Quang Duc Truong

Direct Observation of Antisite Defects in LiCoPO4 Cathode Materials by Annular Dark- and Bright-Field Electron Microscopy

LiCoPO4 cathode materials have been synthesized by a sol-gel route. X-ray diffraction analysis confirmed that LiCoPO4 was well-crystallized in an orthorhombic structure in the Pmna space group. From the high-resolution transmission electron microscopy (HR-TEM) image, the lattice fringes of {001} and {100} are well-resolved. The HR-TEM image and selected area electron diffraction pattern reveal the highly crystalline nature of LiCoPO4 having an ordered olivine structure. The atom-by-atom structure of LiCoPO4 olivine has been observed, for the first time, using high-angle annular dark-field (HAADF) and annual bright-field scanning transmission electron microscopy. We observed the bright contrast in Li columns in the HAADF images and strong contrast in the ABF images, directly indicating the antisite exchange defects in which Co atoms partly occupy the Li sites. The LiCoPO4 cathode materials delivered an initial discharge capacity of 117 mAh/g at a C/10 rate with moderate cyclic performance. The discharge profile of LiCoPO4 shows a plateau at 4.75 V, revealing its importance as a potentially high-voltage cathode. The direct visualization of atom-by-atom structure in this work represents important information for the understanding of the structure of the active cathode materials for Li-ion batteries.

Controlling the shape of LiCoPO4 nanocrystals by supercritical fluid process for enhanced energy storage properties

Lithium-ion batteries offer promising opportunities for novel energy storage systems and future application in hybrid electric vehicles or electric vehicles. Cathode materials with high energy density are required for practical application. Herein, high-voltage LiCoPO4 cathode materials with different shapes and well-developed facets such as nanorods and nanoplates with exposed {010} facets have been synthesized by a one-pot supercritical fluid (SCF) processing. The effect of different amines and their roles on the morphology-control has been investigated in detail. It was found that amine having long alkyl chain such as hexamethylenediamine played important roles to manipulate the shape of the nanocrystals by selective adsorption on the specific {010} facets. More importantly, the nanorods and nanoplates showed better electrochemical performance than that of nanoparticles which was attributed to their unique crystallographic orientation with short Li ion diffusion path. The present study emphasizes the importance of crystallographic orientation in improving the electrochemical performance of the high voltage LiCoPO4 cathode materials for Li-ion batteries.

Synthesis of nitrogen-doped graphene by pyrolysis of ionic-liquid-functionalized graphene

Nitrogen-doped graphene with up to 22.1% N/C atom and 7.15 × 104 Sm−1 electrical conductivity was synthesized by ionic-liquid-assisted electrolysis with subsequent thermal annealing of the resultant ionic-liquid-functionalized graphene sheet.

Fluoride-free self-templated synthesis of hollow TiO2 nanostructures for hydrogen evolution

Hollow TiO2 nanostructures have been fabricated by a facile self-templated hydrothermal synthesis without the assistance of any fluoride compound. The hollow nanostructures with diameters of approximately 2 μm are composed of mesoporous shell walls of 200 nm in thickness. On the basis of the investigation result, it was found that the inside-out Ostwald ripening was responsible for the hollowing process in which the evacuation of solid cores was mainly driven by the minimization of overall surface energy. The hollow nanostructures exhibited enhanced photocatalytic activity by means of hydrogen evolution from methanol solution which is attributed to their unique structural features.

Synthesis of Li2CoSiO4 nanoparticles and structure observation by annular bright and dark field electron microscopy

Nanocrystalline Li2CoSiO4 particles have been successfully synthesized via a supercritical fluid process. The as-synthesized and heated particles were 50–250 nm in diameter and were well dispersed as observed by high resolution transmission electron microscopy (HRTEM). Energy dispersive spectroscopy (EDS) and elemental mapping by scanning transmission electron microscopy (STEM) show the purity and homogenous elemental distribution of the Li2CoSiO4 particles. The high resolution transmission electron microscopy (HRTEM) images show well resolved lattice fringes of the crystalline Li2CoSiO4 particles. The structure of the Li2CoSiO4 particles has been determined for the first time by annular bright field-scanning transmission electron microscopy (ABF-STEM) and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The results revealed a tetrahedral arrangement of CoSiO4 in the Li2CoSiO4 structure. The electrochemical performance results support the idea that this material has potential as a positive electrode for use in a lithium ion battery.

Benzylamine-directed growth of olivine-type LiMPO4 nanoplates by a supercritical ethanol process for lithium-ion batteries

Olivine-type LiMPO4 (M = Fe, Mn, Co and Ni) cathode materials hold promise for next-generation of lithium-ion batteries and future applications as hybrid electric vehicles or electric vehicles. In lithium intercalation olivine compounds, the lithium diffusion along a channel is highly anisotropic which is mainly confined to the channel along the [010] direction. Thus, nanosheets or nanoplates with shortened Li ion diffusion distance along the [010] direction are enabled to accelerate the lithium intercalation rate and improve power density of the batteries. Herein, we report the production of high-quality thin LiMPO4 nanoplates with exposed {010} facets by a rapid supercritical fluid processing. The unique structure of the olivine nanocrystals with a shortened Li ion diffusion pathway allows fast extraction/insertion of Li ions in the structures. Thus, the LiMPO4 nanoplates provide high capacity and high rate capacity and excellent cyclability.

Disulfide-Bridged (Mo3S11) Cluster Polymer: Molecular Dynamics and Application as Electrode Material for a Rechargeable Magnesium Battery

Exploring novel electrode materials is critical for the development of a next-generation rechargeable magnesium battery with high volumetric capacity. Here, we showed that a distinct amorphous molybdenum sulfide, being a coordination polymer of disulfide-bridged (Mo3S11) clusters, has great potential as a rechargeable magnesium battery cathode. This material provided good reversible capacity, attributed to its unique structure with high flexibility and capability of deformation upon Mg insertion. Free-terminal disulfide moiety may act as the active site for reversible insertion and extraction of magnesium.

Synthesis, characterization and observation of antisite defects in LiNiPO4 nanomaterials

Structural studies of high voltage cathode materials are necessary to understand their chemistry to improve the electrochemical performance for applications in lithium ion batteries. LiNiPO4 nanorods and nanoplates are synthesized via a one pot synthesis using supercritical fluid process at 450 oC for 10 min. The X-ray diffraction (XRD) analysis confirmed that LiNiPO4 phase is well crystallized, phase purity supported by energy dispersive spectroscopy (EDS) and elemental mapping by scanning electron transmission electron microscopy (STEM). For the first time, we have carried out direct visualization of atom-by-atom structural observation of LiNiPO4 nanomaterials using high-angle annular dark-field (HAADF) and annular bright-field (ABF) scanning transmission electron microscopy (STEM) analysis. The Rietveld refinement analysis was performed to find out the percentage of antisite defects presents in LiNiPO4 nanoplates and about 11% of antisite defects were found. Here, we provide the direct evidence for the presence of Ni atoms in Li sites and Li in Ni sites as an antisite defects are provided for understanding of electrochemical behavior of high voltage Li ion battery cathode materials.

Supercritical fluid methods for synthesizing cathode materials towards lithium ion battery applications

Lithium ion battery materials improve the current technology to store electrical energy for the creation of green environment. The synthesis of lithium ion battery materials is crucial for energy applications in mobile electronic devices to plug-in hybrid electric vehicles. This review summarizes the recent progress made to synthesize lithium ion battery materials via supercritical fluid methods, and particularly, its application towards the synthesis of layered transition metal oxides, spinel structured cathodes, lithium metal phosphates, lithium metal silicates and lithium metal fluorophosphates. The structure, particle size, morphology and electrochemical properties of cathode materials are discussed. From the perspective of material synthesis, supercritical fluid methods are economical and have several advantages such as phase purity, morphology control and size tuning down to 5 nm, which would significantly impact the performance of lithium ion batteries.

Hierarchical ZnO nanostructures: controlling the synthesis and photocatalytic decomposition of nitrogen monoxide

Hierarchical metal oxide nanostructures that possess unique chemical and physical properties have attracted widespread interest recently because of their potential applications in catalysis, biological engineering, and photoelectronic devices. Herein, hierarchical ZnO nanostructures (ZONS) with unique structural features such as dumbbells, twin-prisms, whiskers, and nanorod bundles have been fabricated using a facile hydrothermal method with lysine as an in situ source of OH− and structure-directing agent. The effect of the lysine amount on structural features was systematically investigated, in order to elucidate the roles of lysine in the growth process. It was found that lysine plays an important role in controlling the shape and assembly of the hierarchical structures of ZONS using its capping and chelating abilities. The photocatalytic activities of different ZONS were evaluated using the photocatalytic decomposition of nitrogen monoxide. The decomposition properties were found to be strongly dependent on the surface area and the assembled features of the structure.