A Highly Dispersed Cobalt Electrocatalyst with Electron-Deficient Centers Induced by Boron toward Enhanced Adsorption and Electrocatalysis for Room-Temperature Sodium-Sulfur Batteries.

As vitally prospective candidates for next-generation energy storage systems, room-temperature sodium-sulfur (RT-Na/S) batteries continue to face obstacles in practical implementation due to the severe shuttle effect of sodium polysulfides and sluggish S conversion kinetics. Herein, the study proposes a novel approach involving the design of a B, N co-doped carbon nanotube loaded with highly dispersed and electron-deficient cobalt (Co@BNC) as a highly conductive host for S, aiming to enhance adsorption and catalyze redox reactions. Crucially, the pivotal roles of the carbon substrate in prompting the electrocatalytic activity of Co are elucidated. The experiments and density functional theory (DFT) calculations both demonstrate that after B doping, stronger chemical adsorption toward polysulfides (NaPSs), lower polarization, faster S conversion kinetics, and more complete S transformation are achieved. Therefore, the as-assembled RT-Na/S batteries with S/Co@BNC deliver a high reversible capacity of 626 mAh g-1 over 100 cycles at 0.1 C and excellent durability (416 mAh g-1 over 600 cycles at 0.5 C). Even at 2 C, the capacity retention remains at 61.8%, exhibiting an outstanding rate performance. This work offers a systematic way to develop a novel Co electrocatalyst for RT-Na/S batteries, which can also be effectively applied to other transition metallic electrocatalysts.

High-Performance Sulfurized Polyacrylonitrile Cathode by Using MXene as a Conductive and Catalytic Binder for Room-Temperature Na/S Batteries.

Sulfurized polyacrylonitrile (PAN@S) is a promising cathode material for room-temperature Na/S batteries but suffers from low conductivity and insufficient electrochemical activity, resulting in unsatisfactory actual capacity and rate performance. Herein, Ti3C2Tx MXene nanosheets are used as a conductive and catalytic binder to establish the PAN@S electrode, wherein MXene constructs a highly conductive framework for fast charge transport and provides high catalytic effect to improve the active material utilization and accelerate the redox kinetics significantly. Therefore, the PAN@S electrode bonded by MXene shows an electronic conductivity of 5.05 S cm-1, 4 orders of magnitude higher than the conventional electrodes bonded by the insulative polymer binders, and much decreased activation energy barrier and resistance. Consequently, the PAN@S electrode displays superior performance in terms of high capacity (697.3 mAh g-1 at 200 mA g-1), unparalleled rate capability (189.0 mAh g-1 at 20 A g-1), and excellent high-rate cycling performance (a capacity decay rate of ∼0.04% per cycle during 1000 cycles at 5 A g-1). This work provides a high-performance electrode for room-temperature Na/S batteries and shows the promising potential of conductive and catalytic MXene binders in boosting the performance of active materials.

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.

Chemical and spatial dual-confinement engineering for stable Na-S batteries with approximately 100% capacity retention.

Sodium-sulfur (Na-S) batteries are attracting intensive attention due to the merits like high energy and low cost, while the poor stability of sulfur cathode limits the further development. Here, we report a chemical and spatial dual-confinement approach to improve the stability of Na-S batteries. It refers to covalently bond sulfur to carbon at forms of C-S/N-C=S bonds with high strength for locking sulfur. Meanwhile, sulfur is examined to be S1-S2 small species produced by thermally cutting S8 large molecules followed by sealing in the confined pores of carbon materials. Hence, the sulfur cathode achieves a good stability of maintaining a high-capacity retention of 97.64% after 1000 cycles. Experimental and theoretical results show that Na+ is hosted via a coordination structure (N···Na···S) without breaking the C-S bond, thus impeding the formation and dissolution of sodium polysulfide to ensure a good cycling stability. This work provides a promising method for addressing the S-triggered stability problem of Na-S batteries and other S-based batteries.

Research on Wide-Temperature Rechargeable Sodium-Sulfur Batteries: Features, Challenges and Solutions.

Sodium-sulfur (Na-S) batteries hold great promise for cutting-edge fields due to their high specific capacity, high energy density and high efficiency of charge and discharge. However, Na-S batteries operating at different temperatures possess a particular reaction mechanism; scrutinizing the optimized working conditions toward enhanced intrinsic activity is highly desirable while facing daunting challenges. This review will conduct a dialectical comparative analysis of Na-S batteries. Due to its performance, there are challenges in the aspects of expenditure, potential safety hazards, environmental issues, service life and shuttle effect; thus, we seek solutions in the electrolyte system, catalysts, anode and cathode materials at intermediate and low temperatures (T < 300 °C) as well as high temperatures (300 °C < T < 350 °C). Nevertheless, we also analyze the latest research progress of these two situations in connection with the concept of sustainable development. Finally, the development prospects of this field are summarized and discussed to look forward to the future of Na-S batteries.

Core-Shell Tandem Catalysis Coupled with Interface Engineering For High-Performance Room-Temperature Na-S Batteries.

The sluggish redox kinetics and shuttle effect seriously impede the large application of room-temperature sodium-sulfur (RT Na-S) batteries. Designing effective catalysts into cathode material is a promising approach to overcome the above issues. However, considering the multistep and multiphase transformations of sulfur redox process, it is impractical to achieve the effective catalysis of the entire S8 →Na2 Sx →Na2 S conversion through applying a single catalyst. Herein, this work fabricates a nitrogen-doped core-shell carbon nanosphere integrated with two different catalysts (ZnS-NC@Ni-N4 ), where isolated Ni-N4 sites and ZnS nanocrystals are distributed in the shell and core, respectively. ZnS nanocrystals ensure the rapid reduction of S8 into Na2 Sx (4 < x ≤ 8), while Ni-N4 sites realize the efficient conversion of Na2 Sx into Na2 S, bridged by the diffusion of Na2 Sx from the core to shell. Besides, Ni-N4 sites on the shell can also induce an inorganic-rich cathode-electrolyte interface (CEI) on ZnS-NC@Ni-N4 to further inhibit the shuttle effect. As a result, ZnS-NC@Ni-N4 /S cathode exhibits an excellent rate-performance (650 mAh g-1 at 5 A g-1 ) and ultralong cycling stability for 2000 cycles with a low capacity-decay rate of 0.011% per cycle. This work will guide the rational design of multicatalysts for high-performance RT Na-S batteries.

High performance sulfur/carbon cathode for Na-S battery enabled by electrocatalytic effect of Sn-doped In2S3.

Room-temperature sodium-sulfur (RT Na-S) batteries have been attracting enormous interests due to their low-cost, high capacity and environmental benignity. However, the shuttle effect and the sluggish electrochemical reaction activity of sodium polysulfides (NaPSs) seriously restrict their practical application. To solve these issues, we rationally designed an advanced Sn-doped In2S3/S/C cathode for RT Na-S batteries by magnetron sputtering in this work, which exhibited a high reversible capacity (1663.5 mAh g-1 at 0.1 A g-1) and excellent cycling performance (902.9 mAh g-1 after 50 cycles). The in situ electrochemical impedance spectroscopy indicated that the Sn-doped In2S3 coating can accelerate charge-transfer kinetics and facilitate the diffusion of Na+. Furthermore, theoretical calculation revealed that doping of Sn into In2S3 can reduce the energy band gap, thus accelerating the electron transfer and promoting the electrochemical conversion of active species. It is demonstrated that adjusting the electronic structure is a reliable method to improve the electrocatalytic effect of catalyst and significantly improve the performance of S cathode in RT Na-S batteries.

Toward Complete Transformation of Sodium Polysulfides by Regulating the Second-Shell Coordinating Environment of Atomically Dispersed Fe.

Room temperature sodium-sulfur (RT Na-S) batteries are highly competitive as potential energy storage devices. Nevertheless, their actually achieved reversible capacities are far below the theoretical value due to incomplete transformation of polysulfides. Herein, atomically dispersed Fe-N/S active center by regulating the second-shell coordinating environment of Fe single atom is proposed. The Fe-N4 S2 coordination structure with enhanced local electronic concentration around the Fermi level is revealed via synchrotron radiation X-ray absorption spectroscopy (XAS) and theoretical calculations, which can not only significantly promote the transformation kinetics of polysulfides, but induce uniform Na deposition for dendrite-free Na anode. As a result, the obtained S cathode delivers a high initial reversible capacity of 1590 mAh g-1 , nearly the theoretical value. This work opens up a new avenue to facilitate the complete transformation of polysulfides for RT Na-S batteries.

Orthorhombic Nb2 O5 Decorated Carbon Nanoreactors Enable Bidirectionally Regulated Redox Behaviors in Room-Temperature Na-S Batteries.

Regulating redox kinetics is able to spur the great-leap-forward development of room-temperature sodium-sulfur (RT Na-S) batteries, especially on propelling their Na-ion storage capability. Here, an innovative metal oxide kinetics accelerator, orthorhombic Nb2 O5 Na-ion conductor, is proposed to functionalize porous carbon nanoreactors (CNR) for S cathodes. The Nb2 O5 is shown to chemically immobilize sodium polysulfides via strong affinity. Theoretical and experimental evidence reveals that the Nb2 O5 can bidirectionally regulate redox behaviors of S cathodes, which accelerates reduction conversions from polysulfides to sulfides as well as promotes oxidation reactions from sulfides to S. In situ and ex situ characterization techniques further verify its electrochemical lasting endurance in catalyzing S conversions. The well-designed S cathode demonstrates a high specific capacity of 1377 mA h g-1 at 0.1 A g-1 , outstanding rate capability of 405 mA h g-1 at 2 A g-1 , and stable cyclability with a capacity retention of 617 mA h g-1 over 600 cycles at 0.5 A g-1 . An ultralow capacity decay rate of 0.0193% per cycle is successfully realized, superior to those of current state-of-the-art RT Na-S batteries. This design also suits emerging Na-Se batteries, which contribute to outstanding electrochemical performance as well.

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 Sx (1 ≤ x ≤ 4) on surface of MoC/Mo2 C is significantly lowered compared to analogous redox in liquid. Strong binding of Na2 Sx (1 ≤ x ≤ 4) on MoC/Mo2 C surfaces results from charge transfer between the sulfur and Mo sites on carbides' surface.

Robust Room-Temperature Sodium-Sulfur Batteries Enabled by a Sandwich-Structured MXene@C/Polyolefin/MXene@C Dual-functional Separator.

Room-temperature sodium-sulfur (RT-Na-S) batteries are attracting increased attention due to their high theoretical energy density and low-cost. However, the traditional RT-Na-S batteries assembled with glass fiber (GF) separators are still hindered by the polysulfide shuttle effect and sodium dendrite growth, limiting the battery's capacity and cycling stability. Here, a facile and effective method toward commercial polyolefin separators for constructing stable RT-Na-S batteries is presented. By coating commercial polypropylene membrane with core-shell structured MXene@C nanosheets, a powerful dual-functional separator with improved electrolyte wettability that can inhibit polysulfide migration and induce uniform sodium disposition is developed. More importantly, the modified separator can also accelerate the conversion kinetics of sodium polysulfides. Benefiting from these characteristics, the as-prepared RT-Na-S battery exhibits a remarkably enhanced capacity (1159 mAh g-1 at 0.2 C) and excellent cycling performance (95.8% of capacity retention after 650 cycles at 0.5 C). This study opens a promising avenue for the development of high-performance Na-S batteries.

General Synthesis of Single-Atom Catalysts for Hydrogen Evolution Reactions and Room-Temperature Na-S Batteries.

Herein, we report a comprehensive strategy to synthesize a full range of single-atom metals on carbon matrix, including V, Mn, Fe, Co, Ni, Cu, Ge, Mo, Ru, Rh, Pd, Ag, In, Sn, W, Ir, Pt, Pb, and Bi. The extensive applications of various SACs are manifested via their ability to electro-catalyze typical hydrogen evolution reactions (HER) and conversion reactions in novel room-temperature sodium sulfur batteries (RT-Na-S). The enhanced performances for these electrochemical reactions arisen from the ability of different single active atoms on local structures to tune their electronic configuration. Significantly, the electrocatalytic behaviors of diverse SACs, assisted by density functional theory calculations, are systematically revealed by in situ synchrotron X-ray diffraction and in situ transmission electronic microscopy, providing a strategic library for the general synthesis and extensive applications of SACs in energy conversion and storage.

Polyolefin-Based Janus Separator for Rechargeable Sodium Batteries.

Rechargeable sodium batteries are a promising technology for low-cost energy storage. However, the undesirable drawbacks originating from the use of glass fiber membrane separators have long been overlooked. A versatile grafting-filtering strategy was developed to controllably tune commercial polyolefin separators for sodium batteries. The as-developed Janus separators contain a single-ion-conducting polymer-grafted side and a functional low-dimensional material coated side. When employed in room-temperature sodium-sulfur batteries, the poly(1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethanesulfonyl)imide sodium)-grafted side effectively enhances the electrolyte wettability, and inhibits polysulfide diffusion and sodium dendrite growth. Moreover, a titanium-deficient nitrogen-containing MXene-coated side electrocatalytically improved the polysulfide conversion kinetics. The as-developed batteries demonstrate high capacity and extended cycling life with lean electrolyte loading.

Design and Construction of Sodium Polysulfides Defense System for Room-Temperature Na-S Battery.

Room-temperature Na-S batteries are facing one of the most serious challenges of charge/discharge with long cycling stability due to the severe shuttle effect and volume expansion. Herein, a sodium polysulfides defense system is presented by designing and constructing the cathode-separator double barriers. In this strategy, the hollow carbon spheres are decorated with MoS2 (HCS/MoS2) as the S carrier (S@HCS/MoS2). Meanwhile, the HCS/MoS2 composite is uniformly coated on the surface of the glass fiber as the separator. During the discharge process, the MoS2 can adsorb soluble polysulfides (NaPSs) intermediates and the hollow carbon spheres can improve the conductivity of S as well as act as the reservoir for electrolyte and NaPSs, inhibiting them from entering the anode to make Na deteriorate. As a result, the cathode-separator group applied to room-temperature Na-S battery can enable a capacity of ≈1309 mAh g-1 at 0.1 C and long cycling life up to 1000 cycles at 1 C. This study provides a novel and effective way to develop durable room-temperature Na-S batteries.

Long-Life Room-Temperature Sodium-Sulfur Batteries by Virtue of Transition-Metal-Nanocluster-Sulfur Interactions.

Room-temperature sodium-sulfur (RT-Na/S) batteries hold significant promise for large-scale application because of low cost of both sodium and sulfur. However, the dissolution of polysulfides into the electrolyte limits practical application. Now, the design and testing of a new class of sulfur hosts as transition-metal (Fe, Cu, and Ni) nanoclusters (ca. 1.2 nm) wreathed on hollow carbon nanospheres (S@M-HC) for RT-Na/S batteries is reported. A chemical couple between the metal nanoclusters and sulfur is hypothesized to assist in immobilization of sulfur and to enhance conductivity and activity. S@Fe-HC exhibited an unprecedented reversible capacity of 394 mAh g-1 despite 1000 cycles at 100 mA g-1 , together with a rate capability of 220 mAh g-1 at a high current density of 5 A g-1 . DFT calculations underscore that these metal nanoclusters serve as electrocatalysts to rapidly reduce Na2 S4 into short-chain sulfides and thereby obviate the shuttle effect.

Binding S0.6 Se0.4 in 1D Carbon Nanofiber with CS Bonding for High-Performance Flexible Li-S Batteries and Na-S Batteries.

A one-step synthesis procedure is developed to prepare flexible S0.6 Se0.4 @carbon nanofibers (CNFs) electrode by coheating S0.6 Se0.4 powder with electrospun polyacrylonitrile nanofiber papers at 600 °C. The obtained S0.6 Se0.4 @CNFs film can be used as cathode material for high-performance Li-S batteries and room temperature (RT) Na-S batteries directly. The superior lithium/sodium storage performance derives from its rational structure design, such as the chemical bonding between Se and S, the chemical bonding between S0.6 Se0.4 and CNFs matrix, and the 3D CNFs network. This easy one-step synthesis procedure provides a feasible route to prepare electrode materials for high-performance Li-S and RT Na-S batteries.