Achieving Ultrafast and Stable Na-Ion Storage in FeSe2 Nanorods/Graphene Anodes by Controlling the Surface Oxide

Construction of Sugar-Gourd-Shaped Carbon Nanofibers Embedded with Heterostructured Zinc-Cobalt Selenide Nanocages for Superior Potassium-Ion Storage

Transition metal selenides are considered as promising anode materials for potassium-ion batteries (PIBs) due to their high theoretical capacities. However, their applications are limited by low conductivity and large volume expansion. Herein, sugar-gourd-shaped carbon nanofibers embedded with heterostructured ZnCo-Se nanocages are prepared via a facile template-engaged method combined with electrospinning and selenization process. In this hierarchical ZnCo-Se@NC/CNF, abundant phase boundaries of CoSe2/ZnSe heterostructure can promote interfacial electron transfer and chemical reactivity. The interior porous ZnCo-Se@NC nanocage structure relieves volume expansion and maintains structural integrity during K+ intercalation and deintercalation. The exterior spinning carbon nanofibers connect the granular nanocages in series, which prevents the agglomeration, shortens the electron transport distance and enhances the reaction kinetics. As a self-supporting anode material, ZnCo-Se@NC/CNF delivers a high capacity (362 mA h g-1 at 0.1 A g-1 after 100 cycles) with long-term stability (95.9% capacity retention after 1000 cycles) and shows superior reaction kinetics with high-rate K-storage. Energy level analysis and DFT calculations illustrate heterostructure facilitates the adsorption of K+ and interfacial electron transfer. The K+ storage mechanism is revealed by ex situ XRD and EIS analyses. This work opens a novel avenue in designing high-performance heterostructured anode materials with ingenious structure for PIBs.

Designing Sphere-like FeSe2-Carbon Composites with Rational Construction of Interfacial Traits towards Considerable Sodium-storage Capabilities

Due to their low cost and high stability, sodium-ion batteries have been increasingly studied. However, their further development is limited by the relative energy density, resulting in the search for high-capacity anodes. FeSe₂ displays high conductivity and capacity but still suffers from sluggish kinetics and serious volume expansion. Herein, through sacrificial template methods, a series of sphere-like FeSe₂-carbon composites are successfully prepared, displaying uniform carbon coatings and interfacial chemical FeOC bonds. Moreover, benefiting from the unique traits of precursor and acid treatment, rich structural voids are prepared, effectively alleviating volume expansion. Utilized as anodes of sodium-ion batteries, the optimized sample displays considerable capacity, achieving 462.9 mAh g^-1, with 88.75% coulombic efficiency at 1.0 A g^-1. Even at 5.0 A g^-1, their capacity can be kept at approximately 318.8 mAh g^-1, while the stable cycling can be prolonged to 200 cycles above. Supported by the detailed kinetic analysis, it can be noted that the existing chemical bonds facilitate the fast shuttling of ions at the interface, and the enhanced surface/near-surface properties are further vitrified. Given this, the work is expected to offer valuable insights for the rational design of metal-based samples toward advanced sodium-storage materials.

CuSe2 Nanocubes Enabling Efficient Sodium Storage

As the most promising candidate for lithium-ion batteries (LIBs), the electrochemical performance of sodium-ion batteries (SIBs) is highly dependent on the electrode materials. Copper selenides have established themselves as potential anode materials for SIBs due to their high theoretical capacity and good conductivity. However, the poor rate performance and fast capacity fading are the major challenges to their practical application in SIBs. Herein, single-crystalline CuSe₂ nanocubes (CuSe₂ NCs) are successfully synthesized via a solvothermal method. As an anode of SIBs, the CuSe₂ NCs render an almost 100% initial Coulombic efficiency, an outstanding long cycle life, e.g., 380 mA h g^-1 after 1700 cycles at 10 A g^-1, and an unprecedented rate performance of 344 mA h g^-1 at 50 A g^-1. Ex situ X-ray diffraction (XRD) patterns reveal the crystalline transformation of energy-storage materials, and the density functional theory (DFT) conclusion suggests that fast and stable ion diffusion kinetics enhances their electrochemical performance upon sodiation/desodiaton. The investigation into the mechanism provides a theoretical basis for subsequent practical applications.

Metal Selenides Anode Materials for Sodium Ion Batteries: Synthesis, Modification, and Application

The powerful and rapid development of lithium-ion batteries (LIBs) in secondary batteries field makes lithium resources in short supply, leading to rising battery costs. Under the circumstances, sodium-ion batteries (SIBs) with low cost, inexhaustible sodium reserves, and analogous work principle to LIBs, have evolved as one of the most anticipated candidates for large-scale energy storage devices. Thereinto, the applicable electrode is a core element for the smooth development of SIBs. Among various anode materials, metal selenides (MSe_x ) with relatively high theoretical capacity and unique structures have aroused extensive interest. Regrettably, MSe_x suffers from large volume expansion and unwished side reactions, which result in poor electrochemistry performance. Thus, strategies such as carbon modification, structural design, voltage control as well as electrolyte and binder optimization are adopted to alleviate these issues. In this review, the synthesis methods and main reaction mechanisms of MSe_x are systematically summarized. Meanwhile, the major challenges of MSe_x and the corresponding available strategies are proposed. Furthermore, the recent research progress on layered and nonlayered MSe_x for application in SIBs is presented and discussed in detail. Finally, the future development focuses of MSe_x in the field of rechargeable ion batteries are highlighted.

Building core-shell FeSe2@C anode electrode for delivering superior potassium-ion batteries

Inferior electrical conductivity and large volume variation are two disadvantages of metal selenides. In this work, we have designed a core-shell structure of FeSe₂@C composite with low cost using facile hydrothermal method. The FeSe₂particles as the 'core' and the carbon layer as the 'shell' displayed good synergistic effect that attributed to alleviate volume expansion of electrode and improving the electrical conductivity, which achieved the fast potassium storage. The core-shell structural FeSe₂@C electrode achieved 286 mA h g^-1at 1 A g^-1over 1000 cycles with 99.8% coulombic efficiency and delivered excellent rate capacity with 273 mA h g^-1at 2 A g^-1, which was ascribed to dispersed FeSe₂particles and the strong carbon shell coating. This work will provide the basis for the further development of the application of metal selenides in the field of flexible electrodes.

Realizing an aqueous sodium-ion battery with super-high discharge voltage based on a novel FeSe2@rGO anode

Aqueous sodium-ion batteries (ASIBs) promise particularly increased operational safety and lower manufacturing cost than current state-of-the-art organic electrolytes-based lithium-ion batteries. However, the output voltages of reported ASIBs are still restricts...

Carbon-Coated Three-Dimensional MXene/Iron Selenide Ball with Core-Shell Structure for High-Performance Potassium-Ion Batteries

Two-dimensional (2D) MXenes are promising as electrode materials for energy storage, owing to their high electronic conductivity and low diffusion barrier. Unfortunately, similar to most 2D materials, MXene nanosheets easily restack during the electrode preparation, which degrades the electrochemical performance of MXene-based materials. A novel synthetic strategy is proposed for converting MXene into restacking-inhibited three-dimensional (3D) balls coated with iron selenides and carbon. This strategy involves the preparation of Fe₂O₃@carbon/MXene microspheres via a facile ultrasonic spray pyrolysis and subsequent selenization process. Such 3D structuring effectively prevents interlayer restacking, increases the surface area, and accelerates ion transport, while maintaining the attractive properties of MXene. Furthermore, combining iron selenides and carbon with 3D MXene balls offers many more sites for ion storage and enhances the structural robustness of the composite balls. The resultant 3D structured microspheres exhibit a high reversible capacity of 410 mAh g^-1 after 200 cycles at 0.1 A g^-1 in potassium-ion batteries, corresponding to the capacity retention of 97% as calculated based on 100 cycles. Even at a high current density of 5.0 A g^-1, the composite exhibits a discharge capacity of 169 mAh g^-1.

Realizing Fast Charge Diffusion in Oriented Iron Carbodiimide Structure for High-Rate Sodium-Ion Storage Performance

Iron carbodiimide (FeNCN) belongs to a type of metal compounds with a more covalent bonding structure compared to common transition metal oxides. It could provide possibilities for various structural designs with improved charge-transfer kinetics in battery systems. Moreover, these possibilities are still highly expected for promoting enhancement in rate performance of sodium (Na)-ion battery. Herein, oriented FeNCN crystallites were grown on the carbon-based substrate with exposed {010} faces along the [001] direction (O-FeNCN/S). It provides a high Na-ion storage capacity with excellent rate capability (680 mAh g^-1 at 0.2 A g^-1 and 360 mAh g^-1 at 20 A g^-1), presenting rapid charge-transfer kinetics with high contribution of pseudocapacitance during a typical conversion reaction. This high rate performance is attributed to the oriented morphology of FeNCN crystallites. Its orientation along [001] maintains preferred Na-ion diffusion along the two directions in the entire morphology of O-FeNCN/S, supporting fast Na-ion storage kinetics during the charge/discharge process. This study could provide ideas toward the understanding of the rational structural design of metal carbodiimides for attaining high electrochemical performance in future.

FeSe2@C Microrods as a Superior Long-Life and High-Rate Anode for Sodium Ion Batteries

Transition-metal selenides have emerged as promising anode materials for sodium ion batteries (SIBs). Nevertheless, they suffer from volume expansion, polyselenide dissolution, and sluggish kinetics, which lead to inadequate conversion reaction toward sodium and poor reversibility during the desodiation process. Therefore, the transition-metal selenides are far from long cycling stability, outstanding rate performance, and high initial Coulombic efficiency, which are the major challenges for practical application in SIBs. Here, an efficient anode material including an FeSe₂ core and N-doped carbon shell with inner void space as well as high conductivity is developed for outstanding rate performance and long cycle life SIBs. In the ingeniously designed FeSe₂@NC microrods, the N-doped carbon shell can facilitate mass transport/electron transfer, protect the FeSe₂ core from the electrolyte, and accommodate volume variation of FeSe₂ with the help of the inner void of the core. Thus, the FeSe₂@NC microrods can maintain strong structural integrity upon long cycling and ensure a good reversible conversion reaction of FeSe₂ during the discharge/charge process. As a result, the as-prepared FeSe₂@NC microrods exhibit excellent sodium storage performance and ultrahigh stability, achieving an excellent rate capability (411 mAh g^-1 at 10.0 A g^-1) and a long-term cycle performance (401.3 mAh g^-1 after 2000 cycles at 5.0 A g^-1).

Rational design of hierarchical FeSe2 encapsulated with bifunctional carbon cuboids as an advanced anode for sodium-ion batteries

Earth-abundant transition-metal selenides (TMSs) have aroused great interest towards their application in sodium-ion batteries (SIBs). Herein, we present Fe-based Prussian blue analogs (PBA) modified by graphene oxide as precursors to synthesize FeSe2 nanoparticles within a nitrogen-doped carbon (NC) matrix and graphene layer (FeSe2/NC@G). The bifunctional carbon wrapped FeSe2/NC@G shows excellent sodium-storage performance with a large reversible capacity of 331 mA h g-1 at 5.0 A g-1 and a high cyclability of 323 mA h g-1 at the current density of 2.0 A g-1 after 1000 cycles (82% capacity retention). Furthermore, full SIBs are also fabricated and exhibit superior capacities and stabilities. The remarkable electrochemical properties result from the formation of an Fe-O-C chemical bond in the composite with enhanced electronic/ionic diffusion kinetics and structural integrity. This study paves the way for the successful synthesis of novel nanostructural TMSs which can be utilized in energy storage system application.

Core-Shell FeSe2 /C Nanostructures Embedded in a Carbon Framework as a Free Standing Anode for a Sodium Ion Battery

Embedding the functional nanostructures into a lightweight nanocarbon framework is very promising for developing high performance advanced electrodes for rechargeable batteries. Here, to realize workable capacity, core-shell (FeSe₂ /C) nanostructures are embedded into carbon nanotube (CNT) framework via a facile wet-chemistry approach accompanied by thermally induced selenization. The CNT framework offers 3D continuous routes for electronic/ionic transfer, while macropores provide adequate space for high mass loading of FeSe₂ /C. However, the carbon shell not only creates a solid electronic link among CNTs and FeSe₂ but also improves the diffusivity of sodium ions into FeSe₂ , as well as acts as a buffer cushion to accommodate the volume variations. These unique structural features of CNT/FeSe₂ /C make it an excellent host for sodium storage with a capacity retention of 546 mAh g^-1 even after 100 cycles at 100 mA g^-1 . Moreover, areal and volumetric capacities of 5.06 mAh cm^-2 and 158 mAh cm^-3 are also achieved at high mass loading 16.9 mg cm^-2 , respectively. The high performance of multi-benefited engineered structure makes it a potential candidate for secondary ion batteries, while its easy synthesis makes it extendable to further complex structures with other morphologies (such as nanorods, nanowires, etc.) to meet the high energy demands.

Building Hierarchical Microcubes Composed of One-Dimensional CoSe2 @Nitrogen-Doped Carbon for Superior Sodium Ion Batteries

Designing and synthesizing highly stable anode materials with high capacity is critical for the practical application of sodium ion batteries (SIBs), however, to date, this remains an insurmountable barrier. The introduction of hierarchical architectures and carbon supports is proving an effective strategy for addressing these challenges. Thus, we have fabricated a hierarchical CoSe₂ @nitrogen-doped carbon (CoSe₂ @NC) microcube composite using the Prussian blue analogue Co₃ [Co(CN)₆ ]₂ as template. The rational combination of the unique hierarchical construction from one to three dimensions and a nitrogen-doped carbon skeleton facilitates sodium ion and electron transport as well as stabilizing the host structure during repeated discharge/charge processes, which contributes to its excellent sodium storage capability. As expected, the as-prepared CoSe₂ @NC composite delivered remarkable reversible capacity and ultralong cycling lifespan even at a high rate of 2.0 A g^-1 (384.3 mA h g^-1 after1800 loops) when serving as the anode material for SIBs. This work shows the great potential of the CoSe₂ -based anode for practical application in SIBs, and the original strategy may be extended to other anode materials.

Conversion Reaction Mechanism of Ultrafine Bimetallic Co-Fe Selenides Embedded in Hollow Mesoporous Carbon Nanospheres and Their Excellent K-Ion Storage Performance

Potassium-ion batteries (KIBs) are considered as promising alternatives to lithium-ion batteries owing to the abundance and affordability of potassium. However, the development of suitable electrode materials that can stably store large-sized K ions remains a challenge. This study proposes a facile impregnation method for synthesizing ultrafine cobalt-iron bimetallic selenides embedded in hollow mesoporous carbon nanospheres (HMCSs) as superior anodes for KIBs. This involves loading metal precursors into HMCS templates using a repeated "drop and drying" process followed by selenization at various temperatures, facilitating not only the preparation of bimetallic selenide/carbon composites but also controlling their structures. HMCSs serve as structural skeletons, conductive templates, and vehicles to restrain the overgrowth of bimetallic selenide particles during thermal treatment. Various analysis strategies are employed to investigate the charge-discharge mechanism of the new bimetallic selenide anodes. This unique-structured composite exhibits a high discharge capacity (485 mA h g^-1 at 0.1 A g^-1 after 200 cycles) and enhanced rate capability (272 mA h g^-1 at 2.0 A g^-1 ) as a promising anode material for KIBs. Furthermore, the electrochemical properties of various nanostructures, from hollow to frog egg-like structures, obtained by adjusting the selenization temperature, are compared.

Nanocubes composed of FeS2@C nanoparticles as advanced anode materials for K-ion storage

The unique core–shell structural FeS₂@C nanocubes display outstanding K-storage performance with impressive specific capacity, excellent cycling stability and superior rate capability with 73% capacity retention at 2 A g⁻¹.

In Situ Fabrication of Carbon-Encapsulated Fe7X8 (X = S, Se) for Enhanced Sodium Storage

Sodium-ion batteries (SIBs) have been regarded as a promising alternative to lithium-ion batteries due to the natural abundance of sodium in the earth's crust. In our work, fusiform Fe₇X₈@C (X = S, Se) composites were obtained via a one-step pyrolysis strategy applied to SIB anode materials. The formed carbon skeleton could prevent the Fe₇X₈ nanoparticles from agglomeration and stabilize the interface of Fe/Na₂X generated in the redox reactions. Fe₇X₈@C (X = S, Se) exhibits excellent reversible specific capacity (1005.3 mAh g^-1 under 0.2 A g^-1 for Fe₇S₈@C and 458.5 mAh g^-1 under 0.5 A g^-1 for Fe₇Se₈@C), outstanding rate performance (654.7 mAh g^-1 for Fe₇S₈@C and 392.9 mAh g^-1 for Fe₇Se₈@C going through 300 loops even under 2 A g^-1), and excellent cycling properties (795.8 mAh g^-1 after 50 loops under 0.2 A g^-1 for Fe7S₈@C and 399.9 mAh g^-1 going through 150 loops under 0.5 A g^-1 for Fe₇Se₈@C). The excellent electrochemical performance of Fe₇X₈@C composites makes them promising anode materials for SIBs.

Applications of MxSey (M = Fe, Co, Ni) and Their Composites in Electrochemical Energy Storage and Conversion

Transition-metal selenides (M_xSe_y, M = Fe, Co, Ni) and their composites exhibit good storage capacities for sodium and lithium ions and occupy a unique position in research on sodium-ion and lithium-ion batteries. M_xSe_y and their composites are used as active materials to improve catalytic activity. However, low electrical conductivity, poor cycle stability, and low rate performance severely limit their applications. This review provides a comprehensive introduction to and understanding of the current research progress of M_xSe_y and their composites. Moreover, this review proposes a broader research platform for these materials, including various bioelectrocatalytic performance tests, lithium-sulfur batteries, and fuel cells. The synthesis method and related mechanisms of M_xSe_y and their composites are reviewed, and the effects of material morphologies on their electrochemical performance are discussed. The advantages and disadvantages of M_xSe_y and their composites as well as possible strategies for improving the storage and conversion of electrochemical energy are also summarized.

A Salt-Templated Strategy toward Hollow Iron Selenides-Graphitic Carbon Composite Microspheres with Interconnected Multicavities as High-Performance Anode Materials for Sodium-Ion Batteries

In this work, a facile salt-templated approach is developed for the preparation of hollow FeSe₂ /graphitic carbon composite microspheres as sodium-ion battery anodes; these are composed of interconnected multicavities and an enclosed surface in-plane embedded with uniform hollow FeSe₂ nanoparticles. As the precursor, Fe₂ O₃ /carbon microspheres containing NaCl nanocrystals are obtained using one-pot ultrasonic spray pyrolysis in which inexpensive NaCl and dextrin are used as a porogen and carbon source, respectively, enabling mass production of the composites. During post-treatment, Fe₂ O₃ nanoparticles in the composites transform into hollow FeSe₂ nanospheres via the Kirkendall effect. These rational structures provide numerous conductive channels to facilitate ion/electron transport and enhance the capacitive contribution. Moreover, the synergistic effect between the hollow cavities within FeSe₂ and the outstanding mechanical strength of the porous carbon matrix can effectively accommodate the large volume changes during cycling. Correspondingly, the composite microsphere exhibits high discharge capacity of 510 mA h g^-1 after 200 cycles at 0.2 A g^-1 with capacity retention of 88% when calculated from the second cycle. Even at a high current density of 5.0 A g^-1 , a high discharge capacity of 417 mA h g^-1 can be achieved.

Two-Dimensional NiSe2/N-Rich Carbon Nanocomposites Derived from Ni-Hexamine Frameworks for Superb Na-Ion Storage

Design of functional carbon-based nanomaterials from metal-organic frameworks (MOFs) has attracted soaring interests in recent years. However, a MOF-derived strategy toward two-dimensional (2D) nanomaterials remains a great challenge. In this work, we develop a layered Ni-hexamine framework as efficient precursor to prepare a 2D NiSe₂/N-rich carbon nanocomposite by a simple pyrolysis and subsequent selenization process. In the 2D NiSe₂/N-rich carbon nanocomposite, NiSe₂ nanoparticles with diameters of ca. 75 nm are homogeneously distributed in the N-rich carbon nanosheets. When serving as anode materials for sodium-ion batteries, the 2D nanocomposites exhibit a high reversible capacity of 410 mAh g^-1 at 1 A g^-1 and maintain a value of 255 mAh g^-1 even at 10 A g^-1. The excellent electrochemical performance can be attributed to the synergistic effects between the N-rich carbon nanosheets and NiSe₂ nanoparticles. More importantly, the hexamine-based MOFs can be regarded as new and powerful platforms for the fabrication of 2D N-rich carbon-based nanomaterials, which is of great importance for various potential applications.

Graphene-Loaded Bi2Se3: A Conversion-Alloying-type Anode Material for Ultrafast Gravimetric and Volumetric Na Storage

Sodium-ion battery (SIB) has been a promising alternative for sustainable electrochemical energy-storage devices. However, it still needs great efforts to develop electrode materials with ultrafast gravimetric and volumetric Na-storage performance, due to difficult balance between Na-ion diffusion kinetics and pressing density of materials. In this work, Bi₂Se₃/graphene composites, synthesized by a selenization reaction, are investigated as anode materials for SIBs. Na-ion storage mechanism of Bi₂Se₃ should be attributed to a combined conversion-alloying one by a series of ex situ measurements. In the composites, Bi₂Se₃ particles with an average diameter of 100 nm are uniformly dispersed onto graphene with strong interfacial interaction. Despite their nanoscale size, the pressing density of Bi₂Se₃/graphene composite could still reach a high value of 2.07 g/cm³. Therefore, the composites can deliver a high gravimetric specific capacity of 346 mAh/g and volumetric specific capacity of 716 mAh/cm³ at a current density of 0.1 A/g. Remarkably, the composites exhibit an ultrafast Na-storage capability and a negligible capacity fading with the increasing of current density from 0.2 to 5 A/g. Even at 10 A/g (≈30 C), the composites still possess a gravimetric capacity of 183 mAh/g and volumetric capacity of 379 mAh/cm³ with ultrastable cyclability up to 1000 cycles. This work introduces a valid route to design electrode materials with both excellent gravimetric and volumetric performance of Na-ion storage.