Sodium-Ion Batteries: Linking Liquid and Solid-State Electrolytes, Electrode Compatibility, and Commercial Viability

Abstract

Sodium-ion batteries are emerging as credible complements to lithium-ion technology for sustainable, safe, and cost-effective energy storage. This critical review links molecular-scale electrolyte solvation and interphase chemistry to electrode compatibility, full-cell engineering, manufacturing constraints, and commercial viability. Organic liquid, aqueous, ionic-liquid, concentrated, inorganic solid, polymer, and composite electrolytes are compared using transport, stability, processing, and interface criteria. The principal cathode and anode families are then evaluated in terms of practical voltage, reversible capacity, cycling stability, raw-material exposure, manufacturability, and end-of-life implications. A distinctive contribution of this work is the explicit separation of thermodynamic predictions, laboratory measurements, prototype demonstrations, and company-reported targets, together with design rules that connect electrolyte chemistry to cell-level performance. Sodium-ion batteries are unlikely to replace lithium-ion batteries universally, but they can occupy a strategic role where cost, safety, abundance, supply-chain resilience, and circularity outweigh maximum energy density.

1. Introduction

Growing awareness of the environmental impact of fossil fuel dependence has accelerated the global shift toward renewable energy sources such as solar and wind [1]. These technologies alone cannot guarantee continuous power generation, as they depend on uncontrollable external factors, like weather and time [2,3]. To bridge this gap and maintain a stable and reliable energy supply, efficient energy storage solutions are essential [2,3,4].

Among energy storage technologies, batteries stand out because their performance is governed by coupled electrochemical processes at the cathode, anode, and electrolyte. The electrolyte transports ions while blocking electronic current, whereas the electrode chemical potentials establish the cell voltage and electrodes the cell’s capacity. Interfaces govern charge-transfer kinetics, passivation, impedance growth, and long-term stability (Figure 1) [4,5,6,7,8,9].

Figure 1. Schematic of the operation of a sodium-ion battery employing a polyanionic compound positive electrode (active cathode) and carbon anode. The designations anode and cathode strictly refer to the electrodes during discharge; during charging, their electrochemical roles are reversed.

At present, lithium-based systems dominate the energy storage field, spanning grid applications, consumer electronics, and electric mobility [2,10,11]. However, their long-term use raises significant challenges, particularly related to raw material scarcity, environmental and ethical concerns, and safety issues [12,13]. The Study on Critical Raw Materials for the EU (2023) highlights that key elements such as lithium, cobalt, nickel and natural graphite, vastly used in lithium-ion batteries (LIBs), are scarce and/or geographically concentrated [14]. This concentration not only exposes the global energy storage industry to geopolitical and economic risks, but also makes the market prices highly dependent on their availability and, by extension, on the efficiency of their recycling processes [15,16,17]. The instability of global supply chains was already demonstrated in 2010, prompting other nations to invest in developing their own rare earth supply chains [15,16].

China, the world’s largest energy producer and consumer, dominates the global critical raw material landscape, processing 56% of the world’s lithium, 60% of its cobalt, and extracting 67% of natural graphite [14]. Its dominance is even more pronounced in rare earth elements (REEs), critical for electric motors and wind turbines, where it processes nearly 85% of light REEs and controls virtually the entire processing of heavy REEs [14]. Meanwhile, the Democratic Republic of Congo (DRC) supplies approximately 63% of global cobalt [18]. These figures illustrate only a fraction of the global monopolies and related issues in the critical raw material (CRM) supply chain, which fundamentally undermines the long-term sustainability of LIBs.

Another factor limiting the long-term feasibility of LIBs is the toxicity of several cell components. Key constituents such as cobalt and nickel, used in the cathode, are classified as carcinogenic, mutagenic, and toxic to reproduction [2,13]. LIBs also contain highly reactive organic and inorganic compounds that can ignite or explode at elevated temperatures and pollute the environment [9,19]. While government regulations and recycling initiatives aim to mitigate these risks and improve sustainability, the recycling of LIBs remains a complex and imperfect process, facing both technical and economic limitations [17,20].

The challenges associated with LIBs have motivated the development of alternative, more sustainable chemistries capable of supporting the growing energy demand [3,4]. Among these, sodium-ion batteries (SIBs) have re-emerged as a promising candidate. Initially explored in the 1970s and 1980s alongside lithium-based systems, SIB research declined after the rapid success of LIBs due to lithium’s superior energy density [9,21]. Interest was renewed around 2000, with the discovery of sodium hard carbon as a suitable anode, shifting attention toward material optimization in response to growing concerns about lithium’s scarcity and uneven resource distribution [9,22].

Sodium is approximately 1300 times more abundant than lithium in earth’s crust (Na 2.36 wt.% vs Li 0.0017 wt.%) and is evenly distributed across the globe, which significantly reduces both material cost and geopolitical exposure [23,24]. It can be extracted from seawater and industrial waste, making it a more sustainable alternative to lithium [23].

Although SIBs exhibit lower energy density due to the higher atomic mass and larger ionic radius of sodium, their advantages in cost, sustainability and operational safety make them particularly attractive for stationary energy storage and grid-scale applications, where weight and energy density are less critical [4,23]. SIBs are also well positioned to replace LIBs in lower-energy-density mobile markets, e.g., low-speed electric vehicles (EVs) and electric bikes, and to serve as a cost-effective complement to LIBs in the broader energy storage landscape [9].

In stationary energy storage systems (ESSs), safety is paramount, and LIBs have already demonstrated significant safety vulnerabilities. Incidents such as the 2017 fire at Belgium’s first grid-connected LIB energy storage facility, along with similar events in South Korea and the United States, emphasize the danger of thermal runaway, where localized heating can trigger cascading cell failures, fires, and toxic gas release [25]. Sodium-ion systems, by contrast, operate at slightly lower cell voltages and use electrolytes with higher thermal stability and less reactive components [9,25,26]. Research indicates that sodium-ion electrolytes can incorporate a higher proportion of heat-resistant solvents, raising the threshold for thermal runaway and reducing the likelihood of combustions [9,26].

A potentially important transport advantage of some SIB designs is the possibility of using aluminum current collectors on both electrodes and shipping cells at very low state of charge. These benefits are chemistry- and design-dependent rather than universal: they require electrode potentials that avoid aluminum alloying or dissolution and must be validated at the complete cell level. Accordingly, zero-volt transport and dual-aluminum architectures should be treated as system-specific engineering options, not intrinsic properties of every sodium-ion cell [26].

On the industrial front, sodium-ion technology has entered early commercialization, but the maturity of individual announcements varies. Figure 2 is therefore date-stamped to August 2026 and distinguishes active production, pilot-scale activity, announced projects, partnerships, and inactive companies. CATL performance values are identified as company-reported claims rather than independently verified cell data; vehicle range, cell-level energy density, and projected cycle life are reported separately. This evidence hierarchy is essential because commercial status and announced specifications can change rapidly [27,28,29].

Despite rapid progress and growing industrial interest in sodium-ion technology, several challenges persist, particularly in material optimization and performance [9,30]. The lower energy density of SIBs relative to LIBs and their limited cycle life remain major obstacles [9,23]. At the electrolyte level, conventional organic liquid electrolytes (OLEs) enable fast ion transport but suffer from flammability, leakage, and narrow electrochemical stability windows (ESWs) [31]. Solid-state electrolytes (SSEs) offer a safer, more thermally stable, and simpler battery design alternative, eliminating issues related to volatility and leakage while providing the mechanical rigidity necessary to prevent dendrite formation [5,31]. Their adoption, however, is still limited by interfacial resistance and ionic conductivity at room temperature, which raise cell impedance and contribute to capacity fading during cycling [30,31]. At the electrode level, the optimization of cathode materials, particularly in architectures that integrate SSEs, is essential for achieving higher capacity, longer cycle life, and improved energy density [9,30].

Figure 2. Global landscape of sodium-ion battery industrial players, and geographical concentration of critical raw materials (CRMs). CRM processing shares from [14]; sodium abundance data from [32].

Figure 2. Global landscape of sodium-ion battery industrial players, and geographical concentration of critical raw materials (CRMs). CRM processing shares from [14]; sodium abundance data from [32].

This review examines sodium-ion batteries across the coupled chain from electrolyte chemistry to electrode selection, cell engineering, commercialization, and end-of-life strategy. Unlike reviews focused exclusively on a single electrolyte or electrode family, it compares liquid, aqueous, ionic-liquid, concentrated, inorganic-solid, polymer, and composite electrolytes under common transport, stability, interface, safety, and processing criteria; it then connects those properties to cathode/anode compatibility and full-cell evidence. Particular emphasis is placed on distinguishing theoretical values, half-cell measurements, prototype full cells, cell-level specifications, and company-reported pack targets. Section 2 addresses electrolytes, Section 3 cathodes, Section 4 anodes, Section 5 cross-cutting cell-design principles, Section 6 future directions, and Section 7 conclusions.

Review Methodology and Evidence Classification

The peer-reviewed literature was identified using Web of Science, Scopus, and Google Scholar with combinations of the terms “sodium-ion battery”, “sodium electrolyte”, “solvation”, “SEI”, “CEI”, “aqueous electrolyte”, “ionic liquid”, “high-concentration electrolyte”, “solid electrolyte”, “NaSICON”, “beta-alumina”, “sulfide”, “halide”, “polymer electrolyte”, “cathode”, “hard carbon”, “commer-cialization”, and “recycling”. The principal publication window was 2012–5 August 2026, complemented by seminal earlier studies. Studies were included when they reported clearly identifiable materials, test conditions, or industrial status. Commercial information was last checked on 5 August 2026, and it prioritized primary corporate, regulatory, or facility sources. Throughout the review, peer-reviewed electrochemical data, independent prototype results, market analyses, and company announcements are labeled separately and are not as-signed equivalent evidentiary weight.

The reference catalog is dominated by the peer-reviewed literature, comprising 189 of the 209 references (90.4%) as journal articles, supplemented by two book chapters (1.0%), two technical reports (1.0%), one conference abstract (0.5%), and 15 web-based-institutional sources (7.1%). Given the early stage of sodium-ion battery commercialization, a limited number of non-peer-reviewed sources were included where equivalent academic information was unavailable, primarily to document commercial cell specifications, product launches, market developments, and regulatory frameworks. These include six company sources (2.9%) from CATL, Hithium, Kuraray, and Stora Enso, eight trade-news and market-intelligence reports (3.8%) from EE Power/Tech Insights, Shanghai Metals Market, IDTechEx, CIC energiGUNE, TYCORUN Energy, BEST Magazine, and Battery-News.de, and three institutional or regulatory reports (1.4%) from the European Commission, the U.S. Geological Survey, and the Batteries European Partnership Association (BEPA) addressing raw materials and battery policy.

2. Sodium-Based Electrolytes for SIBs

As in LIBs, the electrolytes play a key role in the operation and safety of SIBs, as they govern the migration of Na⁺ ions between the electrodes while maintaining electronic insulation. Their composition directly influences ionic conductivity, interfacial stability, and the electrochemical durability of a battery [5]. Based on their physical state and chemistry, electrolytes are classified as non-aqueous (organic), aqueous and solid-state (including polymers) and all-solid-state electrolytes (inorganic) [5,30].

2.1. Organic Liquid Electrolytes

Organic liquid electrolytes (OLEs) have been the dominant electrolyte choice in sodium-ion systems due to their high ionic conductivity [5]. OLEs for SIBs typically consist of a sodium-based salt dissolved in aprotic solvents, with small amounts of additives incorporated to improve electrochemical performance and stabilize electrode/electrolyte interphases (EEIs) [33]. These EEIs, commonly known as the solid electrolyte interphase (SEI) on the anode side and cathode electrolyte interphase (CEI) on the cathode side, act as passivating layers that form from the decomposition of the electrolyte components, as shown in Figure 3 [34]. The SEI is particularly important when using Na metal anode because of its high reactivity [5]. An inhomogeneous and unstable SEI can lead to uneven Na metal deposition and dendrite formation, which hinder the cell’s behavior [5]. The nature and stability of these interphases are therefore strongly dependent on the choice of salt and solvent.

Figure 3. Schematic of the electrolyte’s electrochemical stability window (ESW). (a) Ideal case: the ESW fully spans the potential range between anode (${\mu }_a$) and cathode (${\mu }_c$). Non-ideal case: (b) the cell voltage exceeds the ESW at the anode, ${\mu }_a$ exceeds the electrolyte’s LUMO and at the cathode the ${\mu }_c$ is below the electrolyte’s HOMO level, likely leading to the formation of an SEI and a CEI and (c) cycling chemical potential variations.

Crystalline sodium salts are described by periodic band structures with valence-band maxima and conduction-band minima. After dissolution, however, liquid-electrolyte reduction and oxidation are governed by the frontier orbitals and redox free energies of solvated ion–solvent complexes, together with electrode surfaces, defects, concentration, and interphase kinetics [34]. Periodic solid-state band edges therefore cannot be interpreted as the HOMO/LUMO levels or practical electrochemical stability window of a liquid electrolyte. In this review, calculated crystalline values are used only as qualitative electronic-structure descriptors and are explicitly separated from experimentally measured solution-phase stability.

2.1.1. Sodium Salts and Solvents

As crystalline materials, sodium salts are properly described in terms of band structure, characterized by the valence maximum energy (E_v) and conduction band minimum energy (E_c). However, once the salt is dissolved, the ESW is governed by the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the solvated ion complexes in the liquid, which depend jointly on the anion and the specific solvent in which it is dissolved, as shown in Figure 3 [34]. For a stable electrolyte, the anode’s electrochemical potential must lie below the LUMO energy level and the cathode’s electrochemical potential above the HOMO energy level (Figure 3a) [34]. The ESW must then fully span the operating potential range between the two electrodes for the electrolyte to remain electrochemically stable. Anions whose solvated HOMO is high are prone to oxidative decomposition at the cathode, while those with a low solvated LUMO are more susceptible to reduction at the anode and can facilitate the formation of the SEI layer (Figure 3b,c) [5,33,34].

Selecting materials, which balance these two parameters, is essential to prevent degradation and preserve the electrolyte’s chemical integrity (Figure 4). The salt should exhibit: (1) high solubility in the solvent in order to facilitate Na⁺ ions transportation, (2) high thermal stability, (3) chemically inertness toward other cell components, and (4) stability across wide voltage ranges to resist oxidative and reductive decomposition [5,33]. The solvent, in turn, must: (1) be polar and have a high dielectric constant ($\epsilon$) to support salt dissociation, (2) have low viscosity to enable better ionic conductivity, (3) remain electrochemically inert at both electrode surfaces, and (4) display a wide liquid-state temperature range defined by a low melting point and a high boiling point [4,5].

Figure 4. Sodium salts comparison map. Color indicates toxicity (green = low; red = high); border style denotes Al corrosion. Corrosion data were compiled from Eshetu et al. [33] Toxicity level was adapted from Li et al. [35] * Calculated herein for the salts in a periodic lattice, not solvated in a liquid electrolyte. For additional information, see Appendix A.

Figure 4. Sodium salts comparison map. Color indicates toxicity (green = low; red = high); border style denotes Al corrosion. Corrosion data were compiled from Eshetu et al. [33] Toxicity level was adapted from Li et al. [35] * Calculated herein for the salts in a periodic lattice, not solvated in a liquid electrolyte. For additional information, see Appendix A.

Figure 4 shows a qualitative comparison of representative sodium salts. Color denotes the toxicity category assigned from the cited safety literature, and border style denotes reported aluminum corrosion behavior. The displayed periodic solid-state band-edge values were calculated for crystalline salts and must not be interpreted as solvated HOMO/LUMO energies or as direct predictions of the practical electrochemical stability window. Corrosion data were compiled from Eshetu et al. [33], and toxicity categories were adapted from Li et al. [35]. Full computational settings and numerical values are provided in Appendix A.

Among the most commonly used salts in non-aqueous liquid SIB electrolytes are NaClO₄, NaPF₆, NaBF₄, NaFSI − NaN(SO₂F)₂, NaTFSI − NaN(SO₂CF₃)₂, and NaOTf − NaCF₃SO₃ (Figure 4) [4,5].

Of these salts, NaPF₆ (Na hexafluorophosphate) is regarded as the most balanced choice in terms of ionic conductivity, cost and safety. It generally exhibits high ionic conductivity; 1 M NaPF₆ solution in propylene carbonate (PC) reaches 8 mS cm⁻¹ at 25 °C [33,36]. It shows good thermal stability, with a decomposition temperature reported to be around 300 °C, which is not the case for its lithium analog LiPF₆, improving safety of the sodium correspondent [5]. NaPF₆ offers low toxicity compared to NaClO₄ and its synthesis route is almost identical to that of LiPF₆, which facilitates cost estimation and scaling [37].

In specific environments, the solvation structure of [PF₆]⁻ favors preferential anion decomposition, leading to the formation of a thin, inorganic compound-rich CEI (Figure 3c) that ensures electrode stability and facilitates charge transfer [5]. However, despite these favorable attributes, NaPF₆ is highly sensitive to moisture, decomposing to form hydrofluoric acid (HF), a highly toxic and aggressively corrosive species that attacks aluminum current collectors and dissolves redox-active transition metals [5,38]. While additives with water-scavenging ability, such as trimethoxy (pentafluorophenyl)silane (TPFS), can reduce HF generation, they do not eliminate it, and their long-term efficacy under real operating conditions remains an open question [5,39].

NaClO₄ (sodium perchlorate) is the most soluble of the commonly studied perchlorate salts and is widely used in research due to its availability and low cost [5]. It shows superior thermal stability, but retains significant water content even after prolonged high-temperature drying, and, in its dry form, poses serious safety risks due to its explosive potential and high toxicity [5,40].

Imide-based salts, such as NaFSI and NaTFSI, exhibit good ionic conductivity. For example, NaTFSI in PC reaches 6 mS cm⁻¹, similar to NaClO₄ (6 mS cm⁻¹) under the same conditions [33,36]. Their main drawbacks include concerns associated with their fluorinated nature, including potential implications under the EU’s proposed class-wide PFAS restriction, corrosion of aluminum current collectors, and, particularly for FSI-based salts, lower oxidative stability than NaPF₆ [5,36]. It was reported that adding a small fraction of NaPF₆ (5 wt.%) to an imide-based electrolyte can suppress Al dissolution and extend the anodic stability window [33,36].

The performance of sodium salts in OLEs is inseparable from the properties of the solvents in which they are dissolved, since ion dissociation, Na⁺ mobility and interphase formation are governed by salt–solvent interactions [5,33]. No single solvent can simultaneously satisfy all electrolyte requirements, so OLEs typically rely on a mixture of two or three solvents [5]. Carbonate esters and ether-based solvents constitute the two main families used in OLEs, selected primarily according to their dielectric constant ($\epsilon$) and viscosity, the two parameters that most directly control salt dissociation and ionic mobility [28]. As illustrated in Figure 5, cyclic carbonates exhibit high dielectric constants but relatively high viscosities, whereas linear carbonates and ethers offer lower viscosity at the expense of reduced dielectric constant.

Figure 5. (a) Representative organic electrolyte solvents used in SIBs and key physicochemical properties: (b) melting, boiling and flash point, (c) viscosity, and (d) dielectric constant of representative organic electrolyte solvents used in sodium-ion batteries. Data compiled from [5].

Cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) form the backbone of most non-aqueous SIB electrolytes due to their high dielectric constants (89.7 for EC and 64.9 for PC at 25 °C, Figure 5d) [5]. Their drawback lies in their high viscosity (1.9 cP for EC at 40 °C and 2.5 cP for PC at 25 °C, Figure 5c) [5]. EC’s high melting point (c.a. 36 °C, Figure 5b) further prevents its use as a single solvent, making blending unavoidable [41].

Linear carbonates, most importantly ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and diethyl carbonate (DEC), exhibit much lower viscosity (e.g., DEC: 0.75 cP at 25 °C, Figure 5c) but also significantly lower dielectric constants (e.g., DMC: 3.1 at 25 °C, Figure 5d) [5]. Evidence from studies using NaPF₆-based electrolytes in Na₃V₂(PO₄)₂F₃ (NVPF)/hard-carbon (HC) full-cell prototypes containing single linear carbonates (DMC, EMC or DEC) indicates that these solvents lack sufficient reductive stability at low potentials, leading to the formation of highly soluble decomposition products at the HC electrode and preventing the development of an effective SEI layer [42].

Carbonate solvents critically influence solvation structure and interphase chemistry in OLEs [33]. At typical concentrations (1 M), Na⁺ is coordinated by approximately five to six solvent molecules, meaning that the initial SEI formed in carbonate electrolytes originates predominantly from the reductive decomposition of solvent molecules, rather than the salt anion [5]. However, it is important to note that deep understanding of the EEI is extremely limited by the available experimental methods [33]. Early research studies conducted by Ponrouch et al. in 2012 showed that choosing NaClO₄ or NaPF₆ as the electrolyte salt resulted in little difference in SEI formation or cycling inefficiencies, emphasizing instead the dominant role of the solvent [41,43].

In SIBs, the reduction in EC and PC on carbon-based anodes (which are the most common for SIBs) produces polymeric carbonates and sodium alkyl carbonates, which generates SEI layers that are more soluble and mechanically weaker than those typically observed in LIBs [33,41,44]. EC is particularly prone to ring-opening reduction, a low-barrier pathway that drives the continuous formation and dissolution of SEI species, sustained electrolyte consumption, and long-term interfacial instability [33,45].

Solvation also governs the electronic stability of the electrolyte [34]. Coordination between Na⁺ and solvent molecules lowers both the LUMO and HOMO levels of the solvated complex due to the donation of lone pair electrons to cations through strong electrostatic interactions, making the electrolyte more susceptible to reduction at the anode, as seen in Figure 3 and Figure 6 (conventional electrolyte) [34,45].

Figure 6. Solvation structures. Schematic comparison of ion coordination environments in conventional electrolytes, highly concentrated electrolytes (HCEs), and localized highly concentrated electrolytes (LHCEs). The blue boxes highlight the corresponding shift in solvation structure, from solvent-separated ion pairs (SSIPs) in conventional electrolytes to contact ion pairs (CIPs) and cation–anion aggregates (AGGs) as salt concentration increases.

Blending cyclic and linear carbonates mitigates the drawbacks of each family [5]. In the 2012 study by Ponrouch et al. referenced earlier, the authors examined several combinations of Na salts (NaClO₄, NaPF₆, and NaTFSI) and solvents (PC, EC, DMC, DME, DEC, tetrahydrofuran (THF) and triglyme) and solvent mixtures (EC:DMC, EC:DME, EC:PC and EC:triglyme) [43]. Among these, 1 M NaPF₆ in EC:PC was identified as a high-performing formulation for HC electrodes, providing a reversible capacity of 200 mAh g⁻¹ over 180 cycles at C/10 with ≥99% capacity retention. The same group later refined these systems by introducing a linear carbonate (DMC) to reduce viscosity and enhance ionic mobility [46]. The resulting ternary mixture EC_0.45:PC_0.45:DMC_0.1 achieved an ionic conductivity of 10.0 mS cm⁻¹ and allowed a higher capacity of 320 mAh g⁻¹ to be obtained in Na/HC cells, which maintained ≥99% capacity retention over 100 cycles at C/10.

Carbonate-based electrolytes have difficulty in commercial SIB applications due to the quality of the SEI layer formed [4]. Ether-based electrolytes have been proposed as an alternative, as they tend to generate thinner and denser SEI layers enriched in inorganic species, improving interfacial stability [33]. Ether solvents also support a unique [Na-ether]⁺ co-intercalation mechanism into graphite, impossible with carbonates [47,48]. The Na⁺ ion does not undergo complete desolvation and instead the solvated [Na-ether]⁺ complex binds reversibly to the graphite layers. Representative ether solvents include 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (DEGDME), and triethylene glycol dimethyl ether (TEGDME). They exhibit higher HOMO and LUMO energy levels than carbonate solvents, indicating higher reductive stability, but inferior oxidative stability, consequently restricting their use with high-voltage Na⁺-insertion cathodes [4,34].

2.1.2. Concentrated Electrolytes

Beyond salt and solvent selection, electrolyte concentration itself has proven to be a powerful lever for widening the ESW. In high-concentration electrolytes (HCEs), the salt-to-solvent ratio increases to the point where free solvent molecules are nearly absent, shifting the solvation structure from solvent-separated ion pairs (SSIPs) toward contact ion pairs (CIPs) and cation-anion aggregates (AGGs), as seen in Figure 6 [34]. This transition lowers the HOMO level of the electrolyte as follows: as ion-solvent complex pairs form, the solvent’s lone pair is drawn into a coordinate bond with Na⁺ instead of remaining non-bonding on the free solvent molecule, improving oxidative stability, as seen in Figure 6 [34]. With few free solvent molecules remaining, the anion with the lowest LUMO is preferentially reduced at the anode, forming an inorganic-rich SEI and improving the reduction stability of the electrolyte system [34]. Geng et al. [49] reported that 3 M NaPF₆ in PC raises the ESW from 5.3 V to 6.3 V relative to the 1 M formulation, and Liang et al. [50] showed that a 3.36 M NaPF₆ in DEGDME with the addition of 1,3-dioxolane (DOL) remained anodically stable at 4.5 V vs. Na⁺/Na, sustaining a Na₃V₂(PO₄)₂O₂F (NVPOF)//HC full cell over 700 cycles at 95% capacity retention.

In parallel, the initial CEI composition is directly governed by the decomposition chemistry of solvated sodium ions during desolvation at the electrode surface (Figure 6). Greater anion participation in the solvation structure, associated with the transition from SSIPs to CIPs and AGGs, as seen in Figure 6, increases the number of anions coordinated to Na⁺ and promotes the formation of inorganic-rich CEIs [34,51]. Low-concentration electrolytes (<1 M) sit at the opposite end with abundance of free solvent favoring SSIP-dominated structures and, consequently, organic-rich CEIs. However, high-voltage interfacial resistance depends more on CEI stability and electrolyte oxidation resistance than on CEI composition alone. As shown by Deng et al. [52], a higher anion/solvent ratio produced a more inorganic-rich (anion-dominated) interphase that nonetheless caused larger interfacial Na⁺ transport resistance and worse gas evolution than a lower anion/solvent ratio.

HCEs carry practical drawbacks, namely high viscosity, high cost, and poor wettability, which limit commercial adoption [35,53]. Localized high-concentration electrolytes (LHCEs) address the viscosity limitation by introducing an “inert” diluent, miscible with the solvent but insoluble in the salt, which dilutes the electrolyte without disrupting the underlying salt–solvent coordination, as seen in Figure 6 [34]. Zheng et al. [54] designed a 2.1 M NaFSI in DME/bis(2,2,2-trifluoroethyl) ether (BTFE) (solvent molar ratio 1:2) LHCE using BTFE as a diluent, achieving >99% Coulombic efficiency for Na plating/stripping, and Na//Na₃V₂(PO₄)₃ (NVP) cells retained 90.8% capacity retention after 40,000 cycles at 20C. Despite reducing viscosity, LHCEs do not resolve the problem of low ionic conductivity, since the inert diluent cannot dissolve the sodium salt, and the high cost of these diluents further limits their practical adoption [34].

2.1.3. Ionic Liquids

Ionic liquids (ILs) constitute a distinct class of liquid electrolytes, consisting entirely of ions rather than a conventional solvent–salt system, as seen in Figure 7. ILs are typically composed of bulky organic cations, such as imidazolium, pyrrolidinium, piperidinium, phosphonium, or quaternary ammonium, paired with charge-delocalized anions including TFSI⁻, FSI⁻, or BF₄⁻ [55], as seen in Figure 7. Therefore, ion correlation cannot be neglected, and their behavior often deviates markedly from that of diluted molecular solutions. Cation–anion interactions are strong and persistent and adding a sodium salt intensifies this effect as follows: Na⁺ coordinates tightly with the surrounding anions, forming ion pairs and aggregates rather than moving as an independently diffusing species [55,56].

Figure 7. Anions and cations in ionic liquid electrolytes for SIBs. Adapted from Zhu et al. [57], licensed under CC BY 4.0.

Figure 7. Anions and cations in ionic liquid electrolytes for SIBs. Adapted from Zhu et al. [57], licensed under CC BY 4.0.

A recent demonstration of this principle is the single-anion IL electrolyte developed by Wang et al. [58], combining NaFSI with PY13FSI (1-methyl-1-propylpyrrolidinium FSI) as follows: concentrated FSI⁻ anions preferentially decomposed during initial Na deposition to form an inorganic, NaF-rich SEI, while FSI⁻-coordinated ion pairs and aggregates facilitated Na⁺ desolvation kinetics, together enabling Na||Na symmetric cells to cycle stably for over 2200 h at 0.5 mA cm⁻² at RT while remaining non-flammable. In contrast to conventional carbonate-based electrolytes, ILs are non-volatile and non-flammable, remaining stable up to 300–400 °C, which represents a significant safety advantage for large-scale energy storage applications [33]. Despite these advantages, ILs present significant limitations that constrain their practical application. The main one is high viscosity, typically one to two orders of magnitude greater than conventional organic solvents, which generally translates into lower ionic conductivity than carbonate-based electrolytes at RT [55,59]. This was demonstrated directly by Do et al. [60], who compared BMPyrTFSI-based IL electrolytes (containing NaClO₄, NaFSI, or NaTFSI at 0.5 M) against a conventional 1.0 M NaClO₄ in EC/PC electrolyte at RT as follows: the carbonate-based electrolyte exhibited both the highest ionic conductivity (7.38 mS cm⁻¹) and the lowest viscosity (6.8 cP), compared to 1.29–1.70 mS cm⁻¹ and 166.3–224.6 cP across the three IL formulations. Considerable effort has been directed towards tailoring IL chemistry, through cation and anion design, salt concentration, and additive engineering, in an attempt to decouple ionic transport from bulk viscosity [61,62,63,64].

2.2. Aqueous Electrolyte

The working principle of aqueous rechargeable sodium-ion batteries is very similar to that of conventional non-aqueous (organic) batteries discussed in Section 2.1. During operation, Na⁺ ions diffuse through the aqueous electrolyte while electrons flow through the external circuit between the anode and the cathode (Figure 8). The main difference lies in the electrolyte, where sodium salts are dissolved in water rather than in organic solvents (Figure 8) (Table 1). Although some salts, including NaClO₄, NaOTf, NaFSI, and NaTFSI, are common to both aqueous and non-aqueous (organic) systems, aqueous electrolytes also frequently employ salts such as Na₂SO₄ and NaNO₃ [65]. The stable operating voltage for aqueous electrolytes is approximately 1.23 V, since at a neutral pH, the redox potentials for H₂ and O₂ evolution are 3.527 and 2.297 V vs. Na⁺/Na, respectively (Figure 8) [65]. Beyond this voltage, electrolysis of water takes place, resulting in H₂ and O₂ evolution, compromising cycle life. This narrow ESW, as discussed in the previous section (Figure 3), contrasts the much wider ESW of non-aqueous (organic) electrolytes of approximately 4 V; therefore, aqueous systems usually display much lower energy density than organic systems [65].

Table 1. Cross-family comparison of representative sodium electrolytes. Values are order-of-magnitude ranges from the studies discussed in Section 2 and are not directly comparable unless temperature, method, pressure, electrode configuration, and thickness are matched.

Electrolyte Family	Representative RT Conductivity	Na+ Transference	Main Stability Limitation	Processing/ Interface Issue	Evidence Level
Carbonate liquid	~8–10 mS cm−1	<1 (mixed-ion)	Flammability; solvent-derived SEI/CEI	Wetting is good; interphase control required	Half-cell and full-cell
Ether liquid	Typically mS cm−1 range	<1 (mixed-ion)	Lower oxidative stability	Graphite co-intercalation possible	Half-cell and full-cell
Ionic liquid	~1.3–1.7 mS cm−1 in cited examples	<1 (correlated ions)	High viscosity; cost	Non-flammable but sluggish at RT	Symmetric and full-cell
β/β″-alumina	~10−3–10−2 S cm−1 for optimized phases	Approaches 1	Na-interface voids; moisture sensitivity	High-temperature densification	Solid-state full cells
NaSICON oxide	Up to ~0.67 mS cm−1 in cited composition	Approaches 1	Interfacial resistance	Hot pressing/coatings often needed	Solid-state full cells
Na3PS4 sulfide	~0.2–4.6 mS cm−1	Approaches 1	H2S release; narrow practical stability	Cold pressing; interface reactivity	Solid-state full cells
PEO-based SPE	Usually <10−6 S cm−1 neat at RT	Variable	Crystallinity; limited RT transport	Flexible interface; elevated T often needed	Mostly lab cells
CPE	~10−6–10−4 S cm−1 in cited examples	Up to ~0.91	Percolation and filler dispersion	Thin-film processing and interface control	Lab full cells

A consequence of using water as the solvent is that aqueous electrolytes generally do not form an SEI. In organic systems, electrolyte decomposition at the anode produces insoluble species that deposit as a passivating interphase, as seen in Figure 3. In water, the decomposition products are volatile (H₂ and O₂) and escape the electrode surface rather than passivating it, so no such kinetic protection is available, and the practical window remains close to the thermodynamic value given above [65]. Concentrated electrolytes have emerged as the principal strategy for overcoming this limitation. In practice, the most widely used aqueous electrolyte for Na-ion batteries remains 1 M Na₂SO₄ at neutral pH, paired with a range of electrode materials [65]. Manganese-based oxides and Prussian blue analogs dominate on the cathode side, while activated carbon and NaTi₂(PO₄)₃ are the most commonly reported anodes [65,66,67,68]. Across these systems, the stability window is consistently found to scale with electrolyte concentration, with more concentrated formulations delivering wider operating potential windows [65]. The origin of this effect lies in the solvation structure. In dilute solutions, Na⁺ ions remain fully hydrated: because water vastly outnumbers the cations, two solvation shells form around each Na⁺, giving a conventional salt-in-water electrolyte (SiW), as seen in Figure 8 [65]. Beyond a certain concentration, however, Na⁺ ions outnumber the available water molecules, yielding a water-in-salt (WiS) electrolyte in which nearly all water is tightly coordinated to the cation, as seen in Figure 8 [65]. This coordination lowers the HOMO of water and suppresses its oxidation, widening the ESW [65].

Figure 8. Schematic representation of an aqueous sodium-ion battery and the electrolyte concentration effect. (Top) Basic cell configuration showing charge/discharge directions, with the electrochemical stability window (ESW) of water at neutral pH (1.23 V), bounded by the H₂ and O₂ evolution reactions. (Bottom) Comparison between “salt-in-water” and “water-in-salt” electrolytes. Adapted from Zhu et al. [69], licensed under CC BY 3.0.

Figure 8. Schematic representation of an aqueous sodium-ion battery and the electrolyte concentration effect. (Top) Basic cell configuration showing charge/discharge directions, with the electrochemical stability window (ESW) of water at neutral pH (1.23 V), bounded by the H₂ and O₂ evolution reactions. (Bottom) Comparison between “salt-in-water” and “water-in-salt” electrolytes. Adapted from Zhu et al. [69], licensed under CC BY 3.0.

2.3. Solid-State Sodium Electrolytes

Despite their widespread use in SIBs, OLEs present inherent safety concerns related to flammability, thermal instability, and interfacial degradation. These limitations become more pronounced paired with high-capacity electrodes, which often undergo large volume changes and/or operate at high potentials, exacerbating SEI/CEI instability and promoting electrolyte decomposition and dendritic growth [33]. To overcome these challenges, solid-state electrolytes (SSEs) (Table 1) have been extensively investigated owing to their enhanced safety, improved thermal and electrochemical stability, reduced risks associated with leakage and volatility, sufficient mechanical rigidity to suppress dendrite growth and enhanced electrochemical stability against metallic anodes or high-voltage cathodes [5,31,70]. Based on chemical composition and ion transport mechanisms, SSEs can be classified into inorganic solid electrolytes (ISEs), solid polymer electrolytes (SPEs), and composite polymer electrolytes (CPEs) (Figure 9) [70].

Figure 9. Classification of solid-state sodium electrolytes (SSEs) into inorganic solid electrolytes (ISEs), solid polymer electrolytes (SPEs), and composite polymer electrolytes (CPEs).

2.3.1. Inorganic Solid-State Electrolytes

Inorganic solid electrolytes (ISEs) are non-volatile and generally non-flammable, eliminating bulk-liquid leakage, as seen in Figure 9 [5,71]. Nevertheless, neither a wide calculated stability window nor mechanical rigidity guarantees practical compatibility with sodium metal or high-voltage cathodes. Thermodynamic decomposition, defect-assisted electronic leakage, catalytic surfaces, grain boundaries, interphase growth, void formation, and local current focusing can all promote degradation or sodium penetration [70,72,73]. Compatibility must therefore be established for each electrolyte–electrode pair using interface-specific electrochemical measurements and full cell cycling [70,74].

Oxide ISEs. Within ISEs, oxide-based solid electrolytes are between the most extensively studied for SIBs, with Na-$\beta$-alumina and NaSICON-type (Sodium Super Ionic Conductor) emerging as representative families [70,75]. The $\beta$-alumina electrolytes, depending on the Na/Al ratio, exist in the following two closely related layered structures (Figure 10): hexagonal $\beta$-Al₂O₃ ($\beta$-Na₂O·xAl₂O₃), where x varies from 8 to 11, and rhombohedral β″-Al₂O₃ (β″-Na₂O·yAl₂O₃), where y varies from 5 to 7, with stoichiometries NaAl_xO₁₇ and NaAl_yO₈, respectively [70,76]. Both structures are built from spinel blocks composed of tetrahedral and octahedral Al-O units, separated by 2D conduction planes that host mobile Na⁺ ions, as seen in Figure 10. While the hexagonal $\beta$-Al₂O₃ phase is built from two spinel blocks, the rhombohedral β″-Al₂O₃ phase comprises three spinel blocks, resulting in an elongated crystallographic c-axis, as seen in Figure 10 [8]. This structural arrangement increases the number of accessible Na⁺ sites per conduction layer and weakens the electrostatic interaction between Na⁺ ions and the surrounding oxygen framework, thereby facilitating ion migration, as seen in Figure 10 [70]. As a result, β″-Al₂O₃ exhibits ionic conductivities ca. 3–4 times higher than those in $\beta$-Al₂O₃, reaching values of the order 10⁻³–10⁻² S cm⁻¹ at room temperature (RT) [77,78]. Despite these favorable transport properties, β″-Al₂O₃ is a thermodynamically metastable phase and it is prone to transform into $\beta$-Al₂O₃ or decompose into alumina-rich mixtures at high temperatures (around 1500 °C, the same temperature for phase formation), making the synthesis and stabilization of the phase-pure material challenging [8,75]. Strategies aimed to enhance the ionic conductivity of Na-$\beta$-Al₂O₃. ISEs by doping and adjusting the ratio of $\beta$-Al₂O₃ to β″-Al₂O₃ have been explored [70]. For example, Lie et al. synthesized Na-$\beta$-Al₂O₃ with high β″-Al₂O₃ phase purity of 95 wt.% by utilizing boehmite as alumina source, which yielded ionic conductivities of 0.63 mS cm⁻¹ at RT and reached 11.6 mS cm⁻¹ at 350 °C [77]. When coupled with a Na₃V₂(PO₄)₃ cathode and a sodium metal anode, the system delivered an initial discharge capacity of 80.5 mAh g⁻¹ at a rate of C/2 [77]. Moreover, pure β″-Al₂O₃ is sensitive to moisture [8,79]. Water molecules can penetrate the conduction planes, leading to ion-exchange reaction where mobile Na⁺ ions are replaced by H⁺ or H₃O⁺ ions. These results in the formation of Al(OH)₃ and AlO(OH), which decrease ionic conductivity and mechanical strength [8].

Figure 10. Crystal structures of $\beta$-Al₂O₃ (left) and β″-Al₂O₃ (right).

NaSICON-type compounds as solid-state electrolytes, first reported by Goodenough and Hong in 1976, constitute another important class of oxide-based solid electrolytes featuring 3D Na⁺ diffusion channels [80,81,82]. NaSICON materials are defined by the general formula Na_1+xZr₂Si_xP_3−xO₁₂ in which x varies between 0 and 3 [71,80,82]. These materials crystallize into two main polymorphs, namely, monoclinic and rhombohedral, both constructed from corner-sharing ZrO₆ octahedra and (Si/P)O₄ tetrahedra that form the 3D Na⁺ diffusion networks (Figure 11) [70,83]. Ionic transport is governed by narrow triangular bottlenecks defined by three oxygen atoms, which constitute the dominant energy barrier for Na⁺ migration between adjacent interstitial sites, as seen in Figure 11 [71]. The monoclinic phase, stabilized at specific compositions, introduces additional sodium sites and yields the highest RT ionic conductivity, reaching 0.67 mS cm⁻¹ when x = 2 [70,82]. Their high thermal and chemical robustness distinguishes this class of materials as one of the few SSEs capable of combining elevated ionic conductivity with strong chemical and electrochemical stability [71]. Interfacial limitations have become the dominant performance bottleneck for $\beta$-Al₂O₃ and NaSICON electrolytes, surpassing constraints associated with bulk ionic conductivity [76,84]. At the cathode/electrolyte interface, insufficient solid–solid contact between the rigid ceramic electrolytes and the cathode active materials leads to high interfacial resistance [70,85]. Various approaches have been proposed and validated to tackle this issue. Improved contact can be promoted by introducing a small amount of liquid solvent during electrode processing, followed by solvent evaporation to form an integrated cathode [76,86]. In oxide-based ISE systems, hot pressing, i.e., applying pressure at elevated temperature, enhances particle–particle contact; by contrast, sulfide and complex hydride electrolytes, owing to their softer mechanical nature, can achieve good interfacial contact via simple cold pressing [76,79,87]. Introducing a trace amount of liquid electrolytes or ionic liquids (i.e., salts composed entirely of ions that remain in the liquid state at or near RT) as an interfacial wetting agent is another effective strategy [71,88]. The rigid “point-to-point” solid contacts transform into a continuous ionic surface, lowering the total cell resistance of the quasi-solid battery by orders of magnitude [88].

Figure 11. Crystal structures of rhombohedral and monoclinic phases of Na₃Zr₂Si₂PO₁₂ and ion transport paths. Adapted from Zhao et al. [8], with permission. Copyright 2018, Wiley.

At the anode/electrolyte interface, poor physical contact and grain boundaries promote high interfacial impedance and facilitate sodium dendrite growth [70,89]. During Na stripping, insufficient replenishment of sodium at the interface leads to void formation; these voids concentrate current during subsequent plating, accelerating dendrite propagation and increasing the risk of short circuits [70,90]. To mitigate these effects, strategies such as surface cleaning, interfacial buffer layers, and stack pressure are employed. For instance, heat treatment of $\beta$-Al₂O₃ effectively removed surface Na₂CO₃ impurities, substantially reducing interfacial resistance by ca. 19,000 $\mathsf{\Omega }$.cm², while coating NaSICON with SnO₂ enabled the formation of a sodiophilic Na-Sn alloy interlayer that improves wetting and lowers impedance by orders of magnitude [91,92].

Sulfide ISEs. Sulfide-based SSEs emerged in the 1980s as sulfur analogs of oxide SSEs [93]. Because sulfur is less electronegative than oxygen, S²⁻ exerts a weaker electrostatic attraction on Na⁺, and its larger ionic radius widens the migration channels within the anionic framework. Consequently, sulfide electrolytes combine high RT ionic conductivity with low grain-boundary resistance and, owing to their soft mechanical nature, can be densified by simple cold pressing rather than hot pressing, simplifying manufacturing [87,94,95]. Na₃PS₄ is the most extensively studied member of this family, as seen in Figure 12a. Its glass-ceramic form exhibits a room-temperature ionic conductivity of approximately 0.2 mS cm⁻¹ [87]. Subsequent optimization of the synthesis route by the same group, consisting of 1.5 h of milling followed by heat treatment at 270 °C for 1 h, increased the ionic conductivity to 4.6 mS cm⁻¹ [94]. Several strategies have since been explored to improve ionic conductivity, including modifications to the synthesis route and elemental doping [87,94,96,97]. The main limitation of sulfide ISEs is their sensitivity to moisture: S²⁻ reacts with H₂O to release toxic H₂S and form alkaline by-products, degrading the material, Figure 12b [98].

Figure 12. (a) Crystal structures of Na₃PS₄-type compounds in the cubic and tetragonal phases, showing the PS₄³⁻ polyhedra and Na⁺ occupation sites. Adapted from Yin et al. [95], licensed under CC BY 4.0. (b) Schematic representation of the microstructural changes in sulfide-based SEs before and after air exposure. Adapted from Yang et al. [99], licensed under CC BY 4.0.

Elemental co-doping and protective coatings substantially suppress H₂S evolution, and several members of the family are also obtained in glass or glass–ceramic form, which favors dendrite suppression. Beyond these intrinsic properties, the practical viability of sulfide SSEs is governed by their compatibility with other battery components [99]. Despite their attractive ionic conductivity and processability, most sulfide SSEs display poor interfacial compatibility with chemical, electrochemical and mechanical incompatibilities arising at both electrode interfaces promoting electrolyte decomposition, capacity fading and shortened cycle life [99]. Most sulfide SSEs are thermodynamically unstable against metallic Na and against oxide cathodes, reacting to form decomposition products that block Na⁺ transport and cause interfacial resistance to grow continuously. Moreover, the practical ESW of sulfide SSEs is narrow and lies below the operating potential of most insertion-type cathodes, which severely restricts the choice of cathode chemistry [99].

Halides. Halide-based ISEs became a focus of attention following the 2018 report of mechanochemically synthesized Li₃YX₆ (X = Cl, Br) [100]. Na-conducting analogs have since attracted growing interest for their balanced ionic conductivity, oxidative stability, and RT processability [101]. Na⁺ transport proceeds through a “soft” halide-ion lattice with a low migration-energy barrier, but early Na₃YX₆ compositions showed conductivities of only 10⁻¹–10⁻³ mS cm⁻¹ at 500 K, prompting the search for new families, such as Na_3−xEr_1−xZr_xCl₆, Na_3−xY_1−xZr_xCl₆, Na_2+xZr_1−xIn_xCl₆, Na₂ZrCl₆, and NaAlCl₄ [102,103,104,105,106,107]. Among these, the synthesis conditions of Na_2+xZr_1−xIn_xCl₆ and NaAlCl₄ were found to play a decisive role as follows: lower-temperature annealing yields higher conductivity and lower activation energy in both systems, indicating that processing control, not only composition, is a lever for optimizing halide ISEs [102,104]. However, like sulfides, chloride ISEs are moisture-sensitive: Cl⁻ hydrolyzes to HCl and metal oxides/carbonates, cracking particles and blocking conduction pathways [108]. Surface coating, structural/doping optimization, improved synthesis routes, and encapsulation are the main mitigation strategies reported to date [109,110,111,112]. Taken together, the oxide, sulfide, and halide subclasses of ISEs trace a relation can be established as follows: oxide electrolytes are the most chemically inert and easiest to decommission but the most costly to densify, while sulfides and halides process more cheaply via cold pressing yet release toxic H₂S or HCI on moisture exposure, shifting the recyclability burden from manufacturing to disposal.

Complex Hydrides. Complex hydrides ISEs pair Na⁺ (or Li⁺) with a polyatomic borohydride-type anion (e.g., BH⁴⁻, B₁₀H₁₀²⁻, and NH²⁻), combining low density with exceptional plasticity [113,114]. Above their order-disorder phase-transition temperature, rapid anion reorientation creates a highly disordered environment that greatly accelerates Na⁺ migration [115]. However, this transition temperature is often too high for practical operation, so most research targets its suppression [115,116]. Carbon incorporation is usually used to lower the transition temperature by modifying local cation–anion interactions [115].

Antiperovskite-derived glassy electrolytes have exhibited polarization-induced feedback currents that oppose the discharge current, thereby sustaining the cell voltage over extended periods even under discharge conditions [8,117,118,119]. In this description, polarization is coupled to redistribution of mobile ions and can produce voltage relaxation or feedback currents after a prior electrical or chemical perturbation [120,121]. These observations should not be interpreted as spontaneous creation of net energy: the recoverable electrical response is bounded by energy previously supplied through polarization, chemical-potential gradients, thermal history, or electrochemical reactions. Reported voltage recovery and sustained-voltage effects depend on cell architecture, load, measurement protocol, relaxation time, and interfacial blocking conditions. Independent replication remains limited, and competing interpretations—including slow ionic redistribution, concentration-cell effects, interfacial redox processes, and dielectric absorption—must be considered. Ferroionic coupling is therefore presented here as an emerging, testable mechanism rather than an established universal property of solid sodium electrolytes. Related ion–polarization coupling has also been discussed in CuInP₂S₆ and hybrid halide perovskite MAPbI₃ [120].

The glassy and semi-amorphous electrolytes reported by Braga et al. were synthesized from crystalline antiperovskite precursors with composition A_3−2xM_xOCl (A = Li⁺, Na⁺, or K⁺, and M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) via a water-mediated process followed by controlled dehydration at 230–250 °C. Partial substitution of A⁺ by divalent M²⁺ ions introduces alkali-ion vacancies, increasing the concentration of mobile charge carriers without relying on thermally activated defect formation, thereby reducing the activation energy for ion migration [118].

Beyond storage applications, this polarization–ion coupling enables the development of non-volatile memories [120]. In these devices, ionic redistribution alters the interfacial resistance between two electrodes, enabling the material to switch between distinct high- and low-resistance states that remain stable even after removal of the applied electric field [120].

2.3.2. Solid Polymer and Composite Electrolytes

Solid polymer electrolytes (SPEs) (Table 1) consist of a polymer host with a dissolved sodium salt and, unlike ISEs, offer mechanical flexibility that lowers electrode–electrolyte interfacial impedance, as seen in Figure 9 and Figure 13 [31]. Ion transport proceeds via segmental motion of polymer chains in the amorphous phase rather than through vacancy or interstitial hopping in a rigid lattice. Consequently, SPE conductivity follows a Vogel–Tammann–Fulcher rather than an Arrhenius temperature dependence [81]. Poly(ethylene oxide) (PEO) remains the most widely used SPE host, owing to its electrochemical stability, mechanical robustness, and capacity to solvate a broad range of sodium salts [81]. However, the ionic conductivity of pristine PEO-based SPEs typically reaches only <10⁻⁶ S cm⁻¹ at RT, since the polymer’s high crystallinity restricts chain mobility and thus ion transport, requiring elevated operating temperatures for practical use [31,122,123]. The principal strategy to overcome this limitation is to reduce crystallinity, via copolymerization, plasticization, or filler addition, thereby expanding the amorphous fraction responsible for fast ion conduction [124,125,126]. This is exemplified by Chen et al. [127], who incorporated an ionic-liquid plasticizer (Pyr13FSI) into a PEO-NaClO₄ host: hydrogen bonding between the FSI- anion and the PEO backbone suppressed crystallinity, raising conductivity from ~10⁻⁷ to 6.8 × 10⁻⁵ S cm⁻¹ at RT while widening the electrochemical stability window from 3.70 to 4.44 V. Poly(vinylidene fluoride) (PVDF)-based SPEs offer an alternative host with higher intrinsic amorphicity and greater polarity than PEO, which favors salt dissociation and reduces the need for crystallinity-suppression strategies. This is illustrated by Bristi et al. [128], who developed a filler-less PVDF/polyvinylpyrrolidone (PVP) composite SPE with NaPF₆, identifying an optimum composition (PVDF:NaPF₆ = 9:1, 11 wt.% PVP) that achieved an ionic conductivity of 8.51 × 10⁻⁴ S cm⁻¹ at 23 °C and a low activation energy (0.19 eV), consistent with fast ion-hopping transport. Electrochemical evaluation used a hybrid symmetric configuration, with carbon cloth soaked in a supporting liquid electrolyte (20 L of 1 M NaClO₄ in EC/PC) inserted at each SPE-electrode interface; in a full cell with an NVP cathode, this SPE delivered a discharge capacity of 93.20 mAh g_-1 at 0.1C, with 86% capacity retention after 100 cycles and 99% Coulombic efficiency [128]. Despite this performance, PVDF is chemically unstable at the sodium interface and, as a per- and polyfluoroalkyl substance (PFAS), faces growing EOL disposal restrictions [31].

Figure 13. Structure of solid polymer electrolytes (SPEs) and composite polymer electrolytes (CPEs), highlighting common host polymer matrixes and filler types.

Composite polymer electrolytes (CPEs) (Table 1) involve the distribution of inorganic filler within a polymer matrix, as seen in Figure 9 and Figure 13. CPEs differ mechanistically from filler-doped SPEs in that the inorganic phase forms a continuous ion-conduction network rather than merely promoting the amorphous phase [129]. Taking advantages of both ISE and organic polymer electrolyte enhances ionic conductivity, mechanical strength and high chemical/thermal stability [130,131,132]. Inorganic fillers in CPEs for SIBs are classified as active or inactive by their role in ion transport [133]. Inactive fillers (e.g., Al₂O₃, SiO₂, TiO₂, and ZrO₂) do not conduct Na⁺ directly; they act interfacially via Lewis acid–base interactions with the polymer and salt anions, reducing crystallinity and freeing Na⁺ from polymer coordination, as seen in Figure 13 [133,134]. Active fillers (e.g., NASICON-type, beta-alumina, and Na₃SbS₄) additionally conduct Na⁺ through their own lattice, and above the percolation threshold form a continuous conduction pathway, yielding higher conductivity than inactive-filler systems, at the cost of stricter dispersion and interfacial control, as seen in Figure 13 [135]. Conductivity rises sharply once filler loading crosses a threshold connecting a continuous filler-polymer interfacial pathway, rather than scaling linearly with content. Below threshold, filler particles behave as isolated defects, as in SPEs; above it, the filler network itself becomes the dominant transport channel. This non-linear behavior is confirmed experimentally. Sahu et al. [136] demonstrated this while characterizing [80PEO:20NaPF₆] + xTiO₂ CPE films, varying the nanofiller content from 1 to 9 wt.%. Conductivity did not increase gradually with filler content: it remained relatively stable up to around 6–7 wt.% but reached a maximum of 3.53 × 10⁻⁶ S cm⁻¹ at 8 wt.% TiO₂, which is roughly one order of magnitude higher than the unfilled SPE (7.78 × 10⁻⁷ S cm⁻¹). More advanced CPE architecture uses an active, ion-conducting filler rather than an inert one, allowing the filler itself to contribute a parallel Na⁺ transport pathway rather than merely suppressing polymer crystallinity. Fang et al. [137] developed an ultrathin single-particle-layer membrane combining a PVDF matrix, NaPF₆, and size-screened sodium $\beta$-alumina ceramic electrolyte (SBACE) particles, reducing membrane thickness to accommodate a single layer of filler particles and thereby minimizing the non-conductive polymer volume the ion must traverse. This design reached 0.19 mS cm⁻¹ at RT and a sodium transference number of 0.91, substantially higher than typical SPE values. Paired with a potassium manganese hexacyanoferrate cathode, the membrane retained 95.5% of its capacity after 100 cycles at 0.5C rate, outperforming a thicker composite membrane with isolated filer particles [137]. Taken together, CPEs currently offer one of the clearest routes within organic solid electrolytes to improve ionic conductivity, mechanical strength, and sodium transference number relative to unfilled SPEs. However, the field remains comparatively less mature for SIBs than for LIBs, and most reported CPEs still fall short of LE conductivities under practical operating conditions.

3. Cathode Materials for SIBs

The performance, cost, and practical viability of SIBs are strongly dictated by the choice of cathode material [138] (Table 2). By serving as the host for reversible Na⁺ storage, the cathode defines the operating voltage window and constrains the energy density and cycling stability of the battery [9]. A typical cathode formulation consists of (1) an active material, (2) a conductive additive to establish an electronically conductive network between the active particles and the current collector, and (3) a binder to hold the electrode materials together and ensure cohesion between the active material particles [2,139]. There are the following four primary cathode materials for SIBs (Figure 14): Layered Transition Metal Oxides (LTMOs), polyanions, Prussian blue analogs (PBAs), and organic compounds.

Table 2. Quantitative and evidentiary comparison of the principal cathode families. Reported values distinguish theoretical or material-level performance from prototype and commercial claims.

Cathode Family	Practical Voltage/Capacity	Key Strength	Key Limitation	Commercial Evidence
Layered oxides	~2.4–3.6 V; ~100–160 mAh g−1	Higher energy density; Fe/Mn-rich options	Phase transitions, air sensitivity, CEI instability	Commercial/pilot use; company-reported data
NVP/NVPF polyanions	~3.4 V; NVP 117 mAh g−1 theoretical	Fast 3D Na transport; thermal stability	Low electronic conductivity; vanadium exposure	High-power prototypes
NFPP	~3.1 V; 129 mAh g−1 theoretical	Abundant Fe; small volume change	Phase purity; low electronic conductivity	Commercial utility-scale product claims
Prussian blue analogs	~3.1–3.5 V; ~160–209 mAh g−1 reported	Open framework; aqueous synthesis	Vacancies, coordinated water, cyanide handling	Early commercialization; mixed market traction
Organic cathodes	Typically lower voltage; up to ~150 mAh g−1 in cited example	Low critical-metal dependence	Dissolution and low conductivity	Primarily laboratory evidence

Figure 14. Comparative map of the different cathode families for SIBs.

Layered oxide cathodes have been the industry standard for most LIBs [2] (Table 2). In LIB technology, nickel-rich materials such as NMC (lithium Nickel Manganese Cobalt oxide) are widely used, although their reliance on cobalt and nickel increases material cost and supply concerns [2]. In SIBs, layered oxides are likewise among the most explored families, with companies such as Faradion and HiNa adopting this chemistry while focusing more on earth-abundant elements like iron (Fe) and manganese (Mn) [140].

Layered sodium transition metal oxides are generally described by the formula Na_xTMO₂, where TM is the transition metal (e.g., Ni, V, Co, Fe, Cu, Ti, etc.), and consist of repeated MO₆ layers with Na ions in-between them [138]. The coordination environment of the sodium ions defines the structural classification [138]. According to the Delmas notation, these materials are subdivided into “O” (octahedral) and “P” (prismatic) phases, referring to the coordination of Na⁺ between adjacent MO₂ layers [138]. The nomenclature also includes a numeral (commonly two or three) indicating the number of transition metal oxide layers within a single unit cell, as shown in Figure 15 [138]. For instance, the O3 phase (Figure 15d) features sodium ions in octahedral sites within a framework characterized by an ABCABC oxygen stacking sequence, whereas the P2 phase (Figure 15a) contains sodium ions in primastic sites within an ABBA stacking sequence [9]. This structural distinction influences electrochemical behavior. In prismatic environments, Na⁺ diffusion follows a direct path, while in octahedral environments migration proceeds through a more indirect zigzag route involving intermediate sites [138,141]. Consequently, P2-type materials often display superior ionic mobility compared to their O3-type counterparts [75,142].

Na_xTMO₂ based cathodes deliver theoretical capacities of 100–160 mAh g⁻¹ and operating voltages of 2.4–3.6 V vs. Na/Na⁺ [75,142]. This attractive combination of capacity and energy density comes at the expense of increased susceptibility to structural phase transitions, surface degradation, and CEI instability, creating a fundamental trade-off between energy density and long-term stability [143,144]. During cycling, reversible and irreversible phase transitions (e.g., O3 $\leftrightarrow$ P3 or P2 $\leftrightarrow$ O2) occur upon Na extraction and insertion, accompanied by slab gliding, lattice strain, and volume changes that can induce long-term degradation [145]. To mitigate these challenges, three optimization strategies are typically combined. Bulk composition engineering employs cation substitution and doping to stabilize the layered framework, improving structural reversibility and suppressing detrimental phase transitions during cycling [144]. Surface engineering reduces surface reactivity, suppresses microcracking, and limits transition-metal dissolution at high SOC [146,147]. Finally, electrolyte and interphase optimization stabilize the CEI and suppress parasitic reactions at elevated potentials [148,149]. These materials are also notably sensitive to moisture and atmospheric air [145]. Layered oxides adsorb H₂O and CO₂, forming NaOH, NaHCO₃ and Na₂CO₃ residues on the particles surface [150,151]. Far from being a mere storage inconvenience, this contamination propagates through the entire electrode fabrication line as follows: the residual alkali reacts with the PVDF binder, promoting dehydrofluorination and subsequent crosslinking that gels the slurry and can render an entire batch unusable [152]. The associated material losses constitute a significant barrier to commercialization, yet they are frequently overlooked in the academic literature, as cathode slurry preparation is rarely performed at industrially relevant scales in laboratory settings. Despite these challenges, and against market tendencies, layered oxide cathodes appear to be CATL’s option of choice. On 21 April 2026, the company announced the launch of its sodium-ion battery, combining a long-cycle hard carbon anode with a layered oxide composite cathode [28].

Figure 15. Schematic illustration of common crystal structures of layered sodium transition metal oxides (Na_xTMO₂). (a) P2 phase, (b) O2 phase, (c) OP4 phase, (d) O3 phase and (e) P3 phase, where the purple, yellow, and red spheres represent, respectively, transition metals, Na⁺, and O²⁻. Adapted from Liu et al. [153], licensed under CC-BY 4.0.

Figure 15. Schematic illustration of common crystal structures of layered sodium transition metal oxides (Na_xTMO₂). (a) P2 phase, (b) O2 phase, (c) OP4 phase, (d) O3 phase and (e) P3 phase, where the purple, yellow, and red spheres represent, respectively, transition metals, Na⁺, and O²⁻. Adapted from Liu et al. [153], licensed under CC-BY 4.0.

In LIBs, polyanionic cathodes play a complementary role to layered oxides [138]. While NMC dominates high-energy applications, the phosphate cathode LFP has been widely adopted for the abundance and low cost of its raw materials (Figure 2) and is now used by automakers including Tesla, Ford and BYD [154,155]. This precedent has since been reproduced in sodium storage, where analogous polyanionic frameworks already underpin the majority of commercial cell shipments [4]. Polyanionic chemistries dominate the wider market, accounting for 77% of shipments from major players such as BYD, HiThium and Peak Energy [156].

Polyanionic materials (Table 2) can be generally expressed by Na_xTM_y(XO₄)_n, where TM represents one or more transition metals and X denotes a central atom of the polyanionic group (e.g., P, S, Si, As, Mo, and W) [138] (Table 2). A defining chemical characteristic of polyanions is the inductive effect, which is absent in simple layered oxides [138,157]. In these compounds, the oxygen atoms are strongly covalently bonded to a highly electronegative central atom X [9]. Because X withdraws electron density, the oxygen 2p orbitals are stabilized (shifted to a lower energy) [9]. Since the redox-active states of the transition metal involve hybridization with these oxygen orbitals, the metal-oxygen antibonding levels are also lowered in energy [9]. As a result, the cathode redox level lies at a lower absolute energy relative to Na/Na⁺, which increases the redox potential and leads to higher operating voltages [9]. Moreover, unlike the 2D sheets of layered oxides that are prone to sliding and gliding, polyanions possess a rigid 3D skeletal framework reinforced by the strong covalent X-O bonds [30]. This robust bonding network enhances structural integrity, suppresses oxygen release, and stabilizes high oxidation states of the transition metal, contributing to the superior thermal stability typically observed in polyanionic cathodes [158,159]. However, this structural robustness comes with performance trade-offs. Polyanionic frameworks generally exhibit low intrinsic electronic conductivity, as the XO₄ units are electronically insulating and electron transport must proceed through extended TM-O-X-O-TM pathways rather than direct metal–metal interaction [160]. In addition, the relatively bulky and heavy polyanionic groups increase the molecular weight of the compound, which often leads to lower gravimetric specific capacities compared to more compact layered oxide structures [30]. While both families often operate within an experimental range of 100 to 160 mAh g⁻¹, polyanionic compounds are generally constrained to the lower end of this spectrum [30].

From an industrial perspective, the selection of a cathode material is driven not only by its electrochemical performance but also by its impact on overall system cost. In this regard, polyanionic cathodes have emerged as particularly attractive for stationary energy storage because their intrinsic safety characteristics simplify system design and reduce deployment costs. Unlike layered oxides, whose highly delithiated/desodiated states can release lattice oxygen under abuse conditions, the strong covalent P-O bonds in polyanionic frameworks suppress oxygen evolution during overcharge, thermal stress, mechanical damage, or external short circuits [161,162]. This enhanced thermal stability has the potential to simplify thermal management strategies, including the use of passive or air cooling, which may reduce auxiliary energy consumption, capital and operating costs, and maintenance requirements. Furthermore, improved fire safety can facilitate permitting and regulatory approval, an increasingly important consideration for large-scale ESS deployment [163]. Additionally, polyanionic cathodes are less susceptible than layered oxides to residual alkali-induced slurry gelation during electrode processing, although they are not entirely moisture-insensitive [164]. Although these materials sacrifice gravimetric energy density and require conductive carbon coatings, because of their intrinsically low electronic conductivity, these limitations are generally acceptable in stationary applications [165].

Among polyanionic cathodes, vanadium (V)-based compounds have attracted disproportionate attention because they combine the inductive effect of polyanionic groups with vanadium redox activity and an open framework that supports fast Na⁺ transport [166,167]. Unlike many transition metals, vanadium can access a wide range of oxidation states, specifically, V²⁺, V³⁺, V⁴⁺, and V⁵⁺, within a single polyanionic host lattice [158,168]. Most vanadium-based polyanions adopt the NaSICON-type structure (Na_xM₂(XO₄)₃), which consists of corner-sharing polyhedra that create open 3D channels, as discussed in Section 2.3.1 [71].

Within V-based phosphates, Na₃V₂(PO₄)₃ (NVP) is widely regarded as a reference cathode in the NaSICON family. It exhibits a theoretical specific capacity of 117 mAh g⁻¹ and a voltage plateau of approximately 3.4 V vs. Na/Na⁺, corresponding to the V⁴⁺/V³⁺ redox couple [166,169]. Figure 16 shows the crystal structure of NVP. Structurally, three PO₄ tetrahedra and two VO₆ octahedra share corners along the c-axis, forming a three-dimensional covalent [V₂(PO₄)₃]³⁻ framework [170]. This robust skeleton can accommodate up to three Na⁺ ions when vanadium is in the trivalent state and undergoes only minimal volume change within the unit cell, unlike what is observed in layered oxide materials [86,171,172].

Figure 16. (a) Crystal structure of NaSICON-type Na₃V₂(PO₄)₃ and local environments of the (b) Na1 site and (c) N2 site. Adapted from Ishado et al. [171], licensed under CC BY-NC-SA 4.0.

The NVP structure contains the following two crystallographically distinct interstitial sodium sites with markedly different coordination environments: the Na1 (or M1) site and the Na2 (or M2) site [171,173]. There is one Na1 site per formula unit, where Na⁺ is six-fold coordinated by oxygen atoms, and three Na2 sites per formula unit, where Na⁺ exhibits eight-fold coordination and longer Na-O bond distances [81,171]. Because the Na1-O bonds are shorter and stronger, sodium ions occupying the Na1 site are considered relatively immobile and electrochemically inactive during cycling [166,169]. As a consequence, only two sodium ions can be reversibly extracted at 3.4 V, giving rise to the theoretical capacity of 117 mAh g⁻¹ associated with the V⁴⁺/V³⁺ redox reaction [166,169]. The removal of the third sodium ion is kinetically and thermodynamically hindered [171].

NVP’s electrochemical performance is still limited by intrinsic material constraints. It behaves as a semiconductor with a wide bandgap, resulting in low intrinsic electronic conductivity [174,175]. Additionally, although the NaSICON framework provides the interconnected 3D diffusion pathways for Na⁺ transport, pristine NVP still exhibits sluggish sodium-ion diffusion kinetics [171,176]. This is attributed to the activation energy barriers associated with the migration through narrow triangular bottlenecks formed by the surrounding oxygen atoms, which restrict Na⁺ mobility within the structure [71]. Optimization strategies for NVP include various dopant/substitution strategies, carbon modification, and nanostructure and morphology design [174].

The industrialization of these materials is already underway. TIAMAT, for example, developed full-cell prototypes based on Na₃V₂(PO₄)₂F₃ (NVPF) for high-power applications, reporting an energy density of 125 Wh kg⁻¹ with at least 3200 cycles at 2C/5C with 80% capacity retention [177]. However, vanadium is a relatively scarce and costly resource, with an average abundance of only ~0.02 wt.% in the Earth’s crust [158,168]. Although it is more abundant than critical battery metals such as Co, Ni, and Li, its availability remains limited, and the supply of vanadium is highly concentrated in China, Russia, and South Africa [178]. In addition, vanadium compounds pose handling and end-of-life challenges, further limiting their attractiveness for large-scale battery applications.

A key objective of sodium-ion battery development is to reduce dependence on critical raw materials [179]. However, vanadium-based cathodes such as NVP and NVPF reintroduce this dependency, as their widespread deployment would effectively replace lithium supply risks with those associated with vanadium.

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP), sodium iron phosphate pyrophosphate, offers several intrinsic advantages, including the use of earth-abundant elements such as Na and Fe, together with favorable safety characteristics, making it well-suited for large-scale stationary energy storage application [165]. Hithium Corporation begun commercializing a prismatic NFPP hard carbon cell for utility-scale energy storage in 2024 [180,181]. The cell delivers a gravimetric energy density of ≥95 Wh kg⁻¹ and a nominal cycle life of $\ge$20,000 cycles [180,181]. It also exhibits an exceptionally wide operating temperature range, supporting charging from 10 to 60 °C and discharging from −40 to 60 °C [180,181].

NFPP (Table 2) is a mixed-polyanion cathode that crystallizes in an orthorhombic structure with the Pn2₁a space group and shares structural features with NaSICON-type materials [182]. The interconnected [PO₄] and [P₂O₇] polyanionic units create an open framework that enables Na⁺ diffusion through 3D pathways, similar to NVP but unlike many other polyanionic cathodes, where ion transport is restricted to one- or two-dimensional channels. However, while these polyanionic units facilitate fast Na⁺ diffusion, they are intrinsically insulating and therefore limit the electronic conductivity and electrochemical performance of NFPP.

NFPP exhibits a theoretical capacity of 129 mAh g⁻¹, based exclusively on the Fe²⁺/Fe³⁺ redox couple, and an average operating voltage of approximately 3.1 V versus Na/Na⁺ [182,183]. During Na⁺ extraction, the Fe oxidation states evolve sequentially, with the electrostatic interactions between the Fe sites and their relative positions to Na⁺ driving the stepwise oxidation of Fe1, followed by Fe2 and finally Fe3 [182]. This highly reversible redox process, combined with the rigid open framework, results in exceptional structural stability. Consequently, NFPP undergoes one of the smallest volume changes (<4%) among sodium-ion cathodes, contributing to its outstanding cycling stability and ultralong cycle life [182]. The minimal volume variation is attributed to the rigidity of the mixed-polyanion framework and the structural flexibility of the [P₂O₇] units, which can rotate and distort to accommodate lattice changes during Na⁺ insertion and extraction [182]. Although NFPP exhibits lower electronic conductivity than NVP due to its insulating polyanionic framework, it retains comparable ionic conductivity. NFPP also exhibits good thermal stability across a wide temperature range. Li et al. [184] reported an NFPP@C/Bi full cell that retained 70.2% of its RT capacity at −70 °C at C/38 while maintaining stable electrochemical performance at 100 °C.

One of the greatest challenges facing NFPP, at both laboratory and industrial scales, is achieving phase purity [182]. Pure-phase NFPP is difficult to synthesize and is frequently accompanied by secondary phases, primarily NaFePO₄ and Na₂FeP₂O₇, which exhibit inferior electrochemical properties and consequently compromise overall cell performance [182]. As a result, many reported NFPP cathodes consist of multiphase mixtures rather than single-phase materials [182]. To address this issue, defect engineering, single and multi-ion doping as well as surface modification particularly carbon modification have been reported has successful strategies [182].

Prussian Blue and its Analogs (PBAs) (Table 2) represent a distinct class of cathode materials for SIBs and constitute a strategic technological direction initially pushed by American companies [30]. Natron Energy, a Stanford spin-off (now inactive), developed aqueous sodium-ion systems employing PBAs at both electrodes with a power density of 775 W kg⁻¹, reporting a cycle life over 25,000 cycles at 12C rate [185].

PBAs are generally described by the formula Na_xTMa[TMb(CN)_y]_z.H₂O where TMa and TMb are transition metals that may be identical or different (e.g., Fe, Mn, Co, Ni, and Cu) [145]. Their primary appeal lies in their structure. The TMa and TMb are connected through linear cyanide (CN⁻) bridges, forming a cage-like structure [9,186]. The face-centered cubic structure contains large interstitial sites that facilitate rapid Na⁺ insertion and extraction [9,187]. Their specific capacity varies based on the transition metals used in the framework. For instance, Prussian blue (standard iron-based) reaches approximately 160 mAh g⁻¹ at a potential of 3.1 V vs. Na/Na⁺ and double Mn analog can reach up to 209 mAh g⁻¹ at 3.5 V [30].

A key disadvantage of PBAs is their aqueous synthesis route, which often leaves coordinated (“zeolitic”) water trapped within the framework. This residual water can compete with Na⁺ for interstitial sites and accelerate structural degradation during cycling [9,188]. Nevertheless, aqueous processing is also one of their main industrial advantages, offering lower manufacturing costs and a reduced environmental footprint compared with many oxide synthesis routes. In addition, PBA production requires careful process and occupational safety management because cyanide-containing precursors must be handled under appropriate industrial safeguards. Although the cyanide ligands are strongly coordinated within the crystal lattice, potential toxicity concerns remain if free cyanide species were released under failure conditions [30,188].

Among the cathode families discussed, PBAs have demonstrated the slowest commercial adoption. Although they were the first cathode chemistry to reach commercialization through Natron Energy and were initially adopted by CATL in its first-generation sodium-ion cells, more recent commercial developments have shifted toward layered oxide-based and polyanionic cathodes, indicating that PBAs have yet to achieve widespread industrial acceptance.

Organic cathodes (Table 2) store Na⁺ through reversible redox reactions at molecular functional groups rather than through intercalation into a rigid inorganic lattice. Charge storage proceeds via electron transfer at these redox-active sites, with Na⁺ coordinating to the reduced (electron-rich) form of the group, so capacity scales with the density of redox-active groups per molecule rather than with lattice vacancy availability [145]. This molecular-level mechanism allows structural design flexibility (small molecules to polymers), avoids reliance on scarce transition metals, and is compatible with more sustainable synthesis routes, aligning with the broader sustainability rationale for SIBs. Wang et al. [189] exploited this by combining perylene and naphthalenetetracarboxylic dianhydride units bearing $\mathrm{C}=\mathrm{O}$ groups that coordinate Na⁺, yielding a polymer cathode with ~150 mAh g⁻¹ capacity at 1.5–3.5 V and 87.5% capacity retention after 5000 cycles at 0.1C. However, the electronically insulating nature of organic frameworks results in intrinsically low conductivity, limiting rate performance [145]. This is typically compensated by incorporating high carbon loadings, which in turn lowers the practical energy density of the electrode [189,190,191,192].

4. Anode Materials for SIBs

Anode materials are essential components in a wide range of electrochemical devices. By definition, the anode is the electrode where the oxidation occurs (donor of electrons). However, rechargeable batteries operate through reversible electrochemical reactions, meaning that the anode and cathode alternate depending on whether the battery is charging or discharging (Figure 3c). During discharge, the negative electrode acts as the anode and undergoes oxidation (Figure 3c). While charging, the same electrode undergoes reduction and therefore acts as the cathode (Figure 3c). Therefore, what characterizes anode material in a battery?

The key characteristic of an anode material is its electrochemical potential relative to the electrolyte. Ideally, the anode chemical potential should lie below the conduction band minimum of the electrolyte, preventing electron transfer from the anode to the electrolyte (Figure 3a) [34]. However, in most cases, the chemical potential of the anode material lies above the electrolyte’s conduction band, enabling electron tunneling from the anode to the electrolyte LUMO in liquid electrolytes or to the conduction band in solid-state electrolytes (Figure 3c). As a result, a SEI, consisting of an insulating solid material, is formed. Once it reaches sufficient thickness, the SEI blocks further electron tunneling. The formed SEI should be electronically insulating, with its conduction band positioned above both the chemical potential of the anode and the conduction band of the electrolyte (Figure 3c).

Anode materials can be broadly classified into the following three main categories: intercalation-type, alloy-type and conversion-type materials (Table 3) [143]. The most widely studied intercalation-type anodes are carbon-based and titanium-based materials, as seen in Figure 1. In these materials, Na⁺ ions are reversibly inserted into the interlayer spacing or crystallographic sites within the host structure, forming stable intercalation compounds that enable sodium storage [4,143]. Among carbon-based materials, graphite and soft carbon typically exhibit poor Na⁺ intercalation in conventional carbonate-based electrolytes [143,193]. Significant sodium storage generally requires alternative electrolyte formulations, particularly ether-based electrolytes, through a co-intercalation mechanism, which introduces additional electrolyte compatibility constraints, as discussed in Section 2.1. Consequently, their practical application relies on specialized electrolyte formulations and effective interphase control. Additional graphite exhibits poor Na⁺ storage because binary Na–graphite intercalation compounds are thermodynamically unstable [4,194]. This instability has been attributed to the size mismatch between Na⁺ and the graphite interlayer spacing, although changes in Na–C chemical bonding may also contribute [4].

Table 3. The principal sodium-ion battery (SIB) anode families are compared in terms of their sodium-storage mechanism, capacity, advantages, limitations, interfacial requirements, and technological maturity. The terms anode and cathode strictly describe electrode functions during discharge; during charging, the electrochemical roles of the electrodes are reversed.

Anode Family	Representative Materials	Na-Storage Mechanism	Typical or Theoretical Capacity	Main Advantages	Main Limitations	Electrolyte and interphase Considerations	Practical and Commercial Status
Hard carbon	Biomass-, lignin-, resin-, pitch-, and coal-derived hard carbons	Combined sodium storage in defects and disordered carbon domains, followed by low-voltage storage commonly associated with nanopores (Figure 1); the voltage profile generally comprises a sloping region and a low-voltage plateau.	~250–350 mAh g−1 reversible capacity	Relatively high reversible capacity; low operating potential; structural tolerance to repeated Na+ insertion; broad precursor availability; potentially low cost and sustainable production.	Low initial Coulombic efficiency; irreversible sodium consumption during SEI formation; precursor-dependent microstructure and performance; batch-to-batch variability; possible cathode oversizing requirement.	SEI chemistry strongly depends on salt, solvent, additives, surface area, and oxygen-containing functional groups. Pre-sodiation or sacrificial sodium sources may be required to compensate for first-cycle sodium loss.	Leading practical anode for SIBs and the material used by most major commercial manufacturers. Industrial production is concentrated mainly in Japan and China, with emerging European production based on lignin-derived precursors.
Graphite	Natural graphite, synthetic graphite, expanded graphite	Limited direct Na+ intercalation in conventional carbonate electrolytes; reversible storage can occur through solvent–Na+ co-intercalation in suitable ether-based electrolytes.	Generally low in carbonate electrolytes; higher reversible storage possible through co-intercalation in ether-based systems.	Established manufacturing infrastructure; high electronic conductivity; well-defined layered structure; relatively low cost.	Thermodynamic instability of binary Na–graphite intercalation compounds; restricted capacity in conventional electrolytes; strong dependence on solvent chemistry; possible structural expansion during co-intercalation.	Requires specialized ether-based electrolytes that stabilize solvated Na+ co-intercalation. Electrolyte oxidation stability and full-cell compatibility may therefore become limiting, particularly with high-voltage cathodes.	Of research interest but currently less broadly adopted than hard carbon because performance depends strongly on specialized electrolyte formulations.
Soft carbon	Petroleum-coke-, pitch-, and polymer-derived soft carbons	Na+ adsorption and insertion within partially graphitizable carbon domains, defects, and interlayer regions.	Usually lower than or comparable to hard carbon, depending strongly on heat-treatment temperature and microstructure.	Good electrical conductivity; tunable graphitic order; relatively straightforward processing; potentially good rate capability.	Lower capacity than optimized hard carbon; limited Na+ intercalation in highly graphitized structures; properties are strongly synthesis-dependent.	Performance depends on interlayer spacing, defect density, surface chemistry, and SEI formation. Ether-based electrolytes may improve storage in more graphitic domains.	Less commercially dominant than hard carbon; mainly considered for high-power or blended-anode formulations.
Titanium-based intercalation materials	Na2Ti3O7, NaTi2(PO4)3, TiO2-based compounds	Reversible insertion of Na+ into crystallographic sites without complete destruction of the host framework.	Generally moderate, commonly below the capacity of hard carbon.	Good structural stability; relatively small volume variation; long cycle life; potentially high rate capability; safer operating potential than very low-voltage carbon anodes.	Lower capacity and higher operating potential reduce full cell energy density; low intrinsic electronic conductivity in some compounds.	Higher operating potentials can reduce electrolyte reduction and excessive SEI formation. Carbon coating, nanostructuring, and conductive matrices are commonly required.	Attractive for long-life and high-power cells, but less suitable where maximum gravimetric energy density is the primary objective.
Alloy-type materials	Sn, Sb, Bi, Ge and related alloys	Sequential formation of Na–M alloy phases during sodiation, followed by dealloying during desodiation.	~500–900 mAh g−1, depending on the element and final alloy phase.	High theoretical capacity; relatively low operating potential; multielectron storage mechanism.	Large volume changes; particle pulverization; loss of electrical contact; repeated SEI rupture and reformation; rapid capacity fading.	Electrolyte salts, solvents, and additives must generate a flexible and mechanically stable SEI. Carbon matrices, nanosizing, void engineering, and elastic binders are generally necessary.	Promising for high-capacity anodes but still limited mainly to laboratory-scale demonstrations because long-term mechanical and interfacial stability remain unresolved.
Phosphorus-based alloy anodes	Red phosphorus, black phosphorus, phosphorus–carbon composites	Multistep alloying with Na+, potentially forming Na3P.	Up to ~2596 mAh g−1 theoretical capacity.	Among the highest theoretical capacities reported for SIB anodes; earth-abundant active element.	Very large volume expansion; intrinsically low electronic conductivity; unstable electrode architecture; severe first-cycle loss and continuous SEI damage.	Requires intimate carbon integration, nanoconfinement, robust binders, controlled porosity, and electrolyte formulations that form mechanically resilient interphases.	Highly attractive at the materials level but not yet commercially mature because of severe expansion and interphase-instability problems.
Conversion-type oxides	Fe2O3, Co3O4, MnOx, CuO, MoO3 and related oxides	Multielectron conversion of the metal oxide into metallic or lower-valence phases and Na2O, followed by partial reconstruction during desodiation.	Material-dependent; often several hundred mAh g−1.	High theoretical capacity; wide compositional diversity; inexpensive and abundant options available.	Large voltage hysteresis; low energy efficiency; sluggish reaction kinetics; substantial volume variation; incomplete reversibility; unstable SEI.	Nanostructuring and carbon-conducting matrices are commonly used to improve electronic transport and buffer volume changes. Electrolyte decomposition may be intensified by catalytically active surfaces.	Primarily at the research stage; practical deployment is limited by hysteresis, efficiency loss, and long-term structural instability.
Conversion-type sulfides	MoS2, FeS2, SnS2, Sb2S3 and related sulfides	Na+ insertion followed by conversion and, in some cases, additional alloying reactions.	Commonly several hundred mAh g−1, depending on the reaction pathway.	Higher electronic conductivity and weaker metal–sulfur bonding than many oxides; potentially faster kinetics and high capacity.	Large volume changes; dissolution or migration of sulfur-containing intermediates; voltage hysteresis; incomplete reaction reversibility.	Carbon confinement, heterostructures, protective coatings, and electrolyte additives are used to stabilize the SEI and suppress dissolution.	Promising experimentally, particularly in nanocomposites, but still limited by cycling stability and reaction complexity.
Conversion/alloying phosphides	Sn4P3, FeP, CoP, Ni2P and related phosphides	Conversion of the metal phosphide followed by alloying of phosphorus-derived phases with Na.	Often high, depending on metal content and achievable Na–P reaction.	Combines conversion and alloying contributions; potentially high capacity; improved conductivity relative to elemental phosphorus.	Large expansion; phase segregation; voltage hysteresis; interfacial instability; complex reaction pathways.	Conductive carbon matrices, nanoscale architectures, and stable SEI-forming electrolytes are essential to preserve electrical contact and reaction reversibility.	Mainly under laboratory development; performance remains strongly dependent on nanostructure and composite design.

Hard carbon (Table 3) remains the leading anode material for SIBs. This trend extends beyond academia to industry, where all major players employ hard carbon-based anodes (Figure 17) [24]. Hard carbon exhibits a practical reversible capacity of approximately 250–350 mAh g⁻¹ [143]. Its sodium storage mechanism is generally described as a combination of a sloping region, commonly attributed to defect and surface storage processes, and a low-voltage plateau frequently associated with sodium storage in nanopores [143]. One of the key remaining challenges is its low initial coulombic efficiency. The first-cycle irreversible capacity loss can be substantial due to SEI formation and parasitic reactions, often requiring cathode oversizing or pre-sodiation strategies in full cells [143]. Several optimization strategies have been proposed to mitigate these limitations [195,196,197]. Industrial hard carbon production is currently dominated by Japan and China. Japanese manufacturers have established the benchmark for high-performance hard carbon materials, some from natural sources, while Chinese companies lead large-scale production and are increasingly adopting lower-cost coal-derived precursors [198,199]. In Europe, Stora Enso has emerged as a key supplier with its lignin-derived Lignode (Natural product), highlighting the growing interest in sustainable hard carbon production [200].

Figure 17. Timeline of representative sodium-ion battery companies (2010–2026), showing founding/launch year, cathode and anode chemistries, and notable industry consolidation event.

Compared with hard carbon, alloy-type anodes such as Sn, Sb, Bi, and especially P offer significantly higher theoretical capacities, ranging from approximately 500 to 2600 mAh g⁻¹ depending on the composition (Table 3) [143]. These high capacities arise from the formation of multiple Na-M alloy phases during sodiation and their reversible dealloying upon desodiation [143]. Despite its exceptional capacity, phosphorus remains challenging to implement in practical applications due to its large volume expansion and intrinsically low electronic conductivity [143]. Electrolyte and additive selection are also critical, as the repeated volume changes continuously challenge SEI stability [44,143]. An unstable or mechanically fragile SEI repeatedly ruptures and reforms, consuming cyclable sodium, increasing interfacial resistance, and accelerating cell polarization. Consequently, the practical viability of alloy anodes depends not only on their high specific capacity but also on maintaining interphase stability and mitigating volume expansion during long-term cycling [143,201].

Conversion-type anodes (Table 3), including transition-metal oxides, sulfides, and phosphides, store Na⁺ through multielectron conversion reactions, in which the host structure is transformed into new phases during sodiation and reversibly reconstructed upon desodiation, rather than through simple ion insertion [143,202]. This mechanism enables high theoretical capacities and large charge storage per formula unit, making these materials attractive anode candidates for SIBs. However, their practical implementation remains challenging due to large voltage hysteresis, severe volume expansion during cycling, sluggish Na⁺ diffusion kinetics, and interfacial instability, all of which accelerate electrode degradation and capacity fading. To address these limitations, strategies such as carbon-conducting matrices have been widely explored to improve electronic conductivity and cycling stability [143,202].

5. From Fundamental Electrolyte Chemistry to Practical Cell Engineering: Design Principles and Remaining Challenges

Battery materials are defined not by perfection but by compromise; every anode, cathode, and electrolyte offers distinct advantages while inevitably imposing limitations, making their selection an exercise in balancing competing electrochemical, engineering, economic, and safety requirements (Table 1, Table 2 and Table 3). Ultimately, battery chemistry is useless if it cannot be scaled. Commercial deployment therefore demands criteria that extend well beyond laboratory electrochemical performance. Although capacity and cycle life remain essential benchmarks, they alone cannot determine commercial viability, and greater emphasis should be placed on realistic testing conditions and long-term calendar aging. Manufacturing considerations are equally important.

Practical cell design requires matching the electrolyte’s solvation/desolvation behavior, interphase-forming tendency, transport properties, and mechanical response to both electrodes. A high bulk ionic conductivity is insufficient when charge transfer is limited by desolvation, poor solid–solid contact, unstable SEI/CEI chemistry, or current constriction. Accordingly, electrolyte screening should be performed in the intended full cell configuration and should report temperature, pressure, electrode loading, areal capacity, electrolyte amount or thickness, formation protocol, and impedance evolution. Excess electrolyte in lab coin-cell architectures compensates for continuous consumption at the interphase and therefore masks exactly the instability that most electrolyte research aims to address. Formulations should be validated at commercially representative electrolyte loading, and the loading should be reported. Results obtained under flooded conditions do not transfer.

For liquid systems, the principal design trade-off is between low viscosity and high salt dissociation on one hand, and oxidative/reductive stability and controlled interphase formation on the other. Hard-carbon anodes are especially sensitive to solvent-derived SEI solubility and first-cycle sodium loss, while high-voltage layered or polyanionic cathodes require oxidation-resistant electrolytes and stable CEIs. Concentrated and localized-concentration formulations can increase anion participation and alter interphase chemistry, but their benefits must be weighed against viscosity, cost, wettability, and reduced low-temperature conductivity. Low-temperature performance is currently the clearest commercial advantage of sodium-ion technology, and it is the basis for its early deployment in cold-climate stationary storage and hybrid vehicle packs. Its practical significance is illustrated by the excellent low-temperature operation reported for NFPP-containing cells down to −70 °C, as well as the already commercial available Hithium design as discussed at the end of Section 3, [180,181,184].

For solid-state systems, the dominant engineering problem shifts from volatility to contact mechanics and interfacial reaction. Oxides offer chemical robustness but generally require high-temperature densification and interface modification; sulfides and halides are more deformable and can be cold-pressed, but moisture sensitivity and electrode reactivity impose dry-room, coating, and end-of-life constraints. Polymer and composite electrolytes improve conformability, yet practical evaluation must account for thickness, active-filler percolation, transference number, mechanical creep, and whether a supporting liquid electrolyte was used. Critical-current-density or dendrite-suppression claims should always be reported with stack pressure, defect density, electrode area, and cycling duration.

6. Future Directions

Sodium-ion batteries have progressed from academic alternatives to early commercial technologies, but their readiness is application- and chemistry-specific. Their strongest proposition is not simply replacing lithium with sodium; it is the possibility of combining geographically broader raw-material availability, aluminum-based current collectors in suitable designs, robust low-cost cathodes, and safer transport or operation under validated cell conditions. SIB advantages make them more compelling for stationary storage, grid services, low-temperature operation, and selected mobility applications where cost, lifetime, and resilience outweigh maximum gravimetric energy density.

To bridge the remaining gap between laboratory research and commercial deployment, future sodium-ion battery studies should adopt standardized reporting practices. At a minimum, commercially relevant investigations should report: (i) complete cell chemistry and N/P balance; (ii) active-material loading and areal capacity; (iii) electrolyte-to-capacity ratio or solid-electrolyte thickness; (iv) formation and temperature/pressure conditions; (v) initial Coulombic efficiency and sodium inventory; (vi) capacity retention at a stated rate and depth of discharge; (vii) impedance or interphase evolution; and (viii) whether performance is measured in a half-cell, laboratory full cell, prototype, commercial cell, or pack.

A second priority is supply-chain and manufacturing maturity, and here the regional contrast is evident. The absence of standardized cell formats further complicates matters, as it limits the direct transfer of manufacturing processes and quality control protocols across production lines [203]. In China, the commercialization is advancing roughly in line with earlier projections: CATL has released its second-generation Naxtra battery, achieving an energy density of 175 Wh kg⁻¹ on par with mainstream LFP cells, with the product line currently comprising a passenger-EV battery and a 24 V integrated battery for heavy-duty trucks; BYD began construction of its first dedicated sodium-ion plant in January 2024 in Xuzhou, targeting EVs, grid storage, and industrial applications [27,28,204]. Outside China, the picture is less favorable. In the United States, Natron Energy ceased operations in September 2025, only months after beginning commercial-scale production, abandoning a planned $1.4 billion, 14 GWh facility. This closure was attributed to the absence of an integrated regional supply chain comparable to China’s, along with pricing pressure from a sharp decline in lithium carbonate prices that further plummeted the cost of LFP cells and narrowed the economic advantages for early SIB contestants [205]. Even within China, not all projects have proceeded on schedule. One example is Veken Teck, which delayed its 2 GWh facility by one year from its original December 2024 completion date, citing uncertainty in energy storage demand, financing constraints, and supply chain scaling difficulties [206]. Closing this gap outside China will likely require building regionally dedicated supply chains for much-solicited hard carbon anodes, sodium-specific salts, and aluminum current collectors, rather than relying on the assumption that existing lithium-ion infrastructure alone is sufficient to support a parallel sodium-ion industry at scale.

Recyclability should also be designed in parallel with cell chemistry from the outset, rather than retrofitted later as has occurred in the LIB industry; doing so would both reduce environmental impact and differentiate SIBs from LIBs on sustainability grounds. Sodium abundance does not by itself establish a low environmental footprint or an economically trivial recovery process: precursor origin, fluorinated salts, solvent production, drying energy, manufacturing yield, lifetime, and electricity mix all affect life-cycle impact, and recovery economics depend on process scale, energy use, reagent consumption, wastewater treatment, and material prices. Future work should therefore report system boundaries and sensitivity analyses, and compare direct regeneration, mechanical separation, hydrometallurgy, and water-assisted disassembly under controlled reaction and effluent-treatment conditions. A study conducted by Liu et al. outlined a water-based disassembly route in which contact with water releases sodium as NaOH and simultaneously corrodes the aluminum foil, successfully separating the two [207]. The same study identified the recovery of transition metals from the cathode as an unresolved step requiring dedicated chemical recycling methods, a gap that warrants further investigation. Direct experimental work on spent commercial NaNi_1/3Fe_1/3Mn_1/3O₂ cells reinforces the following: aqueous pretreatment proved more effective at separating and cleaning the cathode than conventional acid leaching, and cathode regeneration via sodium replenishment and thermal treatment yielded a profit of $3.76 kg⁻¹ battery, surpassing the $2.64 kg⁻¹ reported for LIB recycling under comparable conditions [208].

The central difficulty, however, is economic motivation. The transition metals most commonly used in SIB cathodes, principally iron and manganese, are inexpensive commodities with well-established production routes. Unlike cobalt and nickel, whose high market value provides a direct financial incentive to recover them from spent LIBs, Fe and Mn offer no comparable pull. This raises legitimate concern. Without an economic driver, recycling rated for SIBs may remain low even if the technical pathways exist. One route forward is regulatory. The EU Battery Regulation (2023) already mandates minimum recycled content and collection targets that will apply to SIBs as volumes grow, which may substitute market incentives where they are absent [209]. A second direction is to shift the economic framing entirely, from metal recovery toward cathode regeneration, which the experimental evidence cited above suggests can be profitable even for Fe-/Mn-based systems precisely because it avoids the energy and chemical-intensive dissolution and re-precipitation steps that make hydrometallurgical recovery of cheap metals unviable. Embedding regeneration-friendly design principles into cathode development from the outset may therefore be a promising path to a self-sustaining recycling economy for SIBs.

7. Conclusions

Sodium-ion batteries have moved beyond their status as a purely academic alternative to lithium-ion systems and are now entering a decisive stage of technological validation and early commercialization. Their main strength is not simply the replacement of lithium by sodium, but the possibility of building a battery ecosystem less dependent on geographically concentrated critical raw materials, more compatible with low-cost large-scale storage, and potentially safer across transport, operation, and end-of-life handling. The advantages of sodium abundance, aluminum current collectors and improved discharge-state safety make SIBs particularly attractive for stationary energy storage, grid balancing, and selected mobility applications where cost and robustness are more important than maximum gravimetric energy density.

Electrolyte development remains one of the central scientific and technological bottlenecks. Organic liquid electrolytes currently offer high ionic conductivity and relatively mature processing, but their flammability, interfacial instability, and sensitivity to salt–solvent interactions continue to limit long-term performance and safety. The design of stable SEI and CEI layers is especially critical in sodium-ion systems, where solvation, desolvation, and electrolyte decomposition pathways differ substantially from those established for Li-ion batteries. Solid-state sodium electrolytes, including polymers, composites, $\beta$/β″-alumina, NaSICON-type oxides, sulfides, halides and antiperovskite-derived systems, offer an important route toward safer and more thermally robust cells. However, their practical implementation is still constrained by solid–solid contact resistance, grain-boundary effects, sodium wetting, dendrite propagation, and the need for scalable processing routes.

Cathode chemistry will largely determine the commercial identity of sodium-ion batteries. Layered transition-metal oxides provide attractive capacities and can be designed with Fe-/Mn-rich and Co-/Ni-poor compositions, but they suffer from phase transitions, air sensitivity, and structural degradation during cycling. Polyanionic compounds, especially NaSICON-type phosphates and fluorophosphates, offer excellent structural and thermal stability, although low electronic conductivity and the frequent use of vanadium raise cost and sustainability concerns. Prussian blue analogs are highly appealing because of their open framework, low-cost synthesis, and fast Na⁺ transport, but coordinated water, vacancies, and potential safety concerns linked to cyanide-containing frameworks must be carefully controlled. Organic cathodes add sustainability potential, but their low conductivity, dissolution, and limited energy density still require substantial material engineering.

The future of SIBs will depend less on a single breakthrough material and more on the integration of chemistry, interfaces, manufacturing, and circularity. Commercial success will require standardized cell formats, dedicated sodium-compatible supply chains, scalable hard-carbon production, stable sodium salts, robust aluminum-based current-collector strategies, and recycling or regeneration routes designed from the beginning rather than retrofitted after deployment. In this context, SIBs should not be presented as universal substitutes for LIBs, but as complementary technologies with a strong strategic role in safe, affordable, and sustainable energy storage. Their greatest opportunity lies in applications where resource security, environmental impact, operational safety, low temperature operation and cost stability are more valuable than achieving the highest possible energy density.