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

Metal Coordination Complex Derived Fe7S8 Particles Embedded in N-doped Carbon Framework with Regulated Porous Structure Enabling High-Rate and Durable Sodium Storage

Iron sulfides with high theoretical capacity confront the challenges of low rate capability and severe capacity fading for sodium storage, which are mainly caused by poor electron/ion transport kinetics and drastic volume fluctuations during cycling. Herein, to mitigate these obstacles, a multi-step synthetic tactic involving solvothermal, carbonization, and subsequent sulfurization is put forward for the construction of wire-like structure by confining Fe7S8 particles in porous N-doped carbon framework (denoted as Fe7S8/PNC) using zinc iron nitrilotriacetate as template. By partially substituting Fe3+ with Zn2+ in the metal coordination complex, the porous structure of coordination complex derived carbon framework can be regulated through pore structure engineering of Zn nanodroplets. The desired porous and robust core/shell structure can not only afford favorable electron/Na+ transport paths and additional active sites for Na+ storage, but also provide reinforced structural integrity of interior Fe7S8 particles by retarding the pulverization and buffering the mechanical stress against volume fluctuations. As anode for sodium-ion batteries, the optimal Fe7S8/PNC delivers a high reversible capacity (743 mAh g-1 at 0.1 A g-1), superior rate capability (553 mAh g-1 at 10 A g-1), and long-term cycling stability (602 mAh g-1 at 5 A g-1 with 98.5% retention after 1000 cycles).

Engineering CSFe Bond Confinement Effect to Stabilize Metallic-Phase Sulfide for High Power Density Sodium-Ion Batteries

Metallic-phase iron sulfide (e.g., Fe7 S8 ) is a promising candidate for high power density sodium storage anode due to the inherent metal electronic conductivity and unhindered sodium-ion diffusion kinetics. Nevertheless, long-cycle stability can not be achieved simultaneously while designing a fast-charging Fe7 S8 -based anode. Herein, Fe7 S8 encapsulated in carbon-sulfur bonds doped hollow carbon fibers (NHCFs-S-Fe7 S8 ) is designed and synthesized for sodium-ion storage. The NHCFs-S-Fe7 S8 including metallic-phase Fe7 S8 embrace higher electron specific conductivity, electrochemical reversibility, and fast sodium-ion diffusion. Moreover, the carbonaceous fibers with polar CSFe bonds of NHCFs-S-Fe7 S8 exhibit a fixed confinement effect for electrochemical conversion intermediates contributing to long cycle life. In conclusion, combined with theoretical study and experimental analysis, the multinomial optimized NHCFs-S-Fe7 S8 is demonstrated to integrate a suitable structure for higher capacity, fast charging, and longer cycle life. The full cell shows a power density of 1639.6 W kg-1 and an energy density of 204.5 Wh kg-1 , respectively, over 120 long cycles of stability at 1.1 A g-1 . The underlying mechanism of metal sulfide structure engineering is revealed by in-depth analysis, which provides constructive guidance for designing the next generation of durable high-power density sodium storage anodes.

Realizing Ultrafast and Robust Sodium-Ion Storage of Iron Sulfide Enabled by Heteroatomic Doping and Regulable Interface Engineering

Fe-based sulfides are a promising type of anode material for sodium-ion batteries (SIBs) due to their high theoretical capacities and affordability. However, these materials often suffer from issues such as capacity deterioration and poor conductivity during practical application. To address these challenges, an N-doped Fe₇S₈ anode with an N, S co-doped porous carbon framework (PPF-800) was synthesized using a template-assisted method. When serving as an anode for SIBs, it delivers a robust and ultrafast sodium storage performance, with a discharge capacity of 489 mAh g^-1 after 500 cycles at 5 A g^-1 and 371 mAh g^-1 after 1000 cycles at 30 A g^-1 in the ether-based electrolyte. This impressive performance is attributed to the combined influence of heteroatomic doping and adjustable interface engineering. The N, S co-doped carbon framework embedded with Fe₇S₈ nanoparticles effectively addresses the issues of volumetric expansion, reduces the impact of sodium polysulfides, improves intrinsic conductivity, and stimulates the dominant pseudocapacitive contribution (90.3% at 2 mV s^-1). Moreover, the formation of a stable solid electrolyte interface (SEI) film by the effect of uniform pore structure in ether-based electrolyte produces a lower transfer resistance during the charge-discharge process, thereby boosting the rate performance of the electrode material. This work expands a facile strategy to optimize the electrochemical performance of other metal sulfides.

In-situ fabrication of active interfaces towards FeSe as advanced performance anode for sodium-ion batteries

Transition metal selenides have gained enormous interest as anodes for sodium ion batteries (SIBs). Nonetheless, their large volume expansion causing poor rate and inferior cycle stability during Na⁺ insertion/extraction process hinders their further applications in SIBs. Herein, a confined-regulated interfacial engineering strategy towards the synthesis of FeSe microparticles coated by ultrathin nitrogen-doped carbon (NC) is demonstrated (FeSe@NC). The strong interfacial interaction between FeSeand NC endows FeSe@NC with fast electron/Na⁺ transport kinetics and outstanding structural stability, delivering unexceptionable rate capability (364 mAh/gat 10 A/g) and preeminent cycling durability (capacity retention of 100 % at 1 A/g over 1000 cycles). Furthermore, variousex situcharacterization techniques and density functional theory (DFT) calculations have been applied to demonstrate the Na⁺ storage mechanism of FeSe@NC. The assembled Na₃V₂(PO₄)₂F₃@rGO//FeSe@NC full cell also displays a high capacity of 241 mAh/gat 1 A/g with the capacity retention of nearly 100 % over 2000 cycles, and delivers a supreme energy density of 135 Wh kg^-1 and a topmost power density of 495 W kg^-1, manifesting the latent applications of FeSe@NC in the fast charging SIBs.

Constructing Heterostructured Bimetallic Selenides on an N-Doped Carbon Nanoframework as Anodes for Ultrastable Na-Ion Batteries

Transition-metal selenides have been recognized as a class of promising anode materials for sodium-ion batteries (SIBs) on account of their high capacity. Nevertheless, the sluggish conversion kinetics and rapid capacity decay caused by insufficient conductivity and volume change restrain their applications. Herein, hollow heterostructured bimetallic selenides embedded in an N-doped carbon nanoframework (H-CoSe₂/ZnSe@NC) were prepared via a facile template-engaged method. Benefiting from the rich defect at the phase boundary of the CoSe₂/ZnSe heterostructure, pre-reserved cavity, and enhanced structure rigidity, the abovementioned issues are resolved at once, and the accelerated charge transportation kinetics traced by spectroscopy techniques and theoretical calculations certify the interface effect in the capacity release. In addition, ex situ X-ray photoelectron spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy all confirm the high-reversible electrochemical conversion mechanism in H-CoSe₂/ZnSe@NC. Together with a reasonable structural architecture and the highly reversible conversion reaction, H-CoSe₂/ZnSe@NC displays a prominent rate capacity (244.8 mA h g^-1 at 10 A g^-1) as well as an ultralong lifespan (10,000 cycles at 10 A g^-1), highlighting the significance of structure control in fabricating high-performance anodes for SIBs.

MOF-Derived Fe7 S8 Nanoparticles/N-Doped Carbon Nanofibers as an Ultra-Stable Anode for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have aroused wide concern due to their potential applications in large-scale energy-storage systems. In this work, a hybrid of Fe₇ S₈ nanoparticles/N-doped carbon nanofibers (Fe₇ S₈ /N-CNFs) is designed and synthesized via electrospinning. As an anode for SIBs, Fe₇ S₈ /N-CNFs exhibit a high reversible capacity of 649.9 mAh g^-1 at 0.2 A g^-1 after 100 cycles, and superior cycling stability for 2000 cycles at 1 A g^-1 with only 0.00302% capacity decay per cycle. Such excellent performance originates from: i) Fe₇ S₈ nanoparticles (average diameter of 17 nm), which shorten the Na⁺ diffusion distance; ii) the unique 3D N-CNFs, which enhance the conductivity, alleviate the self-agglomeration and large volume change of Fe₇ S₈ nanoparticles, and offer numerous active sites for Na⁺ adsorption and paths for electrolyte diffusion. The fascinating structure and superior electrochemical properties of Fe₇ S₈ /N-CNFs shed light on developing high-performance SIBs anode materials.

A Hierarchically Ordered Mesoporous-Carbon-Supported Iron Sulfide Anode for High-Rate Na-Ion Storage

Sodium-ion batteries (SIBs) are promising alternatives to lithium-based energy storage devices for large-scale applications, but conventional lithium-ion battery anode materials do not provide adequate reversible Na-ion storage. In contrast, conversion-based transition metal sulfides have high theoretical capacities and are suitable anode materials for SIBs. Iron sulfide (FeS) is environmentally benign and inexpensive but suffers from low conductivity and sluggish Na-ion diffusion kinetics. In addition, significant volume changes during the sodiation of FeS destroy the electrode structure and shorten the cycle life. Herein, we report the rational design of the FeS/carbon composite, specifically FeS encapsulated within a hierarchically ordered mesoporous carbon prepared via nanocasting using a SBA-15 template with stable cycle life. We evaluated the Na-ion storage properties and found that the parallel 2D mesoporous channels in the resultant FeS/carbon composite enhanced the conductivity, buffered the volume changes, and prevented unwanted side reactions. Further, high-rate Na-ion storage (363.4 mAh g⁻¹ after 500 cycles at 2 A g⁻¹, 132.5 mAh g⁻¹ at 20 A g⁻¹) was achieved, better than that of the bare FeS electrode, indicating the benefit of structural confinement for rapid ion transfer, and demonstrating the excellent electrochemical performance of this anode material at high rates.

General Synthesis of Sulfonate-Based Metal-Organic Framework Derived Composite of Mx Sy @N/S-Doped Carbon for High-Performance Lithium/Sodium Ion Batteries

A general and simple strategy is realized for the first time for the preparation of metal sulfide (M_x S_y ) nanoparticles immobilized into N/S co-doped carbon (NSC) through a one-step pyrolysis method. The organic ligand 1,5-naphthalenedisulfonic acid in the metal-organic framework (MOF) precursor is used as a sulfur source, and metal ions are sulfurized in situ to form M_x S_y nanoparticles, resulting in the formation of M_x S_y /NSC (M=Fe, Co, Cu, Ni, Mn, Zn) composites. Benefiting from the M_x S_y nanoparticles and conductive carbon, a synergistic effect of the composite is achieved. For instance, the composite of Fe₇ S₈ /NSC as an anode displays excellent long-term cycling stability in lithium/sodium ion batteries. At 5 A g^-1 , large capacities of 645 mA h g^-1 and 426.6 mA h g^-1 can be retained after 1500 cycles for the lithium-ion battery and after 1000 cycles for the sodium-ion battery, respectively.

Fe7Se8 encapsulated in N-doped carbon nanofibers as a stable anode material for sodium ion batteries

Transition metal chalcogenides especially Fe-based selenides for sodium storage have the advantages of high electric conductivity, low cost, abundant active sites, and high theoretical capacity. Herein, we proposed a facile synthesis of Fe₇Se₈ embedded in carbon nanofibers (denoted as Fe₇Se₈-NCFs). The Fe₇Se₈-NCFs with a 1D electron transfer network can facilitate Na⁺ transportation to ensure fast reaction kinetics. Moreover, Fe₇Se₈ encapsulated in carbon nanofibers, Fe₇Se₈-NCFs, can effectively adapt the volume variation to keep structural integrity during a continuous Na⁺ insertion and extraction process. As a result, Fe₇Se₈-NCFs present improved rate performance and remarkable cycling stability for sodium storage. The Fe₇Se₈-NCFs exhibit practical feasibility with a reasonable specific capacity of 109 mA h g^-1 after 200 cycles and a favorable rate capability of 136 mA h g^-1 at a high rate of 2 A g^-1 when coupled with Na₃V₂(PO₄)₃ to assemble full sodium ion batteries.

Promoting Ge Alloying Reaction via Heterostructure Engineering for High Efficient and Ultra-Stable Sodium-Ion Storage

Germanium (Ge)-based materials have been considered as potential anode materials for sodium-ion batteries owing to their high theoretical specific capacity. However, the poor conductivity and Na+ diffusivity of Ge-based materials result in retardant ion/electron transportation and insufficient sodium storage efficiency, leading to sluggish reaction kinetics. To intrinsically maximize the sodium storage capability of Ge, the nitrogen doped carbon-coated Cu3Ge/Ge heterostructure material (Cu3Ge/Ge@N-C) is developed for enhanced sodium storage. The pod-like structure of Cu3Ge/Ge@N-C exposes numerous active surface to shorten ion transportation pathway while the uniform encapsulation of carbon shell improves the electron transportation, leading to enhanced reaction kinetics. Theoretical calculation reveals that Cu3Ge/Ge heterostructure can offer decent electron conduction and lower the Na+ diffusion barrier, which further promotes Ge alloying reaction and improves its sodium storage capability close to its theoretical value. In addition, the uniform encapsulation of nitrogen-doped carbon on Cu3Ge/Ge heterostructure material efficiently alleviates its volume expansion and prevents its decomposition, further ensuring its structural integrity upon cycling. Attributed to these unique superiorities, the as-prepared Cu3Ge/Ge@N-C electrode demonstrates admirable discharge capacity, outstanding rate capability and prolonged cycle lifespan (178 mAh g-1 at 4.0 A g-1 after 4000 cycles).

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.

MnSe embedded in carbon nanofibers as advanced anode material for sodium ion batteries

MnSe with high theoretical capacity and reversibility is considered as a promising material for the anode of sodium ion batteries. In this study, MnSe nanoparticles embedded in 1D carbon nanofibers (MnSe-NC) are successfully prepared via facile electrospinning and subsequent selenization. A carbon framework can effectively protect MnSe dispersed in it from agglomeration and can accommodate volume variation in the conversion reaction between MnSe and Na⁺ to guarantee cycling stability. The 1D fiber structure can increase the area of contact between electrode and electrolyte to shorten the diffusion path of Na⁺ and facilitate its transfer. According to the kinetic analysis, the storage process of sodium by MnSe-NC is a surface pseudocapacitive-controlled process with promising rate capability. Impressively, An MnSe-NC anode in sodium ion full cells is investigated by pairing with an Na₃V₂(PO₄)₂@rGO cathode, which exhibits a reversible capacity of 195 mA h g^-1 at 0.1 A g^-1.

Surface Modification of Fe7 S8 /C Anode via Ultrathin Amorphous TiO2 Layer for Enhanced Sodium Storage Performance

Iron sulfides with high theoretical capacity and low cost have attracted extensive attention as anode materials for sodium ion batteries. However, the inferior electrical conductivity and devastating volume change and interface instability have largely hindered their practical electrochemical properties. Here, ultrathin amorphous TiO₂ layer is constructed on the surface of a metal-organic framework derived porous Fe₇ S₈ /C electrode via a facile atomic layer deposition strategy. By virtue of the porous structure and enhanced conductivity of the Fe₇ S₈ /C, the electroactive TiO₂ layer is expected to effectively improve the electrode interface stability and structure integrity of the electrode. As a result, the TiO₂ -modified Fe₇ S₈ /C anode exhibits significant performance improvement for sodium-ion batteries. The optimal TiO₂ -modified Fe₇ S₈ /C electrode delivers reversible capacity of 423.3 mA h g^-1 after 200 cycles with high capacity retention of 75.3% at 0.2 C. Meanwhile, the TiO₂ coating is conducive to construct favorable solid electrolyte interphase, leading to much enhanced initial Coulombic efficiency from 66.9% to 72.3%. The remarkable improvement suggests that the interphase modification holds great promise for high-performance metal sulfide-based anode materials for sodium-ion batteries.

The encapsulation of MnFe2O4 nanoparticles into the carbon framework with superior rate capability for lithium-ion batteries

Binary transition metal oxides (BTMOs) have been regarded as one of the most hopeful anode materials for lithium-ion batteries (LIBs) owing to their high theoretical capacity, excellent electrochemical activity and abundant electrochemical reactions. However, BTMOs still suffer from two main problems, which are poor conductivity and large volume expansion during the charge/discharge processes. In order to address the above-mentioned problems, mesoporous MnFe2O4@C nanorods have been successfully synthesized in this work. The synergistic effect of the cross-linked carbon framework and mesoporous structure greatly improves the electrochemical performances. As expected, the mesoporous MnFe2O4@C electrode manifests discharge capacities of 987.5 and 816.6 mA h g-1 at the current densities of 100 and 2000 mA g-1, respectively, with the capacity retention ratio of 82.7%, exerting distinguished rate capabilities for LIBs.

Cu2Se Nanoparticles Encapsulated by Nitrogen-Doped Carbon Nanofibers for Efficient Sodium Storage

Cu₂Se with high theoretical capacity and good electronic conductivity have attracted particular attention as anode materials for sodium ion batteries (SIBs). However, during electrochemical reactions, the large volume change of Cu₂Se results in poor rate performance and cycling stability. To solve this issue, nanosized-Cu₂Se is encapsulated in 1D nitrogen-doped carbon nanofibers (Cu₂Se-NC) so that the unique structure of 1D carbon fiber network ensures a high contact area between the electrolyte and Cu₂Se with a short Na⁺ diffusion path and provides a protective matrix to accommodate the volume variation. The kinetic analysis and D_Na+ calculation indicates that the dominant contribution to the capacity is surface pseudocapacitance with fast Na⁺ migration, which guarantees the favorable rate performance of Cu₂Se-NC for SIBs.

Tailoring Coral-Like Fe7Se8@C for Superior Low-Temperature Li/Na-Ion Half/Full Batteries: Synthesis, Structure, and DFT Studies

The intrinsic charge-transfer property bears the primary responsibility for the sluggish redox kinetics of the common electrode materials, especially operated at low temperatures. Herein, we report the crafting of homogeneously confined Fe₇Se₈ nanoparticles with a well-defined graphitic carbon matrix that demonstrate a highly efficient charge-transfer system in a designed natural coral-like structure (cl-Fe₇Se₈@C). Notably, the intricate architecture as well as highly conductive peculiarity of C concurrently satisfy the demands of achieving fast ionic/electrical conductivities for both Li/Na-ion batteries in a wide temperature range. For example, when cl-Fe₇Se₈@C is employed as the anode material to assemble full batteries with the cathode of Na₃V₂(PO₄)₂O₂F (NVPOF), decent capacities of 323.1 and 175.9 mA h g^-1 can be acquired at temperatures of 25 and -25 °C, respectively. This work is significant for further developing potential anode materials for advanced energy storage and conversion under low-temperature conditions.