Synthesis of Iron Sulfide Nanocrystals Encapsulated in Highly Porous Carbon-Coated CNT Microsphere as Anode Materials for Sodium-Ion Batteries.

Highly porous carbon materials with a rationally designed pore structure can be utilized as reservoirs for metal or nonmetal components. The use of small-sized metal or metal compound nanoparticles, completely encapsulated by carbon materials, has attracted significant attention as an effective approach to enhancing sodium ion storage properties. These materials have the ability to mitigate structural collapse caused by volume expansion during the charging process, enable short ion transport length, and prevent polysulfide elution. In this study, a concept of highly porous carbon-coated carbon nanotube (CNT) porous microspheres, which serve as excellent reservoir materials is suggested and a porous microsphere is developed by encapsulating iron sulfide nanocrystals within the highly porous carbon-coated CNTs using a sulfidation process. Furthermore, various sulfidation processes to determine the optimal method for achieving complete encapsulation are investigated by comparing the morphologies of diverse iron sulfide-carbon composites. The fully encapsulated structure, combined with the porous carbon, provides ample space to accommodate the significant volume changes during cycling. As a result, the porous iron sulfide-carbon-CNT composite microspheres exhibited outstanding cycling stability (293 mA h g-1 over 600 cycles at 1 A g-1 ) and remarkable rate capability (100 mA h g-1 at 5 A g-1 ).

Highly Selective and Reversible Detection of Simulated Breath Hydrogen Sulfide Using Fe-Doped CuO Hollow Spheres: Enhanced Surface Redox Reaction by Multi-Valent Catalysts.

The precise and reversible detection of hydrogen sulfide (H2 S) at high humidity condition, a malodorous and harmful volatile sulfur compound, is essential for the self-assessment of oral diseases, halitosis, and asthma. However, the selective and reversible detection of trace concentrations of H2 S (≈0.1 ppm) in high humidity conditions (exhaled breath) is challenging because of irreversible H2 S adsorption/desorption at the surface of chemiresistors. The study reports the synthesis of Fe-doped CuO hollow spheres as H2 S gas-sensing materials via spray pyrolysis. 4 at.% of Fe-doped CuO hollow spheres exhibit high selectivity (response ratio ≥ 34.4) over interference gas (ethanol, 1 ppm) and reversible sensing characteristics (100% recovery) to 0.1 ppm of H2 S under high humidity (relative humidity 80%) at 175 °C. The effect of multi-valent transition metal ion doping into CuO on sensor reversibility is confirmed through the enhancement of recovery kinetics by doping 4 at.% of Ti- or Nb ions into CuO sensors. Mechanistic details of these excellent H2 S sensing characteristics are also investigated by analyzing the redox reactions and the catalytic activity change of the Fe-doped CuO sensing materials. The selective and reversible detection of H2 S using the Fe-doped CuO sensor suggested in this work opens a new possibility for halitosis self-monitoring.

Magnetic Control and Real-Time Monitoring of Stem Cell Differentiation by the Ligand Nanoassembly.

Native extracellular matrix (ECM) exhibits dynamic change in the ligand position. Herein, the ECM-emulating control and real-time monitoring of stem cell differentiation are demonstrated by ligand nanoassembly. The density of gold nanoassembly presenting cell-adhesive Arg-Gly-Asp (RGD) ligand on Fe3 O4 (magnetite) nanoparticle in nanostructures flexibly grafted to material is changed while keeping macroscale ligand density invariant. The ligand nanoassembly on the Fe3 O4 can be magnetically attracted to mediate rising and falling ligand movements via linker stretching and compression, respectively. High ligand nanoassembly density stimulates integrin ligation to activate the mechanosensing-assisted stem cell differentiation, which is monitored via in situ real-time electrochemical sensing. Magnetic control of rising and falling ligand movements hinders and promotes the adhesion-mediated mechanotransduction and differentiation of stem cells, respectively. These rising and falling ligand states yield the difference in the farthest distance (≈34.6 nm) of the RGD from material surface, thereby dynamically mimicking static long and short flexible linkers, which hinder and promote cell adhesion, respectively. Design of cytocompatible ligand nanoassemblies can be made with combinations of dimensions, shapes, and biomimetic ligands for remotely regulating stem cells for offering novel methodologies to advance regenerative therapies.

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.

Golden Bristlegrass-Like Hierarchical Graphene Nanofibers Entangled with N-Doped CNTs Containing CoSe2 Nanocrystals at Each Node as Anodes for High-Rate Sodium-Ion Batteries.

Golden bristlegrass-like unique nanostructures comprising reduced graphene oxide (rGO) matrixed nanofibers entangled with bamboo-like N-doped carbon nanotubes (CNTs) containing CoSe2 nanocrystals at each node (denoted as N-CNT/rGO/CoSe2 NF) are designed as anodes for high-rate sodium-ion batteries (SIBs). Bamboo-like N-doped CNTs (N-CNTs) are successfully generated on the rGO matrixed nanofiber surface, between rGO sheets and mesopores, and interconnected chemically with homogeneously distributed rGO sheets. The defects in the N-CNTs formed by a simple etching process allow the complete phase conversion of Co into CoSe2 through the efficient penetration of H2 Se gas inside the CNT walls. The N-CNTs bridge the vertical defects for electron transfer in the rGO sheet layers and increase the distance between the rGO sheets during cycles. The discharge capacity of N-CNT/rGO/CoSe2 NF after the 10 000th cycle at an extremely high current density of 10 A g-1 is 264 mA h g-1 , and the capacity retention measured at the 100th cycle is 89%. N-CNT/rGO/CoSe2 NF has final discharge capacities of 395, 363, 328, 304, 283, 263, 246, 223, 197, 171, and 151 mA h g-1 at current densities of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 A g-1 , respectively.

Hierarchically Well-Developed Porous Graphene Nanofibers Comprising N-Doped Graphitic C-Coated Cobalt Oxide Hollow Nanospheres As Anodes for High-Rate Li-Ion Batteries.

Hierarchically well-developed porous graphene nanofibers comprising N-doped graphitic C (NGC)-coated cobalt oxide hollow nanospheres are introduced as anodes for high-rate Li-ion batteries. For this, three strategies, comprising the Kirkendall effect, metal-organic frameworks, and compositing with highly conductive C, are applied to the 1D architecture. In particular, NGC layers are coated on cobalt oxide hollow nanospheres as a primary transport path of electrons followed by graphene-nanonetwork-constituting nanofibers as a continuous and secondary electron transport path. Superior cycling performance is achieved, as the unique nanostructure delivers a discharge capacity of 823 mAh g-1 after 500 cycles at 3.0 A g-1 with a low decay rate of 0.092% per cycle. The rate capability is also noteworthy as the structure exhibits high discharge capacities of 1035, 929, 847, 787, 747, 703, 672, 650, 625, 610, 570, 537, 475, 422, 294, and 222 mAh g-1 at current densities of 0.5, 1.5, 3, 5, 7, 10, 12, 15, 18, 20, 25, 30, 40, 50, 80, and 100 A g-1 , respectively. In view of the highly efficient Li+ ion/electron diffusion and high structural stability, the present nanostructuring strategy has a huge potential in opening new frontiers for high-rate and long-lived stable energy storage systems.

Investigation of Binary Metal (Ni, Co) Selenite as Li-Ion Battery Anode Materials and Their Conversion Reaction Mechanism with Li Ions.

Highly efficient anode materials with novel compositions for Li-ion batteries are actively being researched. Multicomponent metal selenite is a promising candidate, capable of improving their electrochemical performance through the formation of metal oxide and selenide heterostructure nanocrystals during the first cycle. Here, the binary nickel-cobalt selenite derived from Ni-Co Prussian blue analogs (PBA) is chosen as the first target material: the Ni-Co PBA are selenized and partially oxidized in sequence, yielding (NiCo)SeO3 phase with a small amount of metal selenate. The conversion mechanism of (NiCo)SeO3 for Li-ion storage is studied by cyclic voltammetry, in situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, in situ electrochemical impedance spectroscopy, and ex situ transmission electron microscopy. The reversible reaction mechanism of (NiCo)SeO3 with the Li ions is described by the reaction: NiO + CoO + xSeO2 + (1 - x)Se + (4x + 6)Li+ + (4x + 6)e- ↔ Ni + Co + (2x + 2)Li2 O + Li2 Se. To enhance electrochemical properties, polydopamine-derived carbon is uniformly coated on (NiCo)SeO3 , resulting in excellent cycling and rate performances for Li-ion storage. The discharge capacity of C-coated (NiCo)SeO3 is 680 mAh g-1 for the 1500th cycle when cycled at a current density of 5 A g-1 .

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.

Synthesis Process of CoSeO3 Microspheres for Unordinary Li-ion Storage Performances and Mechanism of Their Conversion Reaction with Li ions.

Multicomponent materials with various double cations have been studied as anode materials of lithium-ion batteries (LIBs). Heterostructures formed by coupling different-bandgap nanocrystals enhance the surface reaction kinetics and facilitate charge transport because of the internal electric field at the heterointerface. Accordingly, metal selenites can be considered efficient anode materials of LIBs because they transform into metal selenide and oxide nanocrystals in the first cycle. However, few studies have reported synthesis of uniquely structured metal selenite microspheres. Herein, synthesis of high-porosity CoSeO3 microspheres is reported. Through one-pot oxidation at 400 °C, CoSex -C microspheres formed by spray pyrolysis transform into CoSeO3 microspheres showing unordinary cycling and rate performances. The conversion mechanism of CoSeO3 microspheres for lithium-ion storage is systematically studied by cyclic voltammetry, in situ X-ray diffraction and electrochemical impedance spectroscopy, and transmission electron microscopy. The reversible reaction mechanism of the CoSeO3 phase from the second cycle onward is evaluated as CoO + xSeO2 + (1 - x)Se + 4(x + 1)Li+ + 4( x + 1)e- ↔ Co + (2x + 1)Li2 O + Li2 Se. The CoSeO3 microspheres show a high reversible capacity of 709 mA h g-1 for the 1400th cycle at a current density of 3 A g-1 and a high reversible capacity of 526 mA h g-1 even at an extremely high current density of 30 A g-1 .

Design and Synthesis of Spherical Multicomponent Aggregates Composed of Core-Shell, Yolk-Shell, and Hollow Nanospheres and Their Lithium-Ion Storage Performances.

Micrometer-sized spherical aggregates of Sn and Co components containing core-shell, yolk-shell, hollow nanospheres are synthesized by applying nanoscale Kirkendall diffusion in the large-scale spray drying process. The Sn2 Co3 -Co3 SnC0.7 -C composite microspheres uniformly dispersed with Sn2 Co3 -Co3 SnC0.7 mixed nanocrystals are formed by the first-step reduction of spray-dried precursor powders at 900 °C. The second-step oxidation process transforms the Sn2 Co3 -Co3 SnC0.7 -C composite into the porous microsphere composed of Sn-Sn2 Co3 @CoSnO3 -Co3 O4 core-shell, Sn-Sn2 Co3 @CoSnO3 -Co3 O4 yolk-shell, and CoSnO3 -Co3 O4 hollow nanospheres at 300, 400, and 500 °C, respectively. The discharge capacity of the microspheres with Sn-Sn2 Co3 @CoSnO3 -Co3 O4 core-shell, Sn-Sn2 Co3 @CoSnO3 -Co3 O4 yolk-shell, and CoSnO3 -Co3 O4 hollow nanospheres for the 200th cycle at a current density of 1 A g-1 is 1265, 987, and 569 mA h g-1 , respectively. The ultrafine primary nanoparticles with a core-shell structure improve the structural stability of the porous-structured microspheres during repeated lithium insertion and desertion processes. The porous Sn-Sn2 Co3 @CoSnO3 -Co3 O4 microspheres with core-shell primary nanoparticles show excellent cycling and rate performances as anode materials for lithium-ion batteries.

Synthesis of Uniquely Structured Yolk-Shell Metal Oxide Microspheres Filled with Nitrogen-Doped Graphitic Carbon with Excellent Li-Ion Storage Performance.

Novel structured composite microspheres of metal oxide and nitrogen-doped graphitic carbon (NGC) have been developed as efficient anode materials for lithium-ion batteries. A new strategy is first applied to a one-pot preparation of composite (FeOx -NGC/Y) microspheres via spray pyrolysis. The FeOx -NGC/Y composite microspheres have a yolk-shell structure based on the iron oxide material. The void space of the yolk-shell microsphere is filled with NGC. Dicyandiamide additive plays a key role in the formation of the FeOx -NGC/Y composite microspheres by inducing Ostwald ripening to form a yolk-shell structure based on the iron oxide material. The FeOx -NGC/Y composite microspheres with the mixed crystal structure of rock salt FeO and spinel Fe3 O4 phases show highly superior lithium-ion storage performances compared to the dense-structured FeOx microspheres with and without carbon material. The discharge capacities of the FeOx -NGC/Y microspheres for the 1st and 1000th cycle at 1 A g-1 are 1423 and 1071 mAh g-1 , respectively. The microspheres have a reversible discharge capacity of 598 mAh g-1 at an extremely high current density of 10 A g-1 . Furthermore, the strategy described in this study is generally applied to multicomponent metal oxide-carbon composite microspheres with yolk-shell structures based on metal oxide materials.

Rational Design and Synthesis of Extremely Efficient Macroporous CoSe2 -CNT Composite Microspheres for Hydrogen Evolution Reaction.

Uniquely structured CoSe2 -carbon nanotube (CNT) composite microspheres with optimized morphology for the hydrogen-evolution reaction (HER) are prepared by spray pyrolysis and subsequent selenization. The ultrafine CoSe2 nanocrystals uniformly decorate the entire macroporous CNT backbone in CoSe2 -CNT composite microspheres. The macroporous CNT backbone strongly improves the electrocatalytic activity of CoSe2 by improving the electrical conductivity and minimizing the growth of CoSe2 nanocrystals during the synthesis process. In addition, the macroporous structure resulting from the CNT backbone improves the electrocatalytic activity of the CoSe2 -CNT microspheres by increasing the removal rate of generated H2 and minimizing the polarization of the electrode during HER. The CoSe2 -CNT composite microspheres demonstrate excellent catalytic activity for HER in an acidic medium (10 mA cm-2 at an overpotential of ≈174 mV). The bare CoSe2 powders exhibit moderate HER activity, with an overpotential of 226 mV at 10 mA cm-2 . The Tafel slopes for the CoSe2 -CNT composite and bare CoSe2 powders are 37.8 and 58.9 mV dec-1 , respectively. The CoSe2 -CNT composite microspheres have a slightly larger Tafel slope than that of commercial carbon-supported platinum nanoparticles, which is 30.2 mV dec-1 .

A New Strategy for Humidity Independent Oxide Chemiresistors: Dynamic Self-Refreshing of In2 O3 Sensing Surface Assisted by Layer-by-Layer Coated CeO2 Nanoclusters.

The humidity dependence of the gas sensing characteristics of metal oxide semiconductors has been one of the greatest obstacles for gas sensor applications during the last five decades because ambient humidity dynamically changes with the environmental conditions. Herein, a new and novel strategy is reported to eliminate the humidity dependence of the gas sensing characteristics of oxide chemiresistors via dynamic self-refreshing of the sensing surface affected by water vapor chemisorption. The sensor resistance and gas response of pure In2 O3 hollow spheres significantly change and deteriorate in humid atmospheres. In contrast, the humidity dependence becomes negligible when an optimal concentration of CeO2 nanoclusters is uniformly loaded onto In2 O3 hollow spheres via layer-by-layer (LBL) assembly. Moreover, In2 O3 sensors LBL-coated with CeO2 nanoclusters show fast response/recovery, low detection limit (500 ppb), and high selectivity to acetone even in highly humid conditions (relative humidity 80%). The mechanism underlying the dynamic refreshing of the In2 O3 sensing surfaces regardless of humidity variation is investigated in relation to the role of CeO2 and the chemical interaction among CeO2 , In2 O3 , and water vapor. This strategy can be widely used to design high performance gas sensors including disease diagnosis via breath analysis and pollutant monitoring.

One-Pot Synthesis of CoSex -rGO Composite Powders by Spray Pyrolysis and Their Application as Anode Material for Sodium-Ion Batteries.

A simple one-pot synthesis of metal selenide/reduced graphene oxide (rGO) composite powders for application as anode materials in sodium-ion batteries was developed. The detailed mechanism of formation of the CoSe(x)-rGO composite powders that were selected as the first target material in the spray pyrolysis process was studied. The crumple-structured CoSe(x)-rGO composite powders prepared by spray pyrolysis at 800 °C had a crystal structure consisting mainly of Co0.85 Se with a minor phase of CoSe2. The bare CoSe(x) powders prepared for comparison had a spherical shape and hollow structure. The discharge capacities of the CoSe(x)-rGO composite and bare CoSe(x) powders in the 50th cycle at a constant current density of 0.3 A g(-1) were 420 and 215 mA h g(-1), respectively, and their capacity retentions measured from the second cycle were 80 and 46%, respectively. The high structural stability of the CoSe(x)-rGO composite powders for repeated sodium-ion charge and discharge processes resulted in superior sodium-ion storage properties compared to those of the bare CoSe(x) powders.

Sodium-Ion Storage Properties of FeS-Reduced Graphene Oxide Composite Powder with a Crumpled Structure.

The sodium-ion storage properties of FeS-reduced graphene oxide (rGO) and Fe3O4 -rGO composite powders with crumpled structures have been studied. The Fe3 O4 -rGO composite powder, prepared by one-pot spray pyrolysis, could be transformed to an FeS-rGO composite powder through a simple sulfidation treatment. The mean size of the Fe3O4 nanocrystals in the Fe3O4 -rGO composite powder was 4.4 nm. After sulfidation, FeS nanocrystals of size several hundred nanometers were confined within the crumpled structure of the rGO matrix. The initial discharge capacities of the FeS-rGO and Fe3O4 -rGO composite powders were 740 and 442 mA h g(-1), and their initial charge capacities were 530 and 165 mA h g(-1), respectively. The discharge capacities of the FeS-rGO and Fe3O4 -rGO composite powders at the 50th cycle were 547 and 150 mA h g(-1), respectively. The FeS-rGO composite powder showed superior sodium-ion storage performance compared to the Fe3O4 -rGO composite powder.

Nanofibers Comprising Yolk-Shell Sn@void@SnO/SnO2 and Hollow SnO/SnO2 and SnO2 Nanospheres via the Kirkendall Diffusion Effect and Their Electrochemical Properties.

Nanofibers with a unique structure comprising Sn@void@SnO/SnO2 yolk-shell nanospheres and hollow SnO/SnO2 and SnO2 nanospheres are prepared by applying the nanoscale Kirkendall diffusion process in conventional electrospinning process. Under a reducing atmosphere, post-treatment of tin 2-ethylhexanoate-polyvinylpyrrolidone electrospun nanofibers produce carbon nanofibers with embedded spherical Sn nanopowders. The Sn nanopowders are linearly aligned along the carbon nanofiber axis without aggregation of the nanopowders. Under an air atmosphere, oxidation of the Sn-C composite nanofibers produce nanofibers comprising Sn@void@SnO/SnO2 yolk-shell nanospheres and hollow SnO/SnO2 and SnO2 nanospheres, depending on the post-treatment temperature. The mean sizes of the hollow nanospheres embedded within tin oxide nanofibers post-treated at 500 °C and 600 °C are 146 and 117 nm, respectively. For the 250th cycle, the discharge capacities of the nanofibers prepared by the nanoscale Kirkendall diffusion process post-treated at 400 °C, 500 °C, and 600 °C at a high current density of 2 A g(-1) are 663, 630, and 567 mA h g(-1), respectively. The corresponding capacity retentions are 77%, 84%, and 78%, as calculated from the second cycle. The nanofibers prepared by applying the nanoscale Kirkendall diffusion process exhibit superior electrochemical properties compared with those of the porous-structured SnO2 nanofibers prepared by the conventional post-treatment process.

General formation of tin nanoparticles encapsulated in hollow carbon spheres for enhanced lithium storage capability.

A new simple process for synthesis of heterogeneous yolk-shell microspheres is introduced. The core/shell-structured microspheres are prepared by a one-pot spray pyrolysis process. The removal of one kind of metal oxide by a dry process produces heterogeneous yolk-shell microspheres. The yolk-shell Sn@C microspheres show superior electrochemical properties as anode materials for lithium-ion batteries.

Superior Lithium-Ion Storage Properties of Mesoporous CuO-Reduced Graphene Oxide Composite Powder Prepared by a Two-Step Spray-Drying Process.

Mesoporous CuO-reduced graphene oxide (rGO) composite powders were prepared by using a two-step spray-drying process. In the first step, hollow CuO powders were prepared from a spray solution of copper nitrate trihydrate with citric acid and were wet milled to obtain a colloidal spray solution. In the second step, spray drying of the colloidal solution that contained dispersed GO nanosheets produced mesoporous CuO-rGO composite powders with particle sizes of several microns. Thermal reduction of GO nanosheets to rGO nanosheets occurred during post-treatment at 300 °C. Initial discharge capacities of the hollow CuO, bare CuO aggregate, and CuO-rGO composite powders at a current density of 2 A g(-1) were 838, 1145, and 1238 mA h g(-1) , respectively. Their discharge capacities after 200 cycles were 259, 380, and 676 mA h g(-1) , respectively, and their corresponding capacity retentions measured from the second cycle were 67, 48, and 76 %, respectively. The mesoporous CuO-rGO composite powders have high structural stability and high conductivity because of the rGO nanosheets, and display good cycling and rate performances.

Superior Electrochemical Properties of Nanofibers Composed of Hollow CoFe2 O4 Nanospheres Covered with Onion-Like Graphitic Carbon.

Nanofibers composed of hollow CoFe2 O4 nanospheres covered with onion-like carbon are prepared by applying nanoscale Kirkendall diffusion to the electrospinning process. Amorphous carbon nanofibers embedded with CoFe2 @onion-like carbon nanospheres are prepared by reduction of the electrospun nanofibers. Oxidation of the CoFe2 -C nanofibers at 300 °C under a normal atmosphere produces porous nanofibers composed of hollow CoFe2 O4 nanospheres covered with onion-like carbon. CoFe2 nanocrystals are transformed into the hollow CoFe2 O4 nanospheres during oxidation through a well-known nanoscale Kirkendall diffusion process. The discharge capacities of the carbon-free CoFe2 O4 nanofibers composed of hollow nanospheres and the nanofibers composed of hollow CoFe2 O4 nanospheres covered with onion-like carbon are 340 and 930 mA h g(-1) , respectively, for the 1000th cycle at a current density of 1 A g(-1) . The nanofibers composed of hollow CoFe2 O4 nanospheres covered with onion-like carbon exhibit an excellent rate performance even in the absence of conductive materials.

Electrochemical properties of yolk-shell-structured Zn-Fe-S multicomponent sulfide materials with a 1:2 Zn/Fe molar ratio.

Yolk-shell-structured Zn-Fe-S multicomponent sulfide materials with a 1:2 Zn/Fe molar ratio were prepared applying a sulfidation process to ZnFe2O4 yolk-shell powders. The Zn-Fe-S powders had mixed sphalerite (Zn,Fe)S and hexagonal FeS crystal structures. The discharge capacities of the Zn-Fe-S powders sulfidated at 350 °C at a constant current density of 500 mA g(-1) for the first, second, and fiftieth cycles were 1098, 912, and 913 mA h g(-1), respectively. The powders exhibited a high discharge capacity of 602 mA h g(-1) even at the high current density of 10 A g(-1). The synergistic effect of yolk-shell structure and multicomponent composition improved the electrochemical properties of Zn-Fe-S powders.