Transition Metal-Based Catalysts Powering Practical Room-Temperature Na-S Batteries: From Advances to Further Perspectives

Abstract

Room-temperature sodium–sulfur (RT Na-S) batteries hold great potential in the field of large-scale energy storage due to their high theoretical energy density and low cost of raw materials. However, the inherent low conductivity, notorious shuttling, and sluggish kinetics of cathode materials cause the loss of active substances and capacity delay, hindering the practical application of RT Na-S batteries. Owing to their low cost, variable oxidation states, and unsaturated d orbitals, transition metal (TM)-based catalysts have been extensively studied in circumventing the above shortcomings. Herein, the review first elaborates on the reaction mechanisms and current challenges of RT Na-S batteries. Subsequently, the role and function mechanism of TM-based catalysts (including single/dual atoms, nanoparticles, compounds, and heterostructures) in RT Na-S batteries are described. Specifically, based on the theories of electronic transfer and atomic orbital hybridization, the interaction mechanism between TM-based catalysts and polysulfides, as well as the catalytic performance, are systematically discussed and summarized. Finally, a discussion on the challenges and future research perspectives associated with TM-based catalysts for RT Na-S batteries is provided.

1. Introduction

With the rapid advancement of the global energy transition, the demand for renewable energy sources (wind, geothermal, and solar) is steadily increasing. However, constrained by their geographical availability, intermittency, and instability, the development of energy storage technologies is crucial for storing these unstable energy sources [1,2]. Battery energy storage systems have emerged as critical grid-stabilizing solutions due to their rapid power regulation capabilities. Among them, lithium-ion batteries have achieved scalable deployment by leveraging fundamental advantages, including high energy density, long-term cycling stability, and a low capacity fade rate [3]. Furthermore, the implementation of electrospinning technology has substantially accelerated the industrial application of lithium-ion batteries through precise electrode microstructure engineering [4]. Although lithium-ion batteries have dominated the energy storage market for the past three decades, their poor energy density makes it more difficult for them to meet societal demands. Owing to their enormous capacity and low anode electrochemical potential, lithium metal batteries have recently attracted considerable interest from both academia and industry. Nevertheless, the scarcity and uneven distribution of lithium resources result in high battery costs, hindering the development and application of lithium metal batteries. Sodium, which is a member of the same periodic table group as lithium, is abundant and inexpensive, making sodium metal anodes a highly promising alternative to lithium metal anodes [5,6]. Coupling an abundant sulfur cathode, which exhibits a high theoretical specific capacity (1675 mAh/g) and low cost, with sodium constitutes sodium–sulfur (Na-S) batteries. These systems achieve a theoretical energy density of up to 1274 Wh/kg and are recognized as a highly promising next-generation technology for large-scale energy storage applications. High-temperature Na-S batteries, operating at 300–350 °C, were deployed in energy storage systems as early as the last century [7]. Nevertheless, sustaining such elevated temperatures necessitates substantial auxiliary energy input, imposing a significant economic burden. Moreover, under these conditions, the molten electrodes (anode and cathode) are partitioned solely by a solid-state β″-alumina electrolyte; fracture of this ceramic separator may permit direct electrode contact, presenting critical safety hazards including combustion or explosion risks [8,9]. Intermediate-temperature Na-S batteries (130–180 °C) similarly utilize β″-alumina solid-state electrolytes. The reduced operational temperature, however, markedly diminishes the ionic conductivity of sodium ions (Na⁺), thereby compromising overall electrochemical performance [10,11]. Given the dual constraints of cost and safety, the vigorous development of room-temperature sodium–sulfur (RT Na-S) batteries represents a critical imperative. Under ambient conditions, sulfur undergoes complete conversion to sodium sulfide (Na₂S), enabling the theoretical capacity to be achieved [12]. These inherent advantages highlight the necessity and significant promise of RT Na-S battery systems.

Since the utilization of polyvinylidene fluoride (PVDF)-based gel polymer electrolytes in 2006, research on RT Na-S batteries has advanced significantly [13]. During battery operation, the sulfur cathode undergoes a stepwise conversion process. The initially formed sodium polysulfide (NaPS) intermediates with varying chain lengths are ultimately reduced to sodium sulfide (Na₂S). Theoretically, the complete reduction of elemental sulfur to Na₂S can deliver a high theoretical specific capacity of 1675 mAh/g. However, this reaction system faces multiple challenges: First, long-chain NaPSs readily dissolve in ether-based electrolytes [14] or undergo nucleophilic side reactions in ester-based electrolytes [15], resulting in active material loss. Second, short-chain NaPSs (such as sodium persulfide, Na₂S₂) and the final product Na₂S exhibit extremely low solubility in electrolytes, typically depositing as poorly conductive solids. This significantly reduces reaction conversion kinetics, leading to battery performance degradation. Furthermore, elemental sulfur suffers from intrinsically poor conductivity, causing inefficient electron transfer between sulfur species during charge/discharge cycles, which severely limits sulfur utilization. These issues become particularly pronounced when the sulfur loading in the cathode exceeds 3.0 mg/cm², making stable battery operation challenging [16].

To address the aforementioned challenges and enhance sulfur loading in the cathode as well as energy density, numerous studies have employed conductive porous materials as hosts or separator modifiers [17,18]. These materials serve a dual purpose, enhancing the intrinsic electronic conductivity of the cathode (as sulfur hosts) or acting as secondary current collectors (as separator coatings); concurrently, they effectively suppress the uncontrolled diffusion and shuttle effect of NaPSs through physical confinement or chemical adsorption. For instance, utilizing porous carbon materials (e.g., graphene [19,20], carbon nanotubes [21], carbon nanofibers [22,23,24,25,26]) and porous organic materials (e.g., metal–organic frameworks [27], covalent organic frameworks [28,29], polyacrylonitrile [30]) as hosts or separator modifiers has improved the performance of RT Na-S batteries. However, porous materials can only partially mitigate, not fundamentally resolve, the issues of the NaPS shuttle effect and sluggish conversion kinetics between sulfides of different chain lengths. Drawing inspiration from the development path of lithium–sulfur batteries, researchers have gradually shifted their focus to catalysts for RT Na-S batteries, aiming to accelerate reaction conversion rates and reduce the residence time of NaPS intermediates in the electrolyte [31,32,33,34]. Compared with other catalysts, e.g., precious metal-based catalysts, transition metal (TM)-based catalysts stand out due to their abundance and low cost. In addition, TM-based catalysts exhibit variable oxidation states and partially filled d orbitals, enabling their exceptional performance in catalysis, materials science, energy storage, and related fields.

The classification and application of TM-based catalysts for RT Na-S batteries have been extensively reviewed [35,36,37]. For instance, synthesis strategies for TM-based catalysts were systematically categorized according to metal element species [35]. However, the fundamental understanding of their intrinsic nature (particularly the electronic structures and orbital interactions governing catalytic behavior) remains insufficiently deep and systematic. Herein, this review systematically elucidates the interaction mechanisms between various TM-based catalysts (e.g., metal single/dual atoms [38], metal nanoparticles, metal compounds, heterostructures [39]) and NaPSs, as illustrated in Figure 1. It conceptualizes the adsorption–conversion principle, major catalyst categories, and key performance-determining factors. Furthermore, by establishing structure–performance relationships between the micro-scale electronic/orbital characteristics and macro-scale electrochemical behavior, it provides a theoretical foundation for clarifying the electron transfer mechanisms as well as the “adsorption–catalysis” reaction pathway. Finally, further directions for the rational design of next-generation, high-efficiency, stable TM-based catalysts are highlighted, which are expected to promote the practical implementation of RT Na-S batteries.

2. Working Mechanisms, Challenges, and Design Principles of RT Na-S Batteries

2.1. Working Mechanisms

The redox reaction of the sulfur cathode involves multi-electron transfer processes, inevitably generating NaPS intermediates. Notably, NaPSs exhibit significant differences in dissolution behavior and chemical stability between ester-based and ether-based electrolytes [41]. Consequently, when these two types of electrolytes are applied to RT Na-S batteries, the electrochemical reaction mechanisms of the sulfur cathode also demonstrate marked distinctions.

In ether-based electrolytes, the solubility of NaPSs decreases dramatically with chain length shortening. During the charge/discharge of RT Na-S batteries, sulfur undergoes a multi-step “solid–liquid–solid” conversion pathway [42]. As illustrated in Figure 2a, in the initial discharge stage, S₈ combines with Na⁺ to form soluble Na₂S₈ (solid-to-liquid conversion), attributed to the voltage plateau at 2.30 V (Process (1) in Figure 2b). Subsequently, Na₂S₈ is further reduced to Na₂S₆ and Na₂S₄ (liquid-to-liquid conversion), with the voltage decreasing along a slope between 2.30 V and 1.60 V (Process (2) in Figure 2b). This stage involves multiple equilibrium conversion steps (Processes (3)–(5) in Figure 2b). Next, Na₂S₄ continues to be reduced to Na₂S₃, Na₂S₂, or Na₂S (liquid-to-solid conversion), forming a low-voltage plateau at 1.60 V (Processes (6)–(8) in Figure 2b). Finally, Na₂S₂ is reduced to the end product Na₂S (solid-to-solid conversion, Process (9) in Figure 2b). However, the poor conductivity of the discharge products (Na₂S₂ and Na₂S) results in relatively sluggish reduction kinetics, often leading to incomplete conversion. Synchronously, this causes a phenomenon during charging where the voltage rises rapidly while the capacity shows a negligible increase, namely polarization.

In ester-based electrolytes, during the initial cycles, S₈ undergoes Processes (1) and (3) shown in Figure 2b, generating long-chain NaPSs (Na₂S_x, 4 < x ≤ 8). However, long-chain NaPSs exhibit significant thermodynamic instability in ester solvents. Through density functional theory (DFT) calculation, the potential energy difference (ΔE) of the reaction between EC and Na₂S₆ was found to be negative. This indicates that the reaction is thermodynamically favorable and tends to proceed spontaneously because the energy of the reactants is greater than that of the products. It was discovered that EC and long-chain NaPSs undergo irreversible adverse reactions [45]. The disappearance of the high-voltage plateau (approximately 2.30 V) during subsequent discharge (Figure 2c) confirms that long-chain Na₂S_x cannot participate in further reversible conversion [47]. Moreover, within the voltage region below 2.0 V, the reaction corresponds to the stepwise conversion of long-chain NaPSs (which do not directly contact the electrolyte) into short-chain NaPSs (Na₂S_x, 2 ≤ x ≤ 4), ultimately yielding Na₂S. Notably, the ΔE between DEGDME and Na₂S₆ consistently exceeds 1 eV (Figure 2d), indicating negligible interaction between the ether solvent and NaPSs. Consequently, long-chain NaPSs exhibit higher solubility in ether-based electrolytes. By contrast, the significantly reduced solubility of NaPSs in ester-based electrolytes leads to sluggish sulfur electrode reaction kinetics and the concurrent occurrence of multiple reaction steps, ultimately manifesting as weakened plateau features in the charge–discharge curves.

Ester-based electrolytes (e.g., EC, PC, DEC, DMC) exhibit high dielectric constants and wide electrochemical stability windows, enabling stable operation at higher voltages. Additionally, their low solubility for NaPSs effectively suppresses NaPS dissolution and the shuttle effect, thereby extending cycle life [48]. By contrast, ether-based electrolytes (e.g., TEGDME, glymes, DEGDME) display high NaPS solubility, which facilitates the “solid–liquid–solid” conversion reaction, endowing it with superior rate capability.

Consequently, the reduction of sulfur cathodes generates long-chain NaPSs in both ether-based and ester-based electrolytes. For an ether-based system, suppressing NaPS shuttling requires cathode host design or separator modification. For ester-based systems, constructing stable interfaces is essential to prevent direct contact between NaPSs and the electrolyte, thereby avoiding irreversible side reactions. Notably, both electrolytes necessitate catalytic materials to accelerate the conversion kinetics of short-chain NaPSs (Na₂S₂/Na₂S) to achieve high specific capacity and long-term cycling stability.

2.2. Challenges and Design Principles

During practical cycling of RT Na-S batteries, the sulfur cathode faces multiple challenges (Figure 2e) that severely constrain its capacity utilization and cycling stability. Firstly, both the initial active material sulfur (S₈) and the discharged products (Na₂S and Na₂S₂) exhibit extremely low electronic and ionic conductivity, leading to impeded electron/ion transport and sluggish electrochemical reaction kinetics [49]. This not only reduces active material utilization but also causes significant polarization. Secondly, during the solid–liquid–solid phase transformation, sluggish kinetics prolong the residence time of NaPS intermediates in the electrolyte, enabling their diffusion to the anode side via concentration gradients. This results in irreversible active material loss, capacity decay, and reduced Coulombic efficiency. In addition, the conversion of sulfur to Na₂S involves substantial volume expansion, causing the cathode to endure repeated expansion–contraction stress cycles, ultimately leading to electrode structure pulverization [50]. Concurrently, continuous volume changes compromise the integrity of the cathode–electrolyte interphase (CEI) layer, exacerbating electrolyte infiltration and triggering NaPS dissolution and side reactions, which eventually cause battery failure.

Increasing the sulfur loading is crucial for enhancing the energy density of RT Na-S batteries, but the thick electrode structure introduces more severe challenges. Under high sulfur loading, increased sulfur particle stacking density disrupts the conductive network, hindering electron transport pathways. Synchronously, thick electrodes significantly extend Na⁺ diffusion paths, intensifying concentration polarization and reducing active material utilization. Additionally, sluggish solid–liquid–solid phase transformation kinetics prolong NaPS residence time, and under high-loading conditions, the shuttle effect intensifies, leading to rapid battery failure.

To address challenges such as the shuttle effect and volume expansion, researchers have pursued multi-dimensional material and structural optimizations. On the one hand, constructing three-dimensional porous conductive scaffolds or introducing polar host materials effectively enhances sulfur loading and suppresses NaPS dissolution and shuttling while mitigating volume changes during cycling [51]. On the other hand, introducing interlayers between the cathode and separator or modifying the separator surface creates physical/chemical barriers that effectively adsorb or block NaPS migration, significantly suppressing the shuttle effect [52]. These approaches have improved sulfur utilization and reduced active material loss to some extent, thereby enhancing electrochemical performance.

However, although the aforementioned strategies based on physical confinement and chemical adsorption can partially alleviate NaPS diffusion and shuttling, they are fundamentally passive restriction mechanisms. They struggle to achieve long-term, effective immobilization of high-concentration, soluble NaPS intermediates under high sulfur loading and fail to fundamentally resolve the core issue of slow NaPS conversion. By contrast, introducing highly efficient catalytic materials (e.g., metals and their compounds) demonstrates significant advantages [53]. These catalysts can not only anchor NaPSs via chemical adsorption but also lower the reaction energy barrier for converting NaPSs into the final discharge products [54], fundamentally accelerating the kinetics for NaPS conversions, improving sulfur utilization, and consequently enhancing battery cycling stability and capacity (Figure 2f). The classification and mechanistic roles of catalysts will be comprehensively reviewed and discussed in the next section.

3. Classifications and Function Mechanisms of Catalytic Materials

Currently, the catalysts applied in RT Na-S batteries are predominantly transition metal-based materials. Leveraging their advantages, including tunable electronic structures (d-orbital characteristics), multivalent flexibility, and strong interfacial interactions, these catalysts can be categorized into four main types: metal single/dual atoms, metal nanoparticles, metal compounds, and heterostructure materials. As summarized in

Table 1. Summary of mechanisms and performance of different TM-based catalysts.

Category	Catalytic Materials	Catalyst Mechanism	Catalyst Content	Electrolyte	Cycling Stability (Sulfur Loading)	Ref.
TM single/dual atoms	Mn/NC	Strong Lewis acid–base interaction promotes the direct conversion of Na2S4 to Na2S	\	1 M NaFSI in TEGDME with 1wt% NaNO3	720 mAh g−1@500th cycle@500 mA g−1 (1.8–2.5 mg cm−2)	[55]
	Con-HC	Polarity–polarity interaction	28%	1 M NaClO4 in PC/EC (1:1 Vol%) with 5 wt% FEC	508 mAh g−1@600th cycle@100 mA g−1	[56]
	Fe1-NMC	Electron transfer	17.5%	1 M NaClO4 in PC/EC (1:1 Vol%) with 5 wt% FEC	540 mAh g−1@500th cycle@0.1 A g−1	[57]
	Fe/NC/700	Metallic bonding enhances electron transfer	28%	1 M NaClO4 in PC/EC (1:1 Vol%)	325 mAh g−1@5000th cycle@10 A g−1 (1.5–2 mg cm−2)	[58]
	Cu SA/NOC-2	Coordination activates S8 cleavage into short-chain sulfur	23%	1 M NaSO3CF3 in DEGDME	776 mAh g−1@100th cycle@0.1 A g−1 (2.0–2.5 mg cm−2)	[59]
	VCo DACs/N-MXene	Electron spillover effect	23%	1 M NaSO3CF3 in DEGDME	1199.3 mAh g−1@100th cycle@0.2C (0.8–1.2 mg cm−2)	[60]
	Fe-Co/NC	Iron atoms induce the delocalization of Co electrons, altering the electron spin state	32%	1.0 M NaClO4 in EC/DMC (1:1 Vol%) with 5.0% FEC	379 mAh g−1@2000th cycle@1C (1 mg cm−2)	[61]
	Co-N2O2/MOFc	Strong d-p hybridization for Co single-atom d-electron density regulation	23%	1 M NaSO3CF3 in DEGDME	425 mAh g−1@1000th cycle@1 A g−1 (1.0–1.2 mg cm−2)	[62]
	Mn-N-C	Strong p-d hybridization	32%	1 M NaClO4 in PC/EC (1:1 Vol%) with 5 wt% FEC	888 mAh g−1@200th cycle@0.2 A g−1 (1.6 mg cm−2)	[63]
	Fe-N1	Unsaturated coordination	35%	2 M NaTFSI in PC/FEC (1:1 Vol%)	647.8 mAh g−1@200th cycle@167.5 mA g−1	[64]
	Zn-N2/CF	Asymmetric electron distribution	\	1 M NaClO4 in EC/DEC (1:1 Vol%) with 5% FEC	317.4 mAh g−1@100th cycle@0.1 A g−1 (1.0 mg cm−2)	[40]
	Zn-N2@NG	Unsaturated coordination	14%	1 M NaClO4 in EC/DEC (1:1 Vol%) with 5% FEC	414 mAh g−1@400th cycle@0.6 A g−1 (1.0 mg cm−2)	[65]
	Ca-O4N-C	Axial-N-ligand-induced Ca site charge localization promotes p-p orbital hybridization	23%	2 M NaTFSI in PC/FEC (1:1 Vol%)	887 mAh g−1@800th cycle@3C (1 mg cm−2)	[66]
	NHC-InN5 SACs	High s-p orbital overlap	23%	1 M NaClO4 in PC/EC (1:1 Vol%) with 5 wt% FEC	384.9 mAh g−1@800th cycle@1 A g−1 (1.5 mg cm−2)	[67]
	Co, N-MPC-10%	Polar interaction	32%	1 M NaClO4 in PC/EC (1:1 Vol%) with 3 wt% FEC	1275.01 mAh g−1@100th cycle@0.1C (1.3 mg cm−2)	[68]
	3D-PNCV	Promote electron transfer	35%	1 M NaTFSI in PC/FEC (1:1 Vol%)	991 mAh g−1@100th cycle@0.2 A g−1	[69]
	Mn-N2/CNs	Regulating the coordination number of Mn single atoms enhances d-p orbital hybridization	23%	2 M NaTFSI in PC/FEC (1:1 Vol%)	926 mAh g−1@100th cycle@0.1C (0.9 mg cm−2)	[70]
	Zn-N3O/HCs	N/O coordination to generate localized electrons at the single-atom center of Zn to enhance d-p hybridization	23%	2 M NaTFSI in PC/FEC (1:1 Vol%)	1155 mAh g−1@100th cycle@0.1C (0.8–1.0 mg cm−2)	[71]
	SA Fe-N/S@CNF	Regulation of the coordination structure of Fe-N4S2	\	1 M NaClO4 in EC/DEC (1:1 Vol%) with 5 wt% FEC	595 mAh g−1@500th cycle@1 A g−1 (4 mg cm−2)	[72]
	Mn1-PNC	Electron transfer	28%	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	344.1 mAh g−1@3000th cycle@2 A g−1 (2.6 mg cm−2)	[73]
	SA Co-N/S	Strong interaction between Co site and sulfur	\	2 M NaTFSI in PC/FEC (1:1 Vol%)	2.54 mAh cm−2@70th@0.5 A g−1 (4.2 mg cm−2)	[74]
TM nanoparticles	Fe1.88C0.12@CNTs	Strong d-p orbital hybridization based on carbon vacancies and iron defects forms Fe-S bonds	\	1 M NaClO4 in TEGDME with 2% NaNO3	486 mAh g−1@1000th cycle@1 A g−1 (0.8–1.4 mg cm−2)	[75]
	Co-NMCN	The generation of Co-S bonds	35%	1 M NaClO4 in PC/EC (1:1 Vol%) with 5 wt% FEC	529.6 mAh g−1@100th cycle@0.5C (3 mg cm−2)	[76]
	CoNC	The conversion of Co nanoparticles and CoN4 sites into CoS	35%	a molar ratio of NaFSI/DME/BTFE = 1:1:0.8	530 mAh g−1@150th cycle@80 mA g−1 (0.6–0.7 mg cm−2)	[77]
	CNTs/Co@NC-0.25	Catalysis based on the Co nanoparticles	\	1 M NaClO4 in EC/DEC (1:1 Vol%) with 2 wt% FEC	634.6 mAh g−1@100th cycle@0.1C (0.7–0.9 mg cm−2)	[54]
	Co@NPCNFs	Co nanoparticles enable rapid sodium intercalation and reduction of NaPSs	\	1 M NaClO4 in EC/DEC (1:1 Vol%)	411 mAh g−1@800th cycle@1C (1 mg cm−2)	[78]
	FeNi3@HC	Alloying regulates the electronic structure	35%	2 M NaClO4 in PC/FEC (1:1 Vol%)	862 mAh g−1@100th cycle@0.2 A g−1	[79]
	Ni/Co-C-12	Adjusting the Ni/Co ratio delays the conversion of Na2S to NaPSs and accelerates the conversion of NaPSs to sulfur	40%	1 M NaClO4 in TEGDME	813.5 mAh g−1@200th cycle@0.5C	[80]
	Co@BNC	The formation of electron defect centers in Co nanoparticles based on B doping	31.5%	1 M NaClO4 in PC/EC (1:1 Vol%) with 3 wt% FEC	416 mAh g−1@600th cycle@0.5C (0.8–1 mg cm−2)	[81]
	MG-Co	High-conductivity MXene in synergy with Co nanoparticles for catalysis	40%	1.0 M NaPF6 in DOL/DIGLYME (1:1 Vol%)	360 mAh g−1@200th cycle@0.5C	[51]
	W@N-G	The reduction of Na2S4 is promoted by embedding W nanoparticles	7.28%	1 M NaClO4 in EC/PC (1:1 Vol%) with 3 wt% FEC	398 mAh g−1@1000th cycle@1C (1 mg cm−2)	[82]
TM compounds	NOC@MoS2	Strong Lewis acid–base interactions	35%	1 M NaCF3SO3 in DEGDME	576 mAh g−1@100th cycle@0.1 A g−1 (0.9–1.5 mg cm−2)	[83]
	YS-Fe2N@NC	N-doped carbon encapsulating Fe2N to promote the transfer of electrons from NaPSs to N in Fe2N	56%	1 M NaClO4 in DEGDME	467 mAh g−1@350th cycle@2C (3.1 mg cm−2)	[84]
	MnO@NACM	The conductive network of carbon nanorods works in synergy with MnO for adsorption and catalysis	44%	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	234 mAh g−1@1000th cycle@2 A g−1 (0.7–0.9 mg cm−2)	[85]
	NiSe-C	Regulating the electronic state of the d/p orbital at the NiSe catalytic site	35%	2 M NaTFSI in PC/FEC (1:1 Vol%)	401.4 mAh g−1@1000th cycle@2 A g−1	[86]
	HPC/Mo2C	Strong adsorption and high catalysis of highly conductive Mo2C	38.5%	2 M NaTFSI in PC/FEC (1:1 Vol%)	1098 mAh g−1@120th cycle@0.2 A g−1	[87]
	Mo5N6	Mo5N6 based on high d bands	5%	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	186 mAh g−1@10000th cycle@1675 mA g−1 (1.2 mg cm−2)	[88]
	2D/3D Co4N-NC@CC	The abundant adsorption and catalytic active sites of 2D/3D CoN4 accelerate charge transfer	\	1 M NaClO4 in EC/PC (1:1 Vol%) with 2M NaNO3	592 mAh g−1@1000th cycle@1.0C (1.0 mg cm−2)	[89]
	Fe3N-NMCN	The Na-N and Fe-S bonds of Fe3N and NaPSs	\	1 M NaPF6 in DOL/DIGLYME (1:1 Vol%)	696 mAh g−1@2800th cycle@8375 mA g−1	[90]
	NCCS	Enhance conductivity	50%	1 M NaClO4 in EC/DEC (1:1 Vol%)	470.3 mAh g−1@500th cycle@1C	[91]
	Nb2O5-CNR	Excellent sodium affinity and sulfur affinity	40%	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	617 mAh g−1@600th cycle@0.5 A g−1	[92]
	GeOx/NC	The O vacancy provides highly active sites	\	1 M NaSO3CF3 in DEGDME	385 mAh g−1@1200th cycle@1 A g−1 (1 mg cm−2)	[93]
	PC/CeO2-x	The O vacancy leads to a reduction in the CeO2-x band gap, increasing the electrical conductivity	35%	2 M NaTFSI in PC/FEC (1:1 Vol%)	906 mAh g−1@200th cycle@0.5 A g−1	[94]
	TiO2@SPC	Valence regulation forms defect sites	35%	2 M NaTFSI in EC/PEC	400 mAh g−1@400th cycle@2A g−1	[95]
	MoTe2	Multiple active sites	8%	\	498 mAh g−1@500th cycle@1C	[96]
	Ti3C2Tx/Ni(OH)2/C	Synergistic effect of physical constraints and MXene adsorption as well as Ni(OH)2 catalysis	28%	1 M NaSO3CF3 in DIGLYME	1144.7 mAh g−1@100th cycle@0.22C	[97]
TM heterostructures	Co3O4-NC@C3N4	Unpaired Co 3d electrons promote electron transfer	24%	1 M NaClO4 in EC/DEC (1:1 Vol%) with 5 wt% FEC	737.2 mAh g−1@1000th cycle@1C (1.5 mg cm−2)	[98]
	Co1-CoS2/NC	CoS2/Co1 dual-terminal electron donor–catalytic synergistic effect	28%	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	642 mAh g−1@150th cycle@0.2 A g−1 (1.6 mg cm−2)	[99]
	P-Fe2O3@Fe-PPy	Series-segmented catalysis	\	1 M NaFSI in TEGDME with 1wt% NaNO3	674 mAh g−1@500th cycle@500 mA g−1 (1.8–2.5 mg cm−1)	[100]
	ZnS-NC@Ni-N4	Series-segmented catalysis	35%	1 M NaClO4 in EC/DEC (1:1 Vol%) with 5 wt% FEC	725 mAh g−1@2000th cycle@1 A g−1 (1.0 mg cm−1)	[101]
	HCS@Ni-MnO2	Heterostructure interface rich in micro/mesoporous hollow structures	35%	1 M NaCF3SO3 in DEGDME	586.8 mAh g−1@1000th cycle@5 A g−1 (1.2–1.4 mg cm−1)	[102]
	MoC-W2C-MCNF	The MoC-W2C heterointerface accelerates the quasi-solid-state conversion of electron/ion transport	\	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	640 mAh g−1@500th cycle@0.2 A g−1 (1.5 mg cm−2)	[103]
	FCNT@Co3C-Co	Strong adsorption and catalysis based on Lewis acid sites of Co3C and Co	\	1 M NaClO4 in EC/DMC (6:4 Vol%)	448 mAh g−1@500th cycle@2C (3.2 mg cm−2)	[104]
	MoN-W2N@PC	The Mo2N-W2N heterojunction	35%	2 M NaTFSI in PC/FEC (1:1 Vol%)	517 mAh g−1@400th cycle@1 A g−1 (0.9 mg cm−2)	[105]
	TiN-TiO2@MCCFs	The synergistic effect of TiN catalysis and TiO2 adsorption	\	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	640 mAh g−1@100th cycle@0.1 A g−1 (1.08 mg cm−2)	[106]
	Ni-B@GO	Catalysis of the interface between amorphous Ni-B and crystalline Ni-Sx	16%	1 M NaTFSI in PC/FEC	470 mAh g−1@1000th cycle@2 A g−1	[107]
	MoS2-Mo1/SGF	Atomic-level dual active site delocalization of electrons optimizes the Mo electronic structure	7%	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	505 mAh g−1@1000th cycle@0.1 A g−1 (0.96 mg cm−2)	[108]
	MoS2/MoSAC/CF	The regulation of the electronic structure at the center of the Mod band promotes the hybridization of the d-p orbitals	35%	1 M NaClO4 in EC/PC (1:1 Vol%) with 5 wt% FEC	441.38 mAh g−1@100th cycle@1 A g−1 (1.0 mg cm−2)	[44]
	Co-S-C@MC	The heterogeneous interface Co-S-C regulates the electronic structure and enhances the center of the Co d band	32%	1 M NaTFSI in TEGDME/FEC (1:1 Vol%)	1215 mAh g−1@500th cycle@0.1C (1.0 mg cm−2)	[109]
	CoS2-CoSe2@CNFs	Strong adsorption of CoS2 and high catalysis of CoSe2	\	2 M NaTFSI in PC/FEC (1:1 Vol%)	749 mAh g−1@200th cycle@1 A g−1	[110]

The “@” symbol denotes “at”. For example, “720 mAh g⁻¹@500th cycle@500 mA g⁻¹” means a capacity of 720 mAh g⁻¹ at the 500th cycle under a current density of 500 mA g⁻¹.

, these materials effectively lower the energy barriers of sulfur redox reactions and accelerate reaction kinetics through diverse function mechanisms, such as electron transfer driven by strong Lewis acid–base interactions, formation of metal–sulfur chemical bonds, and regulation of reaction pathways. As directly demonstrated in Figure 3, single/dual atoms exhibit notable cycling stability at low current densities, while heterostructure catalysts display superior long-term cycling stability under high-current-density conditions.

3.1. TM Single/Dual-Atom Catalysts

TM single/dual-atom catalysts, leveraging their unique atomically dispersed characteristics and tunable electronic structures, offer novel solutions to address NaPS shuttling and sluggish redox kinetics. However, their inherent high surface energy endows them with agglomeration, necessitating the introduction of electron-rich heteroatoms (e.g., N, P, O, S) to construct Lewis base sites. These sites promote the formation of stable M-N coordination bonds with Lewis acidic metal centers (M), enabling robust anchoring within carbon matrices [111,112]. Such coordination not only significantly enhances metal atom stability but, more critically, allows precise modulation of the electronic structure of central metals by tailoring the coordination microenvironment of M-N_x sites (including ligand types, coordination number, and spatial configuration). This optimization ultimately governs their interaction mechanisms with NaPSs.

3.1.1. TM Single-Atom Catalysts

TM single-atom catalysts (SACs) establish strong interactions with NaPSs through isolated metal sites (e.g., Mn, Fe, Co). This interaction not only physically anchors NaPSs to suppress their dissolution in electrolytes but also reduces reaction energy barriers. More critically, electron transfer from the TM single-atom center to sulfur species alters the electronic state of sulfur. This charge transfer process fundamentally arises from electron redistribution between atoms/molecules, originating directly from orbital hybridization that modifies electron cloud density and energy states—a quantum process where energetically proximate valence atomic orbitals (e.g., s, p, d) reconfigure into degenerate hybrid orbitals with new spatial orientations. This orbital reorganization occurs during molecular formation, driven by interatomic interactions that mix disparate orbital types into isoenergetic hybrid sets. Hybrid orbitals affect the adsorption of polysulfides by altering the electron occupation state of the eg orbitals. Meanwhile, they lower the dissociation energy barrier of the S-S bond and enhance the M-S chemical bond, jointly reducing the system energy and accelerating the kinetics of sulfur redox. The degree of orbital hybridization dictates charge transfer intensity: stronger hybridization enhances orbital overlap and energy-level matching, leading to more pronounced charge transfer, and vice versa. As a key regulatory mechanism, the coordination environment directly influences hybridization patterns by modulating the electronic structure and orbital energy levels of central atoms, thereby enabling precise control over charge transfer intensity. Specifically, coordination environment modulation can be achieved through the following four mechanisms.

Regulation of central metal atoms: In RT Na-S batteries, SACs potentiate the adsorption–catalysis synergy of sulfur species by modulating orbital hybridization between metal centers and NaPSs. The electronic structure of metal sites (e.g., d-band center position, occupied state density) directly governs interfacial electron interactions with NaPSs. Tian et al. [63] systematically demonstrated that Mn-N₄ sites, with abundant unoccupied antibonding orbitals, significantly enhance d-p orbital hybridization, thereby improving electrochemical performance. Lei et al. [73] further revealed that Mn single-atom sites exhibit d-band states closer to the Fermi level (indicating fewer electrons in antibonding orbitals) and the lowest d-band center, which boosts electron supply capacity to cleave S-S bonds and accelerate short-chain NaPS/Na₂S formation. Their strong electron-trapping capability also facilitates reversible Na₂S conversion, ultimately enhancing cycling stability. To efficiently screen ideal SACs for high-energy-density Na-S batteries, Bai et al. [74] innovatively integrated natural language processing (NLP, Figure 4a) with catalytic descriptors (the Gibbs free energy of capacity-determining step and d-band center of absorption-determining ability, Figure 4b). This strategy employs NLP for precise central-atom screening, constructs binary descriptors to optimize candidate materials, and ultimately identifies cobalt SACs anchored by nitrogen/sulfur (SA Co-N/S). This approach pioneers new pathways for catalyst selection and provides novel insights into interfacial catalytic kinetics in Na-S systems.

Regulation of the coordination number: In single-atom catalyst studies, metal centers are typically anchored to carbon substrates via coordination with four nitrogen atoms (M-N₄). While this symmetric coordination ensures uniform electron distribution, it constrains catalytic activity. Precise modulation of the coordination number emerges as an effective strategy to overcome this limitation: reduced coordination induces asymmetric structures that enhance charge localization and adsorption strength. Yao et al. [40] demonstrated that Zn-N₂ sites accelerate sulfur conversion and lower Na₂S decomposition barriers, enabling the Zn-N₂/CF/S cathode to achieve a minimal capacity decay rate of 0.006% per cycle after 4000 cycles at 10 A g⁻¹. Song et al. [64] established a dual-descriptor model: the geometric descriptor γ (l_Na-S/l_Fe-N) negatively correlates with adsorption strength, while the electronic descriptor φ (e_g/t_2g) positively correlates with Na₂S decomposition barriers. The Fe-N₁ configuration with minimal γ (0.824) and maximal φ (0.664) exhibits optimal catalytic activity. Li et al. [70] confirmed Mn-S bond formation in Mn-N₂ via Mn 2p XPS (Figure 4c), where enhanced d-p hybridization improves S₈ chemisorption energy, delivering a faster response (114 s) and higher peak current (0.104 mA) of Na₂S precipitation (Figure 4d).

Notably, p-block metals are conventionally considered catalytically inert due to filled d orbitals, but combining with electron-rich atoms can induce activity [113]. Guo et al. [114] engineered In-N₃ sites (ε_d = −5.256 eV) with elevated d-band centers versus In-N₄ (ε_d = −5.427 eV), raising p-d hybridized antibonding orbital levels and reducing bonding energy, thereby lowering NaPS reaction barriers. When steric hindrance impedes metal–sulfur hybridization, alkali metal cations’ (Li⁺/Na⁺) s orbitals interact with catalyst d/p orbitals, altering reaction pathways [115]. Wu et al. [67] introduced a fifth axial N atom to planar InN₄, forming InN₅ configurations that strengthen s-orbital overlap with Na, significantly enhancing charge transfer.

Doping of Coordination Atoms: While reducing the coordination number enhances active site exposure, it may compromise metal center stability. Introducing heteroligands (e.g., O, S) provides an effective alternative for electronic structure modulation. Xiao et al. [59] coordinated Cu atoms with two N and two O atoms, forming Cu sites that weaken S-S bonds in cyclic S₈ molecules, catalyzing the conversion of elemental sulfur to short-chain S₂₋₄ within pores and facilitating mutual transformation between short-chain sulfur species and Na₂S. Zheng et al. [71] synthesized asymmetrically coordinated Zn single-atom sites via template sacrifice (Figure 4e). Compared to symmetric Zn-N₄ sites, Zn-N₃O exhibits superior catalytic activity for NaPSs, maintaining 1155 mAh g⁻¹ reversible capacity after 100 cycles at 0.1C. UV-vis spectra show weakened absorption peaks and clarified Na₂S₆ solution (Figure 4f), confirming strong NaPS adsorption attributed to N/O ligands, inducing localized charges at Zn centers. This triggers intense Zn-S d-p orbital hybridization, forming bonding orbitals below the Fermi level (E_F) and antibonding orbitals above E_F (Figure 4g). Such electronic restructuring elongates Na-S bonds in adsorbed Na₂S_x by 0.18 Å, reducing reaction barriers during charge/discharge and accelerating sulfur conversion kinetics. Similarly, Hu et al. [62] derived N/O dual-coordinated Co single-atom catalysts from MOF precursors. Oxygen incorporation modulates Co d-orbital electron density, strengthening Co-Na₂S_x d-p hybridization. This significantly lowers the Gibbs free energy change (ΔG) of the rate-limiting step (Na₂S₄→Na₂S₂) and reduces Na₂S decomposition barriers by 0.17 eV, demonstrating that enhanced d-p hybridization effectively accelerates sulfur redox kinetics.

Spatial configuration engineering: Spatial configuration engineering serves as a critical strategy for optimizing the electronic structure of metal single-atom catalysts. Beyond modulating the primary coordination shell, secondary coordination shell design proves equally essential. Bai et al. [72] engineered a single-atom catalyst (SA Fe-N/S@CNF) anchored on 3D interconnected carbon nanofibers, featuring an Fe-N₄S₂ active center where two S atoms reside in the secondary coordination shell—bonded to primary-shell N atoms without direct Fe coordination. These secondary-shell S atoms regulate the electronic structure of Fe via d-p orbital hybridization, elevating the Fe d-band center by Δα toward the Fermi level (Figure 4h). This shift raises antibonding orbital energies above the Fermi level, triggering electron back-donation to bonding orbitals and consequently enhancing NaPS binding and catalytic capability. Furthermore, introducing axial ligands overcomes planar coordination constraints to optimize metal centers. Zheng et al. [66] incorporated an axial N ligand to planar Ca-O₄ sites, inducing charge localization that strengthens p-p hybridization between Ca 3p and S 3p orbitals in NaPSs. The higher adsorption energy of NaPSs on Ca-O₄N-C-SAC versus Ca-O₅-C-SAC originates from Ca-S bond formation, which effectively traps NaPS intermediates, suppresses shuttling, and boosts both NaPS affinity and conversion kinetics.

3.1.2. Dual-Atom Catalysts

Although single-atom catalysts demonstrate distinct advantages in enhancing NaPS adsorption, suppressing shuttle effects, and improving conversion kinetics through precise coordination environment modulation, their catalytic performance remains constrained by the inherent limitation of single active sites in simultaneously achieving strong adsorption and efficient catalytic conversion of NaPSs. To overcome this fundamental constraint, researchers have shifted focus to bimetallic dual-atom catalysts (BDACs). These diatomic sites establish bifunctional active centers via electronic coupling and geometric synergy between adjacent metal atoms, thus providing expanded atomic-level design flexibility for synergistic optimization of adsorption–catalysis performance toward NaPSs.

Li et al. [61] developed an Fe-Co heteronuclear dual-atom catalyst embedded in nitrogen-doped hollow carbon nanospheres (Fe-Co/NC). Aberration-corrected HAADF-STEM characterization (Figure 4i) confirmed the existence of Fe-Co atomic pairs. Compared to the monometallic systems, the Fe-Co/NC cathode has a lower concentration (43%), showing a lower polarization voltage and a larger redox peak current (Figure 4j), indicating a significantly enhanced sulfur conversion kinetics at the diatomic position. During the sulfur reduction process, the Fe-Co/NC cathode exhibits the strongest adsorption of NaPSs, and the change in the free energy of the rate-determining step (Na₂S₄→Na₂S) is also the smallest (Figure 4k), further demonstrating that the conversion of NaPSs is accelerated. Moreover, Fe-Co/NC demonstrates a remarkably low Na₂S decomposition energy barrier of 0.89 eV (Figure 4l), confirming superior catalytic efficiency for Na₂S reduction. Mechanistic studies reveal that Fe introduction induces Co 3d-Fe 3d orbital coupling, narrowing the energy gap between Co d_xy and d_z² orbitals by Δo. This promotes electron transfer to e_g orbitals, shifting Co(II) from a low-spin to a high-spin state due to limited e_g occupancy and reduced d_z² occupation. Consequently, enhanced hybridization between Co d_z² orbitals and antibonding π* orbitals of sulfur in NaPSs occurs, substantially boosting catalytic performance.

3.2. TM Nanoparticle Catalysts

Different from atomically dispersed single/dual-atom catalysts, metal nanoparticles (e.g., Fe [116], Co [78,117], Ni [118]) comprise aggregates of tens to thousands of atoms. These nanoparticle catalysts effectively reduce energy barriers for sulfur redox reactions, enhance NaPS interactions, and accelerate conversion kinetics. During catalytic processes in RT Na-S batteries, electron transfer between metal surfaces and adsorbed NaPSs induces localized charge polarization at metal–sulfur interfaces. This charge redistribution provides the driving force for metal–sulfur bond formation. The anchoring and catalytic capabilities of metal nanoparticles toward NaPSs are governed by two critical factors: (1) support structures dictate catalyst–NaPS contact efficiency and mass transport, and (2) intrinsic activity, determined by electronic structure, is primarily regulated by the charge state of metal centers. Strategic approaches, including doping engineering, defect engineering, and alloying, enable precise modulation of nanoparticle electronic structures, thereby optimizing metal charge states for enhanced performance.

3.2.1. Regulation of Catalyst Carriers

The structure of catalyst carriers plays dual roles in regulating NaPS conversion: physically confining NaPS diffusion through adsorption while enhancing active site utilization by optimizing catalyst dispersion. Precise support design significantly boosts catalytic performance: Du et al. [78] embedded cobalt nanoparticles in N-doped porous carbon nanofibers (Co@NPCNFs), where the structure facilitates electron transfer between Co and Na₂S_x, accelerating NaPS–sodium-ion binding. This enables the Co@NPCNFs/S cathode to achieve an ultralow capacity decay rate of 0.038% per cycle over 800 cycles at 0.1C. Yang et al. [51] developed a 3D porous MG-Co aerogel (Figure 5a) integrating MXene nanosheets, reduced graphene oxide (rGO), and Co nanoparticles. Compared to MXene, it exhibits a larger pore volume (0.32 cm³ g⁻¹) and higher specific surface area (143 m² g⁻¹), significantly enhancing Co utilization. The MG-Co@S cathode maintains 360 mAh g⁻¹ after 200 cycles at 0.5C (Figure 5b). Electrochemical tests reveal that symmetric cells with MG-Co electrodes in Na₂S₆ electrolyte show a higher current response and larger CV-integrated area (Figure 5c), confirming that Co has a significant catalytic effect on the redox reactions of sulfur species. EIS analysis demonstrates a reduction in charge transfer resistance (R_ct) for MG-Co versus MG (Figure 5d), indicating optimized interfacial kinetics. This enhancement facilitates more efficient sulfur conversion, reduces polarization, and ultimately improves the energy conversion efficiency and reaction reversibility of sodium–sulfur batteries. XPS verifies Ti-S (458.6 eV and 464.5 eV) and Co-S (785.4 eV and 802.1 eV) bond formations, strengthening chemical adsorption of NaPSs.

3.2.2. Doping Engineering

The electronic structure modulation of metal nanoparticles via doping engineering can effectively optimize their catalytic performance. Tang et al. [81] employed a boron (B) doping strategy to reconstruct the electronic structure of cobalt nanoparticles, successfully inducing the formation of electron-deficient Co sites (Figure 5e). DFT calculations revealed that the Bader charge of Co in Co@BNC reached 1.84, significantly higher than that of undoped Co@NC (1.52), theoretically confirming the formation of electron-deficient Co centers. XPS characterization further elucidated the charge transfer mechanism: Upon Na₂S₆ adsorption, the Co 2p₃/₂ binding energy of Co@BNC shifted by 0.6 eV toward lower energy, indicating electron transfer from Na₂S₆ to the electron-deficient Co sites. Meanwhile, the emergence of a new Co-S bond peak at 777.4 eV (with a 1.8 eV increase in bond energy) further verified the formation of strong interfacial chemical interactions (Figure 5f).

3.2.3. Defect Engineering

Defect engineering optimizes the electronic distribution of active sites by constructing unsaturated bonds in the carbon skeleton to induce localized charge enrichment. Yang et al. [75] fabricated Fe₁.₈₈C₀.₁₂@CNTs catalysts (Figure 5g) that significantly enhanced d-p orbital hybridization through precisely controlled Fe defects, facilitating Fe-S bond formation and accelerating reaction kinetics. The cathode material maintained a reversible capacity of 802.93 mAh g⁻¹ after 200 cycles at 0.2C. A solid–liquid conversion pathway of Na₃FeS₃↔Na₆Fe₁.₈₈S₆ was confirmed by Raman spectroscopy (Figure 5h), which substantially shortened the reaction pathway compared to conventional conversion mechanisms.

3.2.4. Alloying

The alloying strategy effectively modulates the electronic structure of active sites through electron transfer between heterometallic elements. Wang et al. [79] employed an Fe-Ni alloying approach, where the electronegativity difference (Fe < Ni) induced electron transfer from Fe to Ni, achieving dual functional regulation: (1) the increased electron density in Ni 3d orbitals enhanced charge transfer efficiency, while (2) the decreased electron density in Fe 3d orbitals improved its capability to accept electrons from NaPSs. Enhanced characteristic peak intensities, which indicated efficient electron transfer from Ni to S species, were significantly revealed by Ni L-edge X-ray absorption near-edge structure (XANES) spectroscopy (Figure 5i). This electronic structure modulation endowed the material with an exceptional precipitation capacity of 166 mAh g⁻¹ (Figure 5j), much higher than that of Ni@HC (129 mAh g⁻¹).

3.3. TM Compound Catalysts

Despite the high activity of single-atom metal sites, they tend to aggregate and deactivate due to excessive surface energy. Meanwhile, metal nanoparticles suffer from low atomic utilization efficiency and uncontrolled sulfidation/oxidation in sulfur-containing environments. By contrast, metal compounds (e.g., sulfides [36,119], oxides [85,94], carbides [120,121]) exhibit superior stability, enabling precise charge state modulation through multi-dimensional strategies including crystal phase engineering, morphological control, defect construction, and composition optimization. This comprehensive modulation not only significantly enhances the chemisorption capability toward NaPSs but also dramatically improves the reaction kinetics of NaPS conversion, thereby comprehensively boosting the electrochemical performance of RT Na-S batteries.

3.3.1. Crystal Phase Engineering

Crystal phase engineering serves as a pivotal strategy for enhancing room-temperature RT Na-S battery performance by optimizing electronic conductivity and catalytic active sites. Huang et al. [92] demonstrated orthorhombic Nb₂O₅ as an efficient bidirectional redox catalyst that significantly improved battery reaction kinetics (Figure 6a). XPS analysis revealed that at a discharge potential of 1.8 V, Nb₂O₅ achieved the dual chemical anchoring of NaPSs through the formation of Nb-S and Na-O bonds, exhibiting exceptional sulfurphilic/sodiophilic characteristics. DFT calculations illustrated continuous charge density distribution between Nb₂O₅ and sulfur species (Na₂S/Na₂S₆/Na₂S₈), establishing efficient electron transport channels. The accumulated charges on Nb-S and Na-O bonds substantially enhanced chemisorption of sulfur species and promoted multisite catalytic conversion from NaPSs to Na₂S (Figure 6b). Crucially, Nb₂O₅ effectively weakened the Na-S bonds in adsorbed Na₂S (0.04 Å and 0.12 Å bond length increase), reducing the decomposition activation energy and thereby dramatically accelerating the oxidation of Na₂S to S₈ (Figure 6c). This unique bidirectional catalytic mechanism concurrently accelerated NaPS reduction and sulfide oxidation, enabling highly efficient multisite catalysis for sulfur species conversion.

3.3.2. Morphology Engineering

Optimizing nanostructure design, particularly through precise construction of specific geometric configurations, represents a crucial strategy for enhancing RT Na-S battery performance by simultaneously maximizing active site exposure and significantly shortening ion/electron transport pathways. Zhang et al. [123] developed a three-dimensional N-doped porous-carbon-nanosheet-supported Fe₃Se₄ nanoparticle interlayer, where Fe active sites provided electrons to NaPSs, achieving a 1.5-fold enhancement in the conversion rate from long-chain NaPSs to Na₂S. Dong et al. [124] employed a 2H-MoSe₂/N-doped hollow carbon sphere/graphene oxide (2H-MoSe₂/N-HCS/GO) composite separator coating that reduced the NaPS diffusion energy barrier by 58%. Aslam et al. [84] designed a yolk–shell-structured Fe₂N-coated N-doped carbon (S/YS-Fe₂N@NC) that combined high sulfur loading (3.1 mg cm⁻²) with effective volume expansion buffering, delivering a high reversible capacity after 200 cycles at 1C (Figure 6d). XPS analysis revealed a 0.8 eV negative shift in S 2p binding energy after Na₂S₆ adsorption (Figure 6e), confirming strong electron interaction. Symmetric cell tests demonstrated a big oxidation–reduction response (Figure 6f), representing a catalytic activity improvement. The performance enhancement fundamentally relies on nanostructures integrating abundant adsorption sites with high catalytic activity. Li et al. [89] developed a 2D/3D hybrid host (2D N-doped carbon nanosheets with Co₄N-embedded 3D N-doped carbon polyhedrons) that achieved faster electron transfer and reduced the NaPS conversion barrier with high shuttle-effect-suppression efficiency.

3.3.3. Vacancy Defect Engineering

Precise modulation of defects (e.g., doping engineering, vacancy creation) in metal compounds can effectively alter the local electronic structure, creating electron-rich or electron-deficient regions. This electronic structure tailoring significantly enhances chemisorption capability through Lewis acid–base interactions with polar NaPS molecules. Specifically, vacancy defects establish localized charge-enriched zones that strongly anchor NaPSs. A representative study by Ma et al. [95] demonstrated that porous carbon-modified TiO₂ nanoparticles underwent in situ reduction of Ti⁴⁺ to Ti³⁺ during sulfur loading, accompanied by decreased Ti-O bond ratios and oxygen vacancy formation (Figure 6g). The emergence and subsequent disappearance of the Na₂S_x peak (2.1 V–1.8 V) during discharge indicated the strong adsorption and conversion of NaPSs by TiO₂@SPC-S (Figure 6h). Most importantly, the defect sites lowered the energy barrier for Na₂S solid-phase conversion, dramatically accelerating the conversion kinetics from S₈ to Na₂S_x.

Beyond incorporating catalytic materials into sulfur hosts, applied as functional coatings on separators, they can simultaneously serve as secondary current collectors and catalytic centers for NaPS conversion. Sun et al. [122] developed an amorphous iron–tin oxide (A-FeSnO_x) nanosheet separator coating with hierarchical oxygen vacancies (O_vs), where Sn doping precisely regulated the oxygen vacancy concentration. Inferred from the Fe²⁺/Fe³⁺ area ratio (Figure 6i), dual functional sites were confirmed for efficient NaPS chemisorption and catalytic conversion. Compared to the Na₂S₆ solution, the peaks corresponding to S-S bonds in the A-FeSnO_x-Na₂S₆ suspension exhibited significant broadening and reduced visibility (Figure 6j), indicating that the abundant O_vs defects promoted the disruption of S-S bonds and facilitated the strong interaction between A-FeSnO_x and Na₂S₆.

3.4. TM-Based Heterostructure Catalysts

Although the aforementioned catalysts demonstrate specific advantages in RT Na-S battery cathodes, they all suffer from intrinsic limitations: single-atom metals are prone to aggregation and deactivation, nanoparticles exhibit phase instability during sulfur redox reactions, and metal compounds inherently show sluggish charge transfer. To overcome these challenges in charge transport efficiency, structural stability, and active site exposure, researchers have recently developed precisely constructed heterostructures that regulate interfacial electronic coupling effects at the atomic/nano-scale, achieving synergistic enhancement across multi-dimensional catalytic components. In particular, metal heterojunction catalysts have established a new paradigm through their unique mechanisms: (1) “adsorption–catalysis division of labor” (interface-induced electron redistribution), (2) “tandem stepwise catalysis” (multi-active-site relay catalysis), and (3) “heterointerface synergy” (built-in-electric-field-enhanced charge transfer). These mechanisms collectively improve NaPS-anchoring capacity and accelerate conversion kinetics, providing groundbreaking strategies for high-performance RT Na-S battery catalysts.

3.4.1. Synergistic Adsorption–Catalysis

Heterostructure catalysts achieve efficient synergy between NaPS chemisorption and electrocatalytic conversion through spatially separated component specialization. Ye et al. [106] constructed a TiN-TiO₂ heterostructure within multichannel carbon fibers (Figure 7a) that demonstrated exceptional electrochemical stability, maintaining 49% capacity retention (640 mAh g⁻¹) after 100 cycles at 0.1 A g⁻¹ (Figure 7b). Mechanistic studies reveal the following: (1) the TiN component exhibits superior ionic conductivity, with a 30.4% lower Na⁺ migration barrier (0.156 eV) compared to TiO₂ (0.224 eV), significantly enhancing NaPS redox kinetics (Figure 7c), and (2) the TiO₂ component shows strong chemical anchoring of intermediates (Na₂S₄, −3.47 eV adsorption energy) and final products (Na₂S, −3.58 eV), effectively suppressing NaPS shuttling (Figure 7d). This cooperative “TiN-dominant catalysis/TiO₂-responsible adsorption” mechanism endows the heterojunction catalyst with both high sulfur utilization and outstanding cycling stability during charge/discharge processes.

3.4.2. Tandem Stepwise Catalysis

Heterojunction catalysts enable stepwise NaPS conversion through component synergy, establishing an efficient “adsorption–reduction/oxidation” tandem catalytic pathway. Zhang et al. [100] designed a core–shell Fe₂O₃@Fe(CN)₆⁴⁻–polypyrrole composite (Figure 7e) with dual catalytic sites: the Fe₂O₃ core preferentially catalyzes long-chain NaPS conversion to Na₂S₄ (the Tafel slope for the C1 peak to 201–227 mV dec⁻¹, Figure 7f), while the Fe(CN)₆⁴⁻–polypyrrole shell specifically reduces Na₂S₄ to Na₂S (the Tafel slope for the C2 peak to 193 mV dec⁻¹). At a high sulfur loading of 5.8 mg cm⁻², the S/P-Fe₂O₃@Fe-PPy cathode delivered an initial capacity of 606 mAh g⁻¹ at 500 mA g⁻¹ with 62% reduced polarization. Similarly, Fang et al. [101] developed a ZnS-NC@Ni-N₄ core–shell catalyst achieving efficient NaPS conversion via tandem catalysis: (1) ZnS nanocrystals reduced the S₈-to-NaPS conversion Gibbs free energy to 0.23 eV; (2) Ni-N₄ single-atom sites not only reduced the short-chain NaPS-to-Na₂S conversion Gibbs free energy to 1.32 eV but also induced a stable CEI layer, increasing Na₂S precipitation by 35% and significantly enhancing interfacial stability.

3.4.3. Heterointerface Catalysisl

Heterostructure catalysts enhance battery performance through dual synergistic mechanisms: (1) adsorption–catalysis cooperation, where distinct components separately handle NaPS chemisorption and catalytic conversion, and (2) stepwise catalysis, achieving progressive NaPS conversion via ordered interfacial reaction pathways. Crucially, strong electronic interactions at heterointerfaces precisely modulate the adsorption free energy and reaction pathways of intermediates, dramatically improving catalytic efficiency. Zhao et al. [102] demonstrated that Ni-MnO₂ heterostructures on N-doped carbon spheres (1) reduce NaPS adsorption energy through MnO₂-Ni electron transfer while optimizing desorption kinetics and (2) lower the reaction barriers for liquid–solid (Na₂S₄→Na₂S₂) and solid–solid (Na₂S₂→Na₂S) conversions to −2.53 eV and −3.43 eV, respectively, boosting conversion rates. Beyond pre-constructed heterostructures, in situ formation of heterointerfaces during electrochemical cycling can simultaneously achieve strong NaPS anchoring and efficient catalytic conversion. Wei et al. [109] confined cobalt nanoparticles within hollow carbon channels, where Co-S-C heterointerfaces were electrochemically generated in situ. This interface upshifted the Co d-band center, reducing the Na₂S decomposition energy barrier to 0.6 eV and improving Na₂S nucleation by 52%. The Co-S-C@MC cathode demonstrated exceptional cycling stability (only 0.999% capacity decay per cycle over 2500 cycles at 0.5C) while maintaining 1300 mAh g⁻¹ reversible capacity even under harsh conditions (6 mg cm⁻² sulfur loading, 9 μL mg⁻¹ E/S ratio). Similarly, Wang et al. [107] revealed that amorphous Ni-B dynamically evolved into NiS_x catalytic phases during cycling (Figure 7g). The cathode delivered 1487 mAh g⁻¹ initial discharge capacity at 0.2 A g⁻¹, retaining 31.6% capacity (470 mAh g⁻¹) after 1000 cycles at 2 A g⁻¹. At an ultrahigh sulfur loading of 8.9 mg cm⁻², it achieved 5.75 mAh cm⁻² areal capacity. Mechanistic studies demonstrated that (1) the peak current (0.312 mA) and deposition capacity (546 mAh g⁻¹) of Na₂S on the amorphous Ni-B surface confirmed better Na₂S nucleation, dramatically boosting liquid–solid conversion kinetics (Figure 7h), and (2) amorphous Ni-B exhibited ultralow Gibbs free energy changes (ΔG) of 2.24 eV and −0.18 eV for Na₂S₄→Na₂S₂ and Na₂S₂→Na₂S conversions, respectively (Figure 7i).

4. Summary and Perspectives

4.1. Summary

RT Na-S batteries have attracted significant interest due to their high theoretical capacity (1675 mAh g⁻¹), low cost (USD 0.02 kWh⁻¹), and elemental abundance (Na: 2.57%; S: 0.05% in crust) [125]. However, practical applications are hindered by NaPS shuttling and sluggish redox kinetics (conversion barriers > 1.5 eV). Over the past decade, transition metal-based catalysts have extended cycle life by 2–3 orders of magnitude through precise d-band modulation and interfacial charge redistribution (>0.3|e|transfer), offering breakthrough solutions. This review analyzes two core challenges: (1) extremely low intrinsic conductivity (<10⁻³⁰ S cm−¹) of S, causing severe polarization (>300 mV) at high sulfur loading (>5 mg cm−²), and (2) capacity decay exacerbated by disproportionation reactions during solid–liquid–solid phase transitions. Establishing quantitative structure–activity relationships between electronic features (d-band center, orbital hybridization) and NaPS adsorption energy (ΔE_ads)/activation barriers (E_a) provides theoretical foundations for catalyst design. Four catalyst categories have emerged: (1) single/dual-atom catalysts (<40 wt.% loading) achieve 100% atomic utilization via unsaturated coordination; (2) nanoparticles (2–30 nm) leverage high-index facets; (3) metal compounds (sulfides/oxides/carbides) regulate electron transfer via anion vacancies; and (4) heterojunctions utilize built-in fields for function separation.

4.2. Future Perspectives

Despite the advances of TM-based catalysts and enhanced performance, the commercialization of RT Na-S batteries has been impeded by the following five critical bottlenecks: (1) Limitations in characterization: Conventional ex situ techniques (e.g., postmortem XPS, SEM) fail to capture transient interfacial reactions occurring at nanosecond timescales during real-time charge/discharge processes, particularly the dynamic evolution of catalyst–NaPS interfaces and metastable intermediates like Na₂S₃. (2) Theoretical modeling gaps: The absence of quantitative electronic–kinetic correlation models prevents precise prediction of how specific electronic structures (e.g., d-band center position, charge transfer amount) govern reaction thermodynamics (adsorption energy ΔE_ads) and kinetics (activation barrier E_a for Na₂S decomposition). (3) Practical energy density trade-offs: Excessive catalyst loading (>20 wt.%) significantly dilutes the energy density of battery systems. (4) Electrolyte sensitivity: Under lean electrolyte conditions (E/S ratio < 3 μL mg⁻¹), the limited ion transport pathways cause rapid performance degradation. (5) Scalability challenges: High-temperature pyrolysis (typically > 800 °C) and inert-atmosphere requirements in synthesis (e.g., for Co@NC catalysts) not only increase manufacturing costs but also induce structural heterogeneity during scale-up, as evidenced by a reduction in active site density. These process-induced defects further accelerate capacity fading rates.

To address the existing challenges and propel RT Na-S batteries toward commercialization, several breakthrough pathways are envisioned for TM-based catalysts: (1) Advanced operando characterization platforms: The development of integrated multi-modal operando analysis systems combining time-resolved Raman spectroscopy (with <10 ns temporal resolution), synchrotron-based XRD (achieving sub-Ångström spatial resolution), and real-time DFT calculations will be crucial. These platforms will enable atomic-scale observation of dynamic interfacial evolution during electrochemical processes, particularly in capturing transient reaction intermediates (e.g., Na₂S₃ with a lifetime of <1 ms) and potential-dependent catalyst reconstruction. (2) AI-driven catalyst design systems: Machine learning-assisted platforms utilizing >10⁴ curated experimental and computational datasets will revolutionize catalyst discovery. These systems will incorporate key descriptors, including d-band center positions (±0.2 eV precision), Bader charge transfer values, and adsorption energy profiles, to predict optimal catalyst compositions with >90% accuracy before synthesis. (3) Precision catalyst engineering: Achieving ultralow loading (<5 wt.%) through atomic-level dispersion techniques such as atomic layer deposition (ALD) and molecular layer deposition (MLD) will be essential. This approach can maximize active site exposure while maintaining exceptional catalytic activity, potentially achieving turnover frequencies (TOFs) >10³ s⁻¹ for NaPS conversion. (4) Green synthesis paradigms: Room-temperature wet-chemical synthesis methods will replace energy-intensive processes, potentially reducing manufacturing costs by >60% while eliminating the need for inert atmospheres. Techniques like electrochemical atomic layer epitaxy (EC-ALE) could enable precise control over catalyst morphology and composition.

Despite notable progress in lithium–sulfur (Li-S) batteries, their commercialization remains impeded by the high cost and poor scalability of key components. Experience from Li-S systems underscores the crucial role of transition metal catalysts, yet research remains largely confined to lab-scale studies. A fundamental insight, therefore, is the imperative to develop cost-effective and scalable synthesis methods for such catalysts to enable the practical deployment of transition metal-based Na-S batteries. The interdisciplinary integration of these approaches for synthesizing TM-based catalysts, combining advanced characterization, computational design, and sustainable manufacturing, will accelerate the transition of RT Na-S batteries from laboratory-scale prototypes (mAh level) to grid-scale energy storage systems (kWh level). Within the next 5–7 years, we anticipate that these developments will enable practical energy densities exceeding 300 Wh kg⁻¹ at the system level, with a cycle life surpassing 2000 cycles at a 1C rate, finally meeting the stringent requirements for commercial energy storage applications.