Critical Challenges and Recent Advances Toward Practical Room-Temperature Sodium–Sulfur Batteries

Abstract

Room-temperature sodium–sulfur (RT Na–S) batteries are gaining renewed interest as a cost-effective, high-energy-density candidate for next-generation storage, owing to their high theoretical specific energy (1274 Wh kg⁻¹) and the earth abundance of sodium. However, their practical application continues to be hampered by persistent challenges such as the polysulfide shuttle effect, uncontrolled growth of sodium (Na) dendrites and electrolyte-induced interfacial instability. This review summarizes recent progress in several key areas: cathode design (including defect engineering, single-atom catalysts, heterostructures), anode interfacial regulation (through SEI/CEI engineering and additive chemistry), and electrolyte formulation (notably localized high-concentration electrolytes and solvation-structure tuning). Although promising results have been achieved at the laboratory scale, critical challenges remain for practical deployment, particularly under high-sulfur-loading and lean-electrolyte conditions, along with fundamental interfacial issues inherent to all-solid-state battery architectures.

1. Introduction

Sodium-sulfur batteries offer a theoretical specific energy of 1274 Wh kg⁻¹—approximately three times that of conventional lithium-ion systems (~420 Wh kg⁻¹) [1]—while relying on sodium, an element about 1180 times more abundant in the earth than lithium [2,3]. Consequently, the raw material cost of sodium is only about 1/25 that of lithium, making RT Na–S batteries chemistry an inherently low-cost, high-energy platform for grid storage and mobility applications. Historically, Na–S batteries technology was demonstrated in the 1960s as a high-temperature (300–350 °C) system employing molten electrodes and β″-alumina solid electrolytes [4,5]. Although commercialized for stationary storage, its widespread adoption has been limited by the need for thermal management, corrosion challenges, and safety concerns associated with molten sulfur and sodium [6,7]. Over the past decade, research efforts have therefore shifted toward room-temperature operation, where polymer-gel or liquid organic electrolytes enable sufficient Na⁺ conductivity under ambient conditions.

A typical room-temperature Na–S cell consists of a metallic sodium anode, a sulfur/carbon composite cathode, and an ion-conducting electrolyte [8]. During discharge, solid S₈ undergoes ring-opening to form soluble long-chain sodium polysulfides (Na₂S₈), which are then progressively reduced to shorter-chain species and eventually to insulating Na₂S [9]. The charge process reverses these steps; however, the intermediate polysulfides tend to shuttle between electrodes, leading to rapid capacity fade and low Coulombic efficiency. Since the first RT Na–S prototype was reported in 2006 [10], considerable advances have been made in cathode nanoengineering, anode interfacial protection, and electrolyte design. Nevertheless, the complex interplay between sulfur redox chemistry and interfacial stability remains a critical challenge for practical implementation. The configuration and working principle of such a typical RT Na–S coin cell are illustrated in Figure 1.

Figure 1. Configuration of a representative coin cell and schematic operation of an RT Na–S batteries. Reproduced with permission [8], Copyright 2020, WILEY–VCH.

Several authoritative reviews have established the fundamental cell chemistry and catalogued cathode, electrolyte, separator, and anode strategies for RT Na–S batteries [2,4]. The present review is differentiated by a problem-centred framework that links molecular-scale sulfur speciation to electrode-scale transport, Na-metal inventory loss, and full-cell constraints. We therefore evaluate not only whether a material improves capacity or cycling in a coin cell, but also whether the underlying strategy remains credible at sulfur loadings ≥ 5 mg cm⁻², E/S ratios ≤ 5 μL mg⁻¹, limited Na excess (preferably N/P ≤ 3), and pouch- or full-cell formats. These thresholds are used as working screening criteria rather than universal standards, because the optimal values remain chemistry- and architecture-dependent.

2. Working Principles and Challenges of RT Na–S Batteries

RT Na–S batteries operate via a reversible two-electron reaction between metallic sodium and elemental sulfur, enabling the interconversion of electrical and chemical energy. This conversion process is inherently complex, involving sequential electron transfers and concurrent phase transitions. Consequently, a majority of the persistent challenges in these batteries arise from the electrochemical evolution of both electrodes during cycling. A primary source of degradation is the dissolution and migration of soluble polysulfides at the cathode.

2.1. Electrochemical Mechanism of RT Na–S Batteries

RT Na–S batteries offer a high theoretical specific capacity of 1675 mAh g⁻¹ and a theoretical energy density of 1274 Wh kg⁻¹, which originate from the reversible electrochemical reactions at both electrodes [11]. The overall cell reaction can be summarized as:

S₈ + 16Na⁺ + 16e⁻ → 8Na₂S

(1)

The cathodic transformation from elemental S₈ to the final discharge product, Na₂S, is intrinsically complex, which typically begins with the ring-opening of S₈ and proceeds through sodium polysulfide intermediates of varying chain lengths. The electrochemical behavior of RT Na–S batteries is highly electrolyte-dependent, exhibiting pronounced differences between ether-based and carbonate-based liquid electrolytes. In ether-based electrolytes, sulfur conversion generally follows a solid–liquid–solid dissolution–precipitation pathway. Crystalline S₈ first forms soluble long-chain Na₂S_n species, which are subsequently reduced through shorter-chain intermediates before Na₂S₂/Na₂S precipitation. Accordingly, the discharge profile of S₈ exhibits two distinct voltage plateaus separated by two sloping regions, as illustrated in Figure 2a.

Figure 2. (a) Typical discharge profile of RT Na–S battery in ether-based electrolyte; (b) Typical discharge-charge profiles for selected cycles of RT Na–S battery in carbonate-based electrolyte. Reproduced with permission [10], Copyright 2024, Elsevier Ltd.

The first plateau, typically near 2.2 V, is associated with S₈ ring opening and formation of dissolved high-order polysulfides, represented by Equation (2).

S₈ + 2Na⁺ + 2e⁻ → Na₂S₈

(2)

The following sloping region (approximately 2.20–1.65 V) reflects solution-phase reduction and redistribution among high- and intermediate-order polysulfides. Equation (3) provides a balanced limiting representation for conversion of Na₂S₈ to Na₂S₄; Na₂S₆ and Na₂S₅ may also be transiently populated.

Na₂S₈ + 2Na⁺ + 2e⁻ → 2Na₂S₄

(3)

The lower-voltage plateau is associated with precipitation of short-chain species and Na₂S. Equations (4)–(6) show representative balanced branches from Na₂S₄ to Na₂S₃, Na₂S₂, or Na₂S. Their relative contributions vary with solvent donor strength, salt concentration, host chemistry, and overpotential.

3Na₂S₄ + 2Na⁺ + 2e⁻ → 4Na₂S₃

(4)

Na₂S₄ + 2Na⁺ + 2e⁻ → 2Na₂S₂

(5)

Na₂S₄ + 6Na⁺ + 6e⁻ → 4Na₂S

(6)

The final sloping region corresponds primarily to solid-state or interfacial conversion of Na₂S₂ to Na₂S, as represented by Equation (7). This step is often kinetically limiting because both products are electronically insulating and because precipitation can passivate catalytic and conductive surfaces.

Na₂S₂ + 2Na⁺ + 2e⁻ → 2Na₂S

(7)

In carbonate-based electrolytes, sulfur chemistry cannot be described by simply transferring the ether-based shuttle mechanism (Figure 2b). High-order Na₂S_n species are nucleophilic and may react with carbonate solvents, while nanoconfinement and an evolving cathode–electrolyte interphase (CEI) can reduce their effective activity and promote quasi-solid or spatially localized conversion [12]. Thus, carbonate media often suppress long-range polysulfide transport kinetically rather than eliminating polysulfide formation thermodynamically. Partial dissolution, short-lived soluble intermediates, and CEI rupture remain possible, particularly during early activation, at high sulfur loading, or when the carbon host is insufficiently confining. The monotonically sloping voltage profile and lower operating voltage commonly observed in carbonate systems therefore reflect a coupled response of sulfur speciation, solvent decomposition, CEI formation, and solid-state transport—not definitive proof of a completely shuttle-free mechanism. This distinction reconciles reports of reduced shuttle with evidence for residual dissolution and interfacial instability.

2.2. Technical Challenges

RT Na–S batteries face mutually reinforcing limitations at the molecular, electrode, and cell levels. While these issues resemble those encountered in Li-S batteries, their severity is notably greater in the Na–S system. Key challenges include: (i) the inherently low electronic conductivity of sulfur (~10⁻³⁰ S cm⁻¹), which restricts its electrochemical activity in liquid electrolytes; (ii) structural degradation of the sulfur electrode due to the formation of Na₂S_n during cycling, disrupting electronic pathways between particles; and (iii) progressive impedance growth caused by electrode passivation [13,14,15]. Compared with Li-S systems, these challenges are exacerbated in Na–S batteries, where sluggish redox kinetics and enhanced polysulfide diffusion combine to make performance improvements particularly difficult.

2.2.1. Shuttle Mechanism

In ether-based electrolytes, long-chain Na₂S_n (typically 4 ≤ n ≤ 8) are thermodynamically soluble and can diffuse across the separator [16,17]. Their chemical reduction at the Na surface and subsequent return to the cathode establish a parasitic redox shuttle, lowering Coulombic efficiency and depleting both sulfur and Na inventory [18,19]. In carbonate electrolytes, by contrast, apparent shuttle suppression may arise from rapid solvent-derived CEI formation, low free-solvent activity, nanoconfinement, or conversion to less-soluble species. These mechanisms are kinetic and interfacial: they can immobilize polysulfides over experimental timescales without proving zero equilibrium solubility. Accordingly, polysulfide control should be discussed through three separate descriptors—thermodynamic solubility, kinetic transport across the separator, and interfacial conversion/immobilization—rather than through a binary ‘shuttle/no-shuttle’ classification. Although this shuttle effect is also observed in Li-S batteries, the stronger nucleophilicity of sodium widens the kinetic differences, making the shuttle more aggressive in room-temperature Na–S cells. This leads to rapid Coulombic efficiency loss and accelerated capacity fade.

2.2.2. Self-Discharge

Self-discharge represents a critical bottleneck for the practical deployment of room-temperature Na–S batteries. Under open-circuit conditions, soluble high-order polysulfides migrate along concentration gradients toward the Na anode, where they undergo spontaneous chemical reduction, leading to irreversible depletion of active species and progressive capacity fading [20,21]. While quasi-solid carbonate and solid-state electrolytes can effectively mitigate long-range polysulfide diffusion, parasitic processes—including local redox redistribution, interphase degradation, mixed ionic-electronic conduction, and direct chemical interactions—continue to drive state-of-charge decay. Consequently, rigorous evaluation of self-discharge requires standardized conditions encompassing temperature, rest duration, sulfur loading, and electrolyte composition.

2.2.3. Electronic/Ionic Transport and Electrode Passivation

Sulfur and Na₂S possess extremely low electronic conductivity, so conversion proceeds only where ion-conducting electrolytes, electronic pathways, and active sulfur remain in contact. At high loading, tortuous diffusion through thick cathodes produces reaction fronts, local salt depletion, and nonuniform Na₂S deposition. Precipitated products can block pores and catalytic sites, while repeated solvent decomposition increases interfacial resistance. High surface area is therefore not intrinsically beneficial: it improves reaction area but can increase electrolyte demand, side reactions, and inactive interphase mass. Practical hosts must preserve continuous electronic percolation while maintaining short Na⁺ transport distances and controlled electrolyte accessibility.

2.2.4. Structural Evolution and Mechanical Degradation

Conversion between S₈ and Na₂S is accompanied by substantial local volume redistribution. In thin electrodes, free pore volume can mask this problem; in high-loading electrodes, however, repeated precipitation and dissolution generate stress gradients, particle detachment, binder fatigue, pore closure, and current-collector delamination. Loss of conductive contact promotes isolated sulfur/Na₂S domains and accelerates impedance growth. Mechanical robustness, pore-size hierarchy, binder chemistry, and electrode density must therefore be optimized together rather than treated as secondary processing variables.

2.2.5. Na-Metal Instability and Safety

The Na-metal anode experiences nonuniform plating, dendrite formation, polysulfide corrosion, and repeated SEI fracture. Excess Na can conceal these losses in coin cells but lowers cell-level energy density and delays rather than resolves failure. Reactive sodium also amplifies safety risks associated with electrolyte leakage, gas evolution, internal short circuit, and heat generation. A credible practical assessment must therefore report Na thickness or N/P ratio, cumulative plated capacity, full-cell inventory balance, and abuse-relevant safety data.

3. Recent Advances in RT Na–S Batteries

Recent advances can be grouped into four coupled design layers: (i) cathode architectures that maintain electronic/ionic percolation and regulate sulfur speciation; (ii) catalytic motifs that lower barriers for Na₂S_n interconversion and Na₂S nucleation/decomposition; (iii) Na-anode interphases that homogenize ion flux and suppress corrosion; and (iv) electrolyte/solid-electrolyte chemistries that control solvation, interphase formation, and transport. Because improvements in one layer can shift failure to another, performance should be compared under matched loading, E/S ratio, Na inventory, and cell format (Table 1).

Table 1. Quantitative comparison of representative RT Na–S strategies.

No.	Cathode or Strategy	Sulfur Loading	E/S Ratio	Current Density/Rate	Cycling Stability	Coulombic Efficiency	N/P Ratio	Cell Configuration/Electrolyte
[22]	Heterointerfaced nanoreactor electrocatalyst for practical RT Na–S batteries	6.2 mg cm−2	9 µL mg−1	0.1 C	1300 mAh g−1 over 100 cycles	Near 100% reported in consulted source	Not reported	Na||S coin and pouch cells; nanoreactor-confined catalytic sulfur cathode
[23]	Binder-free S@Fe3N-NMCN electrocatalytic cathode	~2.6 mg cm−2	18.2 µL mg−1	long cycling at 8375 mA g−1	almost no attenuation after 2800 cycles at 8375 mA g−1;	Initial CE 92%; near-100% CE during long cycling	Not reported	CR2032 Na||S coin cell; glass-fiber separator; 1 M NaPF6 in DOL/DIGLYME (1:1); RT
[24]	MoTe2 nanosheets vertically grown on graphene coupled with sulfur (MTG/S)	3.5 mg cm−2	7 µL mg−1	0.1 C for high-loading cycling	1081 mAh g−1 initial at 0.1 C; 901 mAh g−1 after 350 cycles at 0.1 C	~100% over 350 cycles; capacity decay ~0.05% per cycle	Not reported	Na||MTG/S coin cell; localized high-concentration electrolyte (NaFSI/DME/TTE); RT
[25]	Solvation inner-sheath engineering/non-coordinating co-solvent electrolyte for practical Na–S cells	1.8 mg cm−2	18 µL mg−1	0.1 C	530 mAh g−1 over 200 cycles	Not reported	Not reported	Na||S pouch/practical-format cells; lean electrolyte; non-coordinating co-solvent electrolyte design
[26]	Free-standing carbon@sulfur framework with NLP-selected single-atom Co-N/S catalyst	Pouch: 4.5 mg cm−2; stricter pouch: 6.67 mg cm−2; high-loading coin: 4.2 mg cm−2	8 µL mg−1 for 4.5 mg cm−2 pouch; 3.2 µL mg−1 for 6.67 mg cm−2 pouch	Pouch rate tests at 0.1–1.0 A g−1; 4.5 mg cm−2 pouch tested under 2 mA	4.5 mg cm−2 pouch: 173.9 Wh kg−1 after 10 cycles; 6.67 mg cm−2 pouch: 173 mAh first cycle; high-loading coin: 2.54 mAh cm−2 after 70 cycles	Close to 100% for 4.2 mg cm−2 coin cell after 70 cycles	3.4:1 for 4.5 mg cm−2 pouch; 2.1:1 for 6.67 mg cm−2 pouch	Na||S pouch cells with thin Na slice; 2 M NaTFSI in PC/FEC; glass-fiber separator
[27]	Ca-O4N-C@S s-block single-atom catalyst with axial-ligand-induced charge localization	1.0 mg cm−2; 3.78 mg cm−2 high-loading test also reported	70 µL mg−1	335 mA g−1 (0.2 C); 3 C long cycling	1.0 mg cm−2 coin: 1211 mAh g−1 after 100 cycles at 0.2 C; 887 mAh g−1 after 800 cycles at 3 C; 3.78 mg cm−2 coin: 978 mAh g−1 at 0.2 C after 100 cycles	Pouch: 99.50% after 50 cycles at 0.1 C	Not reported	CR2032 Na||S; GF/D separator; 2 M NaTFSI in PC/FEC (1:1); 0.5–2.8 V

Mechanistic assignment increasingly relies on complementary operando methods. Operando Raman spectroscopy and synchrotron X-ray diffraction have directly followed sulfur-species evolution and phase formation in catalytic RT Na–S cells [28]. More recently, operando transmission X-ray microscopy combined with sulfur K-edge X-ray absorption spectroscopy resolved spatially heterogeneous sulfur conversion, cathode expansion, and active-material dissolution in an ether electrolyte [29], while operando microbeam XRD, XPS, and ex situ XAS exposed pressure-dependent polysulfide formation in solid-state cells. These studies illustrate why ex situ end-point characterization alone can misidentify transient intermediates or overlook reaction heterogeneity.

3.1. Cathode: From Host Design to Catalytic Mechanism Innovation

Cathode-host engineering remains the most direct and widely explored approach for mitigating polysulfide shuttling. Conventional strategies based on physical adsorption in porous carbon matrices rely on weak, non-selective van der Waals interactions; under high sulfur loading and lean-electrolyte conditions, however, pore saturation occurs, and polysulfide shuttling continues largely unchecked [30]. Yet pore architecture also imposes fundamental trade-offs. Micropores strengthen confinement and favour short-chain or quasi-solid conversion, but restrict sulfur loading and electrolyte infiltration; mesopores shorten ion-transport paths and accommodate products, but can permit polysulfide escape; macropores improve wetting and buffer volume change, but lower volumetric energy density. A hierarchical pore network is therefore preferable only when pore volume, sulfur distribution, tortuosity, and electrolyte inventory are co-optimized. Furthermore, the non-polar surfaces of pristine carbon materials exhibit limited electronic coupling with polar polysulfides, resulting in sluggish conversion kinetics.

Conductive additives must form a continuous network through the full electrode thickness, while binders must sustain adhesion during repeated conversion. Recent SPAN studies show that poly(acrylic acid) can outperform conventional PVDF by hydrogen-bonding with polar C=N motifs, distributing stress more uniformly and sustaining an areal capacity of 2.6 mAh cm⁻² for more than 200 cycles [31]. This example underscores that binder chemistry, slurry rheology, calendering density, and current-collector adhesion can determine whether a catalytic material remains accessible under practical loading.

Defect engineering and heteroatom coordination modify the local electronic density of states and thereby tune the adsorption–conversion balance for Na₂S_n. Pentagon defects disrupt the π-conjugated carbon lattice, localize charge, and lower barriers for S–S bond cleavage, as shown in Figure 3a; Zheng et al. reported 1275 mAh g⁻¹ after 100 cycles at 0.1 C and a decay rate of 0.035% per cycle over 600 cycles at 3 C, as shown in Figure 3b [32]. The mechanistic objective is not maximal adsorption: excessively strong binding can immobilize intermediates and retard product desorption. Effective hosts instead approach a Sabatier-type optimum in which adsorption suppresses escape while allowing rapid interconversion and Na₂S release.

Figure 3. (a) Schematic illustration of the synthesis process for PCs; (b) Long-term cycling stability of S/PCs, S/NNCs and S/NCs at 3 C. The red hollow circle represent the Coulombic efficiency. Reproduced with permission [32], Copyright 2024, Wiley-VCH; (c) Schematic illustration of TM-S-C@MC structure and its sulfur reduction reaction catalytic process. Reproduced with permission [22], Copyright 2025, Wiley-VCH.

Wei et al. integrated catalytic sites and spatial confinement in a heterointerfaced nanoreactor [22]. Interfacial electronic coupling shifts the transition-metal d-band centre, strengthens selective chemisorption, and accelerates reduction, while the surrounding carbon shell limits species loss and accommodates expansion, as shown in Figure 3c. Importantly, the system was also evaluated at 6.2 mg cm⁻² sulfur loading and an E/S ratio of 9 μL mg⁻¹, and in a pouch-cell configuration. Although this remains above the stringent E/S ≤ 5 μL mg⁻¹ benchmark adopted here, it represents a more informative practical test than conventional low-loading coin cells.

The quintessential challenge in cathode design lies in simultaneously suppressing polysulfide dissolution/shuttling and accelerating sulfur redox kinetics. Zhang et al. [33] promoted a quasi-solid-state conversion pathway by embedding MoC-W₂C heterostructures within multichannel carbon nanofibers. This hetero-interface facilitates rapid electron/ion transport, enabling the electrode to retain a capacity of 200 mAh g⁻¹ after 3500 cycles at 4 Ag⁻¹. In parallel, Li et al. [34] engineered an asymmetric Mn-N₂ coordination motif that intensifies d-p orbital hybridization, which elevated the catalytic turnover of polysulfides, resulting in a retained capacity of 458 mAh g⁻¹ after 2300 cycles at 3 C. More strikingly, Li et al. [35] developed a Fe–Co dual-atom catalyst in which Fe incorporation switches Co(II) from a low-spin to a high-spin state, markedly accelerating sodium polysulfide interconversion (Figure 4a,b). The Fe–Co/NC/S electrode exhibits a voltage gap of merely 0.87 V between the second cathodic and anodic peaks—significantly lower than those of Co/NC (0.92 V) and Fe/NC (1.04 V) counterparts—indicating substantially reduced polarization (Figure 4c). Fe–Co dual-atom sites altered the Co spin state and reduced polarization; high-loading cells containing 5.6 mg cm⁻² sulfur were also demonstrated. Figure 4d further confirms a discharge capacity of 341.1 mAh g⁻¹ at 5 A g⁻¹. Collectively, these studies underscore that electronic-structure modulation and interfacial engineering can fundamentally alter polysulfide reaction pathways. Extending this paradigm, Gao et al. [36] constructed a heterostructured Na₂S/Na₂Te@C cathode mediated by a sodium thiotellurate (Na₂TeS₃) interphase. The abundant electron/ion channels within this architecture enhance active-material utilization while curbing polysulfide shuttling (Figure 4e), enabling an initial discharge capacity of 634 mAh g⁻¹ in an anode-free configuration. Nevertheless, the majority of these advances remain confined to laboratory-scale evaluations under low sulfur loadings. Their operative mechanisms under practical conditions such as high sulfur mass loading and lean electrolyte—await systematic elucidation.

Figure 4. (a,b) Spin-state modulation and d–d interaction in Co/NC and Fe–Co/NC; (c) cyclic-voltammetry comparison and (d) rate performance of Fe–Co/NC/S, Co/NC/S, and Fe/NC/S cathodes. Reproduced with permission [35], Copyright 2025, American Chemical Society; (e) Schematic mechanism for enhancing the electrochemical performance of Na₂S/Na₂Te@C in RT Na–S batteries. Reproduced with permission [36], Copyright 2024, Wiley-VCH.

Dynamic active-site reconstruction must nevertheless be considered: electrochemically generated Mo single atoms on MoS₂, for example, can become the true catalytic centres rather than the pristine phase identified before cycling [37]. Operando spectroscopy, post-mortem mass balance, and matched-loading controls are therefore required to establish causal structure–activity relationships.

3.2. Anode: Interface Engineering and Deposition Behaviour Regulation

The metallic sodium anode confronts a bifurcated challenge—dendrite growth and parasitic interfacial reactions—both of which are intensified by the polysulfide-rich environment. To address this challenge, Liu et al. [38] used a nonflammable ionic-liquid electrolyte containing FSI⁻/TFSI⁻ anions and FEC to generate an inorganic-rich, F-containing inorganic-rich solid-electrolyte interphase (SEI) on Na surface, which promotes uniform sodium deposition and suppresses dendrite propagation (Figure 5a). Leveraging the synergistic effect of FSI^-/TFSI⁻ anions and fluoroethylene carbonate (FEC), the electrolyte enables a capacity retention of 565 mAh g⁻¹ after 500 cycles at 0.2 A g⁻¹ (Figure 5b,c). By contrast, Wang et al. [39] introduced tin trifluoromethanesulfonate (Sn(OTf)₂) into a non-flammable phosphate-based electrolyte. By modifying the Na⁺ solvation sheath and interfacial chemistry, the additive facilitates the formation of Na–Sn-alloy-containing SEI together with a cathodic CEI layers, which increase the cell capacity from 322 to 906 mAh g⁻¹ (Figure 5d). The resulting Na–S full cell also demonstrates superior rate capability, further confirming the significant electrochemical improvement imparted by Sn(OTf)₂ (Figure 5e).

Figure 5. (a) Schematic diagram of S@C||Na cell with the IL-FE electrolyte. (b) Cycle performance at 0.2A g⁻¹. (c) Initial charge–discharge profiles at 0.2A g⁻¹. Reproduced with permission [38], Copyright 2025, WILEY-VCH. (d) Cycling performance at 0.5A g⁻¹ and (e) rate performance of Na–S full cells using the electrolyte with and without Sn (OTf)₂ additive. The red hollow circle represent the Coulombic efficiency. Reproduced with permission [39], Copyright 2024, WILEY-VCH.

These interphases should be compared by composition, transport, and mechanics rather than by the generic label ‘stable SEI/CEI’. NaF-rich layers offer high interfacial energy and mechanical stiffness but can become resistive if excessively thick; alloy-containing SEIs can improve sodiophilicity and lateral Na diffusion but consume additive and active Na; organic-rich components can enhance elasticity yet remain susceptible to nucleophilic attack. Long-term validation therefore requires impedance evolution, Na Coulombic efficiency, gas analysis, and morphology after a fixed cumulative areal capacity—not only short symmetric-cell tests.

Beyond additive-based approaches, interphase design can be advanced through solvent engineering. Weng et al. [40] revealed a two-stage CEI formation mechanism when FEC is used as the sole solvent (Figure 6a): rapid nucleophilic attack combined with electrochemical cleavage produces a distinctive “chocolate-biscuit” bilayer CEI rich in NaF. Coupled with a fluorine-enriched SEI on the anode, the FEC-only S@MRP||Na full cell outperformed its PCE-containing counterpart across rates from 0.1 C to 2 C (Figure 6b) and retained 456 mAh g⁻¹ after 1000 cycles at 0.5 C, with Coulombic efficiency approaching unity (Figure 6c). These results highlight that constructing an interphase with high ionic conductivity and mechanical robustness is crucial for dendrite suppression. However, most current studies are still based on half-cell evaluations. Stabilizing Na-metal anodes in full-cell configurations under practical conditions—such as low N/P ratio and lean electrolyte—remains a significant challenge.

Figure 6. (a) Schematic diagram of complete CEI formation mechanism in S@MRP||Na cells with two electrolytes. (b) Rate capability. (c) Long-cycle performance at 0.5 C. The red hollow circle represent the Coulombic efficiency. Reproduced with permission [40], Copyright 2025, WILEY-VCH.

3.3. Electrolyte: Precise Design from Solvation Structure to Interfacial Chemistry

The electrolyte is simultaneously an ion-transport medium, a solvent for sulfur species, and a precursor to SEI/CEI formation. Its effect cannot be reduced to polysulfide solubility alone. Donor strength and salt association determine equilibrium speciation; viscosity and conductivity control kinetic transport; preferential interfacial decomposition determines whether dissolved species are immobilized or whether fresh surfaces remain exposed. Electrolyte design must therefore balance thermodynamic solubility, kinetic shuttle suppression, interfacial passivation, Na plating efficiency, wetting of thick electrodes, and safety.

Pai et al. [41] innovatively employed toluene—a non-fluorinated, low-cost diluent—to construct a localized high-concentration electrolyte (LHCE). Toluene not only provides reductive stability but also modulates the solvation structure, promoting anion-dominated clusters that foster a dense, inorganic-rich SEI on the Na metal surface (Figure 7a). This interphase effectively suppresses dendrite growth and minimizes parasitic electrolyte–anode interactions. As shown in Figure 7b, polysulfide attack leads to rugged dendrites in 2 M-DME electrolytes, whereas the LHCE-cycled electrode retains a smooth, planar morphology. The approach trades some solvation capacity for enhanced interfacial stability, simultaneously mitigating polysulfide shuttle and dendrite formation. Extending this paradigm, Yao et al. [42] systematically investigated solvent molecular frameworks, revealing a decisive link between solvation structure and electric-double-layer (EDL) chemistry. An LHCE formulated with tetrahydropyran (THP) not only reduces polysulfide solubility through solvation restructuring but also promotes a durable inorganic SEI, enabling stable cycling over 500 cycles at 1.0 C with a minimal capacity fade rate of 0.07% per cycle (Figure 7c). These results show that bulk solvation and interfacial composition must be designed together.

Figure 7. (a) Schematic illustration of the electrolyte solvation structure with a non-fluorinated diluent, toluene; (b) SEM images of sodium metal from the Na||Na symmetric cells after cycling for 300 h. Reproduced with permission [41], Copyright 2024, WILEY-VCH; (c) Long-cycle performances of Na–S cells with LHCE-THP/ETFE electrolyte at 1 C rate. The orange hollow circle represent the Coulombic efficiency. Reproduced with permission [42], Copyright 2025, WILEY-VCH.

In a subsequent study, Yao et al. [43] proposed an “inner–outer sheath synergistic shielding” strategy. By employing sterically hindered 1, 2-dimethoxypropane (DMP) as the inner-shell solvent, the additional methyl group effectively excludes polysulfides from the primary solvation sheath (Figure 8a). Complementarily, a fluorinated ether (TFTFE) with low solvation power serves as the outer diluent, further suppressing polysulfide dissolution. This dual-sheath design offers a new perspective for inhibiting the shuttle effect at its origin. Parallel to this, Zou et al. [44] developed a weakly solvating anion-stabilized (WSAS) electrolyte that re-balances Na⁺-solvent and Na⁺-anion interactions, generating abundant contact ion pairs (CIPs) and aggregates (AGGs). The resulting electrolyte supports a 4.0 V-class layered-oxide cathode across a wide temperature range (−40 °C to 60 °C), and achieves an average Na plating/stripping Coulombic efficiency of 99.89% (Figure 8b). These advances highlight the progression of electrolyte design from simple component selection toward precise solvation-structure engineering. Such LHCEs, however, carry nontrivial penalties: concentrated salts and fluorinated diluents increase cost and environmental burden; viscosity can impair wetting and low-temperature transport; reduced ionic conductivity increases polarization in thick electrodes; and limited free solvent can hinder complete utilization at high sulfur loading. Developing cost-effective and recyclability yet inherently safe formulations thus remains crucial for commercial adoption.

Figure 8. (a) Schematic diagram of Na–S batteries through different LHCE. Reproduced with permission [43], Copyright 2025, American Chemical Society; (b) Na plating/stripping Columbic efficiency (CE) of cycling performance in Na||Cu asymmetric cells at 1.0 mA cm⁻². Reproduced with permission [44], Copyright 2024, WILEY-VCH.

3.4. All-Solid-State Na–S Batteries: Suppressed Long-Range Shuttle, Persistent Interfacial Chemistry

All-solid-state Na–S batteries replace liquid-mediated long-range polysulfide transport with solid–solid or locally confined conversion. This architecture can suppress macroscopic dissolution and separator-crossing shuttle, but it does not eliminate sulfur intermediates, interfacial side reactions, or chemo-mechanical failure. Sulfide electrolytes provide high Na⁺ conductivity and intimate chemical affinity with sulfur, yet they are moisture-sensitive and can undergo redox decomposition; borohydride or halide-containing conductors may offer wider stability windows but introduce processing, pressure, or cost constraints. In all cases, reaction kinetics depend on a percolating three-phase network of active sulfur, ionic conductor, and electronic conductor.

Recent research has shifted toward all-solid-state Na–S batteries, based on the premise that solid electrolytes can effectively suppress the long-range dissolution and diffusion of polysulfides that characterize liquid electrolyte systems. However, this does not imply that sulfur intermediate formation or interfacial side reactions are entirely eliminated. Nguyen et al. [45] detected “clandestine polysulfides” in solid-state cells via operando scanning microbeam XRD, operando XPS, and ex situ XAS (Figure 9a). Within Na₃PS₄-type electrolytes, they identified various crystalline and amorphous polysulfides—including Na₂S₃ phases previously stabilized only under elevated temperature and pressure—that persist throughout cycling (Figure 9b). This finding challenges the conventional understanding of interfacial chemistry in solid-state batteries, demonstrating that sulfur conversion pathways persist even without liquid media, albeit constrained by local stress, interfacial contact, and ionic percolation. Moreover, their results also demonstrate that applied pressure is both a thermodynamic variable and a transport parameter: insufficient pressure causes contact loss and ionic isolation, whereas excessive pressure can alter phase selection, densify pathways, and aggravate local stress. In essence, while solid electrolytes act as physical barriers, they cannot prevent interfacial complications arising from sulfur-redox intermediates.

Figure 9. (a) Time-dependent evolution of stress profiles across the cross-section of the cell during charge and discharge; (b) Summary of reaction pathway during charge and discharge. Reproduced with permission [45], Copyright 2025, American Chemical Society; (c) Cycling performance of 5%SP-TiS₂//Na₅Sn ASSSBs at 1 C using NSS2 electrolyte exposed to 50% RH air for different times. The hollow circle represent the Coulombic efficiency. Reproduced with permission [46], Copyright 2024, WILEY-VCH.

Practical all-solid-state development must address cathode composite homogeneity, stack-pressure retention, interfacial resistance, electronic leakage, volume-change accommodation, and environmental stability during manufacturing. Addressing the moisture sensitivity of sulfide electrolytes, Guo et al. [46] designed a humid-air-tolerant Na_3−xSb_1−4x (SnWCaTi)_xS₄ phase through vacancy engineering and configurational-entropy enhancement. The corresponding solid-state cell retained stable cycling for 100 cycles at 1 C after 30 min exposure to air with a dew-point of 13.8 °C (Figure 9c), underscoring that environmental stability is as critical as electrochemical performance for the practical deployment of solid-state batteries. Vacancy and entropy engineering have improved humid-air tolerance of sulfide electrolytes, but performance should be validated after realistic dry-room exposure and in cells with industrially relevant areal capacity. Operando diffraction, XAS, Raman spectroscopy, acoustic/mechanical diagnostics, and cryogenic electron microscopy will be essential for correlating phase evolution with contact loss and fracture [47].

4. Conclusions and Outlook

Substantial progress in room-temperature sodium–sulfur (RT Na–S) batteries has been enabled by catalytic host design, Na-anode interfacial regulation, and solvation engineering. Single-atom catalysts, heterostructures, and defect-rich scaffolds suppress polysulfide shuttling and accelerate sulfur redox, while SEI regulation and tailored solvation improve Na stability. However, as summarized in Table 1, many record performances rely on low sulfur loading, excess electrolyte, high Na excess, and incomplete reporting, and the operando mechanisms of polysulfide conversion and interface evolution remain insufficiently resolved. The field must therefore shift from maximizing specific capacity toward coordinated control of sulfur chemistry, transport, finite Na inventory, interfacial mechanics, and safety.

4.1. Practical Benchmarks and Failure Modes

For the purpose of cross-study evaluation, we recommend that ‘practical’ RT Na–S cells be screened at sulfur loadings ≥ 5 mg cm⁻², E/S ratios ≤ 5 μL mg⁻¹, areal capacities ≥ 4 mAh cm⁻², and limited Na excess (preferably N/P ≤ 3), followed by validation in pouch or otherwise externally pressurized full cells. These values are not rigid standards, but they expose failure modes that thin, flooded coin cells conceal. At high loading, electrolyte depletion and tortuous transport generate through-thickness reaction gradients; at low E/S, salt concentration and viscosity change dynamically as sulfur species form; and at low N/P, even small parasitic currents rapidly exhaust the Na inventory.

Future studies should report the complete mass and capacity balance: sulfur areal loading, sulfur fraction, electrode density and thickness, electrolyte volume, separator thickness, Na foil thickness or N/P ratio, formation protocol, stack pressure, temperature, self-discharge, and cumulative plated capacity. Capacity retention should be accompanied by Coulombic efficiency with sufficient precision, impedance evolution, and post-mortem quantification of inactive sulfur and Na. Gradient electrodes—combining high electronic conductivity near the current collector, controlled catalytic/polar functionality through the reaction zone, and mechanically compliant interfaces—offer a promising route to reconcile transport and stress at high loading.

4.2. Safety, Manufacturability, and Outlook

Safety assessment must extend beyond electrolyte flammability. Metallic Na can react vigorously with trace moisture and leaked electrolyte; internal short circuits can couple dendritic heating with exothermic interphase reactions; and solvent decomposition or polysulfide reactions can generate gas and pressure. Practical studies should therefore include accelerating-rate calorimetry or differential scanning calorimetry of charged components, gas analysis, nail/overcharge and external-short tests where appropriate, leak and seal validation, and procedures for Na handling, module isolation, and end-of-life deactivation.

The most credible path forward is an integrated design loop linking computation, operando characterization, scalable processing, and full-cell validation. Cathode hosts must preserve percolation and mechanical integrity under lean electrolyte; electrolytes must balance solvation, conductivity, interphase chemistry, cost, and environmental impact; and Na anodes must operate with finite inventory over industrially relevant cumulative capacity. For all-solid-state cells, the priority is not simply higher ionic conductivity but stable three-phase contact under controlled pressure and ambient exposure. Only studies that align mechanistic claims with complete practical metrics can establish whether RT Na–S batteries can deliver competitive energy density, long life, and inherent safety.