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

Mo2C-Co heterostructure with carbon nanosheets decorated carbon microtubules: Different means for high-performance lithium-sulfur batteries

The practical applications of lithium sulfur batteries (LSBs) are hindered by notorious shuttle effect and sluggish conversion kinetics of intermediate polysulfides. Herein, Mo2C-Co heterogeneous particles decorated two-dimensional (2D) carbon nanosheets grown on hollow carbon microtubes (CCC@MCC) are synthesized. Three-dimensional (3D) carbon framework with Mo2C-Co heterogeneous particles combines the conductivity, adsorption and catalysis, effectively trapping and accelerating the conversion of polysulfides. As evidenced experimentally, the hetero-structured Mo2C-Co with high Li+ diffusion coefficient enables uniform precipitation and complete oxidation of Li2S. Meanwhile, CCC@MCC is found to have multiple application possibilities for lithium-sulfur batteries. As an interlayer, the cells deliver an excellent capacity of 881.1 mAh/g at 2C and still retain 438.2 mAh/g after 500 cycles under the low temperature of 0 ℃. As a sulfur carrier, the cell with a sulfur loading of 7.0 mg cm-2 exhibits a high area capacity of 5.3 mAh cm-2. This work provides an effective strategy to prepare heterostructured material and imaginatively exploit the application potential of it.

The design of chemisorption and catalysis synergistic defender for efficient room temperature sodium-sulfur batteries

Room temperature sodium-sulfur (RT-Na/S) batteries are a promising candidate for large-scale energy storage systems owing to their low manufacturing cost and high energy density. However, the severe shuttle effects and sluggish reaction kinetics hinder their practical application. Here, a Fe3Se4 nanoparticle anchored three-dimensional nitrogen-doped porous carbon nanosheet was designed as a functional defender to inhibit the shuttle effect and achieve high sulfur utilization. The porous carbon nanosheet builds a fast platform for electron and ion transport and acts as a limiting barrier for polysulfide dissolution and shuttling. Additionally, Fe3Se4 nanoparticles are incorporated to enhance the chemical anchoring and catalytic activity of polysulfides. The ex-situ characterization revealed that the Fe sites can feed electrons to polysulfides, thus facilitating the conversion of long-chain polysulfides to Na2S, resulting in high sulfur availability (323 mAh/g at 2 A/g) and long-term cycle life (72 % capacity retention at 1 A/g for 500 cycles).

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.

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.

3D Flower-like Nanospheres Constructed by Transition Metal Telluride Nanosheets as Sulfur Immobilizers for High-Performance Room-Temperature Na-S Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries hold immense promise as next-generation energy storage systems, owing to their exceptionally high theoretical capacity, abundant resources, eco-friendliness, and affordability. Nevertheless, their practical application is impeded by the shuttling effect of sodium polysulfides (NaPSs) and sluggish sulfur redox kinetics. In this study, an advanced strategy by designing 3D flower-like molybdenum telluride (MoTe2) as an efficient catalyst to promote sulfur redox for RT Na-S batteries is presented. The unique 3D flower-like MoTe2 effectively prevents NaPS shuttling and simultaneously offers abundant active catalytic sites facilitating polysulfide redox. Consequently, the obtained MoTe2/S cathode delivers an outstanding initial reversible capacity of 1015 mAh g-1 at 0.1 C, along with robust cycling stability of retaining 498 mAh g-1 at 1 C after 500 cycles. In addition, pouch cells are fabricated with the MoTe2 additive to deliver an ultrahigh initial discharge capacity of 890 mAh g-1 and remain stable over 40 cycles under practically necessary conditions, demonstrating the potential application in the commercialization of RT Na-S batteries.

Defect engineering of a TiO2 anatase/rutile homojunction accelerating sulfur redox kinetics for high-performance Na-S batteries

Room-temperature sodium-sulfur (RT Na-S) batteries have the drawbacks of the poor shuttle effect of soluble sodium polysulfides (NaPSs) as well as slow sulfur redox kinetics, which result in poor cycling stability and low capacity, seriously affecting their extensive application. Herein, defect engineering is applied to construct rich oxygen vacancies at the interface of a TiO2 anatase/rutile homojunction (OV-TRA) to enhance sulfur affinity and redox reaction kinetics. Combining structural characterizations with electrochemical analysis reveals that OV-TRA well alleviates the shuttle effect of NaPSs and precipitates the deposition and diffusion kinetics of Na2S. Consequently, S/OV-TRA provides excellent electrochemical performance with a reversible capacity of 870 mA h g-1 at 0.1 C after 100 cycles and a long-term cycling capability of 759 mA h g-1 at 1 C after 1000 cycles. This work provides an effective interfacial defect engineering strategy to promote the application of metal oxides in RT Na-S batteries.

Linearly Interlinked Fe-Nx-Fe Single Atoms Catalyze High-Rate Sodium-Sulfur Batteries

Linearly interlinked single atoms offer unprecedented physiochemical properties, but their synthesis for practical applications still poses significant challenges. Herein, linearly interlinked iron single-atom catalysts that are loaded onto interconnected carbon channels as cathodic sulfur hosts for room-temperature sodium-sulfur batteries are presented. The interlinked iron single-atom exhibits unique metallic iron bonds that facilitate the transfer of electrons to the sulfur cathode, thereby accelerating the reaction kinetics. Additionally, the columnated and interlinked carbon channels ensure rapid Na+ diffusion kinetics to support high-rate battery reactions. By combining the iron atomic chains and the topological carbon channels, the resulting sulfur cathodes demonstrate effective high-rate conversion performance while maintaining excellent stability. Remarkably, even after 5000 cycles at a current density of 10 A g-1, the Na-S battery retains a capacity of 325 mAh g-1. This work can open a new avenue in the design of catalysts and carbon ionic channels, paving the way to achieve sustainable and high-performance energy devices.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

Reversible Carbon Dioxide/Lithium Oxalate Regulation toward Advanced Aprotic Lithium Carbon Dioxide Battery

Li-CO2 batteries have received significant attention owing to their advantages of combining greenhouse gas utilization and energy storage. However, the high kinetic barrier between gaseous CO2 and the Li2CO3 product leads to a low operating voltage (<2.5 V) and poor energy efficiency. In addition, the reversibility of Li2CO3 has always been questioned owing to the introduction of more decomposition paths caused by its higher charging plateau. Here, a novel "trinity" Li-CO2 battery system was developed by synergizing CO2, soluble redox mediator (2,2,6,6-tetramethylpiperidoxyl, as TEM RM), and reduced graphene oxide electrode to enable selective conversion of CO2 to Li2C2O4. The designed Li-CO2 battery exhibited an output plateau reaching up to 2.97 V, higher than the equilibrium potential of 2.80 V for Li2CO3, and an ultrahigh round-trip efficiency of 97.1 %. The superior performance of Li-CO2 batteries is attributed to the TEM RM-mediated preferential growth mechanism of Li2C2O4, which enhances the reaction kinetics and rechargeability. Such a unique design enables batteries to cope with sudden CO2-deficient environments, which provides an avenue for the rationally design of CO2 conversion reactions and a feasible guide for next-generation Li-CO2 batteries.

Temporally Decoupled Ammonia Splitting by a Zn-NH3 Battery with an Ammonia Oxidation/Hydrogen Evolution Bifunctional Electrocatalyst as a Cathode

Ammonia splitting to hydrogen is a decisive route for hydrogen economy but is seriously limited by the complex device and low efficiency. Here, we design and propose a new rechargeable Zn-NH3 battery based on temporally decoupled ammonia splitting to achieve efficient NH3-to-H2 conversion. In this system, ammonia is oxidized into nitrogen during cathodic charging (2NH3 + 6OH- → N2 + 6H2O + 6e-) with external electrical energy conversion and storage, while during cathodic discharging, water is reduced to hydrogen (2H2O + 2e- → H2 + 2OH-) with electrical energy generation. In this loop, continuous and efficient H2 production without separation and purification is achieved. With the help of the ammonia oxidation reaction (AOR) and hydrogen evolution reaction (HER) bifunctional catalyst of Mo2C/NiCu@C, a rechargeable Zn-NH3 battery is realized that exhibits a high NH3-to-H2 FE of 91.6% with outstanding durability for 900 cycles (300 h) at 20 mA/cm2, enabling efficient and continuous NH3-to-H2 conversion.

Unleashing the high energy potential of zinc-iodide batteries: high-loaded thick electrodes designed with zinc iodide as the cathode

The realization of high energy is of great importance to unlock the practical potential of zinc-iodine batteries. However, significant challenges, such as low iodine loading (mostly less than 50 wt%), restricted iodine reutilization, and severe structural pulverization during cycling, compromise its intrinsic features. This study introduces an optimized, fully zincified zinc iodide loaded onto a hierarchical carbon scaffold with high active component loading and content (82 wt%) to prepare a thick cathode for enabling high-energy Zn-I2 batteries. The synergistic interactions between nitrogen heteroatoms and cobalt nanocrystals within the porous matrix not only provide forceful chemisorption to lock polyiodide intermediates but also invoke the electrocatalytic effects to manipulate efficient iodine conversion. The ZnI2 cathode could effectively alleviate continuous volumetric expansion and maximize the utilization of active species. The electrochemical examinations confirm the thickness-independent battery performance of assembled Zn-I2 cells due to the ensemble effect of composite electrodes. Accordingly, with a thickness of 300 μm and ZnI2 loading of up to 20.5 mg cm-2, the cathode delivers a specific capacity of 92 mA h gcathode-1 after 2000 cycles at 1C. Moreover, the Zn-I2 pouch cell with ZnI2 cathode has an energy density of 145 W h kgcathode-1 as well as a stable long cycle life.

Zero-Strain Sodium Lanthanum Titanate Perovskite Embedded in Flexible Carbon Fibers as a Long-Span Anode for Lithium-Ion Batteries

"High-capacity" graphite and "zero-strain" spinel Li4Ti5O12 (LTO) occupy the majority market of anode materials for Li+ storage in commercial applications. Nevertheless, their intrinsic drawbacks including the unsafe potential of graphite and unsatisfactory capacity of LTO limit the further development of lithium-ion batteries (LIBs), which is unable to satisfy the ever-increasing demands. Here, a novel Na0.35La0.55TiO3 perovskite embedded in multichannel carbon fibers (NLTO-NF) is rationally designed and synthesized through an electrospinning method. It not only has the advantages of a respectable specific capacity of 265 mAh g-1 at 0.1 A g-1 and superb rate capability, but it also possesses the zero-strain characteristic. Impressively, an ultralong cycling life with 96.3% capacity retention after 9000 cycles at 2 A g-1 is achieved in the half cell, and 90.3% of capacity retention ratio is obtained after even 2500 cycles at 1 A g-1 in the coupled LiFePO4/NLTO-NF full cell. This study introduces a new member with excellent performance to the zero-strain materials family for next-generation LIBs.

Interfacial Synergy in Mo2C/MoC Heterostructure Promoting Sequential Polysulfide Conversion in High-Performance Lithium-Sulfur Battery

A rational design of sulfur host is the key to conquering the"polysulfide shuttle effects" by accelerating the polysulfide conversion. Since the process involves solid-liquid-solid multistep phase transitions, purposely-engineered heterostructure catalysts with various active regions for catalyzing conversion steps correspondingly are beneficial to promote the overall conversion process. However, the functionalities of the materials surface and interface in heterostructure catalysts remain unclear. In this work, an Mo2C/MoC catalyst with abundant Mo2C surface-interface-MoC surface tri-active-region is developed by in situ converting the MoZn-metal organic framework. The experimental and simulation studies demonstrate the interface can catch long-chain polysulfides and promote their conversion. Instead, the Mo2C and MoC tend to accommodate the short-chain polysulfide and accelerate their conversion and the Li2S dissociation. Benefitting from the high catalytic ability, the Li-S battery assembled with the Mo2C/MoC-S cathode shows more discrete redox reactions and delivers a high initial capacity of 1603.6 mAh g-1 at 1 C charging-discharging rate, which is over twofolds of the one assembled using individual hosts, and 80.4% capacity can be maintained after 1000 cycles at 3 C rate. This work has demonstrated a novel synergy between the interface and material surface, which will help the future design of high-performance Li-S batteries.

Electronic configuration regulation of single-atomic Mn sites mediated by Mo/Mn clusters for an efficient hydrogen evolution reaction

Tuning the electron distribution of metal single-atom active sites via bimetallic clusters is an effective way to enhance their hydrogen evolution reaction (HER) activity, but remains a great challenge. A biochar-based electrocatalyst (BCMoMn800-2) with both MnN4 active sites and Mo2C/Mn7C3 clusters was synthesized using in situ enriched Mo/Mn biomass as a precursor to trigger the HER. Various characterization and density functional theory (DFT) calculation results indicated that the presence of Mo2C/Mn7C3 clusters in BCMoMn800-2 effectively induced the redistribution of charges at MnN4 sites, reducing the energy of H* activation during the HER. In 0.5 M H2SO4, the overpotential was 27.4 mV at a current density of 10 mA cm-2 and the Tafel slope was 31 mV dec-1, and its electrocatalytic performance was close to that of Pt/C. The electrocatalyst also exhibited excellent electrocatalytic stability and durability. This work might provide a new strategy for solid waste recycling and constructing efficient HER electrocatalysts.

Sodium-Selenium Batteries with Outstanding Rate Capability by Introducing Cubic Mn2 O3 Electrocatalyst

With their high volumetric capacity and electronic conductivity, sodium-selenium (Na-Se) batteries have attracted attention for advanced battery systems. However, the irreversible deposition of sodium selenide (Na2 Se) results in rapid capacity degradation and poor Coulombic efficiency. To address these issues, cubic α-Mn2 O3 is introduced herein as an electrocatalyst to effectively catalyze Na2 Se conversion and improve the utilization of active materials. The results show that the addition of 10 wt% Mn2 O3 in the selenium/Ketjen black (Se/KB) composite enhances the conversion from Na2 Se to Se by lowering activation energy barrier and leads to fast sodium-ion kinetics and low internal resistance. Consequently, the Mn2 O3 -based composite delivers a high specific capacity of 635 mAh ⋅ g-1 at 675 mA ⋅ g-1 after 250 cycles as well as excellent cycling stability for 800 cycles with a high specific capacity of 317 mAh ⋅ g-1 even at the high current density of 3375 mA ⋅ g-1 . Due to the cubic Mn2 O3 electrocatalyst, the performance of the composites is superior to existing state-of-the-art Na-Se batteries reported in the literature.

Using Highly Electronegative Zn to Regulate the Superlattice Structure for the Na-Ion Layered Oxide Cathode with Superior Electrochemical Performance

The introduction of a superlattice structure into layered oxide cathode materials is a novel strategy to improve the structural stability of sodium-ion batteries (SIBs). However, the superlattice structure gradually disappears during cycling, which shortens the long life of SIBs. Here, the highly electronegative Zn is introduced into a P2-type layered oxide to regulate the superlattice structure. The obtained P2-Na0.80Li0.13Ni0.20Zn0.03Mn0.64O2 exhibits excellent cycling performance (the capacity retention is 96.7% after 100 cycles at 0.5C) and rate capability (95.8 mAh g-1 at 5C). Zn effectively inhibits the Li migration and the Mn dissolution, which ensures the integrity of the Li/Mn superlattice structure during long cycling, thus achieving an ultralong cycling life of SIBs. The introduction of Zn dramatically increases the length of the c-axis, leading to a faster de-embedding rate of Na+ and a better diffusion kinetics. Meanwhile, the larger pristine volume can withstand more stress/strain due to the sharp increase in the level of O-O repulsion during the desodiation process. In addition, Raman test results show that Zn can inhibit the Na+/vacancy ordering transition and improve the structural stability. This study confirms the feasibility of a Zn-regulated superlattice structure. It provides inspiration for the construction of stable layered oxide cathode materials for SIBs.

Intercalation-type catalyst for non-aqueous room temperature sodium-sulfur batteries

Ambient-temperature sodium-sulfur (Na-S) batteries are potential attractive alternatives to lithium-ion batteries owing to their high theoretical specific energy of 1,274 Wh kg-1 based on the mass of Na2S and abundant sulfur resources. However, their practical viability is impeded by sodium polysulfide shuttling. Here, we report an intercalation-conversion hybrid positive electrode material by coupling the intercalation-type catalyst, MoTe2, with the conversion-type active material, sulfur. In addition, MoTe2 nanosheets vertically grown on graphene flakes offer abundant active catalytic sites, further boosting the catalytic activity for sulfur redox. When used as a composite positive electrode and assembled in a coin cell with excess Na, a discharge capacity of 1,081 mA h gs-1 based on the mass of S with a capacity fade rate of 0.05% per cycle over 350 cycles at 0.1 C rate in a voltage range of 0.8 to 2.8 V is realized under a high sulfur loading of 3.5 mg cm-2 and a lean electrolyte condition with an electrolyte-to-sulfur ratio of 7 μL mg-1. A fundamental understanding of the electrocatalysis of MoTe2 is further revealed by in-situ synchrotron-based operando X-ray diffraction and ex-situ time-of-flight secondary ion mass spectrometry.

Coupling of Nd doping and oxygen-rich vacancy in CoMoO4@NiMoO4 nanoflowers toward advanced supercapacitors and photocatalytic degradation

In this paper, we successfully prepared rare earth element-doped 0.8% Nd-CoMoO4@NiMoO4 nanoflowers with a large specific surface area using the sol-gel method for the first time. In the experiment, we added a structure-directing agent to successfully assemble the nanosheets into a three-dimensional ordered micro-flower shape. By using the strategy of forming a flower-shaped morphology with a structure-directing agent and doping Nd elements to generate oxygen vacancies, the problems of the collapse of the active material structure and slow reaction kinetics were solved. Through relevant electrochemical performance tests, it was found that when the rare earth element Nd was doped at a concentration of 0.8%, the material exhibited exceptional specific capacitance (2387 F g-1 at 1 A g-1) and cycling stability (99.3% after 10 000 cycles at 5 A g-1). These performance characteristics far surpassed those of the other synthesized products. We assembled 0.8% Nd-CoMoO4@NiMoO4 with hydrophilic CNTs into an asymmetric device, 0.8% Nd-CoMoO4@NiMoO4//CNTs. This device exhibited high specific capacitance (262 F g-1 at 1 A g-1) and cycling stability (99.2% after 3000 cycles), with a good energy storage effect. In addition, 0.8% Nd-CoMoO4@NiMoO4 has a low band gap, which broadens the absorption range of the product and improves the utilization rate of visible light. The photocatalyst showed good degradation efficiency (all exceeding 96%) and cycling stability (96%) for all four dyes. This paper provides a new strategy and method for preparing doped polymetallic mixtures, which has potential application value.

High-Performance Alkali Metal Ion Storage in Bi2Se3 Enabled by Suppression of Polyselenide Shuttling Through Intrinsic Sb-Substitution Engineering

Bismuth selenide holds great promise as a kind of conversion-alloying-type anode material for alkali metal ion storage because of its layered structure with large interlayer spacing and high theoretical specific capacity. Nonetheless, its commercial development has been significantly hammered by the poor kinetics, severe pulverization, and polyselenide shuttle during the charge/discharge process. Herein, Sb-substitution and carbon encapsulation strategies are simultaneously employed to synthesize Sb_xBi_2-xSe₃ nanoparticles decorated on Ti₃C₂T_x MXene with encapsulation of N-doped carbon (Sb_xBi_2-xSe₃/MX⊂NC) as anodes for alkali metal ion storage. The superb electrochemical performances could be assigned to the cationic displacement of Sb³⁺ that effectively inhibits the shuttling effect of soluble polyselenides and the confinement engineering that alleviates the volume change during the sodiation/desodiation process. When used as anodes for sodium- and lithium-ion batteries, the Sb_0.4Bi_1.6Se₃/MX⊂NC composite exhibits superior electrochemical performances. This work offers valuable guidance to suppress the shuttling of polyselenides/polysulfides in high-performance alkali metal ion batteries with conversion/alloying-type transition metal sulfide/selenide anode materials.

Ultrafine Electrospun Cobalt-Molybdenum Bimetallic Nitride as a Durable Electrocatalyst for Hydrogen Evolution

Transition metal nitrides are promising electrocatalysts for hydrogen evolution reaction (HER) owing to their Pt-like electronic structure. However, the harsh nitriding conditions greatly limit their large-scale applications. Herein, ultrafine Co₃Mo₃N-Mo₂C (<1 nm)-decorated carbon nanofibers (Co₃Mo₃N-Mo₂C/CNFs) were prepared by electrostatic spinning followed by pyrolysis treatment, in which the MoCo-MOF simultaneously serves as the precursor and nitrogen source. The generated synergistic interactions between Mo₂C and Co₃Mo₃N significantly adjust the electronic structure of Mo₂C and afford a fast charge transfer, which endows the resultant hybrid with superior HER electrocatalytic performances. Specifically, the as-obtained Co₃Mo₃N-Mo₂C/CNF delivers a low overpotential of only 76 mV to achieve a current density of 10 mA cm^-2 and superior durability with no obvious degradation for 200 h in acidic media. This performance outperforms most of the transition metal-based electrocatalysts reported to date. This work paves a new way for the design of catalysts with ultrasmall size and high efficiency in energy conversion.

Polar Electrocatalysts for Preventing Polysulfide Migration and Accelerating Redox Kinetics in Room-Temperature Sodium-Sulfur Batteries

Due to the high theoretical energy density, low cost, and rich abundance of sodium and sulfur, room-temperature sodium-sulfur (RT Na-S) batteries are investigated as the promising energy storage system. However, the inherent insulation of the S₈ , the dissolution and shuttle of the intermediate sodium polysulfides (NaPSs), and especially the sluggish conversion kinetics, restrict the commercial application of the RT Na-S batteries. To address these issues, various catalysts are developed to immobilize the soluble NaPSs and accelerate the conversion kinetics. Among them, the polar catalysts display impressive performance. Polar catalysts not only can significantly accelerate (or alter) the redox process, but also can adsorb polar NaPSs through polar-polar interaction because of their intrinsic polarity, thus inhibiting the notorious shuttle effect. Herein, the recent advances in the electrocatalytic effect of polar catalysts on the manipulation of S speciation pathways in RT Na-S batteries are reviewed. Furthermore, challenges and research directions to realize rapid and reversible sulfur conversion are put forward to promote the practical application of 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.

Double design of host and guest synergistically reinforces the Na-ion storage of sulfur cathodes

Development of room-temperature sodium-sulfur batteries is significantly hampered by the shuttle effect of soluble intermediates and intrinsically sluggish conversion kinetics. In this work, a double design host and guest strategy (i.e., implantation of a polar V₂O₃ adsorbent into a carbon substrate and selenium doping of a sulfur guest) is proposed to synergistically reinforce the electrochemical properties of sulfur electrodes in sodium ion storage. The V₂O₃ adsorbent efficiently immobilizes sulfur species via strong polar-polar interactions, while the selenium dopant improves the electronic conductivity of sulfur cathodes and accelerates the redox conversion of sulfur cathodes. The synergistic effect between the V₂O₃ adsorbent and selenium dopant is shown to inhibit the shuttle effect and improve the redox kinetics, thus realizing greatly enhanced Na-ion storage properties of sulfur cathodes. The as-designed sulfur cathode delivers a superior rate capability of 663 mA h g^-1 at 2.0 A g^-1 and demonstrates excellent cyclability of 405 mA h g^-1 over 700 cycles at 1.0 A g^-1.

Tuning the Local Coordination of CoP1-xSx between NiAs- and MnP-Type Structures to Catalyze Lithium-Sulfur Batteries

The slow conversion and rapid shuttling of polysulfides remain major challenges that hinder the practical application of lithium-sulfur (Li-S) batteries. Efficient catalysts are needed to accelerate the conversion and suppress the shuttling. However, the lack of a rational understanding of catalysis poses obstacles to the design of catalysts, thereby limiting the rapid development of Li-S batteries. Herein, we theoretically analyze the modulation of the electronic structure of CoP_1-xS_x caused by the NiAs-to-MnP-type transition and its influence on catalytic activity. We found that the interacting d-orbitals of the active metal sites play a determining role in adsorption and catalysis, and the optimal d_z²-, d_xz-, and d_yz-orbitals in an appropriately distorted five-coordinate pyramid enable higher catalytic activity compared with their parent structures. Finally, rationally designed catalysts and S were electrospun into carbonized nanofibers to form nanoreactor chains for use as cathodes. The resultant Li-S batteries exhibited superior properties over 1000 cycles with only a decay rate of 0.031% per cycle and demonstrated a high capacity of 887.4 mAh g^-1 at a high S loading of 10 mg cm^-2. The structural modulation and bonding analyses in this study provide a powerful approach for the rational design of Li-S catalysts.

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.

Recent Progress in Rechargeable Sodium Metal Batteries: A Review

Sodium metal batteries (SMBs) have been widely studied owing to their relatively high energy density and abundant resources. However, they still need systematic improvement to fulfill the harsh operating conditions for their commercialization. In this review, we summarize the recent progress in SMBs in terms of sodium anode modification, electrolyte exploration, and cathode design. Firstly, we give an overview of the current challenges facing Na metal anodes and the corresponding solutions. Then, the traditional liquid electrolytes and the prospective solid electrolytes for SMBs are summarized. In addition, insertion- and conversion-type cathode materials are introduced. Finally, an outlook for the future of practical SMBs is provided.

Atomically Dispersed Dual-Site Cathode with a Record High Sulfur Mass Loading for High-Performance Room-Temperature Sodium-Sulfur Batteries

Room-temperature sodium-sulfur (RT-Na/S) batteries possess high potential for grid-scale stationary energy storage due to their low cost and high energy density. However, the issues arising from the low S mass loading and poor cycling stability caused by the shuttle effect of polysulfides seriously limit their operating capacity and cycling capability. Herein, sulfur-doped graphene frameworks supporting atomically dispersed 2H-MoS₂ and Mo₁ (S@MoS₂ -Mo₁ /SGF) with a record high sulfur mass loading of 80.9 wt.% are synthesized as an integrated dual active sites cathode for RT-Na/S batteries. Impressively, the as-prepared S@MoS₂ -Mo₁ /SGF display unprecedented cyclic stability with a high initial capacity of 1017 mAh g^-1 at 0.1 A g^-1 and a low-capacity fading rate of 0.05% per cycle over 1000 cycles. Experimental and computational results including X-ray absorption spectroscopy, in situ synchrotron X-ray diffraction and density-functional theory calculations reveal that atomic-level Mo in this integrated dual-active-site forms a delocalized electron system, which could improve the reactivity of sulfur and reaction reversibility of S and Na, greatly alleviating the shuttle effect. The findings not only provide an effective strategy to fabricate high-performance dual-site cathodes, but also deepen the understanding of their enhancement mechanisms at an atomic level.

Graphite Nanoflake-Modified Mo2C with Ameliorated Interfacial Interaction as an Electrocatalyst for Hydrogen Evolution Reaction

Molybdenum carbide (Mo₂C) is anticipated to be a promising electrocatalyst for electrocatalytic hydrogen production due to its low cost, resourceful property, prominent stability, and Pt-like electrocatalytic activity. The rational design of Mo₂C-based electrocatalysts is expected to improve hydrogen evolution reaction (HER) performance, especially by constructing ultrasmall Mo₂C particles and appropriate interfaces. Herein, composites of molybdenum carbide (Mo₂C) quantum dots anchored on graphite nanoflakes (Mo₂C/G) were fabricated, which realized a stable overpotential of 136 mV at 10 mA cm^-2 for the HER with a small Tafel slope of 76.81 mV dec^-1 in alkaline media, and operated stably over 10 h and 2000 cycles. The superior HER performance can be attributed to the fact that graphite nanoflakes could act as a matrix to disperse Mo₂C as quantum dots to expose more active sites and guarantee high electronic conductivity and, more importantly, provide ameliorated interfacial interaction between Mo₂C and graphite nanoflakes with appropriate hydrogen binding energy and charge density distribution. To further explore which kind of interfacial interaction is more favorable to improve the HER performance, density functional theory calculations and corresponding contrast experiments were also performed, and it was interesting to prove that Mo₂C quantum dots anchored to the basal planes of defective graphite nanoflakes exhibit better electrochemical performance than those anchored on the edges.

Catalytic Effects of Electrodes and Electrolytes in Metal-Sulfur Batteries: Progress and Prospective

Metal-sulfur (M-S) batteries are promising energy-storage devices due to their advantages such as large energy density and the low cost of the raw materials. However, M-S batteries suffer from many drawbacks. Endowing the electrodes and electrolytes with the proper catalytic activity is crucial to improve the electrochemical properties of M-S batteries. With regard to the S cathodes, advanced electrode materials with enhanced electrocatalytic effects can capture polysulfides and accelerate electrochemical conversion and, as for the metal anodes, the proper electrode materials can provide active sites for metal deposition to reduce the deposition potential barrier and control the electroplating or stripping process. Moreover, an advanced electrolyte with desirable design can catalyze electrochemical reactions on the cathode and anode in high-performance M-S batteries. In this review, recent progress pertaining to the design of advanced electrode materials and electrolytes with the proper catalytic effects is summarized. The current progress of S cathodes and metal anodes in different types of M-S batteries are discussed and future development directions are described. The objective is to provide a comprehensive review on the current state-of-the-art S cathodes and metal anodes in M-S batteries and research guidance for future development of this important class of batteries.

Decoupled Electrochemical Hydrazine "Splitting" via a Rechargeable Zn-Hydrazine Battery

Hydrogen generation via electrochemical splitting plays an important role to achieve hydrogen economy. However, the large-scale application is highly limited by high cost and low efficiency. Herein, a new type of rechargeable Zn-hydrazine (Zn-Hz) battery is proposed and realized by a bifunctional electrocatalyst based on two separate cathodic reactions of hydrogen evolution (discharge: 2H₂ O + 2e^-  → H₂  + 2OH^- ) and hydrazine oxidation (charge: 1 / 2   N 2 H 4 + 2 OH - → 1 / 2   N 2 + 2 H 2 O + 2 e - $1{\rm{/}}2\,{{\rm{N}}_2}{{\rm{H}}_4}{\bm{ + }}2{\rm{O}}{{\rm{H}}^{\bm{ - }}}{\bm{ \to }}1{\rm{/}}2\,{{\rm{N}}_2}{\bm{ + }}2{{\rm{H}}_2}{\rm{O}}{\bm{ + }}2{e^{\bm{ - }}}$ ). This Zn-Hz battery, driven by temporally decoupled electrochemical hydrazine splitting on the cathode during discharge and charge processes, can generate separated hydrogen without purification. When the highly active bifunctional cathode of 3D Mo₂ C/Ni@C/CS is paired with Zn foil, the Zn-Hz battery can achieve efficient hydrogen generation with a low energy input of less than 0.4 V (77.2 kJ mol^-1 ) and high energy efficiency of 96%. Remarkably, this battery exhibits outstanding long-term stability for 600 cycles (200 h), achieving continuous hydrogen production on demand, which presents great potential for practical application.

Porosity Engineering towards Nitrogen-Rich Carbon Host Enables Ultrahigh Capacity Sulfur Cathode for Room Temperature Potassium-Sulfur Batteries

Potassium-sulfur batteries (KSBs) are regarded as a promising large-scale energy storage technology, owing to the high theoretical specific capacity and intrinsically low cost. However, the commercialization of KSBs is hampered by the low sulfur utilization and notorious shuttle effect. Herein, we employ a porosity engineering strategy to design nitrogen-rich carbon foam as an efficient sulfur host. The tremendous micropores magnify the chemical interaction between sulfur species and the polar nitrogen functionalities decorated carbon surface, which significantly improve the sulfur utilization and conversion. Meanwhile, the abundant mesopores provide ample spaces, accommodating the large volume changes of sulfur upon reversible potassation. Resultantly, the constructed sulfur cathode delivers an ultrahigh initial reversible capacity of 1470 mAh g^-1 (87.76% of theoretical capacity) and a superior rate capacity of 560 mAh g^-1 at 2 C. Reaching the K₂S phase in potassiation is the essential reason for obtaining the ultrahigh capacity. Nonetheless, systematic kinetics analyses demonstrate that the K₂S involved depotassiation deteriorates the charge kinetics. The density functional theory (DFT) calculation revealed that the nitrogen-rich micropore surface facilitated the sulfur reduction for K₂S but created a higher energy barrier for the K₂S decomposition, which explained the discrepancy in kinetics modification effect produced by the porosity engineering.

Single-Atom Yttrium Engineering Janus Electrode for Rechargeable Na-S Batteries

The development of rechargeable Na-S batteries is very promising, thanks to their considerably high energy density, abundance of elements, and low costs and yet faces the issues of sluggish redox kinetics of S species and the polysulfide shuttle effect as well as Na dendrite growth. Following the theory-guided prediction, the rare-earth metal yttrium (Y)-N₄ unit has been screened as a favorable Janus site for the chemical affinity of polysulfides and their electrocatalytic conversion, as well as reversible uniform Na deposition. To this end, we adopt a metal-organic framework (MOF) to prepare a single-atom hybrid with Y single atoms being incorporated into the nitrogen-doped rhombododecahedron carbon host (Y SAs/NC), which features favorable Janus properties of sodiophilicity and sulfiphilicity and thus presents highly desired electrochemical performance when used as a host of the sodium anode and the sulfur cathode of a Na-S full cell. Impressively, the Na-S full cell is capable of delivering a high capacity of 822 mAh g^-1 and shows superdurable cyclability (97.5% capacity retention over 1000 cycles at a high current density of 5 A g^-1). The proof-of-concept three-dimensional (3D) printed batteries and the Na-S pouch cell validate the potential practical applications of such Na-S batteries, shedding light on the development of promising Na-S full cells for future application in energy storage or power batteries.

Room-Temperature Sodium-Sulfur Batteries: Rules for Catalyst Selection and Electrode Design

Seeking an optimal catalyst to accelerate conversion reaction kinetics of room-temperature sodium-sulfur (RT Na-S) batteries is crucial for improving their electrochemical performance and promoting the practical applications. Herein, theoretical calculations of interfacial interactions of catalysts and polysulfides in terms of the surface adsorption state, interfacial ions migration, and electronic concentration around the Fermi level are systematically proposed as guiding principles of catalyst selection for RT Na-S batteries. As a case, MoN catalyst is accurately selected from transition metal nitrides with different d orbital electrons, and for experiment, it is introduced into the carbon nanofibers as a dual-functioning host (MoN@CNFs). The MoN@CNFs can effectively anchor polysulfides and accelerate their conversion reaction. In addition, for the sodium anode, the MoN@CNFs can also induce uniform deposition of Na and inhibit dendrite growth, which are supported by in situ characterizations and finite element simulation technique. As a result, the as-prepared RT Na-S battery displays high reversible capacity of 990 mAh g^-1 at 0.2 A g^-1 after 100 cycles and long lifespan over 1500 cycles at 2 A g^-1 . Even with high S loading of 5 mg cm^-2 , the RT Na-S battery still exhibits a high areal capacity of 2.5 mAh cm^-2 .

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.

In situ implanting MnO nanoparticles into carbon nanorod-assembled microspheres enables performance-enhanced room-temperature Na–S batteries

In situ implanting MnO fine nanoparticles into carbon nanorod-assembled microspheres enables improved electrode stability and electrochemical performance via structural and compositional synergy.