Nickel Hollow Spheres Concatenated by Nitrogen-Doped Carbon Fibers for Enhancing Electrochemical Kinetics of Sodium-Sulfur Batteries

Phosphorus and Nitrogen Dual-Doped Hollow Porous Carbon Spheres toward Enhanced Cycling Stability of Room-Temperature Na-S Batteries

Development of room-temperature sodium-sulfur (RT Na-S) batteries with satisfactory cycling life and rate capability remains challenging due to the unfavorable electric conductivity from S species, sluggish redox kinetics of S conversion, and serious shuttle effects of sodium polysulfides (NaPSs). To address these issues, a phosphorus and nitrogen dual-doped hollow porous carbon sphere (PN-HPCs) is synthesized as the S hosts, which enhances the electric conductivity, ion diffusion, and conversion of polysulfides. Such a hollow hierarchically porous structure is beneficial to accommodate the volume variations of S species and shorten the ion/electron transfer distances during electrochemical reaction process. As a result, the S@PN-HPCs600 cathode delivers noticeable cycling performance (313 mAh g-1 after 4500 cycles at 5.0 C, and capacity degeneration of only 0.01% per cycle) and rate capability (646.4 mAh g-1@1.0 and 527.5 mAh g-1@3.0 C). This work presents an efficient strategy based on structural confinement and dual-heteroatom doping engineering for long-life RT Na-S batteries.

Designing Urchin-Like S/SiO2 with Regulated Pores Toward Ultra-Fast Room Temperature Sodium-Sulfur Battery

Captured by high theoretical capacity and low-cost, Sodium-Sulfur (Na-S) batteries have been deemed as promising energy-storage systems. However, their electrochemical properties, containing both cycling and rate properties, still suffer from the notorious "shuttle effect" of polysulfide. Herein, through the effective regulation of pore sizes, a series of S/SiO2 cathode materials are obtained. Benefitting from the abundant pore channels of SiO2 particles, the sulfur loading is as high as 76.3%. Importantly, a suitable pore size can lead to adequate reaction and rapid diffusion behaviors, resulting in excellent electrochemical performances. Specifically, at 2.0 A g-1, the initial capacity of the as-optimized sample can be up to 1370.6 mAh g-1. Surprisingly, even after 1050 cycles, it could achieve a high reversible capacity of 1280.8 mAh g-1 with an attenuation rate of 0.089%. At 5.0 A g-1, after 500 cycles, the capacity can still remain ≈ 1132.6 mAh g-1 (capacity retention rate, 97.5%). Given this, the work is anticipated to offer an effective strategy for advanced electrodes for Na-S batteries.

A Critical Review on Room-Temperature Sodium-Sulfur Batteries: From Research Advances to Practical Perspectives

Room-temperature sodium-sulfur (RT-Na/S) batteries are promising alternatives for next-generation energy storage systems with high energy density and high power density. However, some notorious issues are hampering the practical application of RT-Na/S batteries. Besides, the working mechanism of RT-Na/S batteries under practical conditions such as high sulfur loading, lean electrolyte, and low capacity ratio between the negative and positive electrode (N/P ratio), is of essential importance for practical applications, yet the significance of these parameters has long been disregarded. Herein, it is comprehensively reviewed recent advances on Na metal anode, S cathode, electrolyte, and separator engineering for RT-Na/S batteries. The discrepancies between laboratory research and practical conditions are elaborately discussed, endeavors toward practical applications are highlighted, and suggestions for the practical values of the crucial parameters are rationally proposed. Furthermore, an empirical equation to estimate the actual energy density of RT-Na/S pouch cells under practical conditions is rationally proposed for the first time, making it possible to evaluate the gravimetric energy density of the cells under practical conditions. This review aims to reemphasize the vital importance of the crucial parameters for RT-Na/S batteries to bridge the gaps between laboratory research and practical applications.

A Mott-Schottky Heterojunction with Strong Chemisorption and Fast Conversion Effects for Room-Temperature Na-S Batteries

The practical application of the room-temperature sodium-sulfur (RT Na-S) batteries is currently limited by low reversible capacity and serious capacity decay due to the sluggish reaction kinetics and shuttle effect. It is necessary to design a suitable sulfur host integrated with electrocatalysts to realize effective chemisorption and catalysis of sodium polysulfides (NaPSs). Herein, under the guidance of theoretical calculation, the Mott-Schottky heterojunction with a built-in electric field composed of iron (Fe) and iron disulfide (FeS2) components anchored on a porous carbon matrix (Fe/FeS2-PC) is designed and prepared. The enhanced chemisorption effect of Fe, the fast electrocatalytic effect of FeS2, and the fast transfer effect of the built-in electric field within the Fe/FeS2 heterojunction in the cathode of RT Na-S batteries work together to effectively improve the electrochemical performance. As a result, the Fe/FeS2-PC@S cathode exhibits high reversible capacity (815 mAh g-1 after 150 cycles at 0.2 A g-1) and excellent stability (516 mAh g-1 after 600 cycles at 5 A g-1, with only 0.07% decay per cycle). The design of the Fe/FeS2 heterojunction electrocatalyst provides a new strategy for the development of highly stable RT Na-S batteries.

Enhancing Conversion Kinetics through Electron Density Dual-Regulation of Catalysts and Sulfur toward Room-/Subzero-Temperature Na-S Batteries

Room-temperature sodium-sulfur (RT Na/S) batteries have received increasing attention for the next generation of large-scale energy storage, yet they are hindered by the severe dissolution of polysulfides, sluggish redox kinetic, and incomplete conversion of sodium polysulfides (NaPSs). Herein, the study proposes a dual-modulating strategy of the electronic structure of electrocatalyst and sulfur to accelerate the conversion of NaPSs. The selenium-modulated ZnS nanocrystals with electron rearrangement in hierarchical structured spherical carbon (Se-ZnS/HSC) facilitate Na+ transport and catalyze the conversion between short-chain sulfur and Na2S. And the in situ introduced Se within S can enhance conductivity and form an S─Se bond, suppressing the "polysulfides shuttle". Accordingly, the S@Se-ZnS/HSC cathode exhibits a specific capacity of as high as 1302.5 mAh g-1 at 0.1 A g-1 and ultrahigh-rate capability (676.9 mAh g-1 at 5.0 A g-1). Even at -10 °C, this cathode still delivers a high reversible capacity of 401.2 mAh g-1 at 0.05 A g-1 and 94% of the original capacitance after 50 cycles. This work provides a novel design idea for high-performance Na/S batteries.

Binary Atomic Sites Enable a Confined Bidirectional Tandem Electrocatalytic Sulfur Conversion for Low-Temperature All-Solid-State Na-S Batteries

The broader implementation of current all-solid-state Na-S batteries is still plagued by high operation temperature and inefficient sulfur utilization. And the uncontrollable sulfur speciation pathway along with the sluggish polysulfide redox kinetics further compromise the theoretical potentials of Na-S chemistry. Herein, we report a confined bidirectional tandem electrocatalysis effect to tune polysulfide electrochemistry in a novel low-temperature (80 °C) all-solid-state Na-S battery that utilizes Na3 Zr2 Si2 PO12 ceramic membrane as a platform. The bifunctional hollow sulfur matrix consisting binary atomically dispersed MnN4 and CoN4 hotspots was fabricated using a sacrificial template process. Upon discharge, CoN4 sites activate sulfur species and catalyze long-chain to short-chain polysulfides reduction, while MnN4 centers substantially accelerate the low-kinetic Na2 S4 to Na2 S directly conversion, manipulating the uniform deposition of electroactive Na2 S and avoiding the formation of irreversible products (e.g., Na2 S2 ). The intrinsic synergy of two catalytic centers benefits the Na2 S decomposition and minimizes its activation barrier during battery recharging and then efficiently mitigate the cathodic passivation. As a result, the stable cycling of all-solid-state Na-S cell delivers an attractive reversible capacity of 1060 mAh g-1 with a high CE of 98.5 % and a high energy of 1008 Wh kgcathode -1 , comparable to the liquid electrolyte cells.

Phosphor-Doped Carbon Network Electrocatalyst Enables Accelerated Redox Kinetics of Polysulfides for Sodium-Sulfur Batteries

Lithium-ion batteries, which have dominated large-scale energy storage for the past three decades, face limitations in energy density and cost. Sulfur, with its impressive capacity of 1675 mAh g-1 and high theoretical energy density of 1274 Wh kg-1, stands out as a promising cathode material, leading to a growing focus on sodium-sulfur (Na-S) batteries as an alternative to address lithium resource scarcity. Nevertheless, the development is restrained by poor conductivity, volume expansion of the sulfur cathode, and the shuttle effect of sodium polysulfides (Na2Sn) in the electrolytes. In this study, a facile method is designed to fabricate phosphor-doped carbon (phos-C), which is then used as a sulfur matrix. This micromesoporous phos-C network enhances sulfur utilization, increases overall cathode conductivity, and effectively mitigates the shuttling of Na2Sn. During the discharge process, phos-C can absorb soluble Na2Sn and increase the conductivity of sulfur, while serving as a reservoir for electrolyte and Na2Sn, thereby preventing their infiltration into the anode and reducing the loss of sodium. As a result, the well-designed sulfur-loaded phos-C (S/phos-C) cathode, employed in the Na-S battery, demonstrates a capacity of 1034 mAh g-1 at 0.1 C (1 C = 1675 mA g-1) and an excellent rate capability of 339 mAh g-1 at 10 C, coupled with a prolonged cycling life up to 2000 cycles at 1 C, exhibiting an ultralow capacity decay rate of 0.013% per cycle. Overall, this study introduces an efficient method for creating long-lasting Na-S batteries.

Efficient Polytelluride Anchoring for Ultralong-Life Potassium Storage: Combined Physical Barrier and Chemisorption in Nanogrid-in-Nanofiber

Metal tellurides (MTes) are highly attractive as promising anodes for high-performance potassium-ion batteries. The capacity attenuation of most reported MTe anodes is attributed to their poor electrical conductivity and large volume variation. The evolution mechanisms, dissolution properties, and corresponding manipulation strategies of intermediates (K-polytellurides, K-pTex) are rarely mentioned. Herein, we propose a novel structural engineering strategy to confine ultrafine CoTe2 nanodots in hierarchical nanogrid-in-nanofiber carbon substrates (CoTe2@NC@NSPCNFs) for smooth immobilization of K-pTex and highly reversible conversion of CoTe2 by manipulating the intense electrochemical reaction process. Various in situ/ex situ techniques and density functional theory calculations have been performed to clarify the formation, transformation, and dissolution of K-pTex (K5Te3 and K2Te), as well as verifying the robust physical barrier and the strong chemisorption of K5Te3 and K2Te on S, N co-doped dual-type carbon substrates. Additionally, the hierarchical nanogrid-in-nanofiber nanostructure increases the chemical anchoring sites for K-pTex, provides sufficient volume buffer space, and constructs highly interconnected conductive microcircuits, further propelling the battery reaction to new heights (3500 cycles at 2.0 A g-1). Furthermore, the full cells further demonstrate the potential for practical applications. This work provides new insights into manipulating K-pTex in the design of ultralong-cycling MTe anodes for advanced PIBs.

Surface Crystal Modification of Na3 V2 (PO4 )3 to Cast Intermediate Na2 V2 (PO4 )3 Phase toward High-Rate Sodium Storage

The two-phase reaction of Na3 V2 (PO4 )3 - Na1 V2 (PO4 )3 in Na3 V2 (PO4 )3 (NVP) is hindered by low electronic and ionic conductivity. To address this problem, a surface-N-doped NVP encapsulating by N-doped carbon nanocage (N-NVP/N-CN) is rationally constructed, wherein the nitrogen is doped in both the surface crystal structure of NVP and carbon layer. The surface crystal modification decreases the energy barrier of Na+ diffusion from bulk to electrolyte, enhances intrinsic electronic conductivity, and releases lattice stress. Meanwhile, the porous architecture provides more active sites for redox reactions and shortens the diffusion path of ion. Furthermore, the new interphase of Na2 V2 (PO4 )3 is detected by in situ XRD and clarified by density functional theory (DFT) calculation with a lower energy barrier during the fast reversible electrochemical three-phase reaction of Na3 V2 (PO4 )3 - Na2 V2 (PO4 )3 - Na1 V2 (PO4 )3 . Therefore, as cathode of sodium-ion battery, the N-NVP/N-CN exhibited specific capacities of 119.7 and 75.3 mAh g-1 at 1 C and even 200 C. Amazingly, high capacities of 89.0, 86.2, and 84.6 mAh g-1 are achieved after overlong 10000 cycles at 20, 40, and 50 C, respectively. This approach provides a new idea for surface crystal modification to cast intermediate Na2 V2 (PO4 )3 phase for achieving excellent cycling stability and rate capability.

Aprotic Sulfur-Metal Batteries: Lithium and Beyond

Metal-sulfur batteries constitute an extraordinary research playground that ranges from fundamental science to applied technologies. However, besides the widely explored Li-S system, a remarkable lack of understanding hinders advancements and performance in all other metal-sulfur systems. In fact, similarities and differences make all generalizations highly inconsistent, thus unavoidably suggesting the need for extensive research explorations for each formulation. Here we review critically the most remarkable open challenges that still hinder the full development of metal-S battery formulations, starting from the lithium benchmark and addressing Na, K, Mg, and Ca metal systems. Our aim is to draw an updated picture of the recent efforts in the field and to shed light on the most promising innovation paths that can pave the way to breakthroughs in the fundamental comprehension of these systems or in battery performance.

Single-Atom Vanadium Catalyst Boosting Reaction Kinetics of Polysulfides in Na-S Batteries

The practical application of the room-temperature sodium-sulfur (RT Na-S) batteries is hindered by the insulated sulfur, the severe shuttle effect of sodium polysulfides, and insufficient polysulfide conversion. Herein, on the basis of first principles calculations, single-atom vanadium anchored on a 3D nitrogen-doped hierarchical porous carbon matrix (denoted as 3D-PNCV) is designed and fabricated to enhance sulfur reactivity, and adsorption and catalytic conversion performance of sodium polysulfide. The 3D-PNCV host with abundant and active V sites, hierarchical porous structure, high electrical conductivity, and strong chemical adsorption/conversion ability of V-N bonding can immobilize the polysulfides and promote reversibly catalytic conversion of polysulfides toward Na₂ S. Therefore, as-fabricated RT Na-S batteries can achieve a high reversible capacity (445 mAh g^-1 over 800 cycles at 5 A g^-1 ) and excellent rate capability (224 mAh g^-1 at 10 A g^-1 ). The electrocatalysis mechanism of sodium polysulfides is further experimentally and theoretically revealed, which provides a new strategy to develop the highly stable RT Na-S batteries.

Bidirectional Tandem Electrocatalysis Manipulated Sulfur Speciation Pathway for High-Capacity and Stable Na-S Battery

The sluggish polysulfide redox kinetics and the uncontrollable sulfur speciation pathway, leading to serious shuttling effect and high activation barrier associated with sulfur cathode. We describe here the use of core-shell structured composite matrixes containing abundant catalytic sites for nearly fully reversible cycling of sulfur cathodes for Na-S batteries. The bidirectional tandem electrocatalysis provide successive reversible conversion of both long- and short-chain polysulfides, whereas Fe₂ O₃ accelerates Na₂ S₈ /Na₂ S₆ to Na₂ S₄ conversion and the redox-active Fe(CN)₆ ^4- -doped polypyrrole shell catalyzes Na₂ S₄ reduction to Na₂ S. The electrochemically reactive Na₂ S can be readily charged back to sulfur with minimal overpotential. Simultaneously, stable cycling of Na-S pouch cell with a high reversible capacity of 696 mAh g^-1 is also demonstrated. The bidirectional confined tandem catalysis renders the manipulation of sulfur redox electrochemistry for practical Na-S cells.

Electrocatalysis in Room Temperature Sodium-Sulfur Batteries: Tunable Pathway of Sulfur Speciation

Benefiting from the merits of natural abundance, low cost, and ultrahigh theoretical energy density, the room temperature sodium-sulfur (RT NaS) batteries are regarded as one of the promising candidates for the next-generation scalable energy storage devices. However, the uncontrollable sulfur speciation pathways severely hinder its practical applications. Recently, various strategies have been employed to tune the conversion pathways of sulfur, such as physical confinement, chemical inhibition, and electrocatalysis. Herein, the recent advances in electrocatalytic effects manipulate sulfur speciation pathways in advanced RT NaS electrochemistry are reviewed, including the promotion of the nearly full conversion of long-chain polysulfides, short-chain polysulfides, and small sulfur molecules. The underlying catalytic modulation mechanism that fundamentally tunes the electrochemical pathway of sulfur species is comprehensively summarized along with the design strategies for catalytic active centers. Furthermore, the challenge and potential solutions to realize the quasi-solid conversion of sulfur are proposed to accelerate the real application of RT NaS batteries.

Molybdenum Carbide Electrocatalyst In Situ Embedded in Porous Nitrogen-Rich Carbon Nanotubes Promotes Rapid Kinetics in Sodium-Metal-Sulfur Batteries

This is the first report of molybdenum carbide-based electrocatalyst for sulfur-based sodium-metal batteries. MoC/Mo₂ C is in situ grown on nitrogen-doped carbon nanotubes in parallel with formation of extensive nanoporosity. Sulfur impregnation (50 wt% S) results in unique triphasic architecture termed molybdenum carbide-porous carbon nanotubes host (MoC/Mo₂ C@PCNT-S). Quasi-solid-state phase transformation to Na₂ S is promoted in carbonate electrolyte, with in situ time-resolved Raman, X-ray photoelectron spectroscopy, and optical analyses demonstrating minimal soluble polysulfides. MoC/Mo₂ C@PCNT-S cathodes deliver among the most promising rate performance characteristics in the literature, achieving 987 mAh g^-1 at 1 A g^-1 , 818 mAh g^-1 at 3 A g^-1 , and 621 mAh g^-1 at 5 A g^-1 . The cells deliver superior cycling stability, retaining 650 mAh g^-1 after 1000 cycles at 1.5 A g^-1 , corresponding to 0.028% capacity decay per cycle. High mass loading cathodes (64 wt% S, 12.7 mg cm^-2 ) also show cycling stability. Density functional theory demonstrates that formation energy of Na₂ S_x (1 ≤ x ≤ 4) on surface of MoC/Mo₂ C is significantly lowered compared to analogous redox in liquid. Strong binding of Na₂ S_x (1 ≤ x ≤ 4) on MoC/Mo₂ C surfaces results from charge transfer between the sulfur and Mo sites on carbides' surface.

Nanostructure Engineering Strategies of Cathode Materials for Room-Temperature Na-S Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries are considered to be a competitive electrochemical energy storage system, due to their advantages in abundant natural reserves, inexpensive materials, and superb theoretical energy density. Nevertheless, RT Na-S batteries suffer from a series of critical challenges, especially on the S cathode side, including the insulating nature of S and its discharge products, volumetric fluctuation of S species during the (de)sodiation process, shuttle effect of soluble sodium polysulfides, and sluggish conversion kinetics. Recent studies have shown that nanostructural designs of S-based materials can greatly contribute to alleviating the aforementioned issues via their unique physicochemical properties and architectural features. In this review, we review frontier advancements in nanostructure engineering strategies of S-based cathode materials for RT Na-S batteries in the past decade. Our emphasis is focused on delicate and highly efficient design strategies of material nanostructures as well as interactions of component-structure-property at a nanosize level. We also present our prospects toward further functional engineering and applications of nanostructured S-based materials in RT Na-S batteries and point out some potential developmental directions.

A High-Efficiency Mo2 C Electrocatalyst Promoting the Polysulfide Redox Kinetics for Na-S Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries, as promising next-generation energy storage candidates, are drawing more and more attention due to the high energy density and abundant elements reserved in the earth. However, the native downsides of RT Na-S batteries (i.e., enormous volume changes, the polysulfide shuttle, and the insulation and low reactivity of S) impede their further application. To conquer these challenges, hierarchical porous hollow carbon polyhedrons embedded with uniform Mo₂ C nanoparticles are designed deliberately as the host for S. The micro- and mesoporous hollow carbon indeed dramatically enhances the reactivity of the S cathodes and accommodates the volume changes. Meanwhile, the highly conductive dispersed Mo₂ C has a strong chemical adsorption to polysulfides and catalyzes the transformation of polysulfides, which can effectively inhibit the dissolution of polysulfides and accelerate the reaction kinetics. Thus, the as-prepared S cathode can display a high reversible capacity (1098 mAh g^-1 at 0.2 A g^-1 after 120 cycles) and superior rate performance (483 mAh g^-1 at 10.0 A g^-1 ). This work provides a new method to boost the performance of RT Na-S batteries.

Ni3FeN functionalized carbon nanofibers boosting polysulfide conversion for Li-S chemistry

Limiting the shuttle effect of polysulfides is an important means to realizing high energy density lithium-sulfur batteries (Li-S). In this study, an efficient electrocatalyst (CNFs@Ni3FeN) is synthesized by anchoring Ni3FeN in the carbon nanofibers (CNFs). The CNFs@Ni3FeN shows electrocatalytic activity and enhances the conversion of polysulfides. After assembling a battery, a high initial capacity (1452 mA h g-1) and favorable long-time cycling stability (100 cycles) with a capacity retention rate of 83% are obtained by the electrocatalysis of Ni3FeN. Compared with unmodified CNFs, the cycling stability of CNFs@Ni3FeN can be greatly improved. The catalytic mechanism is further deduced by X-ray photoelectron spectroscopy (XPS). Our work will inspire the rational design of CNFs@support hybrids for various electrocatalysis applications.

Polysulfide Catalytic Materials for Fast-Kinetic Metal-Sulfur Batteries: Principles and Active Centers

Benefiting from the merits of low cost, ultrahigh-energy densities, and environmentally friendliness, metal-sulfur batteries (M-S batteries) have drawn massive attention recently. However, their practical utilization is impeded by the shuttle effect and slow redox process of polysulfide. To solve these problems, enormous creative approaches have been employed to engineer new electrocatalytic materials to relieve the shuttle effect and promote the catalytic kinetics of polysulfides. In this review, recent advances on designing principles and active centers for polysulfide catalytic materials are systematically summarized. At first, the currently reported chemistries and mechanisms for the catalytic conversion of polysulfides are presented in detail. Subsequently, the rational design of polysulfide catalytic materials from catalytic polymers and frameworks to active sites loaded carbons for polysulfide catalysis to accelerate the reaction kinetics is comprehensively discussed. Current breakthroughs are highlighted and directions to guide future primary challenges, perspectives, and innovations are identified. Computational methods serve an ever-increasing part in pushing forward the active center design. In summary, a cutting-edge understanding to engineer different polysulfide catalysts is provided, and both experimental and theoretical guidance for optimizing future M-S batteries and many related battery systems are offered.

Tessellated N-doped carbon/CoSe2 as trap-catalyst sulfur hosts for room-temperature sodium-sulfur batteries

The construction of highly conductive structure with excellent adsorption-catalytic properties to accelerate electron transfer and suppress polysulfides shuttle is considered as an effective strategy to achieve well-behaved sodium-sulfur batteries. Herein,...

MXene nanofibers confining MnOx nanoparticles: a flexible anode for high-speed lithium ion storage networks

The electron and ion conductivities of anode materials such as MnO_x affect critically the properties of anodes in Li-ion batteries. Herein, a three-dimensional (3D) nanofiber network (MnO_x-MXene/CNFs) for high-speed electron and ion transport with a MnO_x surface anchored and embedded inside is designed via electrospinning manganese ion-modified MXene nanosheets and subsequent carbonization. Ion transport analysis reveals improved Li⁺ transport on the MnO_x-MXene/CNF electrode and first-principles density functional theory (DFT) calculation elucidates the Li⁺ adsorption and storage mechanism. The surface-anchored MnO_x nanoparticles form extremely strong bonds with the nanofibers, and the internally embedded MnO_x nanoparticles, due to the fiber confinement effect, ensure the structural stability during charging and discharging, achieving the so-called dual stabilization strategies for cyclic fluctuation. By electrospinning, self-restacking of MXene flakes can be prevented, thereby giving rise to a larger surface area and more accessible active sites on the flexible anode. Benefiting from the 3D network with excellent conductivity and stability, at high current densities, the MnO_x-MXene/CNF anode exhibits outstanding electrochemical characteristics. Even after 2000 cycles, a reversible capacity of 1098 mA h g^-1 can be obtained at 2 A g^-1 with only 0.007208% decay rate. The MnO_x-MXene/CNF anode also shows a significant rate performance such as 1268 mA h g^-1 at 2 A g^-1 and 1137 mA h g^-1 at 5 A g^-1 corresponding to an area specific capacity of 2.536 mA h cm^-2 at 4 mA cm^-2 and 2.274 mA h cm^-2 at 10 mA cm^-2, respectively. The results indicate that the MnO_x-MXene/CNF anode has excellent Li-ion storage properties and great commercial potential.

A Fe3N/carbon composite electrocatalyst for effective polysulfides regulation in room-temperature Na-S batteries

The practical application of room-temperature Na-S batteries is hindered by the low sulfur utilization, inadequate rate capability and poor cycling performance. To circumvent these issues, here, we propose an electrocatalyst composite material comprising of N-doped nanocarbon and Fe3N. The multilayered porous network of the carbon accommodates large amounts of sulfur, decreases the detrimental effect of volume expansion, and stabilizes the electrodes structure during cycling. Experimental and theoretical results testify the Fe3N affinity to sodium polysulfides via Na-N and Fe-S bonds, leading to strong adsorption and fast dissociation of sodium polysulfides. With a sulfur content of 85 wt.%, the positive electrode tested at room-temperature in non-aqueous Na metal coin cell configuration delivers a reversible capacity of about 1165 mA h g-1 at 167.5 mA g-1, satisfactory rate capability and stable capacity of about 696 mA h g-1 for 2800 cycles at 8375 mA g-1.

Metal-based electrocatalysts for room-temperature Na-S batteries

Room-temperature sodium-sulfur (RT Na-S) batteries have recently captured intensive research attention from the community and are regarded as one of promising next-generation energy storage devices since they not only integrate the advantages in high abundance and low commercial cost of elemental Na/S but also exhibit exceptionally high theoretical capacity and energy density. Whereas, the notorious shuttle effect of soluble intermediates and sluggish kinetics remain two main obstacles for RT Na-S batteries to step into new developmental stage. Recently, impressive advancements of metal-based electrocatalysts have offered a viable solution to stabilize S cathodes and unlocked new opportunities for RT Na-S batteries. Here, we underline the recent progress on metal-based electrocatalysts for RT Na-S batteries for the first time by shedding light on this emerging but promising field. The involved metal-based electrocatalysts include metals, metal oxides, metal sulfides, metal carbides, and other metal-based catalytic species. Our emphasis is focused on the discussion of design, fabrication, and properties of these electrocatalysts as well as interactions between electrocatalysts and sodium polysulfides. Otherwise, some potential electrocatalysts for RT Na-S batteries are pointed out as well. At last, perspectives for the future development of RT Na-S batteries with S cathode electrocatalysts are offered.

Ni@Ni3N Embedded on Three-Dimensional Carbon Nanosheets for High-Performance Lithium/Sodium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are recognized as one of the most promising next-generation energy storage devices, but their practical application is greatly limited by several obstacles, such as the highly insulating nature and sluggish redox kinetics of sulfur and the dissolution of lithium polysulfides. Herein, three-dimensional carbon nanosheet frameworks anchored with Ni@Ni₃N heterostructure nanoparticles (denoted Ni@Ni₃N/CNS) are designed and fabricated by a chemical blowing and thermal nitridation strategy. It is demonstrated that the Ni@Ni₃N heterostructure can effectively accelerate polysulfide conversion and promote the chemical trapping of polysulfides. Meanwhile, the carbon nanosheet frameworks of Ni@Ni₃N/CNS establish a highly conductive network for fast electron transportation. The cells with Ni@Ni₃N heterostructures as the catalyst in the cathode show excellent electrochemical performance, revealing stable cycling over 600 cycles with a low-capacity fading rate of 0.04% per cycle at 0.5 C and high-rate capability (594 mAh g^-1 at 3 C). Furthermore, Ni@Ni₃N/CNS can also work well in room-temperature sodium-sulfur (RT-Na/S) batteries, delivering a high specific capacity (454 mAh g^-1 after 400 cycles at 0.5 C). This work provides a rational way to prepare the metal-metal nitride heterostructures to enhance the performance both of Li-S and RT-Na/S batteries.

Manipulating the Electronic Structure of Nickel via Alloying with Iron: Toward High-Kinetics Sulfur Cathode for Na-S Batteries

The sluggish conversion kinetics and severe shuttle effect in room-temperature Na-S (RT Na-S) batteries cause knotty issues, such as poor rate performance, fast capacity decay as well as low Coulombic efficiency, which seriously impede their practical application. Therefore, exploiting cost-effective and efficient electrocatalysts for absorbing soluble long-chain sodium polysulfides (NaPSs) and expediting NaPSs conversion is of paramount importance. Herein, catalyst mining is first conducted by density functional theory calculations, which reveal that the alloying of Fe into Ni can tailor the electronic structure, leading to lower reaction energy barrier for polysulfide conversion. Based on this, FeNi₃@hollow porous carbon spheres (FeNi₃@HC) as a promising sulfur host for RT Na-S batteries are rationally designed and fabricated. As expected, the S@FeNi₃@HC cathode exhibits an excellent cycling stability (591 mAh g^-1 after 500 cycles at 2 A g^-1) and outstanding rate performance (383 mAh g^-1 at 5 A g^-1). Our work demonstrates an effective strategy (i.e., alloying strategy with a rich electron state) to design superior electrocatalysts for RT Na-S batteries.

Recent Advances of Catalytic Effects in Cathode Materials for Room-Temperature Sodium-Sulfur Batteries

Electrocatalysts in room-temperature sodium-sulfur (RT-Na/S) have captured numerous attention. But, they suffered from shuttle effect and surface passivation. RT-Na/S show inferior energy-storage abilities, ascribed to the larger radii of Na-ions. Herein, the vigorous review is displayed from different kinds of metal-based traits, containing single metal, metal-based samples, and multifunctional hybrids. Through the controlling of structures and composition, the conversion reaction about liquid/solid phases would be enhanced, accompanied by the enhancements of cycling stabilities and rate properties, which enables the break-through of practical applications. The in-depth influences of catalytic effects on the Na-S reaction mechanism and the corresponding electrochemical performance in recently representative works are systematically reviewed. Particularly, this review is anticipated to propose potential research directions for further enhancement of RT-Na/S batteries.

Flower-Like Interlayer-Expanded MoS2- x Nanosheets Confined in Hollow Carbon Spheres with High-Efficiency Electrocatalysis Sites for Advanced Sodium-Sulfur Battery

The room-temperature sodium-sulfur (RT-Na/S) battery is one of the most promising technologies for low-cost energy storage. However, application of RT-Na/S batteries is currently impeded by severe shuttle effects and volume expansion that limits both energy density and cycling stability. Herein, first, the first-principal calculation is used to find that the introduction of sulfur vacancies in MoS₂ can effectively enhance polysulfide adsorption and catalytic ability as well as both the ion and electron conductivities. Then, unique MoS_{2- x} /C composite spheres are further designed and synthesized with flower-like few-layer and interlayer-enlarged MoS_{2- x} nanosheets space-confined in hollow carbon nanospheres by a "ship-in-a-bottle" strategy. With this novel design, the mass loading of S in the MoS_{2- x} /C composite can be reached to as high as 75 wt%. Owing to the synergetic effect of interlayer-expanded and few-layer MoS_{2- x} nanosheets and hollow carbon spheres matrix with high electronic/Na⁺ conductivity, the RT-Na/S batteries deliver highly stable cycle durability (capacity retention of 85.2% after 100 cycles at 0.1 A g^-1 ) and remarkable rate capability (415.7 mAh g^-1 at 2 A g^-1 ) along with high energy density. This design strategy of defect- and interlayer-engineering may find wide applications in synthesizing electrode materials for high-performance RT-Na/S batteries.

Strategies for Polysulfide Immobilization in Sulfur Cathodes for Room-Temperature Sodium-Sulfur Batteries

Room-temperature sodium-sulfur batteries are one of the most attractive energy storage systems due to their low cost and ultrahigh energy density (2600 W h kg^-1 ). During the charge/discharge process, the sulfur can react with sodium via a multistep redox reaction to obtain a high specific capacity (1675 mA h g^-1 ). However, these batteries face the difficult challenge of the "shuttle effect," which hinders their practical application. Many strategies have been employed to address this issue on sulfur electrodes, such as intact physical confinement, chemical inhibition, and electrocatalysis. In this review, the mechanisms of the abovementioned strategies are summarized, the remaining issues are clarified, and research directions are proposed for developing advanced sodium-sulfur batteries.

Metal-Organic Frameworks-Derived Nitrogen-Doped Porous Carbon Nanocubes with Embedded Co Nanoparticles as Efficient Sulfur Immobilizers for Room Temperature Sodium-Sulfur Batteries

Room temperature sodium-sulfur (RT Na-S) batteries are considered a promising candidate for energy-storage due to their high energy-density and low-cost. However, the shutting effect of polysulfides and sluggish kinetics of sulfur redox reactions still severely limit their practical implementation. Herein, a new type of 3D hierarchical porous carbonaceous nanocubes is reported as efficient sulfur hosts, composed of carbon nanotubes (CNT) and Co nanoparticles (NPs) uniformly embedded into a nitrogen-doped carbon matrix (NC). Because of the high specific surface area, large degree of graphitization, and the synergetic effects between Co NPs and N-doping, the as-designed CNTs/Co@NC electrodes not only significantly increase polysulfides immobilization, but also efficiently catalyze sulfur redox reactions, as confirmed by experimental results and DFT calculations. When tested in a RT Na-S battery, the S@CNTs/Co@NC-0.25 cathode demonstrates outstanding electrochemical performance, achieving high initial specific capacity of 1200.3 mAh g^-1 at 0.1 C, remarkable rate capability up to 5.0 C (474.2 mAh g^-1 ), and superior cyclic performance of 450.5 mAh g^-1 (292 mAh g^-1 ) after 400 cycles at 1.0 C (5.0 C). The integration of a 3D hierarchical porous architecture with well-dispersed Co NPs of an electro-catalyst provides valuable insights based on structure-adsorption-catalysis engineering for advanced RT Na-S batteries.

Towards high performance room temperature sodium-sulfur batteries: Strategies to avoid shuttle effect

Room temperature sodium-sulfur battery has high theoretical specific energy and low cost, so it has good application prospect. However, due to the disadvantageous reaction between soluble intermediate polysulfides and sodium anode, the capacity drops sharply, which greatly limits its practical application. In recent years, various strategies have been formulated to address the problem of polysulfides dissolution. This perspective article provides an overview of the research progress on research progress of novel cathode materials, multifunctional host, new electrolyte systems and modified separator/interlayer/anode. The challenge and prospect of the advanced strategies to suppress the polysulfides shuttle for long-life and high-efficiency room temperature sodium-sulfur batteries are proposed.

Ultrafine Sodium Sulfide Clusters Confined in Carbon Nano-polyhedrons as High-Efficiency Presodiation Reagents for Sodium-Ion Batteries

Sodium loss at the anode in the initial sodiation process significantly reduces the energy density of sodium-ion batteries (SIBs). Here, a high-capacity Na₂S/C nanocomposite featuring ultrafine Na₂S nanoclusters (<2 nm) confined in ZIF-8-derived microporous N-doped carbon is fabricated and employed as a cathode presodiation reagent to compensate for this sodium loss and increase the energy density of SIBs. The ultrafine size of Na₂S enables fast reaction kinetics for sodium extraction and the carbon matrix provides good electronic conductivity. Also, the overall particle size of the Na₂S/C nanocomposite (∼40 nm) is close to that of conductive additive. The above features enable it to replace a partial amount of conductive additive and compensate for the sodium loss at the anode concurrently. As a demonstration, the Na₃V₂(PO₄)₃ electrode with 5 wt % Na₂S/C and 5 wt % carbon black was fabricated, and it displayed a 19 mAh g^-1 higher initial charge specific capacity than that of the counterpart with 10% carbon black without the addition of Na₂S/C, realizing an increased energy density from 178 to 263 Wh kg^-1 in the full cell configuration pairing with a hard carbon anode. Moreover, a stable cycling performance up to 200 cycles with an average capacity loss of 0.024 mAh g^-1 per cycle was achieved for the presodiated Na₃V₂(PO₄)₃ electrode.

Transition metal sulfides meet electrospinning: versatile synthesis, distinct properties and prospective applications

One-dimensional (1D) electrospun nanomaterials have attracted significant attention due to their unique structures and outstanding chemical and physical properties such as large specific surface area, distinct electronic and mass transport, and mechanical flexibility. Over the past years, the integration of metal sulfides with electrospun nanomaterials has emerged as an exciting research topic owing to the synergistic effects between the two components, leading to novel and interesting properties in energy, optics and catalysis research fields for example. In this review, we focus on the recent development of the preparation of electrospun nanomaterials integrated with functional metal sulfides with distinct nanostructures. These functional materials have been prepared via two efficient strategies, namely direct electrospinning and post-synthesis modification of electrospun nanomaterials. In this review, we systematically present the chemical and physical properties of the electrospun nanomaterials integrated with metal sulfides and their application in electronic and optoelectronic devices, sensing, catalysis, energy conversion and storage, thermal shielding, adsorption and separation, and biomedical technology. Additionally, challenges and further research opportunities in the preparation and application of these novel functional materials are also discussed.

Tunable Electrocatalytic Behavior of Sodiated MoS2 Active Sites toward Efficient Sulfur Redox Reactions in Room-Temperature Na-S Batteries

Room-temperature (RT) sodium-sulfur (Na-S) batteries hold great promise for large-scale energy storage due to the advantages of high energy density, low cost, and resource abundance. The research progress on RT Na-S batteries, however, has been greatly hindered by the sluggish kinetics of the sulfur redox reactions. Herein, an elaborate multifunctional architecture, consisting of N-doped carbon skeletons and tunable MoS₂ sulfiphilic sites, is fabricated via a simple one-pot reaction followed by in situ sulfurization. Beyond the physical confinement and chemical binding of polarized N-doped carbonaceous microflowers, the MoS₂ active sites play a key role in catalyzing polysulfide redox reactions, especially the conversion from long-chain Na₂ S_n (4 ≤ n ≤ 8) to short-chain Na₂ S₂ and Na₂ S. Significantly, the electrocatalytic activity of MoS₂ can be tunable via adjusting the discharge depth. It is remarkable that the sodiated MoS₂ exhibits much stronger binding energy and electrocatalytic behavior compared to MoS₂ sites, effectively enhancing the formation of the final Na₂ S product. Consequently, the S cathode achieves superior electrochemical performance in RT Na-S batteries, delivering a high capacity of 774.2 mAh g^-1 after 800 cycles at 0.2 A g^-1 , and an ultrahigh capacity retention with a capacity decay rate of only 0.0055% per cycle over 2800 cycles.

The promises, challenges and pathways to room-temperature sodium-sulfur batteries

Room-temperature sodium-sulfur batteries (RT-Na-S batteries) are attractive for large-scale energy storage applications owing to their high storage capacity as well as the rich abundance and low cost of the materials. Unfortunately, their practical application is hampered by severe challenges, such as low conductivity of sulfur and its reduced products, volume expansion, polysulfide shuttling effect and Na dendrite formation, which can lead to rapid capacity fading. The review discusses the Na-S-energy-storage chemistry, highlighting its promise, key challenges and potential strategies for large-scale energy storage systems. Specifically, we review the electrochemical principles and the current technical challenges of RT-Na-S batteries, and discuss the strategies to address these obstacles. In particular, we give a comprehensive review of recent progresses in cathodes, anodes, electrolytes, separators and cell configurations, and provide a forward-looking perspective on strategies toward robust high-energy-density RT-Na-S batteries.

Multi-step Controllable Catalysis Method for the Defense of Sodium Polysulfide Dissolution in Room-Temperature Na-S Batteries

Room-temperature (RT) sodium-sulfur batteries hold great promise for the development of efficient, low-cost, and environmentally friendly energy storage systems. Nevertheless, the dissolution of long-chain polysulfides is a huge obstacle. In this work, a composite cathode which integrates Ni/Co bimetal nanoparticles as the catalyst and carbon spheres with abundant channels as the host is prepared for RT Na-S batteries. Moreover, a valuable strategy to reduce the dissolution of polysulfides by accurately regulating the two-step reaction kinetics of polysulfide transformation (from Na₂S to long-chain polysulfides and then from polysulfides to sulfur) is presented. Through adjusting the ratio of Ni and Co, the optimal cathode with a Ni/Co ratio of 1:2 can retard the first conversion of Na₂S to polysulfides and simultaneously accelerate the subsequent transformation of polysulfides to sulfur. In this case, the soluble polysulfides can immediately transform to solid sulfur as soon as it appears, thus avoiding the shuttle of polysulfides. The galvanostatic intermittent titration method and in situ Raman are employed to supervise the transformation of polysulfides during the discharge/charge process. As a result, the composite shows excellent performance as the cathode of RT liquid/quasi-solid-state Na-S batteries in terms of specific capacities, rate capability, and cycle stability.

MXene-encapsulated hollow Fe3O4 nanochains embedded in N-doped carbon nanofibers with dual electronic pathways as flexible anodes for high-performance Li-ion batteries

Fe3O4 is one of the promising anode materials in Li-ion batteries and a potential alternative to graphite due to the high specific capacity, natural abundance, environmental benignity, non-flammability, and better safety. Nevertheless, the sluggish intrinsic reaction kinetics and huge volume variation severely limit the reversible capacity and cycling life. In order to overcome these hurdles and enhance the cycling life of Fe3O4, a one-dimensional (1D) nanochain structure composed of 2D Ti3C2-encapsulated hollow Fe3O4 nanospheres homogeneously embedded in N-doped carbon nanofibers (Fe3O4@MXene/CNFs) is designed and demonstrated as a high-performance anode in Li-ion batteries. The distinctive 1D nanochain structure not only inherits the high electrochemical activity of Fe3O4, but also exhibits excellent electron and ion conductivity. The Ti3C2 layer on the Fe3O4 hollow nanospheres forms the primary electron transport pathway and the N-doped carbon nanofiber network provides the secondary transport pathway. At the same time, Ti3C2 flakes partially accommodate the large volume change of Fe3O4 during Li+ insertion/extraction. Density functional theory (DFT) calculations demonstrate that the Fe3O4@MXene/CNFs electrode can efficiently enhance the adsorption of Li+ to promote Li+ storage. As a result of the electrospinning process, self-restacking of Ti3C2 flakes and aggregation of Fe3O4 nanospheres can be prevented resulting in a larger surface area and more accessible active sites on the flexible anode. The Fe3O4@MXene/CNFs anode has remarkable electrochemical properties at high current densities. For example, a reversible capacity of 806 mA h g-1 can be achieved at 2 A g-1 even after 500 cycles, corresponding to an area specific capacity of 1.612 mA h cm-2 at 4 mA cm-2 and a capacity as high as 613 mA h g-1 is retained at 5 A g-1, corresponding to an area capacity of 1.226 mA h cm-2 at 10 mA cm-2. The results indicate that the Fe3O4@MXene/CNFs anode has excellent properties for Li-ion storage.

Efficient Catalytic Conversion of Polysulfides by Biomimetic Design of "Branch-Leaf" Electrode for High-Energy Sodium-Sulfur Batteries

Rechargeable room temperature sodium-sulfur (RT Na-S) batteries are seriously limited by low sulfur utilization and sluggish electrochemical reaction activity of polysulfide intermediates. Herein, a 3D "branch-leaf" biomimetic design proposed for high performance Na-S batteries, where the leaves constructed from Co nanoparticles on carbon nanofibers (CNF) are fully to expose the active sites of Co. The CNF network acts as conductive "branches" to ensure adequate electron and electrolyte supply for the Co leaves. As an effective electrocatalytic battery system, the 3D "branch-leaf" conductive network with abundant active sites and voids can effectively trap polysulfides and provide plentiful electron/ions pathways for electrochemical reaction. DFT calculation reveals that the Co nanoparticles can induce the formation of a unique Co-S-Na molecular layer on the Co surface, which can enable a fast reduction reaction of the polysulfides. Therefore, the prepared "branch-leaf" CNF-L@Co/S electrode exhibits a high initial specific capacity of 1201 mAh g^-1 at 0.1 C and superior rate performance.

An MXene-based aerogel with cobalt nanoparticles as an efficient sulfur host for room-temperature Na–S batteries

A network-like MG-Co composite with adsorption and catalysis for Na₂S_x is synthesized as a S host for room temperature Na–S batteries, exhibiting excellent electrochemical performance.

Polyacrylamide hydrogel-derived three-dimensional hierarchical porous N,S co-doped carbon frameworks for electrochemical capacitors

Polyacrylamide hydrogel-derived 3D porous hierarchical N,S co-doped carbon frameworks are purposefully fabricated, and exhibit superior electrochemical capacitance in both alkaline and acidic electrolytes.