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

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

Designing a Magnesium/Sodium Hybrid Battery Using Hierarchical Iron Selenide Architecture as Cathode Material and Modified Dual-Ion Salts in Ether as Electrolyte

We developed a magnesium/sodium (Mg/Na) hybrid battery using a hierarchical disk-whisker FeSe2 architecture (HD-FeSe2) as the cathode material and a modified dual-ion electrolyte. The polarizable Se2- anion reduced the Mg2+ migration barrier, and the 3D configuration possessed a large surface area, which facilitated both Mg2+/Na+ cation diffusion and electron transport. The dual-ion salts with NaTFSI in ether reduced the Mg plating/stripping overvoltage in a symmetric cell. The hybrid battery exhibited an energy density of 260.9 Wh kg-1 and a power density of 600.8 W kg-1 at 0.2 A g-1. It showed a capacity retention of 154 mAh g-1 and a Coulombic efficiency of over 99.5% under 1.0 A g-1 after 800 long cycles. The battery also displayed outstanding temperature tolerance. The findings of 3D architecture as cathode material and hybrid electrolyte provide a pathway to design a highly reliable Mg/Na hybrid battery.

Deciphering the Performance Enhancement, Cell Failure Mechanism, and Amelioration Strategy of Sodium Storage in Metal Chalcogenides-Based Andes

Transition metal chalcogenides (TMCs) emerge as promising anode materials for sodium-ion batteries (SIBs), heralding a new era of energy storage solutions. Despite their potential, the mechanisms underlying their performance enhancement and susceptibility to failure in ether-based electrolytes remain elusive. This study delves into these aspects, employing CoS2 electrodes as a case in point to elucidate the phenomena. The investigation reveals that CoS2 undergoes a unique irreversible and progressive solid-liquid-solid phase transition from its native state to sodium polysulfides (NaPSs), and ultimately to a Cu1.8S/Co composite, accompanied by a gradual morphological transformation from microspheres to a stable 3D porous architecture. This reconstructed 3D porous structure is pivotal for its exceptional Na+ diffusion kinetics and resilience to cycling-induced stress, being the main reason for ultrastable cycling and ultrahigh rate capability. Nonetheless, the CoS2 electrode suffers from an inevitable cycle life termination due to the microshort-circuit induced by Na metal corrosion and separator degradation. Through a comparative analysis of various TMCs, a predictive framework linking electrode longevity is established to electrode potential and Gibbs free energy. Finally, the cell failure issue is significantly mitigated at a material level (graphene encapsulation) and cell level (polypropylene membrane incorporation) by alleviating the NaPSs shuttling and microshort-circuit.

Coral-like CoSe2@N-doped carbon with a high initial coulombic efficiency as advanced anode materials for Na-ion batteries

Na-ion batteries (NIBs) have attracted great interest as a possible technology for grid-scale energy storage for the past few years owing to the wide distribution, low cost and environmental friendliness of sodium resources and similar chemical mechanisms to those of established Li-ion batteries (LIBs). Nonetheless, the implementation of NIBs is seriously hindered because of their low rate capability and cycling stability. This is mainly because the large ionic size of Na+ can reduce the structural stability and cause sluggish reaction kinetics of electrode materials. Herein, three-dimensional nanoarchitectured coral-like CoSe2@N-doped carbon (CL-CoSe2@NC) was synthesized through solvothermal and selenizing techniques. As a result, CL-CoSe2@NC for NIBs at 2 A g-1 exhibits an ultrahigh specific capacity of 345.4 mA h g-1 after 2800 cycles and a superhigh initial coulombic efficiency (ICE) of 93.1%. Ex situ XRD, HRTEM, SAED and XPS were executed to study the crystal structure evolution between Na and CoSe2 during sodiation/de-sodiation processes. The aforementioned results indicate that the improved sodium storage property of CL-CoSe2@NC could be attributed to better electrode kinetics and a stable SEI film because of the 3D nanoarchitecture and the existence of the NC layer.

Three-Dimensional (3D) Ordered Macroporous Bimetallic (Mn,Fe) Selenide/Carbon Composite with Heterojunction Interface for High-Performance Sodium Ion Batteries

Transition-metal selenides have captured significant research attention as anode materials for sodium ion batteries (SIBs) due to their high theoretical specific capacities and excellent electronic conductivity. However, volumetric expansion and inferior cycle life still hinder their practical application. Herein, a three-dimensional (3D) ordered macroporous bimetallic (Mn,Fe) selenide modified by a carbon layer (denoted as 3DOM-MnFeSex@C) composite containing a heterojunction interface is fabricated through selenizing a 3D ordered macroporous Mn-based Prussian Blue analogue single crystal. The 3DOM-MnFeSex@C exhibits hierarchically porous architecture with enhanced mass-transfer efficiency; MnSe and FeSe2 particles are encapsulated into macroporous carbon framework, which can significantly promote the electronic conductivity and maintain the structural integrity. The density functional theory calculation indicates that the heterojunction interface between MnSe and FeSe2 has been successfully engineered so that Na+ can be readily adsorbed and rapidly converted, thus promoting the reaction kinetics and extending the cyclic life. As expected, the 3DOM-MnFeSex@C composite delivers excellent rate performance (277.6 mA h g-1 at 10 A g-1), and prolonged cycling life (191.6 mA h g-1 even after 1000 cycles at 2 A g-1) as a sodium storage anode. The sodium storage mechanism of the composite was further investigated by in situ X-ray diffraction and ex situ high-resolution transmission electron microscopy characterization techniques.

Self-assembled nanoflower-like FeSe2/MoSe2 heterojunction anode with enhanced kinetics for superior-performance Na-ion half/full batteries

Transition metal selenides are a research hotspot in sodium-ion batteries (SIBs). However, slow kinetics and rapid capacity decay due to volume changes during cycling limit their commercial applications. Heterostructures have the ability to accelerate charge transport and are widely used in energy storage devices due to their abundant active sites and lattice interfaces. A rational design of heterojunction electrode materials with excellent electrochemical performance is essential for SIBs. Herein, a novel anode material heterostructured FeSe₂/MoSe₂ (FMSe) nanoflower for SIBs was successfully prepared through a facile co-precipitation and hydrothermal route. The as-prepared FMSe heterojunction exhibits excellent electrochemical performance, including a high invertible capacity (493.7 mA h g^-1 after 150 cycles at 0.2 A g^-1), long-term cycling stability (352.2 mA h g^-1 even after 4200 cycles at 5.0 A g^-1) and competitive rate capability (361.2 mA h g^-1 at 20 A g^-1). By matching with a Na₃V₂(PO₄)₃ cathode, it can even exhibit ideal cycling stability (123.5 mA h g^-1 at 0.5 A g^-1 after 200 cycles). Furthermore, the sodium storage mechanism of the FMSe electrodes was systematically determined by ex situ electrochemical techniques. Theoretical calculation also reveals that the heterostructure on the FMSe interface enhances charge transport and promotes reaction kinetics.

Nickel sulfide/nickel phosphide heterostructures anchored on porous carbon nanosheets with rapid electron/ion transport dynamics for sodium-ion half/full batteries

Nickel-based materials have been extensively deemed as promising anodes for sodium-ion batteries (SIBs) owing to their superior capacity. Unfortunately, the rational design of electrodes as well as long-term cycling performance remains a thorny challenge due to the huge irreversible volume change during the charge/discharge process. Herein, the heterostructured ultrafine nickel sulfide/nickel phosphide (NiS/Ni₂P) nanoparticles closely attached to the interconnected porous carbon sheets (NiS/Ni₂P@C) are designed by facile hydrothermal and annealing methods. The NiS/Ni₂P heterostructure promotes ion/electron transport, thus accelerating the electrochemical reaction kinetics benefited from the built-in electric field effect. Moreover, the interconnected porous carbon sheets offer rapid electron migration and excellent electronic conductivity, while releasing the volume variance during Na⁺ intercalation and deintercalation, guaranteeing superior structural stability. As expected, the NiS/Ni₂P@C electrode exhibits a high reversible specific capacity of 344 mAh g^-1 at 0.1 A g^-1 and great rate stability. Significantly, the implementation of NiS/Ni₂P@C//Na₃(VPO₄)₂F₃ SIB full cell configuration exhibits relatively satisfactory cycle performance, which suggests its widely practical application. This research will develop an effective method for constructing heterostructured hybrids for electrochemical energy storage.

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.

Thermodynamic Transformation of Crystalline Organic Hybrid Iron Selenide to FexSey@CN Microrods for Sodium Ion Storage

Carbon-coated metal chalcogenide composites have been demonstrated as one type of promising anode material for sodium-ion batteries (SIBs). However, combining carbon materials with micronanoparticles of metal chalcogenide always involve complicated processes, such as polymer coating, carbonization, and sulfidation/selenization. To address this issue, herein, we reported a series of carbon-coated FexSey@CN (FexSey = FeSe2, Fe3Se4, Fe7Se8) composites prepared via the thermodynamic transformation of a crystalline organic hybrid iron selenide [Fe(phen)2](Se4) (phen = 1,10-phenanthroline). By pyrolyzing the bulk crystals of [Fe(phen)2](Se4) at different temperatures, FexSey microrods were formed in situ, where the nitrogen-doped carbon layers were coated on the surface of the microrods. Moreover, all the as-prepared FexSey@CN composites exhibited excellent sodium-ion storage capabilities as anode materials in SIBs. This work proves that crystalline organic hybrid metal chalcogenides can be used as a novel material system for the in situ formation of carbon-coated metal chalcogenide composites, which could have great potential in the application of electrochemical energy storage.

A novel exfoliated manganese phosphoselenide as a high-performance anode material for lithium ions storage

Layered manganese phosphoselenide (MnPSe3) is expected to be a potential anode for Li ions storage due to it combines the merits of phosphorus with metal selenide. It promotes charge transfer and ensures a high theoretical capacity of up to 746 mA h g−1. In this work, a comprehensive study clearly demonstrated that bulk MnPSe3 electrode is the inability to maintain the integrity of the structure with severe detectable fracture or pulverization after full lithiation/delithiation, resulting in poor rate capability and cycling stability. Additionally, exfoliated few-layered MnPSe3 nanoflakes by the ultrasonic method show enhanced electrical conductivity and resistance to volume expansion. It has a high initial discharge/charge capacity reaching to 524/796 mA h g−1 and outstanding cycling stability with charge capacities of 709 mA h g−1 after 100 cycles at 0.2 A g−1 within the potential window of 0.005–3 V vs. Li+/Li. While further improving the cycles, the retention rate was still held at ∼72% after 350 cycles. This work provides new insights into exploiting new novel layered materials, such as MnPSe3 as anodes for lithium-ion batteries.

Innovative Materials for Energy Storage and Conversion

The metal chalcogenides (MCs) for sodium-ion batteries (SIBs) have gained increasing attention owing to their low cost and high theoretical capacity. However, the poor electrochemical stability and slow kinetic behaviors hinder its practical application as anodes for SIBs. Hence, various strategies have been used to solve the above problems, such as dimensions reduction, composition formation, doping functionalization, morphology control, coating encapsulation, electrolyte modification, etc. In this work, the recent progress of MCs as electrodes for SIBs has been comprehensively reviewed. Moreover, the summarization of metal chalcogenides contains the synthesis methods, modification strategies and corresponding basic reaction mechanisms of MCs with layered and non-layered structures. Finally, the challenges, potential solutions and future prospects of metal chalcogenides as SIBs anode materials are also proposed.

Rational nanostructured FeSe2 wrapped in nitrogen-doped carbon shell for high-rate capability and long cycling sodium-ion storage

Transition metal selenides (TMSs) have drawn substantial attention as promising anode materials for sodium-ion batteries (SIBs) on account oftheir rapid reaction kinetics and high reversible capacity. However, the undesirable capacity decay and inferior rate performance still hamper their large-scale application. Herein, an anode material comprising combination of olivary nanostructure FeSe₂ core and nitrogen-doped carbon shell (designated as FeSe₂@NC) is well designed by in-situ polymerization and selenization method. The well-designed nitrogen-doped carbon shell can not only alleviate the volume variation during the electrode cycling but also provide an optimized ion/electron transport pathway. The resulting FeSe₂@NC electrodes exhibit a superior rate capability of 228.4 mA h g^-1 at 10 A g^-1 and a long cycling performance of 246.5 mA h g^-1 at 5 A g^-1 after 1000 cycles, which can be assigned to the enhanced structural integrity and improved electrical conductivity. The strategy would present a promising thought for structure design of TMSs as anode materials, which could enhance high-rate and long-lasting cycle performances for SIBs.

Iron Selenide-Based Heterojunction Construction and Defect Engineering for Fast Potassium/Sodium-Ion Storage

Suitable anode materials with high capacity and long cycling stability, especially capability at high current densities, are urgently needed to advance the development of potassium ion batteries (PIBs) and sodium ion batteries (SIBs). Herein, a porous Ni-doped FeSe₂ /Fe₃ Se₄ heterojunction encapsulated in Se-doped carbon (NF₁₁ S/C) is designed through selenization of MOFs precursor. The porous composite possesses enriched active sites and facilitates transport for both ion and electron. Ni-doping is adopted to enrich the lattice defects and active sites. The Se-C bond and carbon framework endow integrity of the composite and hamper aggregation of selenide nano-particles during potassiation/de-potassiation. The NF₁₁ S/C exhibits exceptional rate performance and ultra-long cycling stability (177.3 mA h g^-1 after 3050 cycles at 2 A g^-1 for PIBs and 208.8 mA h g^-1 after 2000 cycles at 8 A g^-1 for SIBs). The potassiation/de-potassiation mechanism is investigated via ex-situ X-ray powder diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectrocopy and Raman analysis. PTCDA//NF₁₁ S/C full cell stably cycles for 1200 cycles at 200 mA g^-1 with a capacity of 103.7 mA h g^-1 , indicating the high application potential of the electrode for highly stable rechargeable batteries.

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...

In Situ Monitoring the Potassium-Ion Storage Enhancement in Iron Selenide with Ether-Based Electrolyte

As one of the promising anode materials, iron selenide has received much attention for potassium-ion batteries (KIBs). Nevertheless, volume expansion and sluggish kinetics of iron selenide result in the poor reversibility and stability during potassiation-depotassiation process. In this work, we develop iron selenide composite matching ether-based electrolyte for KIBs, which presents a reversible specific capacity of 356 mAh g^-1 at 200 mA g^-1 after 75 cycles. According to the measurement of mechanical properties, it is found that iron selenide composite also exhibits robust and elastic solid electrolyte interphase layer in ether-based electrolyte, contributing to the improvement in reversibility and stability for KIBs. To further investigate the electrochemical enhancement mechanism of ether-based electrolyte in KIBs, we also utilize in situ visualization technique to monitor the potassiation-depotassiation process. For comparison, iron selenide composite matching carbonate-based electrolyte presents vast morphology change during potassiation-depotassiation process. When changing to ether-based electrolyte, a few minor morphology changes can be observed. This phenomenon indicates an occurrence of homogeneous electrochemical reaction in ether-based electrolyte, which results in a stable performance for potassium-ion (K-ion) storage. We believe that our work will provide a new perspective to visually monitor the potassium-ion storage process and guide the improvement in electrode material performance.

Three-dimensional microspheres constructed with MoS2 nanosheets supported on multiwalled carbon nanotubes for optimized sodium storage

Molybdenum disulfide (MoS2) has been regarded as a promising anode material in the field of sodium-ion batteries (SIBs), with the advantages of high theoretical capacity and large interlayer spacings. Unfortunately, its intrinsic poor electrical conductivity and large volume changes during the sodiation/desodiation reactions still limit its practical application. To deal with this shortcoming, we built MoS2 nanosheet/multiwalled carbon nanotube (denoted as MoS2-MSs/MWCNTs) composites with a three-dimensional (3D) micro-spherical structure, assembled in situ from MoS2 nanosheets. These nanosheets are connected to each other by the MWCNTs network, which provides a highly conductive pathway for electrons/ions through interparticle and intraparticle interfaces, accelerating charge transfer and ion diffusion capabilities. More importantly, the carbon network can boost electrical conductivity and relieve structural strain. Consequently, the as-prepared MoS2-MSs/MWCNTs composite presents a high reversible specific capacity of 519 mA h g-1 at 0.1 A g-1 after 100 cycles with a capacity retention of 94.4% and excellent rate performance (227 mA h g-1 at 10 A g-1). Outstanding cycling stability was also achieved (327.1 mA h g-1 over 1000 cycles at 2 A g-1) and was characterized by scanning electron microscopy (SEM) analysis. Our findings provide a simple and effective strategy to explore anode materials with advanced sodium storage properties.