Yun Chan Kang

One‐Pot Facile Synthesis of Double‐Shelled SnO2 Yolk‐Shell‐Structured Powders by Continuous Process as Anode Materials for Li‐ion Batteries

Hollow and yolk-shell metal oxide powders used as energy storage materials exhibit good electrochemical properties at high current density because of their shortened diffusion length and increased amount of contact area between the electrolyte and the electrode for Li + insertion/extraction. [ 1–25 ] Although various types of hollow-structured oxide materials have been studied as anode and cathode electrode materials for lithium secondary batteries, [ 13–25 ] the hollow-structured powders cannot be readily applied as battery materials because of their low energy densities due to low tab density. The disadvantages of the hollow materials can be overcome using core@void@shellconfi gured yolk-shell-structured powder particles. The core of such yolk-shell-structured powder particles will improve the rate capability as well as the energy density of the powders by increasing the weight fraction of the electrochemically active component. [ 12 , 15 ]

YAG:Ce phosphor particles prepared by ultrasonic spray pyrolysis

Abstract Spray pyrolysis was applied to the preparation of fine YAG:Ce phosphor particles. The characteristics of particles such as crystallinity, morphology, and photoluminescence were investigated. Phase-pure YAG:Ce particles with high crystallinity were obtained after annealing at lower temperatures than those of conventional preparation processes. The mean size of the particles increased from 0.46 to 1.2 μm when the overall solution concentrations were changed from 0.03 to 1 mol/L. The YAG:Ce particles annealed at 1300°C were nonaggregated and spherical. The particles absorbed excitation energy in the range 403–510 nm, and the maximum excitation wavelength was near 470 nm. The YAG:Ce particles showed broad emission peaks in the range 480–650 nm and had maximum intensity at 528 nm. The photoluminescence intensity of the particles increased monotonically with increasing annealing temperature and showed the maximum value at 1 at% Ce.

Yolk-shell, hollow, and single-crystalline ZnCo(2)O(4) powders: preparation using a simple one-pot process and application in lithium-ion batteries.

The electrochemical properties of yolk-shell-structured, multi-component, transition-metal oxides have not yet been properly compared to those of hollow-structured or nanoscale powders. In this study, yolk-shell, hollow, and single-crystalline ZnCo2 O4 powders with uniform compositions are prepared by using simple gas-phase reaction methods. Double-shelled ZnCo2 O4 yolk-shell powder is prepared directly from the spray solution by using spray pyrolysis. Single-crystalline ZnCo2 O4 nanopowder is prepared by means of flame spray pyrolysis. The yolk-shell ZnCo2 O4 powder shows higher charge and discharge capacities than the hollow and single-crystalline powders. The yolk-shell, hollow, and single-crystalline ZnCo2 O4 powders deliver discharge capacities of 753, 586, and 206 mAh g(-1) , respectively, after 200 cycles at a charge/discharge rate of 3 A g(-1) , and the corresponding capacity retentions measured after the first cycle are 99, 74, and 27 %, respectively. The yolk-shell ZnCo2 O4 powders are structurally stable during cycling and have good electrochemical properties even at high current densities.

Crumpled Graphene–Molybdenum Oxide Composite Powders: Preparation and Application in Lithium‐Ion Batteries

Crumpled graphene-MoO2 composite powders are directly prepared by means of spray pyrolysis and from a stable graphene oxide colloidal solution in the presence of Mo ions. The crumpled graphene-MoO2 composite powders are transformed into MoO3 -based composite powders after post-treatment at 300 °C. The transmission electron microscopy and dot-mapping images of the post-treatment composite powders show uniform distribution of MoO3 nanocrystals in the crumpled graphene powders. The two typical D and G bands of graphene are observed at 1350 and 1590 cm(-1) , respectively, in the Raman spectrum of the graphene-MoO3 composite. In addition, the crumpled graphene-MoO3 powders exhibit superior electrochemical behavior compared to that of pure MoO3 as an anode material for lithium-ion batteries. The initial discharge capacities of the graphene-MoO3 composite and bare MoO3 powders at a current density of 2 A g(-1) are 1490 and 1225 mA h g(-1) , respectively. The capacity retention of the graphene-MoO3 composite is 87 % after the first cycle, whereas that of bare MoO3 is 47 %, as measured after 100 cycles. The reversible discharge capacity of the graphene-MoO3 composite decreases slightly from 1228 to 845 mA h g(-1) as the current density increases from 0.5 to 3 A g(-1) .

UV and VUV characteristics of (YGd)2O3:Eu phosphor particles prepared by spray pyrolysis from polymeric precursors

Red-emitting (YGd){sub 2}O{sub 3}:Eu phosphor particles, with high luminescence efficiency under vacuum ultraviolet (VUV) and ultraviolet (UV) excitation, were prepared by a large-scale spray pyrolysis process. To control the morphology of phosphor particles under severe preparation conditions, spray solution with polymeric precursors were introduced in spray pyrolysis. The prepared (YGd){sub 2}O{sub 3}:Eu phosphor particles had spherical shape and filled morphology even after post-treatment irrespective of Gd/Y ratio. In the case of solution with polymeric precursors, long polymeric chains formed by esterification reaction in a hot tubular reactor; the droplets turned into viscous gel, which retarded the precipitation of nitrate salts and promoted the volume precipitation of droplets. The brightness of (YGd){sub 2}O{sub 3}:Eu phosphor particles increased with increasing gadolinium content, and the Gd{sub 2}O{sub 3}:Eu phosphor had the highest luminescence intensity under UV and VUV excitation. The maximum peak intensity of Gd{sub 2}O{sub 3}:Eu phosphor particles under UV and VUV were 118 and 110% of the commercial Y{sub 2}O{sub 3}:Eu phosphor particles, respectively.

Preparation of nonaggregated Y2O3 : Eu phosphor particles by spray pyrolysis method

Ministry of Education, Culture and Science of Japan, Hiroshima Industrial Technology Organization, KOSEF (Korea Science and Engineering Foundation)

Luminescence characteristics of Y2SiO5 : Tb phosphor particles directly prepared by the spray pyrolysis method

Tb-doped Y 2 SiO 5 phosphor particles were prepared directly by a continuous droplet-to-particle preparation process using an ultrasonic spray generator. The photoluminescence characteristics of as-prepared and calcined particles were investigated. The particles of Y 2 SiO 5 prepared below 1100°C had poor crystallinity because of the short residence time of the particles inside the hot wall reactor, while the particles of Y 2 SiO 5 prepared at 1200°C had sharp crystallinity. The crystal structure of Y 2 SiO 5 particles was changed from the X1 to X2 type after calcining above 1400°C. Both prepared and calcined particles had spherical morphology, submicron size, and narrow size distribution. Agglomeration of particles did not occur even after calcining at 1200°C. The prepared Y 2 SiO 5 :Tb particles absorbed excitation energy in the range from 220 to 325 nm, and the maximum excitation wavelength was near 240 nm. The main emission peak was 538 nm. The optimum brightness was obtained at the doping concentration of 4 atom %. The activation of terbium occurred directly even at short reaction times inside the hot wall reactor. The brightness of as-prepared particles increased with increasing temperature because of good activation and crystallization in high temperatures. The PL intensity of as-prepared particles at 1200°C was 96% in comparison with maximum intensity of calcined particles.

Electrochemical properties of yolk-shell structured ZnFe2O4 powders prepared by a simple spray drying process as anode material for lithium-ion battery.

ZnFe2O4 yolk–shell powders were prepared by applying a simple spray-drying process. Dextrin was used as a drying additive and carbon source material, and thus played a key role in the preparation of the powders. The combustion of precursor powders consisting of zinc and iron salts and dextrin obtained by a spray-drying process produced the yolk–shell-structured ZnFe2O4 powders even at a low post-treatment temperature of 350°C. The ZnFe2O4 powders prepared from the spray solution without dextrin had a filled and pockmarked structure. The initial discharge capacities of the ZnFe2O4 yolk–shell and filled powders post-treated at 450°C at a current density of 500 mA g−1 were 1226 and 993 mA h g−1, respectively, and the corresponding initial Coulombic efficiencies were 74 and 58%. The discharge capacities of the ZnFe2O4 powders with yolk–shell and filled structures post-treated at 450°C after 200 cycles were 862 and 332 mA h g−1, respectively. The ZnFe2O4 yolk–shell powders with high structural stability during cycling had superior electrochemical properties to those of the powders with filled structure.

Ultrafast Synthesis of Yolk-Shell and Cubic NiO Nanopowders and Application in Lithium Ion Batteries

A continuous one-pot method was employed to synthesize yolk-shell and single-crystalline cubic NiO powders in a few seconds. Submicrometer-sized NiO yolk-shell particles were prepared by spray pyrolysis at 900 °C. Single-crystalline cubic NiO nanopowders were prepared by one-pot flame spray pyrolysis from NiO vapors. Particle surface areas of the yolk-shell and single-crystalline cubic NiO powders as obtained using the Brunauer-Emmett-Teller method were 8 and 5 m(2) g(-1), respectively. The mean crystallite sizes of the yolk-shell-structured and cubic NiO powders were 50 and 80 nm, respectively. The yolk-shell and single-crystalline cubic NiO powders delivered discharge capacities of 951 and 416 mA h g(-1), respectively, after 150 cycles, and the corresponding capacity retentions measured after the first cycle were 106 and 66%, respectively. The yolk-shell-structured NiO powders showed rate performance better than that of the single-crystalline cubic NiO nanopowders. Even at a high current density of 1 A g(-1), the discharge capacity of the yolk-shell-structured NiO powders was as high as 824 mA h g(-1) after 50 cycles, in which the current densities were increased stepwise.

First Introduction of NiSe2 to Anode Material for Sodium-Ion Batteries: A Hybrid of Graphene-Wrapped NiSe2/C Porous Nanofiber.

The first-ever study of nickel selenide materials as efficient anode materials for Na-ion rechargeable batteries is conducted using the electrospinning process. NiSe2-reduced graphene oxide (rGO)-C composite nanofibers are successfully prepared via electrospinning and a subsequent selenization process. The electrospun nanofibers giving rise to these porous-structured composite nanofibers with optimum amount of amorphous C are obtained from the polystyrene to polyacrylonitrile ratio of 1/4. These composite nanofibers also consist of uniformly distributed single-crystalline NiSe2 nanocrystals that have a mean size of 27 nm. In contrast, the densely structured bare NiSe2 nanofibers formed via selenization of the pure NiO nanofibers consist of large crystallites. The initial discharge capacities of the NiSe2-rGO-C composite and bare NiSe2 nanofibers at a current density of 200 mA g−1 are 717 and 755 mA h g−1, respectively. However, the respective 100th-cycle discharge capacities of the former and latter are 468 and 35 mA h g−1. Electrochemical impedance spectroscopy measurements reveal the structural stability of the composite nanofibers during repeated Na-ion insertion and extraction processes. The excellent Na-ion storage properties of these nanofibers are attributed to this structural stability.