Boosting High Energy Density Lithium-Ion Storage via the Rational Design of an FeS-Incorporated Sulfurized Polyacrylonitrile Fiber Hybrid Cathode

Nanocrystalline iron oxide and sulfide by the thermal decomposition of cyclohexylammonium hexaisothiocyanatoferrate(III) 2.5H2O

We reported for the first time the establishment of a simple, room-temperature synthesis protocol for the new organic-inorganic hybrid salt of cyclohexylammonium hexaisothiocyanatoferrate(III) 2.5H2O, which was found a convenient single-source precursor for the synthesis of nanocrystalline iron oxide or sulfide. The formation of this salt was spectrophotometrically confirmed by FT-IR and UV-Vis. In addition, SCXRD revealed that this salt had the trigonal space group R-3 with disorder of some isothiocyanate sulfur atoms. The thermal stability and the thermal decomposition products of this salt were atmosphere-dependent (air: 169 °C; α-Fe2O3 at 550 °C; helium: 154 °C; FeS at 800 °C). The thermal decomposition impacted the textural properties of α-Fe2O3 (an average crystallite size of ~ 41 nm and SBET = ~ 4.0 m2/g) and FeS (~ 14 nm and ~ 80 m2/g, respectively). The nanoparticulate nature affected the magnetic behavior of α-Fe2O3 and FeS, as revealed by ac-susceptibility. They showed widen maximum at ~ 55 K due to increasing disorder effect by particle sizes. However, below 40 K, the susceptibility increased sharply, indicating a ferromagnetic ordering. In comparison, the ac-susceptibility of the salt exhibited a broad maximum at ~ 130 K with an inflection point at ~ 180 K. No transition to spin-flip was detected for all three materials.

An innovative double-Shell layer nitrogen and sulfur co-doped carbon-Encapsulated FeS composite for enhanced lithium-Ion battery performance

FeS, with its high theoretical capacity and natural abundance, holds significant promise as an anode material for lithium-ion batteries (LIBs). However, its practical application is constrained by poor electrical conductivity and substantial volume expansion during cycling, which impair charge-discharge efficiency and cycling stability. To overcome these challenges, we developed a nitrogen and sulfur co-doped carbon-encapsulated FeS composite with a hollow double-layer structure (HDL-FeS@NSC). Utilizing sulfur spheres as a sacrificial template, our inside-out synthesis strategy produces a unique material design. The HDL-FeS@NSC composite exhibits significant improvements in electrochemical performance compared to pure FeS. These enhancements are due to its increased specific surface area, which facilitates lithium-ion diffusion; a shortened Li+ diffusion pathway; structural stability that mitigates volume expansion; and an optimized carbon layer that boosts conductivity. The HDL-FeS@NSC-70 anode demonstrates a specific capacity of 879.6 mAh/g after 600 cycles at 1.0 A/g and retains 558.0 mAh/g at 5.0 A/g. Additionally, the lithium storage mechanism has been thoroughly investigated using in-situ techniques. These results suggest that the HDL-FeS@NSC composite anode has the potential to significantly enhance lithium-ion battery performance, offering a promising solution for next-generation energy storage systems.

Macroscale preparation of CoS2 nanoparticles for ultra-high fast-charging performance in sodium-ion batteries

Improving the fast-charging capabilities and energy storage capacity of electric vehicles presents a feasible strategy for mitigating the prevalent concern of range anxiety in the market. Nanostructure electrode materials play a crucial role in this process. However, the current method of preparation is arduous and yields restricted quantities. In view of this, we have devised an innovative approach that provides convenience and efficacy, facilitating the large-scale synthesis of CoS2 nanoparticles, which exhibited exceptional performance. When the current density was 1000 mA g-1, the discharging capacity reached 760 mAh g-1 after 400 cycles. Remarkably, even at an increased current density of 5000 mA g-1, the discharging capacity of CoS2 remained at 685.5 mAh g-1. The ultra-high performance could be attributed to the specific surface area, which minimized the diffusion distance of sodium-ions during the charging and discharging processes and mitigated the extent of structural damage. Our straightforward preparation techniques facilitate the mass production and present a novel approach for the development of cost-effective and high-performing anode materials for sodium-ion batteries.

Multifunctional Asymmetric Separator Constructed by Polyacrylonitrile-Derived Nanofibers for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries hold great promise as next-generation high-energy storage devices owing to the high theoretical specific capacity of sulfur, but polysulfide shuttling and lithium dendrite growth remain key challenges limiting cycling life. In this work, we propose a polyacrylonitrile-derived asymmetric (PDA) separator to enhance Li-S battery performance by accelerating sulfur redox kinetics and guiding lithium plating and stripping. A PDA separator was constructed from two layers: the cathode-facing side consists of polyacrylonitrile nanofibers carbonized at 800 °C and doped with titanium nitride, which can achieve rapid polysulfide conversion via electrocatalysis to suppress their shuttling; the anode-facing side consists of polyacrylonitrile oxidized at 280 °C, on which the abundant electronegative groups guide uniform lithium ion plating and stripping. Li-S batteries assembled with the PDA separator exhibited enhanced rate performance, cycling stability, and sulfur utilization, retaining 426 mA h g-1 capacity at 1 C over 1000 cycles and 632 mA h g-1 at 4 C over 200 cycles. Attractively, the PDA separator showed high thermal stability, which could mitigate the risk of internal short circuits and thermal runaway. This work demonstrates an original path to addressing the most critical issues with Li-S batteries.

Boosting Fast Sodium Ion Storage by Synergistic Effect of Heterointerface Engineering and Nitrogen Doping Porous Carbon Nanofibers

Heterointerface engineering with multiple electroactive and inactive supporting components is considered an efficient approach to enhance electrochemical performance for sodium-ion batteries (SIBs). Nevertheless, it is still a challenge to rationally design heterointerface engineering and understand the synergistic effect reaction mechanisms. In this paper, the two-phase heterointerface engineering (Sb₂ S₃ and FeS₂ ) is well designed to incorporate into N-doped porous hollow carbon nanofibers (Sb-Fe-S@CNFs) by proper electrospinning design. The obtained Sb-Fe-S@CNFs are used as anode in SIBs to evaluate the electrochemical performance. It delivers a reversible capacity of 396 mA h g^-1 after 2000 cycles at 1 A g^-1 and exhibits an ultra-long high rate cycle life for 16 000 cycles at 10 A g^-1 . The admirable electrochemical performance is mainly attributed to the following reasons: The porous carbon nanofibers serve as an accelerator of the electrons/ions and a buffer to alleviate volume expansion upon long cyclic performance. The abundant phase boundaries of Sb₂ S₃ /FeS₂ exert low Na⁺ adsorption energy and greatly promote the charge transfer in the internal electric field calculated by first-principle density functional theory. Therefore, the as-prepared Sb-Fe-S@CNFs represents a promising candidate for an efficient anode electrode material in SIBs.

Feasible Catalytic-Insoluble Strategy Enabled by Sulfurized Polyacrylonitrile with In Situ Built Electrocatalysts for Ultrastable Lithium-Sulfur Batteries

To date, elemental sulfur has been considered as a prospective cathode material for exploring high-energy power systems with low cost and sustainability. However, its practical commercialization has been impeded by inherent drawbacks of notorious capacity decay, unsatisfied insulating nature, and sluggish conversion chemistry. To address these issues, for the first time, freestanding nanofibrous networks with hierarchical nanostructures are facilely constructed by inlaying electrocatalytic bimetallic chalcogenides (Fe_xMn_1-xS nanoparticles) into conductive graphene nanosheet (GN)-doped sulfurized polyacrylonitrile (SPAN) fiber matrices. Covalent-bonded SPAN featuring an insoluble mechanism serves as a reliable cathode substrate with enhanced electrostability and high sulfur utilization, while high-surface-area GN dopants promote conductivity improvement and rapid electron transfer. Meanwhile, the results prove that sulfiphilic Fe_xMn_1-xS nanoparticles with abundant electrochemically active sites facilitate construction of uniform deposition interfaces and efficient electrocatalysis conversion toward lithium polysufides. This feasible catalytic-insoluble cathode strategy drives the Li-S battery, which exhibits excellent electrochemical performances with a remarkable reversible discharge capacity of 967 mA h g^-1 and a capacity retention of 623 mA h g^-1 after 500 cycles. Moreover, the corresponding lithiation/delithiation mechanisms are systematically investigated through complementary morphological and spectral analyses, providing valuable insights into advanced metal-sulfur batteries.

Organic/Inorganic Hybrid Fibers: Controllable Architectures for Electrochemical Energy Applications

Organic/inorganic hybrid fibers (OIHFs) are intriguing materials, possessing an intrinsic high specific surface area and flexibility coupled to unique anisotropic properties, diverse chemical compositions, and controllable hybrid architectures. During the last decade, advanced OIHFs with exceptional properties for electrochemical energy applications, including possessing interconnected networks, abundant active sites, and short ion diffusion length have emerged. Here, a comprehensive overview of the controllable architectures and electrochemical energy applications of OIHFs is presented. After a brief introduction, the controllable construction of OIHFs is described in detail through precise tailoring of the overall, interior, and interface structures. Additionally, several important electrochemical energy applications including rechargeable batteries (lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries), supercapacitors (sandwich-shaped supercapacitors and fiber-shaped supercapacitors), and electrocatalysts (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction) are presented. The current state of the field and challenges are discussed, and a vision of the future directions to exploit OIHFs for electrochemical energy devices is provided.

Multiscale Understanding of Covalently Fixed Sulfur-Polyacrylonitrile Composite as Advanced Cathode for Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) provide high specific capacity due to the reversible redox mechanism based on conversion reaction that makes this battery a more promising candidate for next-generation energy storage systems. Recently, along with elemental sulfur (S8 ), sulfurized polyacrylonitrile (SPAN), in which active sulfur moieties are covalently bounded to carbon backbone, has received significant attention as an electrode material. Importantly, SPAN can serve as a universal cathode with minimized metal-polysulfide dissolution because sulfur is immobilized through covalent bonding at the carbon backbone. Considering these unique structural features, SPAN represents a new approach beyond elemental S8 for MSBs. However, the development of SPAN electrodes is in its infancy stage compared to conventional S8 cathodes because several issues such as chemical structure, attached sulfur chain lengths, and over-capacity in the first cycle remain unresolved. In addition, physical, chemical, or specific treatments are required for tuning intrinsic properties such as sulfur loading, porosity, and conductivity, which have a pivotal role in improving battery performance. This review discusses the fundamental and technological discussions on SPAN synthesis, physicochemical properties, and electrochemical performance in MSBs. Further, the essential guidance will provide research directions on SPAN electrodes for potential and industrial applications of MSBs.

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.

Transition metal sulfides meet electrospinning: versatile synthesis, distinct properties and prospective applications

One-dimensional (1D) electrospun nanomaterials have attracted significant attention due to their unique structures and outstanding chemical and physical properties such as large specific surface area, distinct electronic and mass transport, and mechanical flexibility. Over the past years, the integration of metal sulfides with electrospun nanomaterials has emerged as an exciting research topic owing to the synergistic effects between the two components, leading to novel and interesting properties in energy, optics and catalysis research fields for example. In this review, we focus on the recent development of the preparation of electrospun nanomaterials integrated with functional metal sulfides with distinct nanostructures. These functional materials have been prepared via two efficient strategies, namely direct electrospinning and post-synthesis modification of electrospun nanomaterials. In this review, we systematically present the chemical and physical properties of the electrospun nanomaterials integrated with metal sulfides and their application in electronic and optoelectronic devices, sensing, catalysis, energy conversion and storage, thermal shielding, adsorption and separation, and biomedical technology. Additionally, challenges and further research opportunities in the preparation and application of these novel functional materials are also discussed.

Realizing High-Performance Li/Na-Ion Half/Full Batteries via the Synergistic Coupling of Nano-Iron Sulfide and S-doped Graphene

Iron sulfide (FeS) anodes are plagued by severe irreversibility and volume changes that limit cycle performances. Here, a synergistically coupled hybrid composite, nanoengineered iron sulfide/S-doped graphene aerogel, was developed as high-capacity anode material for Li/Na-ion half/full batteries. The rational coupling of in situ generated FeS nanocrystals and the S-doped rGO aerogel matrix boosted the electronic conductivity, Li⁺ /Na⁺ diffusion kinetics, and accommodated the volume changes in FeS. This anode system exhibited excellent long-term cyclability retaining high reversible capacities of 422 (1100 cycles) and 382 mAh g^-1 (1600 cycles), respectively, for Li⁺ and Na⁺ storage at 5 A g^-1 . Full batteries designed with this anode system exhibited 435 (FeS/srGOA||LiCoO₂ ) and 455 mAh g^-1 (FeS/srGOA||Na_0.64 Co_0.1 Mn_0.9 O₂ ). The proposed low-cost anode system is competent with the current Li-ion battery technology and extends its utility for Na⁺ storage.

Elucidating the Mechanism of Li Insertion into Fe1-xS/Carbon via In Operando Synchrotron Studies

The detailed understanding of kinetic and phase dynamics taking place in lithium-ion batteries (LIBs) is crucial for optimizing their properties. It was previously reported that Fe_1-xS/C nanocomposites display a superior performance as anode materials in LIBs. However, the underlying lithium storage mechanism was not entirely understood during the 1st cycle. In this work, in operando synchrotron techniques are used to track lithium storage mechanisms during the 1st (de)-lithiation process in the Fe_1-xS/C nanocomposite. The combination of in operando techniques enables the uncovering of the phase fraction alternations and crystal structural variations on different length-scales. Additionally, the investigation of kinetic processes, morphological changes, and internal resistance dynamics is discussed. These results reveal that the phase transition of Fe_1-xS → Li₂Fe_1-xS₂ → Fe⁰ + Li₂S occurs during the 1st lithiation process. The redox reaction of Fe²⁺ + 2e^- ⇌ Fe⁰ and the Fe K-edge X-ray absorption spectroscopy (XAS) transformation process are confirmed by in operando XAS. During the 1st de-lithiation process, Fe⁰ and Li₂S convert to Li_2-yFe_1-xS₂ and Li⁺ is extracted from Li₂S to form Li_2-yS. The phase transition from Li₂S to Li_2-yS is not detected in previous reports. After the 1st de-lithiation process, amorphous lithiated iron sulfide nanoparticles are embedded within the remaining Li₂S matrix.

Interfacial and Electronic Modulation via Localized Sulfurization for Boosting Lithium Storage Kinetics

Structural modulation endows electrochemical hybrids with promising energy storage properties owing to their adjustable interfacial and/or electronic characteristics. For MXene-based materials, however, the facile but effective strategies for tuning their structural properties at nanoscale are still lacking. Herein, 3D crumpled S-functionalized Ti₃ C₂ T_x substrate is rationally integrated with Fe₃ O₄ /FeS heterostructures via coprecipitation and subsequent partial sulfurization to induce a highly active and stable electrode architecture. The unique heterostructures with tuned electronic properties can induce improved kinetics and structural stability. The surface engineering by S terminations on the MXene further unlocks extra (pseudo)capacitive lithium storage. Serving as anode for lithium storage, the optimized electrode delivers an excellent long-term cycling stability (913.9 mAh g^-1 after 1000 cycles at 1 A g^-1 ) and superior rate capability (490.4 mAh g^-1 at 10 A g^-1 ). Moreover, the (de)lithiation pathways associated with energy storage mechanisms are further revealed by operando X-ray diffraction, in situ electroanalytical techniques, and first-principles calculations. The hybrid electrode is proved to undergo stepwise phase transformations during discharging but a relatively uniform reconversion during charging, suggesting an asymmetric conversion mechanism. This work provides a novel strategy for designing high-performance hybrids and paves the way for in-depth understanding of complex lithium intercalation and conversion reactions.