Yanwei Zeng

Porous Fe3O4–NCs-in-Carbon Nanofoils as High-Rate and High-Capacity Anode Materials for Lithium-Ion Batteries from Na–Citrate-Mediated Growth of Super-Thin Fe–Ethylene Glycolate Nanosheets

Porous Fe3O4/C composite nanofoils, characterized by a thickness of ∼20 nm and with ∼8 nm open pores and ∼5 nm Fe3O4 nanoparticles embedded in the carbon matrix, were prepared for the first time using Na-citrate to mediate the growth of hexagonal Fe-ethylene glycolate nanosheets and subsequently annealing them at 350 °C in N2. It has been found that the Fe-ethylene glycolate nanosheets can be effectively slimmed by increasing the concentration of Na-citrate, and the microstructures of Fe3O4/C nanocomposites may be tailored by the annealing temperature. When tested as the anode materials in LIBs, the Fe3O4/C nanofoils obtained after annealing at 350 °C were found to exhibit superior electrochemical performance due to its optimal microstructure, featured by a reversible capacity of 1314.4 mAh g(-1) at 0.4 A g(-1) over 100 cycles, 1034.2 mAh g(-1) at 1 A g(-1), and 686.4 mAh g(-1) at 5 A g(-1) after 500 cycles, whereas the annealing treatments at 450 and 550 °C render the Fe3O4/C nanocomposites with the inferior electrochemical performances as a result of shrinking porous microstructures and coarsening of Fe3O4 nanoparticles in the carbon matrix. With a particle-size control model proposed herein, the cycle discharging behaviors of the Fe3O4/C nanocomposites with different microstructures are well explained from the perspective of the local confinement of Fe3O4 nanoparticles inside the carbon matrix and their evolution in size and composite microstructure during the charge/discharge cycling.

Investigation of Sm0.2Ce0.8O1.9/Na2CO3 nanocomposite electrolytes: preparation, interfacial microstructures, and ionic conductivities.

With the analytical grade Ce(NO3)3·6H2O, Sm(NO3)3·6H2O, and Na2CO3 as starting materials, Sm0.2Ce0.8O1.9(SDC)/Na2CO3 nanocomposite electrolytes were prepared through a rare-earth/sodium carbonate complex precipitation, prefiring, and sintering operations. The phase components and microstructures were studied and characterized by XRD, FESEM, TEM, and TG-DSC. In particular, the interfacial interactions between the phases of SDC crystallites and amorphous Na2CO3 were deliberately probed by Raman and infrared spectroscopies. It has been found that the amorphous carbonates in the SDC/Na2CO3 composites are tightly bound to the surface of SDC nanocrystals to form an intimate shell-layer via a long-range interface interaction, characterized by ∼8 nm in thickness and a red-shift of 15 cm(-1) for the Raman symmetrical vibration mode of carbonate ions with reference to the crystalline Na2CO3, which is practically enabled to frustrate the crystallization of Na2CO3 and enhance the transport properties of oxide ions in the SDC/Na2CO3 composite electrolytes because of the disordered interface microstructures. Moreover, smaller SDC nanocrystals were found to achieve higher conductivity enhancements for the SDC/Na2CO3 composite electrolytes and the {100} facets on the surface of SDC nanocrystals are believed to be more important than the other facets because of their strong electropositivity. This effect makes the SDC/Na2CO3 composite sample prefired at 600 °C realize a much higher ionic conductivity than the samples prefired at the other temperatures.

Study of Sm0.2Ce0.8O1.9 (SDC) electrolyte prepared by a simple modified solid-state method

Abstract Sm 0.2 Ce 0.8 O 1.9 (SDC) electrolyte was prepared by a modified solid state method at relatively low sintering temperatures without any sintering promoters. The phase composition and microstructure of the electrolytes were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) technologies. A relative density of SDC electrolyte sintered at 1300 °C reached 97.3% and the mean SDC grain size was about 770 nm. Their ionic conductivity and thermal expansion coefficient were also measured by electrochemical workstation and dilatometer. The electrolyte attained a high conductivity of 5×10 −2 S/cm at 800 °C with an activation energy of 1.03 eV and a proper thermal expansion coefficient of 12.6×10 −6 K −1 .

Preparation of smxCe1-xO2(SDC) electrolyte film with gradient structure via a gas-phase controlling convection-diffusion approach on porous substrate

A SDC electrolyte film with gradient structure rooted on porous alumina substrate has been prepared by using a gas-phase controlling convection-diffusion approach. Investigation on the fabrication principles and the co-precipitation kinetics turned out the gradient distribution of hydroxide product of Ce(OH)(3) and Sm(OH)(3) in a porous substrate could be formed as induced by the down-toward diffusion of NH(3)·H(2)O in polar solvent along vertical direction and the up-toward convection of Sm(3+) and Ce(3+) ions over the cross-section of porous substrate, and the aim ratio of Ce to Sm of 4:1 in the sediment phase would be achieved by controlling component concentration in bulk solution. As a result, Sm(0.2)Ce(0.8)O(2.0)(SDC) electrolyte film with gradient microstructure could be fabricated after a subsequent sintering treatment at a high temperature. Investigation of crystal phase, structural, compositional characteristics of the sintered SDC/substrate specimens proved that a uniform and dense SDC film with an average grain size of ~500 nm spread over on the surface of substrate, and a correct cubic fluorite phase has been formed. Gradient variation presented in both the microstructure of SDC/substrate and the component contents over the cross-section of the SDC/substrate. Numerical analysis on the EDX data presented three component parts were sectioned, including a dense SDC layer of ~25 μm, a uniform filling layer of ~140 μm and a successive diffuse layer stretching as far as ~250 μm. Effect of bulk pH on thickness and surface microstructure of SDC film has been discussed. This microstructure-optimization approach will be applicable to fabricate electrode-supported gradient electrolyte films for IT-SOFC.

Preparation of 2D α-Fe2O3 platelets via a hydrothermal heterogeneous growth approach and study of their magnetic properties

We report a novel strategy to modify the morphology of hydrothermal α-Fe2O3 particles via a heterogeneous growth process of BaFe12O19 crystal nuclei based on their structural similarity to α-Fe2O3 as well as their excellent crystallization habit to hexaplates. Various (001)-exposed single phase α-Fe2O3 platelets with diameters of 2.23–11.99 μm and thicknesses of 0.24–2.21 μm were successfully prepared without surfactants by using Fe(NO3)3·9H2O and Ba(NO3)2 as starting materials with NaOH as a precipitant in a facile hydrothermal process. The microstructural characterization by XRD, FESEM and EDS indicates that Ba2+ ions play an essential role in directing the formation of α-Fe2O3 platelets by producing BaFe12O19 nuclei as heterogeneous growth cores. This outcome has been supported by a series of experimental results, such as composition and cell parameter changes, morphology evolution, and the generation of double-layer α-Fe2O3 platelets. To facilitate the nucleation of BaFe12O19 while preventing its further growth, an appropriate NaOH concentration and barium-poor stoichiometry were required. Furthermore, the magnetic hysteresis measurements demonstrated that these 2D α-Fe2O3 platelets exhibit strong shape- and orientation-dependent magnetic properties.

Dopant-induced shape evolution of polyhedral magnetite nanocrystals and their morphology/component-dependent high-rate electrochemical performance for lithium-ion batteries

Monodisperse MnxFe3−xO4 (x = 0, 0.3, 0.6) polyhedrons enclosed by {100}/{111} facets with different area ratios were synthesized through the thermolysis of Fe(acac)3 and Mn(acac)2 by effectively tuning the Mn/Fe ratio to mediate the adsorption properties of oleic acid (OA) on crystal surfaces after annealing treatment in N2, and studied as high rate (≥1 A g−1) anode materials for lithium ion batteries (LIBs). The electrochemical results show that Mn0.6Fe2.4O4 octahedra possess the best rate cycling performance compared to that of Mn0.3Fe2.7O4 cuboctahedra and Fe3O4 cubes, characterised by a 500th discharge capacity of 803.5 mA h g−1 at 1 A g−1 and a rate capability of 661.5 mA h g−1 when cycled at 4 A g−1, as a result of high electrochemical activity of {111} facets with the highest Fe atom surface density. The present results prove that the substitution of Fe by Mn in the spinel-type anode materials can result in better cycle stability and it would be helpful for the further understanding of Fe3O4 based anode materials and provide a simple and practical route to design high rate anode materials for lithium-ion batteries.

Preparation and characterization of SDC nanorods/LNC nanocomposite electrolyte

The nanocomposite electrolytes composed of Sm0.2Ce0.8O1.9 (SDC) nanorods enclosed by {110} and {100} facets and a binary carbonate ((Li0.52Na0.48)2CO3, LNC) were prepared by a wet mixing method to investigate the conduction mechanism. The X-ray diffraction (XRD), scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) techniques were employed to characterize the phase components and microstructures of SDC nanorods and SDC nanorods/LNC composite electrolytes. X-ray powder diffraction showed that a well-cubic fluorite structure was formed. The AC impedance spectroscopy and DC polarization method were used to measure the electrical conductivities of nanocomposite electrolytes under different conditions. The overall ionic conductivities of nanocomposite electrolytes in the air and hydrogen atmospheres were measured up to 82 and 96 mS/cm at 650 °C, respectively. Additionally, the protonic and oxide ionic conductivities of nanocomposite electrolytes were found to reach 20 and 18 mS/cm at 650 °C, respectively. The conduction mechanism was discussed in detail by comparing the conductivities of nanocomposite electrolytes. The protonic conductivity of SDC nanorods/LNC nanocomposite was higher than oxide ionic conductivity. The melt of LNC and the interface layer may make a dominant contribution to oxide ions and protonic conductivity in air and hydrogen atmosphere, respectively.

Preparation of SDC–NC nanocomposite electrolytes with elevated densities: influence of prefiring and sintering treatments on their microstructures and electrical conductivities

Sm0.2Ce0.8O1.9–Na2CO3 (SDC–NC) nanocomposite powders and electrolytes were prepared through the precipitation of Sm-doped cerium/sodium complex carbonate and its prefiring and sintering operations. Their phase components and microstructures were characterized by XRD, FT-IR, TG-DSC, SEM and TEM, respectively, and, in particular, the sintering performance and oxide ionic and protonic conductivities of SDC–NC nanocomposite electrolytes prepared by prefiring and sintering at different temperatures were studied. It has been found that the SDC–NC nanocomposite powders derived from pre-firing treatments of non-crystalline carbonate precipitates are composed of SDC/NC nanocomposite core–shell structured particles. Moreover, the as-sintered SDC–NC nanocomposite electrolytes are generally made up of densely compacted SDC particles bound by NC phase, while their sintering performances and microstructures are significantly affected by the prefiring and sintering temperatures due to the differences in structural homogeneity and continuity of the NC phase. In addition, the oxide ionic and protonic conductivities of SDC–NC nanocomposite electrolytes can be strongly dependent upon the prefiring and sintering treatments, with the sample S-500-800 (prefired at 500 °C and sintered at 800 °C) showing the highest conductivities, 9.11 and 3.27 mS cm−1 at 600 °C in H2 and air, respectively. The single cell based on the electrolyte of S-500-800 showed an OCV of 0.99 V and a peak power density of 342 mW cm−2 at 550 °C. More interestingly, the dependence of electrical performance on the prefiring and sintering temperatures is discussed from the perspective of the significant effects of the prefiring and sintering treatments on the microstructures and interfacial interactions between the phases of disperse SDC nanoparticles and NC, which is homogeneously and continuously filled in between them.