Kai-Xue Wang

Extended Structures and Physicochemical Properties of Uranyl–Organic Compounds

The ability of uranium to undergo nuclear fission has been exploited primarily to manufacture nuclear weapons and to generate nuclear power. Outside of its nuclear physics, uranium also exhibits rich chemistry, and it forms various compounds with other elements. Among the uranium-bearing compounds, those with a uranium oxidation state of +6 are most common and a particular structural unit, uranyl UO(2)(2+) is usually involved in these hexavalent uranium compounds. Apart from forming solids with inorganic ions, the uranyl unit also bonds to organic molecules to generate uranyl-organic coordination materials. If appropriate reaction conditions are employed, uranyl-organic extended structures (1-D chains, 2-D layers, and 3-D frameworks) can be obtained. Research on uranyl-organic compounds with extended structures allows for the exploration of their rich structural chemistry, and such studies also point to potential applications such as in materials that could facilitate nuclear waste disposal. In this Account, we describe the structural features of uranyl-organic compounds and efforts to synthesize uranyl-organic compounds with desired structures. We address strategies to construct 3-D uranyl-organic frameworks through rational selection of organic ligands and the incorporation of heteroatoms. The UO(2)(2+) species with inactive U═O double bonds usually form bipyramidal polyhedral structures with ligands coordinated at the equatorial positions, and these polyhedra act as primary building units (PBUs) for the construction of uranyl-organic compounds. The geometry of the uranyl ions and the steric arrangements and functionalities of organic ligands can be exploited in the the design of uranyl--organic extended structures, We also focus on the investigation of the promising physicochemical properties of uranyl-organic compounds. Uranyl-organic materials with an extended structure may exhibit attractive properties, such as photoluminescence, photocatalysis, photocurrent, and photovoltaic responses. In particular, the intriguing, visible-light photocatalytic activities of uranyl-organic compounds are potentially applicable in decomposition of organic pollutants and in water-splitting with the irradiation of solar light. We ascribe the photochemical properties of uranyl-organic compounds to the electronic transitions within the U═O bonds, which may be affected by the presence of organic ligands.

Surface and Interface Engineering of Electrode Materials for Lithium-Ion Batteries

Lithium-ion batteries are regarded as promising energy storage devices for next-generation electric and hybrid electric vehicles. In order to meet the demands of electric vehicles, considerable efforts have been devoted to the development of advanced electrode materials for lithium-ion batteries with high energy and power densities. Although significant progress has been recently made in the development of novel electrode materials, some critical issues comprising low electronic conductivity, low ionic diffusion efficiency, and large structural variation have to be addressed before the practical application of these materials. Surface and interface engineering is essential to improve the electrochemical performance of electrode materials for lithium-ion batteries. This article reviews the recent progress in surface and interface engineering of electrode materials including the increase in contact interface by decreasing the particle size or introducing porous or hierarchical structures and surface modification or functionalization by metal nanoparticles, metal oxides, carbon materials, polymers, and other ionic and electronic conductive species.

Surface Binding of Polypyrrole on Porous Silicon Hollow Nanospheres for Li‐Ion Battery Anodes with High Structure Stability

Uniform porous silicon hollow nano-spheres are prepared without any sacrificial templates through a magnesio-thermic reduction of mesoporous silica hollow nanospheres and surface modified by the following in situ chemical polymerization of polypyrrole. The porous hollow structure and polypyrrole coating contribute significantly to the excellent structure stability and high electrochemical performance of the nanocomposite.

3D-hierarchical NiO–graphene nanosheet composites as anodes for lithium ion batteries with improved reversible capacity and cycle stability

3D-hierarchical NiO–graphene nanosheet (GNS) composites as high performance anode materials for lithium-ion batteries (LIBs) were synthesized through a simple ultrasonic method, and characterized by X-ray diffraction, Raman spectrum, field emission scanning electron microscopy and transmission electron microscopy. The results show that the 3D-hierarchical NiO carnations with nanoplates as building blocks are homogeneously anchored onto GNS and act as spacers to reduce the stacking of GNS. Electrochemical performances reveal that the obtained 3D-hierarchical NiO–GNS composites exhibit remarkably high reversible lithium storage capacity, good rate capability and improved cycling stability, e.g. approximate 1065 mA h g−1 of reversible capacity is retained even after 50 cycles at a current density of 200 mA g−1. The remarkable improvement of electrochemical performances of the obtained composites could be attributed to the decrease of the volume expansion and contraction of NiO and the improvement of the electronic conductivity of composites during the cycling process.

Sol–gel preparation of efficient red phosphor Mg2TiO4:Mn4+ and XAFS investigation on the substitution of Mn4+ for Ti4+

An efficient red phosphor based on the substitution of Mn4+ for Ti4+ in the lattice of Mg2TiO4 was prepared through a sol–gel route. The phosphor was characterized by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and UV-vis spectroscopy and the luminescent properties of the samples with varying Mn4+ concentrations were investigated. The photoluminescence of the sol–gel prepared Mg2TiO4:Mn4+ phosphor was compared with the corresponding sample prepared via solid-state reaction and the commercial CaAlSiN3:Eu2+. X-ray absorption fine structure (XAFS) investigation reveals the oxidation state and coordination environment of the Mn ion in the crystal structure of Mg2TiO4 and indicates the substitution of Mn4+ for Ti-sites. Moreover, a decrease in lattice parameter was observed with the increase of Mn4+ content in Mg2TiO4, further confirming the replacement of Ti4+ ions by smaller Mn4+ ions in the host lattice.

Carbon nanocages with nanographene shell for high-rate lithium ion batteries

Carbon nanocages with a nanographene shell have been prepared by catalytic decomposition of p-xylene on a MgO supported Co and Mo catalyst in supercritical CO2 at a pressure of 10.34 MPa and temperatures ranging from 650 to 750 °C. The electrochemical performance of these carbon nanocages as anodes for lithium ion batteries has been evaluated by galvanostatic cycling. The carbon nanocages prepared at a temperature of 750 °C exhibited relatively high reversible capacities, superior rate performance and excellent cycling life. The advanced performance of the carbon nanocages prepared at 750 °C is ascribed to their unique structural features: (1) nanographene shells and the good inter-cage contact ensuring fast electron transportation, (2) a porous network formed by fine pores in the carbon shell and the void space among the cages facilitating the penetration of the electrolyte and ions within the electrode, (3) thin carbon shells shortening the diffusion distance of Li ions, and (4) the high specific surface area providing a large number of active sites for charge-transfer reactions. These carbon nanocages are promising candidates for application in lithium ion batteries.

Strategies to succeed in improving the lithium-ion storage properties of silicon nanomaterials

Silicon is of scientific and practical interest in lithium-ion batteries (LIBs) due to its natural abundance, low toxicity, moderate working potential, and high theoretical capacity. However, its huge volume variation during lithiation and delithiation processes leads to the pulverization of silicon particles and subsequently results in fast capacity fade of the electrodes. Furthermore, the intrinsic electric conductivity of Si is relatively low and lithium diffusion in Si is rather slow. These issues hinder the practical application of Si in LIBs. In the past decade, significant improvements in the anodes cycleability and rate capability have been achieved by the control of the nanostructure and morphology of the Si electrode and incorporation of conductive species. In this review, the preparation methods and electrochemical performance of these Si electrode nanomaterials are summarised. The mechanisms behind the performance enhancement are illustrated. Moreover, factors that affect the performance of Si anodes, such as electrolyte additives, binders, and current collectors, are also discussed. We aim to shed some light on some emerging directions for future research on Si anodes in LIBs.

Hierarchical Li4Ti5O12/TiO2 composite tubes with regular structural imperfection for lithium ion storage.

Hierarchical Li4Ti5O12/TiO2 tubes composed of ultrathin nanoflakes have been successfully fabricated via the calcination of the hydrothermal product of a porous amorphous TiO2 precursor and lithium hydroxide monohydrate. The hierarchical tubes are characterized by powder X-ray diffraction, nitrogen adsorption/desorption, scanning electron microscopy and transmission electron microscopy techniques. These nanoflakes exhibit a quite complex submicroscopic structure with regular structural imperfection, including a huge number of grain boundaries and dislocations. The lithium ion storage property of these tubes is evaluated by galvanostatic discharge/charge experiment. The product shows initial discharge capacities of 420, 225, and 160 mAh g−1 at 0.01, 0.1, and 1.0 A g−1, respectively. After 100 cycles, the discharge capacity is 139 mAh g−1 at 1.0 A g−1 with a capacity retention of 87%, demonstrating good high-rate performance and good cycleability. The high electrochemical performance is attributed to unique structure and morphology of the tubes. The regular structural imperfection existed in the nanoflakes also benefit to lithium ion storage property of these tubes. The hierarchical Li4Ti5O12/TiO2 tubes are a promising anode material for lithium-ion batteries with high power and energy densities.

A graphene-wrapped silver–porous silicon composite with enhanced electrochemical performance for lithium-ion batteries

A graphene-wrapped silver–porous silicon composite has been prepared and used as the anode material in lithium-ion batteries (LIBs). Porous silicon (pSi) prepared through a magnesiothermic reduction of mesoporous silica MCM-41 was surface-modified by Ag nanoparticles formed via the thermal decomposition of AgNO3 and then graphene nanosheets (GNS) generated through the thermal reduction of graphene oxide. The obtained Ag–pSi/GNS composite exhibits a distinctly high reversible specific capacity, long cycling performance and super rate capability. An initial capacity as high as 3531 mA h g−1 is delivered at 0.1 A g−1, whereas at a current density of 32 A g−1, a specific capacity of 1241 mA h g−1, more than three times higher than the theoretical capacity of the graphite anode, is still retained for 50 cycles. The superior electrochemical performance demonstrates that this Ag–pSi/GNS composite is a promising electrode material for high-performance LIBs.

In situ catalytic growth of large-area multilayered graphene/MoS2 heterostructures

Stacking various two-dimensional atomic crystals on top of each other is a feasible approach to create unique multilayered heterostructures with desired properties. Herein for the first time, we present a controlled preparation of large-area graphene/MoS2 heterostructures via a simple heating procedure on Mo-oleate complex coated sodium sulfate under N2 atmosphere. Through a direct in situ catalytic reaction, graphene layer has been uniformly grown on the MoS2 film formed by the reaction of Mo species with S pecies, which is from the carbothermal reduction of sodium sulfate. Due to the excellent graphene “painting” on MoS2 atomic layers, the significantly shortened lithium ion diffusion distance and the markedly enhanced electronic conductivity, these multilayered graphene/MoS2 heterostructures exhibit high specific capacity, unprecedented rate performance and outstanding cycling stability, especially at a high current density, when used as an anode material for lithium batteries. This work provides a simple but efficient route for the controlled fabrication of large-area multilayered graphene/metal sulfide heterostructures with promising applications in battery manufacture, electronics or catalysis.