than ISEs. NASICON is one of the suitable candidates for ceramic filler in CPEs. The research related to NASICON-based solid electrolytes is increasing every year, as depicted in Figure 2. Given the increasing concerns about the environmental impact of plastic waste, recent research efforts have focused on developing biodegradable, recyclable, or renewable polymer-based composites [33]. Per- and polyfluoroalkyl substances (PFASs), such as PVDF, are a broad class of synthetic chemicals that contain several fluorine atoms attached to an alkyl chain. They are often referred to as “forever chemicals” due to their persistence in the environment and accumulation in living organisms, causing significant health and environmental concerns. Within the LIB industry, there has already been a shift towards developing PFAS-free batteries [34]. It is hoped that the SIB industry will similarly advance in creating PFAS-free solutions. Consequently, biodegradable polymers like poly(lactic acid) (PLA) are being explored, offering sustainable alternatives for various applications [35].

A new area of research focuses on hybrid ceramic–polymer electrolyte membranes, which combine inorganic solid electrolytes with polymers. CPE includes all inorganic fillers such as metal oxides and metal sulfide, whereas ceramic–polymer electrolytes focus on ceramic fillers like alumina. These materials aim to achieve high ionic conductivity, good mechanical strength, and easy room-temperature processing [36].

• In ceramic-in-polymer systems, ceramic particles are added in lesser amounts to a polymer matrix. This improves ionic conductivity by reducing polymer crystallization or creating a conductive interface between the polymer and ceramic fillers while maintaining easy processability [36].
• In polymer-in-ceramic systems, the ceramic content exceeds 50%, making ceramics the dominant phase. These systems offer higher mechanical strength and safety, making them ideal for large solid-state battery packs, such as those in electric vehicles [36].

While many reviews on NASICON-based materials have been published, there is more emphasis on NASICON properties and their use as electrodes or inorganic electrolytes for SIBs. For instance, Anantharamulu et al. [37] explored crystalline and glassy NASICON composites, emphasizing their 3D framework structure for high sodium-ion conductivity, synthesis routes, and characterization. Singh et al. [38] provided an overview of NASICON-type oxides, addressing their chemistry, phase formation, conduction mechanism, and fabrication challenges like high-temperature sintering and large interfacial impedance. Cui et al. [39] discussed the first-principle calculations in phosphate-based NASICON materials, highlighting their structural stability and sodium-ion transport performance. Risvi et al. [40] reviewed doping advances in NASICON, focusing on strategies and impacts on electrochemical properties. Li et al. [22] summarized the findings on NASICON derivatives, aiming to improve ion conductivity and interface charge transfer kinetics. However, the use of NASICON as fillers in composite polymer electrolytes (CPEs) remains underexplored and warrants further investigation.

Thus, this review aims to outline recent developments related to NASICON fillers incorporated into polymer electrolytes for SIB application. This review begins by introducing NASICON’s general crystal structure and ion transport mechanisms. It then provides an overview of common preparation methods of NASICON, including both solid-state and liquid-phase approaches. Additionally, the various compositions of NASICON and their ionic conductivities are summarized. Finally, perspectives on the future development of NASICON for SIB application are also provided.

2. NASICON Fillers: Preparation, Structure, and Morphology

NASICON was first studied and developed by Goodenough in the year 1976 [22]. Understanding the structure and morphology of NASICON is required to tune the electrical and mechanical properties of CPEs.

2.1. General Structure of NASICON

The general formula A_nMM′(XO₄)₃ is a broader representation of a NASICON-like structure. In this formula:

i.

A represents alkali cations (e.g., Li, Na, K);

ii.

M and M′ are transition metals (e.g., Fe, Ti, Zr);

iii.

X denotes elements such as phosphorus or silicon;

iv.

n can vary from 1 to 4.

The 3D framework is made up of “lantern” units, in which two MO₆ octahedra (metal–oxygen groups) are linked to three XO₄ tetrahedra (non-metal–oxygen groups). These units are arranged sequentially along the c-axis (a direction in the crystal structure). Sodium (Na⁺) ions are distributed in two types of spaces, called M₁ and M₂ sites (which are also referred to as Na₁ and Na₂ sites), within the NASICON framework [7]:

a.

M₁ sites (octahedral) are tightly packed, with sodium ions that move very little. The sodium ions located at the M1 sites are tightly bound to the surrounding coordinated oxygen atoms.

b.

M₂ sites are larger and can accommodate more sodium ions, which can move more freely.

Sodium ions migrate through the structure by hopping between sites, passing through a “bottleneck” region formed by MO₆ and XO₄ units. This movement allows the battery to work as sodium is inserted and removed during charging and discharging. M₁ sites are not very active in the movement of sodium ions due to their confined space in bottleneck, so most of the action happens at the M₂ sites. However, the contribution of M₁ sites in facilitating sodium storage is still being investigated.

The rhombohedral (R-3c) symmetry is created by corner-sharing metal octahedra and polyanion tetrahedra, resulting in two distinct Na sites, M₁ and M₂, forming a 3D diffusion network, as shown in Figure 3a [41]. This structure allows Na ions to migrate between sites through two triangular bottlenecks, creating a three-dimensional percolating channel for rapid ion conduction. Additionally, NASICON compounds can adopt monoclinic symmetry (C2/c) depending on their composition and temperature. The monoclinic structure is formed due to the distortion of the rhombohedral structure [42], where M₂ sites are split into $M_2^{\alpha }$ and $M_2^{\beta }$ sites (as shown in Figure 3b).

Description of ‘Bottleneck’ Structure

For x = 2 in the general formula Na_1+xZr₂Si_xP_3−xO₁₂ of NASICON [44], the compound Na₃Zr₂Si₂PO₁₂ (NZSP) exhibits two distinct NASICON phases, consisting of corner-sharing tetrahedra ([SiO₄] and [PO₄]) and octahedra ([ZrO₆]). These structural units form a “hexagonal bottleneck” configuration, facilitating ion transport. This bottleneck has the shortest diameter of 4.6 Å (specific for NZSP), which is nearly twice the combined ionic radii of Na⁺ (0.98 Å) and O²⁻ (1.4 Å) [45]. The diameter of the “bottleneck” in NASICON materials can vary depending on the specific composition and structure of the material. The large space allows smooth Na⁺ movement through the bottleneck, significantly improving the conductivity. In the rhombohedral structure of the NASICON, the sodium ions occupy two different sites, namely Na₁ and Na₂, both coordinated to oxygen atoms from three [Si/PO₄] tetrahedra. For the monoclinic structure of the NASICON, the sodium ions occupy three sites: Na₁, Na₂, and Na₃, (where Na₂ and Na₃ sites are referred to as $M_2^{\alpha }$ and $M_2^{\beta }$ sites in general). Na₃ sites are coordinated to oxygen atoms from three [ZrO₆] octahedra. In the rhombohedral phase, there are one Na₁ and three Na₂ sites per formula unit, with Na₁ being fully occupied. Upon transformation to the monoclinic phase, the Na₂ sites split into one Na₂ and two Na₃ sites, and both Na₁ and Na₂ become fully occupied, which is depicted in Figure 4. This rearrangement affects the Na⁺ diffusion pathway.

As discussed earlier, in both the monoclinic and rhombohedral, sodium ions migrate through triangular “bottleneck” regions formed by three oxygen atoms from the (Si/P)O₄ tetrahedra and ZrO₆ octahedra pockets. The diffusion pathways for sodium ions occur between Na₁ and either Na₂ or Na₃ sites, creating four distinct bottleneck areas: two along the Na₁–Na₂ path and two along the Na₁-Na₃ path (shown in Figure 4). These bottlenecks, labeled ‘A’ and ‘B’ in the Na₁-Na₂ channel and ‘C’ and ‘D’ in the Na₁-Na₃ channel, correspond to triangular regions. The bottleneck centers ‘A’ and ‘C’ are positioned closer to the Na₁ site, whereas bottlenecks ‘B’ and ‘D’ are located nearer to the Na₂ and Na₃ sites, respectively [46].

The activation energy (E_a) for ion conduction is primarily influenced by the small-sized bottleneck, with bottleneck ‘B’ in the pathway of Na₁-Na₂ likely being the impeding factor for Na ion conduction [46]. In general, in the monoclinic structure of NZSP, where Na sites are 75% occupied per formula unit, four Na sites remain vacant across four formula units. The Na₁ sites, being thermodynamically more stable, are typically fully occupied, while the remaining sodium ions are distributed randomly between the Na₂ and Na₃ sites [46].

NASICON materials typically have a rhombohedral structure that is thermally stable. The rhombohedral phase exhibits superior ionic conductivity due to its enhanced symmetrical structure, which facilitates higher sodium-ion mobility. Thus, the rhombohedral structure is preferred over the monoclinic structure [47]. Research is ongoing to identify rhombohedral structures of NASICON. The doping of NASICON with different transition metals like chromium (Cr), iron (Fe), or zirconium (Zr), affects the crystal structure of NASICON so that a transition occurs from the monoclinic phase to the rhombohedral phase. In some cases, doping can also lead to temporary change to a monoclinic form at RT, but it reverts to the rhombohedral phase at high temperatures [7].

As cathodes, different NASICONs such as Na₃V₂(PO₄)₃, Na₃V₂(PO₄)₂F₃, Na₂FeTi(PO₄)₃, etc., have been particularly well investigated, whereas NASICON materials with low redox potential such as NaTi₂(PO₄)₃, NaZr₂(PO₄)₃, NaV₂(PO₄)₃, etc., serve as anodes [48]. However, the Zr-based NASICON with general formula Na_1+xZr₂Si_xP_3–xO₁₂ (NZSP) is widely used as the solid electrolyte for SIBs [49] and is considered a promising electrolyte for SIBs owing to its excellent ionic conductivity (in the order of 10⁻³ S/cm at RT) [25]) and stability. Doping can enhance its conductivity by modifying the crystal structure. Recent studies using impedance analysis, first-principle calculations, and in situ X-ray photoelectron spectroscopy confirmed that NZSP forms a kinetically stable interphase with sodium metal within 15 min at RT [50]. The galvanostatic cycling of Na|NZSP|Na cells demonstrated stable plating behavior (plating refers to the process where sodium ions are deposited onto the anode during charging), confirming its chemical and electrochemical stability. This makes NZSP suitable for solid-state NVP (Na₃V₂(PO₄)₃)|NZSP|Na batteries, achieving high-capacity retention over 120 cycles [50]. The cell is capable of delivering a reversible capacity of 50 mAh/g even when operated at a high rate of 1C. Thus, the upcoming section details the structure, synthesis routes, and doping of NZSP-NASICON with dopants like Sc³⁺ and Yb³.

2.2. Na₃Zr₂Si₂PO₁₂-Based NASICON-Outline

Na₃Zr₂Si₂PO₁₂ (x = 2) exhibits high Na⁺ conductivity (with the order of 10⁻³ S/cm) and moderate activation energy (0.36 eV) due to its favorable monoclinic structure at RT, crystallizing in the C2/c space group [51]. Sodium ions diffuse through bottleneck areas formed by ZrO₆ octahedra and (Si/P)O₄ tetrahedra connected by corner-sharing oxygen atoms. The bottleneck size ~5.223 Å is sensitive to both sodium content and temperature, which affects ionic mobility [15]. The monoclinic structure of NZSP transforms into a rhombohedral structure at higher temperatures, typically in the range of 160 °C to 167 °C [52], thus reducing the activation energy for ion conduction, as observed by a phase transition around 147 °C, which enhances Na-ion mobility. The unit cell comprises Na, Zr, Si, P, and O atoms, with Na⁺ diffusion occurring between the Na₁–Na₂ and Na₁–Na₃ sites. Two-thirds of the Na sites are occupied, while one-third remain vacant, creating ion diffusion pathways. The optimal ionic radius for doping cations is approximately 0.72 Å [53]. This value is especially significant when doping the Zr site in NASICON (NZSP) with cations that have lower valencies, as it improves the ionic transport properties of the material [53]. While doping enhances bulk conductivity, grain boundaries continue to influence overall ionic conductivity. The increase in the size of grains can reduce the number of grain boundaries, thereby improving total ionic conductivity by reducing the resistance at the grain boundaries [54]. This comprehensive strategy of structural tuning and doping is crucial for improving the performance of NASICON-type materials to be utilized as solid electrolytes in sodium-ion battery applications [16]. The overall conductivity of NASICON-type materials is influenced by both bulk and grain boundary contributions, which can be distinguished and quantified through impedance spectroscopy analysis, where the total resistance (R = R_b + R_gb) can be separated by fitting the impedance spectroscopy data with the suitable equivalent circuit. The presence of bulk (R_b) and grain boundary (R_gb) resistances are represented by two semicircles, while the electrode polarization appears as a spike at low frequencies. The lower frequency semicircle in the Argand plane will be associated with resistance due to the grain boundary. The synthesis methods and dopant selection can enhance the ionic conductivity of NZSP [55]. This material shows strong potential to be used as an electrolyte in SIBs due to its high Na-ion conductivity, stable structure, and temperature-dependent phase behavior [15]. The different physiochemical properties and their characterization methods are discussed below for further understanding of tuning the ionic conductivity of NZSP.

2.3. Synthesis Routes and Post-Processing of Na₃Zr₂Si₂PO₁₂

The synthesis method and conditions used for preparing NZSP materials have a significant impact on their phase purity, density, grain boundaries, and, ultimately, their ionic conductivity. Both solid- and liquid-based methods are widely used in the preparation of NZSP. A significant challenge in developing NZSP-based materials is achieving a highly dense, stoichiometric phase. Problems such as the formation of different phases such as glassy and secondary phases (e.g., monoclinic zirconium dioxide) and the volatilization of sodium and phosphorus during synthesis often impede the production of the NZSP in its pure form [15]. Various methods have been explored to address these challenges. This section briefly overviews these approaches, such as solid-state synthesis, coprecipitation method, hydrothermal route, and sol–gel techniques, as summarized in Figure 5.

After the synthesis of NASICON, sintering is required to achieve high density and enhanced ionic conductivity. During sintering, the powdered material is heated to high temperatures, causing the particles to fuse together and form a solid, dense material [56]. This process eliminates porosity and improves the material’s structural integrity, which is crucial for its performance as a solid electrolyte in SIBs.

For instance, sintering at high temperatures enhances the density and crystallinity of NASICON-type materials, but exceeding certain temperatures can alter the Si and P arrangement, causing Na loss (around ~1200 C°) or forming secondary phases, all of which reduce ionic conductivity. Narayanan et al. [57] found that sintering NZSP at 1100 °C resulted in the highest conductivity (1.13 mS cm⁻¹) but increasing sintering time can increase grain size which can result in reduced ionic conductivity. The spark plasma sintering (SPS) of NZSP is shown to increase ionic conductivity (1.7 mS cm⁻¹) more than conventional sintering (1.3 mS cm⁻¹) [58]. Adding excess Na content can increase ionic conductivity, as seen in Na_3.3Zr₂Si₂PO₁₂ (10 wt.% excess Na) due to enhanced Na-ion mobility from larger bottleneck areas [46]. Liquid phase sintering [59], using Na₂SiO₃ as an additive improves the ionic conductivity by facilitating Na-ion movement along grain boundaries. Thus, it is evident that the synthesis routes play a vital role in tuning the properties of NASICON.

2.3.1. Liquid Phase Synthesis

In contrast to solid-state synthesis, liquid-based approaches require lower heat treatment temperatures, allowing for the synthesis of NASICON-type solid-state electrolytes (SSEs) with more controlled particle size distribution and well-defined morphology.

• Sol–Gel Method

The synthesis process was carried out in three stages [60]. In the first stage, distilled water, ethanol (C₂H₅OH), tetraethyl orthosilicate (TEOS), and nitric acid were mixed at RT, with the proportions selected based on the TEOS, C₂H₅OH, and H₂O phase diagram to enhance homogenization and limit zirconia segregation. The pH was adjusted to 1 using HNO₃ solution. In the second stage, stoichiometric amounts of sodium nitrate and ZrO(NO₃)₂ were dissolved in distilled water and added to the prepared solution with continuous stirring until a clear phase was obtained. In the final stage, a diammonium hydrogen phosphate solution was quickly introduced, leading to the formation of a white gelatinous precipitate. The mixture was subsequently dried at 100 °C to evaporate the solvent, yielding a dehydrated compound.

b.

Hydrothermal Method

Hydrothermal method refers to a synthesis method that involves using water as a solvent under elevated temperature and pressure conditions, typically in a sealed autoclave.

Shimanouchi-Futagami et al. [61] prepared boron-substituted NaTi₂(PO₄)₃ (NTP) NASICON by hydrothermal synthesis. The starting materials were prepared via the coprecipitation method where TiCl₄ was added to distilled water, forming a yellow TiCl₄ solution that, upon addition of NH₃, resulted in a Ti(OH)₄ gel. This gel was mixed with 85% H₃PO₄, H₃BO₃, and NaOH aqueous solutions in a Teflon-lined autoclave, maintaining a constant Na concentration, and heated at 250 °C for 5 h. The obtained white powdered product was filtered, washed, and dried. XRD patterns revealed that a single-phase NTP structure was achieved for P:B ratios between 9:1 and 7:3, while sintering at 1000 °C for 3 h resulted in a single-phase structure at a P:B ratio of 6:4. This indicates that high-temperature sintering can facilitate the incorporation of boron into the crystal lattice, thus altering the P:B ratio. TG-DTA analysis showed water loss at ~100 °C and a solid-state reaction at ~830 °C, forming Na_2.2Ti₂B_0.6P_2.4O₁₂·0.97H₂O. Lattice parameters indicated a systematic change with increasing P:B ratio, while Arrhenius plots showed enhanced conductivity and reduced activation energy due to Na⁺ enrichment and structural adjustments, resulting from the substitution of smaller B³⁺ ions (in BO₄ units) for larger P⁵⁺ ions (in PO₄ units). The substitution reduces ionic size and localized distribution charge, causing lattice adjustments that accommodate the smaller tetrahedral units. The increase in the a-parameter and cell volume below the 6:4 ratio is due to the incorporation of BO₄ units, which leads to a denser cationic network. The contraction of the c-parameter reflects the rearrangement of TiO₆ octahedra and PO₄/BO₄ tetrahedra to accommodate new units. Larger cell volumes resulting from boron substitution decrease the activation energy for ionic migration. This is due to the reduced electrostatic potential barriers for sodium-ion conduction within the NASICON framework.

c.

Coprecipitation Method

The coprecipitation method, a common wet chemical synthesis technique, enables precise control over the material’s composition and morphology at the molecular level [62]. In this process, a solute that would typically remain dissolved in a solution is precipitated out by interacting with a carrier such as hydroxide [63]. The carrier provides nucleation sites, causing the solute to bind together rather than remaining dispersed in the solution. It is an efficient approach for producing inorganic powders on a large scale, within a short duration, and under low-temperature conditions.

NASICON materials with varying compositions, including Na_1+xSi_xZr₂P_3−xO₁₂, Y-doped NASICON, and Fe-doped NASICON, were synthesized using the coprecipitation method [55]. The aqueous solutions of precursors like ZrO(NO₃)₂, NaOH, SiO₂, (NH₄)₂HPO₄, and dopants (Y(NO₃)₃·6H₂O or Fe(NO₃)₃·9H₂O) were mixed to form a gel-like precipitate. The precipitate was dried at temperatures gradually increasing to 150 °C and was calcined in two steps: at 750 °C or 600 °C for 24 h, followed by calcination at higher temperatures (900 °C for pure/Y-doped or 700 °C for Fe-doped NASICON). The powders were ground, pressed into disks, and sintered at specific temperatures (1175 °C for pure/Y-doped and 800 °C for Fe-doped NASICON).

A comparison study between the coprecipitation and sol–gel methods revealed that the powder synthesized via coprecipitation exhibited higher purity and better crystallinity compared to that obtained through the sol–gel process [58]. Coprecipitation minimizes the formation of undesirable phases like ZrO₂ compared to the sol–gel method. The sol–gel process showed a greater tendency for ZrO₂ segregation due to the rapid hydrolysis and condensation of zirconium alkoxide [58].

2.3.2. Solid State Synthesis

Solid-state synthesis is a method of preparing materials where all reactants exist in a solid state. This technique involves chemical reactions occurring at elevated temperatures to form new solid products. It is widely used in the production of inorganic compounds, including ceramics and semiconductors such as zirconium dioxide (ZrO₂), allowing for the direct conversion of solid precursors into desired materials while minimizing the need for solvents. In the solid-state synthesis process, precursors are typically mixed through either mechanical mixing or ball milling, followed by solid-state reactions and sintering methods, which require elevated temperatures (>1000 °C) and extended sintering times (>10 h). The resulting products generally exhibit a highly crystalline structure with high density, though they also experience significant grain boundary effects. To minimize energy consumption and improve the performance of SPEs, various low-temperature sintering methods have been developed alongside traditional high-temperature sintering. Therefore, this section will discuss both high and low temperature sintering techniques utilized in the synthesis of NASICON structures for sodium-ion battery (SIB) applications.

To synthesize NZSP through solid-state synthesis, Jalalian-Khakshour et al. [64] mixed precursors such as silicon dioxide, zirconium dioxide, and sodium phosphate dodecahydrate in stoichiometric amounts and wet-milled them in isopropanol. The dried mixture was pressed into pellets and subjected to a pre-calcination step at 400 °C to initiate solid-state reactions. The pellets were then re-milled and subjected to high-temperature sintering at 1230 °C for various durations (10, 24, or 40 h), which allowed densification and grain growth, resulting in improving the material’s ionic conductivity. This study highlights the critical role of precursor powder particle size in determining the microstructure of NZSP. ZrO₂ and SiO₂ were employed in nano- and macro-forms, while sodium phosphate dodecahydrate was used in its macro-form due to its consistent decomposition at relatively low temperatures. Nano-precursors were chosen for their larger surface area, which enhances reactivity during the solid-state reaction, potentially leading to improved homogeneity, a reduction in porosity, and finer grain structures. This improved microstructure minimized grain boundary impurities and porosity, leading to higher ionic conductivity. In contrast, macro-precursors showed slower reaction kinetics and resulted in coarser microstructures. NZSP synthesized using nanoparticle precursors achieved a high ionic conductivity of 1.16 mS cm⁻¹ when sintered at a temperature of 1230 °C for a time period of 40 h, resulting in NZSP synthesized via solid state synthesis. In comparison, macron-sized precursors yielded a lesser ionic conductivity of 0.62 mS/cm under identical conditions as used for NZSP.

2.3.3. Sintering Techniques

Sintering is a process that results in the densification of compacted solid materials (shown in Figure 6) such as NASICON, employing only heat with no pressure. It involves heating the material to a temperature below the melting point of its primary constituent, enabling atomic diffusion and bonding between particles. This process leads to grain growth, strength enhancement, and densification. Sintering progresses through three distinct stages. In the early stage, atomic mobility initiates bonding, forming concave necks between particles, resulting in minimal densification and high porosity (>10%). The middle stage involves significant densification as the interconnected pore network starts breaking down, leaving channel-like pores with reduced porosity (5–10%). The middle stage of sintering is characterized by the onset of grain growth or coarsening and the development of grain boundaries. Finally, the last stage features the closure of voids (pore healing) and pronounced grain growth, resulting in a dense microstructure with minimal residual porosity.

Solid-state sintering is the most widely used technique used to densify different ceramic electrolytes. However, the requirement of elevated temperatures (typically above 1200 °C) and long sintering times (~up to 10 h) often lead to irregular grain growth and the depletion of volatile components. These effects can ultimately reduce ionic conductivity, while making the process both energy- and cost-intensive. Alternatively, several advanced sintering technologies, such as microwave-assisted sintering, spark plasma sintering, hot press sintering, and cold sintering, have been developed to densify ceramic electrolytes.

The effect of low-temperature sintering techniques, such as field-assisted sintering technology (FAST) or spark plasma sintering (SPS) and cold sintering, for densifying scandium-substituted NASICON (Na_3.4Sc_0.4Zr_1.6Si₂PO₁₂) has been reported elsewhere [66]. FAST/SPS is a technique that combines high heating rates with the application of uniaxial pressure to achieve the rapid densification of ceramic and metallic powders. The process involves passing a pulsed direct current through the sample and the die, which generates joule heating and facilitates sintering at comparatively low temperatures and for shorter durations compared to conventional methods. FAST/SPS is considered superior in achieving higher densification and conductivity, as it utilizes electrical currents to rapidly heat materials, enabling densification at temperatures of around 700 °C with improved grain growth and phase formation. Cold sintering, in contrast, utilizes a combination of aqueous solutions as sintering aids and high uniaxial pressure to achieve high densification at low temperatures (<300 °C). The process relies on the dissolution–precipitation mechanism [67] where, initially, a suitable amount of aqueous solution is added to homogeneously wet the ceramic particles, forming a liquid film around them.

The solution facilitates the local dissolution of the sharp particle surfaces and acts as a lubricant, aiding in particle rearrangement and sliding. As external pressure compacts the particles, the aqueous solution redistributes, filling the spaces between them. In the second and third stages, dissolution–precipitation and crystal growth occur, with the liquid phase evaporating at a temperature slightly above the boiling point of the solution. This evaporation creates a supersaturated state in the interspaces, where the chemical potential at the contact points is higher than at the crystal sites. As a result, dissolved atomic clusters and ionic species precipitate onto the crystal sites, contributing to the densification of the ceramics. Strong acids or bases enhance this mechanism by promoting incongruent dissolution, leading to densification up to 86.8% of theoretical density. However, the grain size remains small (23–114 nm), resulting in high grain boundary resistance and limited ionic conductivity. Post-sintering thermal treatments are often necessary to improve crystallinity and further densify the material. Despite these challenges, cold sintering is a promising low-energy method, especially for integrating materials sensitive to high temperatures. While cold sintering offers significant energy savings and the potential for integrating sensitive materials, FAST/SPS is more effective in producing higher-density materials with better electrical properties, as high densification achieved with FAST/SPS often eliminates the need for secondary thermal treatments, saving processing time. This makes FAST/SPS the preferred method for optimizing solid electrolyte performance in energy storage applications.

Thus, sintering methods also influence the properties of NASICON-type structures, such as ionic conductivity. Thus, understanding the different types of sintering techniques is very important. A detailed explanation of each type of sintering technique is given below.

• Spark Plasma Sintering (SPS)

Spark plasma sintering (SPS) is a cutting-edge technology used for sintering materials. It leverages microscopic electrical discharges between particles under applied pressure to achieve rapid densification. This method allows materials to be compacted to nearly their theoretical density quickly and efficiently [68]. The theoretical density refers to the maximum possible density of a material if it is fully dense, meaning it contains no pores or voids. It is calculated based on the atomic or molecular structure of the material, assuming that all the atoms are perfectly packed together. Using conventional sintering (CS), a NASICON sample was produced by calcining a powder mixture of ZrSiO₄ and Na₃PO₄ at the temperature of 1125 °C for a period of 11 h. The calcined mixture was then milled for 24 h using a zirconia ball mill. The resulting powder was pressed into a disk at 100 MPa and further sintered in air at 1220 °C for a period of 14 h [69]. Lee et al. [68] used spark plasma sintering (SPS) technique, where 1.5–2.0 g of the precursor powders, such as Na₂CO₃, ZrO₂, SiO₂, and NH₄H₂PO₄, were placed in a 10 mm graphite die. The heating rate was 100 °C/min, with sintering temperatures ranging from 900 °C to 1200 °C for up to 30 min. Once the predetermined time elapsed, the electric current was turned off and the pressure was released.

The SPS method was employed to fabricate dense NASICON ceramics with higher ionic conductivity, which were compared with those obtained through the conventional sintering process [68]. It has been reported that fully dense NASICON can be achieved at 1100 °C using the SPS method. In comparison, conventional sintering only reached one-third of the theoretical density (for NZSP, the theoretical density is 3.27 g/cm³ [70]). It showed ionic conductivity of 1.8 mS cm⁻¹ at RT, using SPS. This improvement in conductivity with increasing sintering temperature in SPS is primarily attributed to the higher sample density, despite limited grain growth.

b.

Hot Press Sintering

During any sintering process at elevated sintering temperatures (>1200 °C), challenges such as the decomposition of NASICON structures, such as NZSP, and the volatilization of sodium (Na) and phosphorus (P) often occur [71]. The hot-pressing method is an effective technique for achieving the high densification of solid electrolytes and is a promising solution to mitigate the above-mentioned issue and involves applying compressive stress during sintering to minimize the glassy phase thickness at the grain boundaries, thereby lowering the overall grain boundary resistance.

NZSP powders were hot-pressed at different temperatures, such as 1150, 1200, and 1250 °C, achieving a total ionic conductivity of 4.2 mS/cm at RT when sintered at 1250 °C, close to bulk conductivity values (5–6 mS/cm) [71]. Impedance spectroscopy, microstructural analysis, and electron microscopy showed that applying stress during the process of hot-pressing at higher temperatures suppresses high-resistance glassy phase thickness at the grain boundaries, lowering grain boundary resistance. This method enhances conductivity compared to sintered samples (<1 mS/cm) where no stress is applied. Hot-pressing is effective in improving the ionic conductivity of polycrystalline solid electrolytes by reducing grain boundary resistance.

c.

Cold Sintering Process (CSP)

Cold sintering is a technique that operates at low temperatures, using chemical interactions between the ceramic surface and a transient solvent [72]. This process enables the densification of materials at much lower temperatures compared to traditional sintering methods by facilitating particle rearrangement and bonding through the solvent’s action.

For NZSP, the aqueous solvent typically used in cold sintering is replaced with molten hydroxide. In this case, NaOH acts as the solvent to densify the NZSP electrolyte at a significantly lower temperature of 375 °C in just three hours; thus, a substantial reduction from the conventional sintering temperature of 1200 °C is achieved.

d.

Microwave-Assisted Sintering

Microwave sintering has emerged as a promising alternative to conventional solid-state sintering, particularly for ceramic materials. This technique offers several key advantages, including lower sintering temperatures, shorter processing times, and higher yields. Microwave sintering promotes uniform and rapid heating through direct interaction with the atoms, ions, and dipoles within the material, ensuring more homogeneous heat distribution. This rapid heating prevents excessive grain growth and results in improved material qualities. Recent studies have demonstrated the successful use of microwave sintering for synthesizing solid electrolytes at lower temperatures and shorter durations.

In the densification of NZSP using microwave sintering [73], NZSP pellets were sintered at 850 °C for just 30 min, achieving a high relative density of 96% and RT ionic conductivity of 2.5 × 10⁻⁴ S/cm, which is comparable to that of samples prepared via conventional solid-state sintering at a temperature of 1200 °C for a time period of 12 h. The fine-grained structure obtained through this method demonstrates that microwave sintering effectively inhibits excessive grain growth. Additionally, the NZSP electrolyte, when used in a NVP/C|NZSP|Na solid-state sodium battery, delivered a discharge-specific capacity of 75.7 mAh/g and maintained 81.97% of its capacity after 100 cycles at 0.5 C rate. These results highlight the energy-saving and time-saving benefits of microwave sintering, paving the way for NZSP’s application in the development of high-performance SIBs.

e.

Liquid Phase Sintering

Liquid phase sintering (LPS) is a sintering technique that utilizes a liquid phase to enhance the binding of solid particles during the sintering process. This method accelerates densification by allowing for rapid mass transport through the liquid, which is generally much faster than diffusion through solids. LPS involves heating a mixture of solid particles and a liquid-forming additive, such as sodium carbonate or boron oxide, to a temperature where the additive melts, creating a liquid phase that facilitates particle rearrangement and densification. Liquid-phase sintering is an effective method for sintering materials that are challenging to densify. This technique involves adding a material with a lower melting point into the matrix. When heated to elevated temperatures, the lower melting point material forms a melt pool, facilitating the sintering process. This results in a lower sintering temperature and a shorter time to achieve a highly dense structure. Liquid-phase sintering is particularly advantageous for fabricating solid-state electrolytes due to these benefits [59].

The liquid-phase sintering (LPS) process for synthesizing NZSP involves adding a lower melting point material, such as sodium metasilicate (Na₂SiO₃), to reduce the sintering temperature and duration [59]. Initially, NZSP precursors, including Na₂CO₃, SiO₂, ZrO₂, and NH₄H₂PO₄, are ball-milled together and dried. These precursors are then calcined at high temperatures (1100 °C) to form NASICON powder. To achieve liquid-phase sintering, different amounts of Na₂SiO₃ (2.5 wt.%, 5 wt.%, 7.5 wt.%) are mixed into the NZSP powder. The mixture is then pressed into pellets and sintered at 1175 °C, which is slightly above the melting point of Na₂SiO₃ (1088 °C). During the sintering process, Na₂SiO₃ creates a melt pool that aids the densification and the diffusion of Na and Si ions into the NASICON structure. This diffusion enhances both grain and grain boundary conductivity, leading to improved ionic conductivity. After sintering, the pellets are polished for further characterization. The addition of Na₂SiO₃ results in the formation of a Si-rich secondary phase along the grain boundaries, further boosting ionic transport properties. The result is a highly dense NASICON electrolyte with improved ionic conductivity, suitable for use in SIBs.

Although various synthesis methods have been outlined, it is imperative to consider the critical factors that significantly influence the electrical properties of NASICON-type materials. These factors include particle size, the concentration of dopants, and sintering temperature. A detailed examination of these parameters is essential for optimizing the performance of NASICON materials. Therefore, these factors are comprehensively discussed in the next section.

2.4. Synthesis Factors Affecting the Properties of NASICON-Type Solid Electrolytes

2.4.1. Calcination

Calcination is a thermal treatment process where a solid material is heated to high temperatures in a controlled environment to induce phase transition by removing the volatile substances. Ruan et al. [74] investigated how calcination temperature affects the structure and ionic conductivity of NZSP. NZSP were synthesized at different calcination temperatures ranging from 1000 to 1200 °C. A gradual increase in crystallinity is observed with higher temperatures. At the relatively low calcination temperature of 1000 °C, the NZSP sample forms the NASICON structure but contains a significant level of ZrO₂ impurities. As the calcination temperature increases, the intensity of the ZrO₂ impurity peaks gradually decreases. However, the relative density reached its maximum at 1150 °C, then declined due to the formation of secondary phases (ZrO₂ and Na₂ZrSi₄O₁₁) and increased porosity. At 1150 °C for 12 h, the NZSP sample exhibited its RT total conductivity of 4.56 × 10⁻⁴ S/cm [22].

2.4.2. Bottleneck Size and Sodium-Ion Concentration

NASICON-structured materials with ionic conductivity (σ) are dependent on the mobile Na⁺ ions concertation and their mobility (µ), which is affected by the bottleneck size for ion migration and the ratio of Na⁺ concentration to vacancy availability. Insufficient vacancies reduce Na⁺ mobility, leading to decreased conductivity [22].

In addition to the bottleneck size, the occupancy of the Na sites affects Na⁺ ion transport. The increase in Na⁺ ion concentration enhances coulombic repulsion, which promotes correlated migration and faster ion diffusion. However, beyond a certain threshold, excess Na⁺ ion concentration can overcrowd the lattice, hindering ion mobility. The optimal Na content for maximizing conductivity was found to be around 3.3-3.55 mol per formula unit [22].

2.4.3. Chemical Substitutions and Doping Methods

To enhance NASICON conductivity, chemical substitutions and doping methods, particularly at the Zr⁴⁺ site, have proven effective. Substitutional doping refers to replacing some of the original cations in the crystal lattice with different cations (dopants). In the case of NASICON, this typically involves replacing zirconium (Zr) with aliovalent cations such as aluminum (Al), iron (Fe), yttrium (Y), cobalt (Co), nickel (Ni), and zinc (Zn). The introduction of dopants can cause slight distortions in the NASICON lattice, which can create pathways that facilitate easier ion transport [53].

When a trivalent cation (like Al, Fe, or Y) replaces a tetravalent cation (like Zr), it creates a charge imbalance. To balance the negative charge introduced by the +3 ion, an extra Na⁺ ion is incorporated into the structure, maintaining overall charge neutrality. This increases the concentration of mobile Na⁺ ions, enhancing the ionic conductivity of the material. This process is called “Aliovalent Doping” which helps to increase the concentration of mobile Na⁺ ions available for ion conduction, enhancing ionic conductivity. Doping can lower the activation energy required for ion migration [53]. The doping of NZSP with divalent ions like Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ has been studied, and with Mg²⁺ doping, the room-temperature (RT) conductivity of 3.5 mS cm⁻¹ is obtained due to enlarged bottleneck when 2.5% of Zr⁴⁺ is replaced with Mg²⁺ [75]. The substitution of Zr⁴⁺ by aliovalent Zn²⁺ [76] in NZSP combined with an increased Si/P ratio, led to a record conductivity of 5.3 mS cm⁻¹ for Na_3.4Zr_1.9Zn_0.1Si_2.2P_0.8O₁₂. The ionic radius of the dopant plays a crucial role.

Jolley et al. [52,53] explored substitutional doping at the zirconium site in NZSP using trivalent cations (Al, Fe, Y) and bivalent cations (Co, Ni, Zn). Impedance analysis demonstrated that aliovalent doping significantly boosts the bulk conductivity of NASICON materials. Notably, Zn-doped NZSP exhibited the RT bulk conductivity, reaching 3.75 mS/cm, while Co-doped NZSP showed only an RT ionic conductivity of 1.55 mS/cm. The DC polarization plot for Co-doped NASICON indicated a transference number greater than 0.99 for both doped and undoped samples, confirming that the conductivity is primarily due to ionic mobility rather than electronic contributions.

2.5. Other Modified Composition of Na_1+xZr₂Si_xP_3−xO₁₂

Element substitution is the most prevalent method for modifying composition. According to the charge balance principle, substituting with suitable low-valence ions can increase lattice defects, thereby improving the concentration of mobile sodium ions and enhancing conductivity. Examples of such substitutions include Ca²⁺, Mg²⁺, Sc³⁺, and Si⁴⁺. Table 3 provides a list of ionic conductivity of NASICON-type structures at RT with various ion substitutions reported in recent years.

Divalent, trivalent, tetravalent, and pentavalent dopants can partially occupy the Zr⁴⁺ sites, leading to enhanced ionic conductivity in NZSP [77]. Substituting with lower valent elements increases the concentration of charge carriers available for ion conduction to maintain charge balance, thereby improving Na⁺ mobility due to reduced coulombic repulsion between sodium ions and dopant ions. Additionally, the size of the bottleneck can be influenced by the radii of the dopants, further affecting Na⁺ mobility and ionic conductivity [77].

Given the similar ionic radii of Mg²⁺ (72 pm) and Zr⁴⁺ (72 pm), Mg²⁺ is an ideal candidate for the partial substitution of Zr⁴⁺. For instance, Mg-doped NZSP demonstrates a RT ionic conductivity of 3.64 mS/cm and a broad electrochemical window of 5 V [78]. Sc³⁺ also has a similar ionic radius (74.5 pm) to Zr⁴⁺ (72.0 pm), making it a suitable substitution ion for NASICONs, which have been extensively studied [79]. Na_3.4Sc_0.4Zr_1.6(SiO₄)₂(PO₄) synthesized via solution-assisted solid-state reaction showed an RT ionic conductivity of 4 mS/cm, whereas pure NZSP showed RT ionic conductivity at around 2 mS/cm only.

In addition to dopants with low-valence substitutions at different sites in NASICON structures, high-valence dopant substitutions can also improve ionic conductivity. For example, Liu et al. [80] introduced Nb⁵⁺ into NASICON-type solid electrolyte Na_3.4Zr₂Si_2.4P_0.6O₁₂, synthesized via a solution-assisted solid-state reaction, and achieved a remarkable ionic conductivity, reaching 5.51 mS/cm at RT due to increased Na⁺ concentration.

Dinachandra Singh et al. [81] introduced trivalent Ru³⁺ cations into the Zr⁴⁺ sites of NZSP electrolytes, significantly enhancing ionic conductivity. Although Ru³⁺ has a smaller ionic radius than Zr⁴⁺, causing bottleneck shrinkage, Ru-doping achieves RT ionic conductivity of 2.1 mS/cm, which is higher in comparison to undoped NZSP; has an ion transference number of approximately 98.9%; and supports stable sodium plating/stripping at 0.5 mA cm⁻² for a period of 100 h with a low overpotential of 20 mV due to its fused microstructure indicating efficient ion transfer and minimal energy loss during sodium plating/stripping, contributing to stable cycling and high capacity retention. In an assembled battery, the Ru-doped electrolyte delivers an initial capacity of 87 mAh/g at 0.3 C, surpassing the undoped electrolyte’s 70 mAh/g, while the pristine sample retains 60% capacity after 100 cycles.

In summary, numerous elements have been explored for substitution at Zr⁴⁺ sites. Substitution alters the bottleneck size in NASICON structures due to varying dopant radii, leading to more Na⁺ mobility through low-valence substitutions, controls the ratio of monoclinic to rhombohedral structures, and modifies grain boundaries via secondary phases, all contributing to enhanced ionic conductivity.

Table 3. List of ionic conductivity of NASICON with various ion substitutions.

Year of Publishing	Chemical Formula	Substituting Ion	Synthetic Method	Ionic Conductivity [S/cm] at RT	Ref.
2019	Na3.2Zr1.9Ca0.1Si2PO12	Ca2+	Sol–gel	1.67 × 10−3	[82]
2020	Na3.2Zr1.9Mg0.1Si2PO12	Mg2+	Solid-state reaction	2.2 × 10−3	[83]
2020	Na3.4Zr1.9Zn0.1Si2.2P0.8O12	Zn2+	Solid-state reaction	5.27 × 10−3	[77]
2020	Na3Zr1.9Ce0.1Si2PO12	Ce4+	Liquid-feed flame spray	6.9 × 10−4	[84]
2021	Na3.3Zr1.7La0.3Si2PO12	La3+	Sol–gel	1.34 × 10−3	[85]
2021	Na3.4Sc0.4Zr1.6Si2PO12	Sc3+	Solid-state reaction	2.6 × 10−3	[86]
2021	Na3.1Zr1.9Ga0.1Si2PO12	Ga3+	Solid-state reaction	1.06 × 10−3	[87]
2022	Na3.2Zr1.8Pr0.2Si2PO12	Pr3+	Solid-state reaction	1.27 × 10−3	[88]
2023	Na3.4Zr1.6Sc0.4Si2PO12	Sc3+	Solid-state reaction	1.77 × 10−3	[89]
2023	Na3.2Zr1.8Tb0.2Si2PO12	Tb3+	Solid-state reaction	6.32 × 10−4	[90]
2024	Na3Zr1.92Ru0.08Si2PO12	Ru3+	Solid-state reaction	8.1 × 10−4	[81]
2024	Na3.5Zr1.75Mg0.25Si2PO12	Mg2+	Solid-state reaction	2.4 × 10−3	[91]

Table 3. List of ionic conductivity of NASICON with various ion substitutions.

Year of Publishing	Chemical Formula	Substituting Ion	Synthetic Method	Ionic Conductivity [S/cm] at RT	Ref.
2019	Na3.2Zr1.9Ca0.1Si2PO12	Ca2+	Sol–gel	1.67 × 10−3	[82]
2020	Na3.2Zr1.9Mg0.1Si2PO12	Mg2+	Solid-state reaction	2.2 × 10−3	[83]
2020	Na3.4Zr1.9Zn0.1Si2.2P0.8O12	Zn2+	Solid-state reaction	5.27 × 10−3	[77]
2020	Na3Zr1.9Ce0.1Si2PO12	Ce4+	Liquid-feed flame spray	6.9 × 10−4	[84]
2021	Na3.3Zr1.7La0.3Si2PO12	La3+	Sol–gel	1.34 × 10−3	[85]
2021	Na3.4Sc0.4Zr1.6Si2PO12	Sc3+	Solid-state reaction	2.6 × 10−3	[86]
2021	Na3.1Zr1.9Ga0.1Si2PO12	Ga3+	Solid-state reaction	1.06 × 10−3	[87]
2022	Na3.2Zr1.8Pr0.2Si2PO12	Pr3+	Solid-state reaction	1.27 × 10−3	[88]
2023	Na3.4Zr1.6Sc0.4Si2PO12	Sc3+	Solid-state reaction	1.77 × 10−3	[89]
2023	Na3.2Zr1.8Tb0.2Si2PO12	Tb3+	Solid-state reaction	6.32 × 10−4	[90]
2024	Na3Zr1.92Ru0.08Si2PO12	Ru3+	Solid-state reaction	8.1 × 10−4	[81]
2024	Na3.5Zr1.75Mg0.25Si2PO12	Mg2+	Solid-state reaction	2.4 × 10−3	[91]

Incorporating NASICON as a filler into polymer matrices to form CPE for sodium-ion battery application has resulted in enhanced performance by combining the high ionic conductivity of NASICON with the flexibility and processability of polymers. This hybrid approach results in solid electrolytes with improved mechanical properties, safety, and overall efficiency. The next section deals with the addition of NASICON-type fillers in various polymer matrices to form CPEs.

3. NASICON as a Filler in Composite Polymer Electrolytes

CPEs are usually composed of polymer matrix and inorganic fillers. Ion conduction in CPEs is influenced by the combination of sodium-ion conductive polymers and ceramic particles, where these ceramic particles can be either passive (non-conductive) or active (conductive). Filler particles such as Al₂O₃, SiO₂, and ZrO₂ do not actively contribute to ion transport in CPEs and are known as passive fillers [92]. In contrast, active fillers like NZSP and Na-β-alumina are directly involved in the ion conduction process, significantly enhancing the electrolyte’s ionic conductivity.

In polymer-passive ceramic systems, non-conductive ceramic particles such as zirconia modify the properties of the polymer matrix by reducing crystallinity [93] and enhancing sodium-ion conductivity. Additionally, these ceramics improve mechanical strength [93]. For polymer-active ceramic composites, fillers are conductive themselves, thus the inclusion of sodium-ion conductive ceramic particles further improves overall ionic conductivity by introducing a higher-conductive ceramic phase [92]. These active ceramics reduce polymer crystallinity in regions near the polymer–ceramic interface and enhance ion transport through both the polymer and filler phases. Nevertheless, the performance is primarily influenced by factors such as the intrinsic ionic conductivity of the NASICON, the interfacial resistance between the polymer and NASICON, and the establishment of continuous sodium-ion pathways. Higher conductivity can be achieved when the active ceramic particles form percolating networks or when the polymer–ceramic interfaces are sufficiently conductive.

Dalvi et al. [94] reported the development of hybrid composites consisting of NaCF₃SO₃, PEO, and NaTi₂(PO₅)₃ (NTP) NASICON, which exhibited high ionic conductivity and electrochemical stability. A maximum conductivity of 3 × 10⁻⁵ S cm⁻¹ at 40 °C is achieved for 10 wt.% NaCF₃SO₃ and 90 wt.% [NTP+PEO] CPE composition along with the enhanced thermal stability of CPE up to 100 °C. Conductivity behavior follows Arrhenius trends, and linear sweep voltammetry shows improved electrochemical stability, with a current initiation at ~3 V. These composites show promise for the development of advanced SIBs.

New attempts have been made to utilize the polymers as wetting agents for NZSP solid electrolytes. A study addressed the performance of NZSP solid electrolytes using polyvinyl acetate (PVAc) as wetting agents [95]. Electrochemical impedance spectroscopy (EIS) reveals that PVAc-based glue significantly reduces interfacial resistance, achieving an RT ionic conductivity of 1.31 mS/cm (when compared to the pristine NZSP, the obtained result is 63.8% higher). The full cell with NVP/C cathode and hard carbon anode showed low resistance (2052 Ω·cm²), highlighting the PVAc-based glue’s potential in fabricating high-conducting NZSP.

Shen et al. [96] demonstrated a NASICON composite solid electrolyte (CSE) with 80 wt.% Na_3.4Zr_1.9Zn_0.1Si_2.2P_0.8O₁₂ (NZP) particles in the PEO and ionic liquid (IL exhibited RT ionic conductivity of 1.48 × 10⁻⁴ S/cm. Solid-state NVP/NZP-PEO@IL/Na batteries showed outstanding performance, retaining 90.0% capacity after 150 cycles at 0.5 C under 60 °C and delivering 109.4 mAh/g with 85.4% retention at 0.2 C under 25 °C.

Wang et al. [97] reported the sodium-ion conduction mechanism in NZSP incorporated PEO-NaTFSI matrix, achieving excellent RT ionic conductivity of 0.14 mS/cm, significantly higher than that of reported PEO-based SPEs. The NASICON framework enhances mechanical strength, suppresses Na dendrites, and ensures stable electrolyte-electrode contact with low interface impedance over 200 cycles (400 h). Scanning probe microscopy reveals fast Na-ion diffusion pathways at filler–polymer interfaces, supporting the superior ionic conductivity. In NASICON and CPEs, sodium-ion conduction occurs through interconnected ceramic–polymer interfaces and NASICON pathways. NASICON ceramics provides a 3D ion-conducting framework with low activation energy, enabling fast Na-ion transport. At the interfaces of NZSP and polymer, the ionic mobility has been enhanced due to the smooth ion diffusion pathways created by thin PEO coatings. Compared to bulk NZSP or crystalline PEO regions, ions diffuse faster at these interfaces. In the PEO phase, Na ions migrate via the segmental motion of amorphous polymer chains. The interfacial network and ceramic framework collectively facilitate continuous pathways for efficient Na-ion conduction, achieving high ionic conductivity. Further details on sodium-ion conduction mechanisms in CPE due to NASICON-type fillers incorporation are discussed in the next section.

Sodium-Ion Conduction Mechanisms in CPE Caused by NASICON Incorporation

Sodium-ion conduction in NASICON-incorporated CPEs depends on the volume fraction of NASICON-type fillers. Three distinct loading regimes significantly affect sodium-ion transport [16]: (i) low ceramic volume fractions where the polymer dominates conduction, (ii) intermediate ceramic volume fractions where polymer–ceramic interfaces form new conduction pathways, and (iii) high ceramic volume fractions where continuous ceramic pathways facilitate easier sodium-ion migration. NASICON falls under the category of active ceramic composites. The design of CPEs, especially in systems with active ceramics, require balancing ceramic volume fraction, particle size, and interface resistance to optimize sodium-ion conductivity.

There are three primary pathways for Na-ion diffusion in CPEs [98], depending on the loading of ionic conductors: (1) ion hopping among the filler particles, (2) transport at the interfaces of both polymer and filler particles, and (3) diffusion through the polymer matrix only. The pathways are shown in Figure 7. Considering the ion-conducting pathways, pathway (2) is more applicable in CPE, where both fillers and polymer matrix contribute to ionic conductivity, but pathways (1) and (3) could still occur in places where numerous NASICON particles are present and in certain regions where the polymer matrix dominates due to lack of NASICON particles, respectively.

In ion hopping mechanisms, Na⁺ ions move directly through the Na-conducting ceramic particles via an ion-hopping process [99]. NASICON ceramics, known for their high Na⁺ conductivity, facilitate ion transport through specific lattice sites where the Na⁺ ions “hop” from one site to another. However, this ion-hopping process is relatively slow compared to the other mechanisms because the crystalline structure and the energy barriers between adjacent Na⁺ sites constrain it. Since NASICON ceramics have a very high transference number for Na⁺ ions (close to 1), this pathway effectively contributes to the selective transport of Na⁺ ions over anions. By optimizing the balance between ceramic content and polymer phase, these pathways work together to maximize the overall performance of Na-conducting hybrid solid electrolytes [99].

4. Effect of NASICON-Type Fillers on Various Properties of Composite Solid Electrolytes

When NASICON-type fillers are introduced into a chosen polymer matrix, the thermal, electrical, and electrochemical properties of the solid electrolyte system are affected. Zhang et al. [100] investigated the electrical properties of the NaTFSI-PEO polymer matrix with NASICON (Na_3.4Zr_1.8Mg_0.2Si₂PO₁₂) as an active ceramic filler. The incorporation of NASICON significantly achieved a high ionic conductivity of 2.8 mS/cm at the temperature of 80 °C with a 50 wt.% NASICON content. NASICON fillers inhibit the decomposition of PEO, as indicated by the lack of weight loss in thermogravimetric analysis (TGA) at higher temperatures (250 °C), compared to pure SPE. This suppression is due to the interaction between the NASICON particles and the polymer matrix which stabilizes the polymer chains and prevents their breakdown at elevated temperatures. In the same study [100], it was observed that the impedance spectra of filler-free SPE and CPE with 50 wt.% NASICON at RT (shown in Figure 8a,b) reveal distinct differences in their ion transfer behaviors. The presence of grain boundary resistance is explicitly seen in this study due to the introduction of NASICON into the polymer matrix. The CPE exhibits two semicircles in the impedance spectrum, suggesting enhanced ion motion in both the ceramic grains and the interface between the filler and the PEO matrix, compared to the single semicircle observed in the SPE.

Electrochemical performance evaluation of NASICON-based CPEs is crucial to assessing their suitability for SIBs. This evaluation helps in understanding key parameters such as overall battery efficiency. By analyzing the factors, future research can be focused on optimizing the composition and structure of NASICON-based CPEs to enhance their performance, ensuring the requirements for practical energy storage applications. Thus, the upcoming section deals with the electrochemical performance of NASICON-based CPEs.

Electrochemical Performance Evaluation of NASICON-Based CPEs

Electrochemical stability windows (ESWs) are often measured using linear sweep voltammetry (LSV). In this method, the current at the working electrode is recorded while the potential is gradually increased from a low to a high value. Initially, the electrolyte produces a small current that is non-faradaic (not caused by a chemical reaction). However, if the electrolyte undergoes reduction or oxidation within the voltage range, the current rises significantly. When the reaction voltage is reached, charge migration occurs at the interface between the electrode and the electrolyte, causing the current increase. As the voltage continues to rise, the current eventually decreases and returns to its non-faradaic state due to the reduction in reactive species.

Cyclic voltammetry (CV) is another common method to measure the ESW of electrolytes. In this technique, the working electrode’s potential is increased from the open circuit voltage to a set potential and then reversed back. When oxidation or reduction reactions occur at the electrode surface, distinct peaks appear in the CV plot. The position and size of these peaks indicate the material’s electrochemical stability and help estimate the ESW of the electrolyte.

Niu et al. [101] synthesized NZSP powders using the sol–gel method and fabricated PEO/NZSP composite solid electrolytes via solution-casting, achieving ionic conductivities greater than 10⁻⁴ S/cm at 55 °C, a broad electrochemical stability window (>4.7 V), and effective sodium dendrite suppression. PEO-NaClO₄-NZSP-based cell exhibits a higher first discharge capacity of 72.9 mAh/g compared to 63.8 mAh/g for PEO-NaClO4-based cell, demonstrating enhanced ion transport due to NZSP. Additionally, the NZSP incorporated CPE cells maintain admirable discharge capacities across various C-rates and retain 98.4% capacity after 100 cycles at 0.1 C rate, significantly outperforming the PEO-NaClO₄-only electrolytes. The impedance analysis shows that the NZSP cell starts with a lower impedance (345 Ω) and stabilizes around 433 Ω after cycling, whereas the PEO-NaClO₄ cells experience a dramatic increase in impedance. The smooth Na surfaces in contact with the NZSP electrolyte indicate the effective suppression of dendrite growth, further contributing to improved cycling stability. Overall, NZSP enhances the performance of solid-state sodium metal batteries by improving discharge capacity, cycling stability, rate capability, and interfacial stability while mitigating dendrite formation.

Table 4. Electrochemical performances of NASICON-incorporated polymer electrolytes.

	Polymer and Sodium Salt	NASICON type	Ionic Conductivity S/cm	Electrochemical Performance	Ref.
1	PEO and sodium bis(trifluoromethanesulfonyl)imide (Na(CF3SO2)2N)	Na3.4Zr1.8Mg0.2Si2PO12 (50 wt.%)	2.8 × 10−3 (at 80 °C)	With Na3V2(PO4)3; Na as the electrode showed a 115.9 mAh/g and 107 mAh/g charge and discharge capacity at a 0.1 C rate with 4.3 V ESW and a 98.6% Coulombic efficiency.	[100]
2	PEO and sodium bis(fluoro sulfonyl)imide (NaFSI)	Na3.4Zr1.8Mg0.2Si2PO12 (NZMSP) (40 wt.%)	4.4 × 10−5 (at RT)	With Na3V2(PO4)3; Na as the electrodes showed a 106.1 mAh/g charge capacity at a 0.1 C rate with >5 V ESW and a 94% Coulombic efficiency.	[102]
3	PVDF–HFP (binder) and sodium triflate (NaSO3CF3)	Na3Zr2Si2PO12 (70 wt.%)	1.4 × 10−3 (at 90 °C)	With NaFePO4; HC as the electrodes showed a 330 mAh/g discharge capacity at a 0.2 C rate.	[99]
4	PEO and sodium perchlorate (NaClO4)	Na3Zr2Si2PO12 (25 wt.%)	5.6 × 10–4 (at 60 °C)	With Na2MnFe(CN)6; Na as the electrode showed a 111 mAh/g and 109 mAh/g charge and discharge capacity at a 0.5 C rate with a 97% Coulombic efficiency and 83% capacity retention over 300 cycles.	[103]
5	PEO and sodium bis(trifluoro methanesulfonyl)imide (NaTFSI)	Na2Zn2TeO6 (50 wt.%)	4 × 10−5 (at 30 °C)	With Na3V2(PO4)3; Na as the electrode showed a 106 mAh/g discharge capacity at a 0.2 C rate with a 4 V ESW.	[104]
6	PE-based macromonomer and sodium srifluoro methanesulfonate (NaTFSA)	Na3Zr2Si2PO12 (30 wt.%)	1.03 × 10–5 (at RT)	With NaCoO2; Na as the electrodes showed a 115.9 mAh/g and a 107 mAh/g charge and discharge capacity at a 0.1 C rate with a 4.3 V ESW and a 98.6% Coulombic efficiency.	[105]
7	PEO and sodium bis(trifluoro methanesulfonyl)imide (NaTFSI)	Na3Zr2Si2PO12 (10 wt.%)	1.4 × 10−4 (at RT)	With a loading of 25 wt.% filler in CPE; Na3V2(PO4)3 and Na as electrodes showed a 96.2 mAh/g discharge capacity at a 0.1 C rate with a 4 V ESW and a 98% Coulombic efficiency.	[97]
8	PVDF-HFP and sodium perchlorate (NaClO4)	Na3Zr2Si2PO12 (10 wt.%)	2.25 × 10−3 (at RT)	With Na3V2(PO4)3; Na as the electrode showed a 98 mAh/g reversible capacity at a 0.2 C rate with a 5 V ESW and a 62.7% Coulombic efficiency	[106]

shows the list of electrochemical performance of NZSP-incorporated polymer electrolytes.

As already mentioned, Wang et al. [97] developed NZSP/PEO composite electrolytes which demonstrated a stable initial discharge capacity of 96.2 mAh/g, which decreases to 49.6 mAh/g by the 50th cycle and further decreases to 39.2 mAh/g by the 100th cycle. Despite the initial rapid capacity fading, a stable reversible capacity of approximately 37 mAh/g (with a capacity retention of about 38.5%) was maintained, along with a high coulombic efficiency of 98% after 100 cycles. This performance illustrates that while the initial cycles are affected by the formation of a passivation layer at the composite electrolyte–Na interface, subsequent cycles stabilize the performance, leading to low impedance at the electrolyte–electrode interface and good durability over long-term cycling. Furthermore, when a symmetric full cell was constructed using the NASICON/PEO electrolyte with NVP electrodes, it exhibited a charge–discharge plateau of between 1.65 V and 1.80 V, with the 100th cycle capacity reaching 34.5 mAh/g (83.7% retention) after continuous cycling. These data demonstrate that the NASICON/PEO composite electrolytes not only enhance initial performance but also ensure consistent cycling stability, positioning them as viable alternatives to traditional liquid electrolytes in all-solid-state Na batteries. Cell performance in NASICON-based CPEs for SIBs is influenced by ionic conductivity, interfacial resistance, thermal stability, mechanical properties, and electrochemical stability. The optimization of the above factors is required to enhance cell performance.

5. Conclusions and Future Directions

Composite polymer electrolytes show promise in improving mechanical flexibility and enhanced sodium-ion conductivities by incorporating NASICON-type fillers. NASICON-type materials used as fillers play a significant role in reducing the crystallinity of the base polymer, thereby improving not only the flexibility but also the ion conduction paths in the chosen polymer matrix. The inherent sodium-ion conductivity of NASICON-type structures is higher, and hence preparing CPEs with NASICON can significantly improve the ionic conductivity. The crystal structures, synthesis routes, and ion conduction mechanisms in NASICON-type structures along with various strategies used to enhance room temperature ionic conductivity of those materials are detailed in this review. The scope of NASICON-incorporated CPEs as the next-generation solid electrolytes are outlined in this review.

Although NASICON nanostructures show promise for the development of flexible next-generation sodium-ion-conducting solid polymer electrolytes, various factors affect the ionic conductivity of CPEs, such as dispersion of NASICON nanostructures in the polymer matrix and grain boundary resistance between the NASICON nanostructures. Therefore, the limitations and the scope for enhancing the inherent sodium-ion conductivity of NASICON structures are elaborated in this review. The effect of sintering in reducing grain boundary resistance, so the effective use of those materials for solid-state sodium-ion battery application has also been detailed in this review. Thus, this review will form a complete guide for tailoring sodium-ion conductivity of NASICON-incorporated solid polymer electrolytes.

Future research should focus predominantly on improving the sodium-ion conductivity of NASIOCN-type structures by doping various metal ions. The interaction between NASICON nanostructures and the chosen polymer matrix must be engineered so that better sodium-ion transport can be expected. Also, the impact of chosen polymer or polymer blend characteristics when incorporated with NASICON nanostructures requires a systematic and thorough investigation.

Further, in pursuit of finding sustainable and eco-friendly CPEs with NASICON nanostructures, the development of PFAS-free polymer materials for polymer electrolyte application is inevitable. To prepare flexible CPEs with NASICONs other than PEO, PFAS-free bio-degradable polymers could be used. Another interesting option could be to modify the structure of the chosen polymer which has branches ending in sodium ions that could be synthesized. With this polymer and NASICON nanostructures as fillers, the CPEs could be tailor-made to exhibit a higher room temperature conductivity of above 10⁻² S/cm. Developing theoretical models for predicting the ionic conductivity of CPEs is one of the challenging areas for future work. The impact of NASICON nanostructures on the development of next-generation sodium-ion conducting CPEs will be ever-increasing.