An Emerging Energy Storage System: Advanced Na-Se Batteries

Iron nanoparticles surface decorated MXene via molten salts etching as selenium host for ultrafast sodium ion storage

Sodium-selenium (Na-Se) batteries have gained attention due to their high energy density and power density, resulting from the liquid-liquid reaction at the interface in the dimethoxyethane electrolyte. Nevertheless, the pronounced shuttle effect of polyselenides causes low coulomb efficiency and inadequate cycling stability for Na-Se batteries. Herein, the iron nanoparticles surface modified accordion-like Ti3C2Tx MXene (MXene/Fe) synthesized via the molten salt etching is utilized as the host of Se species for high-performance Na-Se battery cathode. Benefiting from the layered structure and chemical adsorption of accordion-like MXene, the shuttle effect of the cathode is effectively inhibited. Simultaneously, electrochemical kinetics is boosted due to the catalytic effect of Fe nanoparticles, which facilitate the transformation of polyselenide from long-chain to short-chain, contributing to pseudocapacitive capacity. Consequently, the Se-based cathode delivers a steady capacity of 575.0 mA h g-1 at 0.2 A/g, and even a high capacity of 500 mAh/g at 50 A/g based on the mass of Se@MXene/Fe electrode, indicating the ultrafast Na+ ion storage. Most notably, this structure demonstrated remarkable long-term cycling stability for 5000 cycles with a high capacity retention of 97.4 %. The electrochemical energy storage mechanism is further revealed by in situ Raman. Herein, the confinement-catalysis structure shines light on inhibiting shuttling and facilitating ultrafast ion storage.

Advanced cathodes for aqueous Zn batteries beyond Zn2+ intercalation

Aqueous zinc (Zn) batteries have attracted global attention for energy storage. Despite significant progress in advancing Zn anode materials, there has been little progress in cathodes. The predominant cathodes working with Zn2+/H+ intercalation, however, exhibit drawbacks, including a high Zn2+ diffusion energy barrier, pH fluctuation(s) and limited reproducibility. Beyond Zn2+ intercalation, alternative working principles have been reported that broaden cathode options, including conversion, hybrid, anion insertion and deposition/dissolution. In this review, we report a critical assessment of non-intercalation-type cathode materials in aqueous Zn batteries, and identify strengths and weaknesses of these cathodes in small-scale batteries, together with current strategies to boost material performance. We assess the technical gap(s) in transitioning these cathodes from laboratory-scale research to industrial-scale battery applications. We conclude that S, I2 and Br2 electrodes exhibit practically promising commercial prospects, and future research is directed to optimizing cathodes. Findings will be useful for researchers and manufacturers in advancing cathodes for aqueous Zn batteries beyond Zn2+ intercalation.

Facilitating Rapid Na+ Storage through MoWSe/C Heterostructure Construction and Synergistic Electrolyte Matching Strategy

The realization of sodium-ion devices with high-power density and long-cycle capability is challenging due to the difficulties of carrier diffusion and electrode fragmentation in transition metal selenide anodes. Herein, a Mo/W-based metal-organic framework is constructed by a one-step method through rational selection, after which MoWSe/C heterostructures with large angles are synthesized by a facile selenization/carbonization strategy. Through physical characterization and theoretical calculations, the synthesized MoWSe/C electrode delivers obvious structural advantages and excellent electrochemical performance in an ethylene glycol dimethyl ether electrolyte. Furthermore, the electrochemical vehicle mechanism of ions in the electrolyte is systematically revealed through comparative analyses. Resultantly, ether-based electrolytes advantageously construct stable solid electrolyte interfaces and avoid electrolyte decomposition. Based on the above benefits, the Na half-cell assembled with MoWSe/C electrodes demonstrated excellent rate capability and a high specific capacity of 347.3 mA h g-1 even after cycling 2000 cycles at 10 A g-1. Meanwhile, the constructed sodium-ion capacitor maintains ∼80% capacity retention after 11,000 ultralong cycles at a high-power density of 3800 W kg-1. The findings can broaden the mechanistic understanding of conversion anodes in different electrolytes and provide a reference for the structural design of anodes with high capacity, fast kinetics, and long-cycle stability.

K-birnessite-MnO2/hollow mulberry-like carbon complexes with stabilized and superior rate performance for aqueous magnesium ion storage

Manganese oxides are commonly employed as a cathode for magnesium ion storage in aqueous magnesium ion hybrid supercapacitors (MHS). However, sluggish reaction kinetics still hinders their practical application. Herein, we designed K-birnessite-MnO2 and electrostatically spun mulberry-like carbon composites (K-MnO2/HMCs) via an in situ growth technique. Benefiting from the 3D conductive carbon network substrate, the in situ fabricated K-MnO2 exhibits more active sites and provides more interfacial contact area between the electrode material and the electrolyte. This improvement enhances its conductivity, facilitating the rapid transfer of electrons, diffusion of ions, and redox reactions. As a result, K-MnO2/HMC-based MHS achieves a specific capacity of 168 mA h g-1 at 0.5 A g-1, simultaneously exhibiting a superior energy density of 111.1 W h kg-1 at a power density of 505 W kg-1. Furthermore, it demonstrates excellent high rate performance and a long cycling life for aqueous magnesium ion storage, offering new insights for MHS applications.

Tetrahedral Bonding Structure (Ni3 -Se) Induced by Lattice-Distortion of Ni to Achieve High Catalytic Activity in Na-Se Battery

Fabrication of transition-metal catalytic materials is regarded as a promising strategy for developing high-performance sodium-selenium (Na-Se) batteries. However, more systematic explorations are further demanded to find out how their bonding interactions and electronic structures can affect the Na storage process. This study finds that lattice-distorted nickel (Ni) structure can form different bonding structures with Na2 Se4 , providing high activity to catalyze the electrochemical reactions in Na-Se batteries. Using this Ni structure to prepare electrode (Se@NiSe2 /Ni/CTs) can realize rapid charge transfer and high cycle stability of the battery. The electrode exhibits high storage performance of Na+ ; i.e., 345 mAh g⁻1 at 1 C after 400 cycles, and 286.4 mAh g⁻1 at 10 C in rate performance test. Further results reveal the existence of a regulated electronic structure with upshifts of the d-band center in the distorted Ni structure. This regulation changes the interaction between Ni and Na2 Se4 to form a Ni3 -Se tetrahedral bonding structure. This bonding structure can provide higher adsorption energy of Ni to Na2 Se4 to facilitate the redox reaction of Na2 Se4 during the electrochemical process. This study can inspire the design of bonding structure with high performance in conversion-reaction-based batteries.

Rational design of MXene-based single atom catalysts for Na-Se batteries from sabatier principle

Na-Se batteries have attracted great attention because of their high-energy density and low cost, though the shuttle effect of polyselenides and sluggish reaction dynamics still limit their practical applications. Herein, MXenes were decorated with single zinc atom as selenium hosts, and the effect of interfacial electrochemical reaction was studied via first-principles simulation. The embedding of single zinc atom into MXenes was found to enhance the anchoring ability to inhibit the shuttle effect. However, Zn-MXenes as single atom catalysts had different effects on interfacial electrochemical reactions, which can be attributed to the increased interaction strengths between Zn-MXenes and polyselenides. For Ti-based MXenes, the enhanced interaction was found to be beneficial for the electrochemical reaction, whereas the overly strong anchoring strength of Zn-Cr2CO2 would inhibit charging-discharging kinetics. Therefore, the matching of MXenes and metal atoms should be considered to adjust the anchoring ability based on the Sabatier principle. This work provides new insights into the design of SACs and high-performance Na-Se batteries.

Facile aqueous synthesis of hollow dual plasmonic hetero-nanostructures with tunable optical responses through nanoscale Kirkendall effects

Herein, we report the colloidal synthesis of hollow dual-plasmonic nanoparticles (NPs) using Au@Cu₂O core-shell NPs as templates and exploiting the nanoscale Kirkendall effect. In our synthesis, we used organic compounds as a source of chalcogenide ions for an anion exchange reaction at elevated temperatures using polyvinylpyrrolidone (PVP) as a capping reagent to transform the solid Cu₂O shell into a hollow copper chalcogenide shell. The resulting structures possess different features depending on the chalcogenide precursor employed. TEM images confirm the complete transformation of Au@Cu₂O templates when 1,1-dimethyl-2-selenourea was added and the formation of hollow Au@Cu_2-x Se nanostructures. In contrast, residues of Cu₂O attached to the Au core were present when thioacetamide was used for the synthesis of Au@Cu_2-x S with all other conditions kept the same. The divergence of architectures caused distinct optical properties of Au@Cu_2-x S and Au@Cu_2-x Se NPs. This synthetic approach is an effective pathway for maneuvering the size of interior voids by varying the concentration of chalcogenide ions in the reaction mixture. The insights gained from this work will enrich the synthetic toolbox at the nanoscale and guide us on the rational design of multicomponent plasmonic nanoparticles with precisely controlled hollow interiors and sophisticated geometries, further enhancing our capabilities to fine-tune the electronic, optical, compositional, and physicochemical properties.

3D Sodiophilic Ti3C2 MXene@g-C3N4 Hetero-Interphase Raises the Stability of Sodium Metal Anodes

Owing to several advantages of metallic sodium (Na), such as a relatively high theoretical capacity, low redox potential, wide availability, and low cost, Na metal batteries are being extensively studied, which are expected to play a major role in the fields of electric vehicles and grid-scale energy storage. Although considerable efforts have been devoted to utilizing MXene-based materials for suppressing Na dendrites, achieving a stable cycling of Na metal anodes remains extremely challenging due to, for example, the low Coulombic efficiency (CE) caused by the severe side reactions. Herein, a g-C₃N₄ layer was attached in situ on the Ti₃C₂ MXene surface, inducing a surface state reconstruction and thus forming a stable hetero-interphase with excellent sodiophilicity between the MXene and g-C₃N₄ to inhibit side reactions and guide uniform Na ion flux. The 3D construction can not only lower the local current density to facilitate uniform Na plating/stripping but also mitigate volume change to stabilize the electrolyte/electrode interphase. Thus, the 3D Ti₃C₂ MXene@g-C₃N₄ nanocomposite enables much enhanced average CEs (99.9% at 1 mA h cm^-2, 0.5 mA cm^-2) in asymmetric half cells, long-term stability (up to 700 h) for symmetric cells, and stable cycling (up to 800 cycles at 2 C), together with outstanding rate capability (up to 20 C), of full cells. The present study demonstrates an approach in developing practically high performance for Na metal anodes.

Strain-regulated Gibbs free energy enables reversible redox chemistry of chalcogenides for sodium ion batteries

Manipulating the reversible redox chemistry of transition metal dichalcogenides for energy storage often faces great challenges as it is difficult to regulate the discharged products directly. Herein we report that tensile-strained MoSe₂ (TS-MoSe₂) can act as a host to transfer its strain to corresponding discharged product Mo, thus contributing to the regulation of Gibbs free energy change (ΔG) and enabling a reversible sodium storage mechanism. The inherited strain results in lattice distortion of Mo, which adjusts the d-band center upshifted closer to the Fermi level to enhance the adsorbability of Na₂Se, thereby leading to a decreased ΔG of the redox chemistry between Mo/Na₂Se and MoSe₂. Ex situ and in situ experiments revealed that, unlike the unstrained MoSe₂, TS-MoSe₂ shows a highly reversible sodium storage, along with an evidently improved reaction kinetics. This work sheds light on the study on electrochemical energy storage mechanism of other electrode materials.

Manipulating the redox chemistry of transition metal dichalcogenides still faces challenges. Here the authors report that tensile-strained MoSe2 can pass on the strain to its sodiated product Mo, and thus regulate the Gibbs free energy in the charging process to enable the reversible sodium storage.

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.

Enabling fast-charging selenium-based aqueous batteries via conversion reaction with copper ions

Selenium (Se) is an appealing alternative cathode material for secondary battery systems that recently attracted research interests in the electrochemical energy storage field due to its high theoretical specific capacity and good electronic conductivity. However, despite the relevant capacity contents reported in the literature, Se-based cathodes generally show poor rate capability behavior. To circumvent this issue, we propose a series of selenium@carbon (Se@C) composite positive electrode active materials capable of delivering a four-electron redox reaction when placed in contact with an aqueous copper-ion electrolyte solution (i.e., 0.5 M CuSO₄) and copper or zinc foils as negative electrodes. The lab-scale Zn | |Se@C cell delivers a discharge voltage of about 1.2 V at 0.5 A g⁻¹ and an initial discharge capacity of 1263 mAh g_Se⁻¹. Interestingly, when a specific charging current of 6 A g⁻¹ is applied, the Zn | |Se@C cell delivers a stable discharge capacity of around 900 mAh g_Se⁻¹ independently from the discharge rate. Via physicochemical characterizations and first-principle calculations, we demonstrate that battery performance is strongly associated with the reversible structural changes occurring at the Se-based cathode.

Aqueous battery Se-based cathodes are based on a two-electron transfer electrochemical reaction and generally show inadequate rate capability behaviour. Here, the authors propose a four-electron Se chemistry with copper ions as charge carriers to enable fast-charging battery cycling.

A Facile Pre-Lithiated Strategy towards High-Performance Li2Se-LiTiO2 Composite Cathode for Li-Se Batteries

Conventional lithium-ion batteries with a limited energy density are unable to assume the responsibility of energy-structure innovation. Lithium-selenium (Li-Se) batteries are considered to be the next generation energy storage devices since Se cathodes have high volumetric energy density. However, the shuttle effect and volume expansion of Se cathodes severely restrict the commercialization of Li-Se batteries. Herein, a facile solid-phase synthesis method is successfully developed to fabricate novel pre-lithiated Li₂Se-LiTiO₂ composite cathode materials. Impressively, the rationally designed Li₂Se-LiTiO₂ composites demonstrate significantly enhanced electrochemical performance. On the one hand, the overpotential of Li₂Se-LiTiO₂ cathode extremely decreases from 2.93 V to 2.15 V. On the other hand, the specific discharge capacity of Li₂Se-LiTiO₂ cathode is two times higher than that of Li₂Se. Such enhancement is mainly accounted to the emergence of oxygen vacancies during the conversion of Ti⁴⁺ into Ti³⁺, as well as the strong chemisorption of LiTiO₂ particles for polyselenides. This facile pre-lithiated strategy underscores the potential importance of embedding Li into Se for boosting electrochemical performance of Se cathode, which is highly expected for high-performance Li-Se batteries to cover a wide range of practical applications.

Hollow carbon spheres loaded with NiSe2 nanoplates as multifunctional SeS2 hosts for Li-SeS2 batteries

Selenium sulfide as a new alternative cathode material can effectively address the inferior electronic conductivity of sulfur, which is the main cause for poor electrochemical reactivity of conventional lithium-sulfur batteries (Li-S batteries). Therefore, in this work, hollow carbon spheres loaded with NiSe₂ nanoplates were prepared as SeS₂ hosts for Li-SeS₂ batteries. The unique micro-mesoporous hollow carbon spheres not only provide channels for the diffusion of SeS₂, but also afford spaces for alleviating the volume expansion of the active substance. Besides, the external polar NiSe₂ nanoplates increase active sites for capturing polysulfides or polyselenides during the charge/discharge process. Meanwhile, the excellent electronic conductivity of NiSe₂ can accelerate the catalytic reaction on the surface, thus reducing the loss of soluble intermediate products and finally suppressing the "shuttle effect". These extraordinary features of the as-proposed cathode offer many superiorities in electrochemical performances in terms of a high initial discharge capacity of 1139 mA h g^-1 at a current rate of 0.1C and an excellent cycling life of up to 1000 cycles at 1C.

Graphene-Supported Atomically Dispersed Metals as Bifunctional Catalysts for Next-Generation Batteries Based on Conversion Reactions

Next-generation batteries based on conversion reactions, including aqueous metal-air batteries, nonaqueous alkali metal-O₂ and -CO₂ batteries, alkali metal-chalcogen batteries, and alkali metal-ion batteries have attracted great interest. However, their use is restricted by inefficient reversible conversion of active agents. Developing bifunctional catalysts to accelerate the conversion reaction kinetics in both discharge and charge processes is urgently needed. Graphene-, or graphene-like carbon-supported atomically dispersed metal catalysts (G-ADMCs) have been demonstrated to show excellent activity in various electrocatalytic reactions, making them promising candidates. Different from G-ADMCs for catalysis, which only require high activity in one direction, G-ADMCs for rechargeable batteries should provide high activity in both discharging and charging. This review provides guidance for the design and fabrication of bifunctional G-ADMCs for next-generation rechargeable batteries based on conversion reactions. The key challenges that prevent their reversible conversion, the origin of the activity of bifunctional G-ADMCs, and the current design principles of bifunctional G-ADMCs for highly reversible conversion, have been analyzed and highlighted for each conversion-type battery. Finally, a summary and outlook on the development of bifunctional G-ADMC materials for next-generation batteries with a high energy density and excellent energy efficiency are given.

The rational design of nickel-cobalt selenides@selenium nanostructures by adjusting the synthesis environment for high-performance sodium-ion batteries

Different selenization products (from the perspectives of micromorphology and sodium storage performance) can be obtained via regulating the selenization environment.

Trimodal hierarchical porous carbon nanorods enable high-performance Na–Se batteries

Technical bottlenecks of polyselenide shuttling and material volume variation significantly hamper the development of emerging sodium–selenium (Na–Se) batteries. The nanopore structure of substrate materials is demonstrated to play a vital role in stabilizing Se cathodes and approaching superior Na-ion storage properties. Herein, an ideal nanorod-like trimodal hierarchical porous carbon (THPC) host is fabricated through a facile one-step carbonization method for advanced Na–Se batteries. The THPC possesses a trimodal nanopore structure encompassing micropores, mesopores, and macropores, and functions as a good accommodator of Se molecules, a reservoir of polyselenide intermediates, a buffer for volume expansion of Se species during sodiation, and a promoter for electron/ion transfer in the electrochemical process. As a result, Na–Se batteries assembled with the Se–THPC composite cathode realize high utilization of Se, fast redox kinetics, and excellent cyclability. Furthermore, the Na-ion storage mechanism of the well-designed Se–THPC composite is profoundly revealed by in situ visual characterization techniques.

Technical bottlenecks of polyselenide shuttling and material volume variation significantly hamper the development of emerging sodium–selenium (Na–Se) batteries.

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.