Xuefei Du

MoS2 Nanosheets Vertically Grown on Graphene Sheets for Lithium-Ion Battery Anodes

A designed nanostructure with MoS2 nanosheets (NSs) perpendicularly grown on graphene sheets (MoS2/G) is achieved by a facile and scalable hydrothermal method, which involves adsorption of Mo7O24(6-) on a graphene oxide (GO) surface, due to the electrostatic attraction, followed by in situ growth of MoS2. These results give an explicit proof that the presence of oxygen-containing groups and pH of the solution are crucial factors enabling formation of a lamellar structure with MoS2 NSs uniformly decorated on graphene sheets. The direct coupling of edge Mo of MoS2 with the oxygen from functional groups on GO (C-O-Mo bond) is proposed. The interfacial interaction of the C-O-Mo bonds can enhance electron transport rate and structural stability of the MoS2/G electrode, which is beneficial for the improvement of rate performance and long cycle life. The graphene sheets improve the electrical conductivity of the composite and, at the same time, act not only as a substrate to disperse active MoS2 NSs homogeneously but also as a buffer to accommodate the volume changes during cycling. As an anode material for lithium-ion batteries, the manufactured MoS2/G electrode manifests a stable cycling performance (1077 mAh g(-1) at 100 mA g(-1) after 150 cycles), excellent rate capability, and a long cycle life (907 mAh g(-1) at 1000 mA g(-1) after 400 cycles).

Investigation of In-doped BaFeO3−δ perovskite-type oxygen permeable membranes

Cobalt-free BaFe1−xInxO3−δ perovskites, with Fe partially substituted by indium at the B-site, were synthesized by a conventional solid state reaction and systematically characterized in terms of their phase composition, crystal structure, thermal reducibility, oxygen permeability, as well as structural stability in order to evaluate their application as oxygen permeation membranes. Introduction of more than 10 at.% of In into BaFe1−xInxO3−δ causes the formation of a single phase material with a cubic perovskite structure, which exhibits no phase transition during the cooling process. The thermal reducibility and thermal expansion coefficient are effectively reduced by indium doping, owing to the less changes of concentration of the oxygen vacancies in these compounds. However, the In occupying B-site breaks the B–O–B double exchange mechanism, and thus results in a gradual decrease of the electrical conductivity upon doping. Rietveld refinement and first principles calculation were performed to get an insight into the In influence on the lattice structure, oxygen migration energy and electron conduction behaviour of BaFe1−xInxO3−δ. When using He/Air as sweep/feed gas, the BaFe0.9In0.1O3−δ dense membrane with 1.0 mm thickness features a high oxygen permeation flux of 1.11 mL cm−2 min−1 at 950 °C. The observed good performance is attributed to the relatively high concentration of oxygen vacancies and low energy barrier for oxygen ion migration. It is also found that for membranes thinner than 0.8 mm, the oxygen flux is no longer limited by the bulk diffusion, while the oxygen surface exchange process becomes the dominant factor.

Novel cobalt-free BaFe1−xGdxO3−δ perovskite membranes for oxygen separation

A cobalt-free perovskite-type mixed ionic and electronic conductor (MIEC) is of technological and economic importance in many energy-related applications. In this work, a new group of Fe-based perovskite MIECs with BaFe1−xGdxO3−δ (0.025 ≤ x ≤ 0.20) compositions was developed for application in oxygen permeation membranes. Slight Gd doping (x = 0.025) can stabilize the cubic structure of the BaFe1−xGdxO3−δ perovskite. The Gd substitution of BaFe1−xGdxO3−δ materials increases the structural and chemical stability in the atmosphere containing CO2 and H2O, and decreases the thermal expansion coefficient. The BaFe0.975Gd0.025O3−δ membrane exhibits fast oxygen surface exchange kinetics and a high bulk diffusion coefficient, and achieves a high oxygen permeation flux of 1.37 mL cm−2 min−1 for a 1 mm thick membrane at 950 °C under an air/He oxygen gradient, and can maintain stability at 900 °C for 100 h. Compared to the pristine BaFeO3−δ and the well-studied Ba0.95La0.05FeO3−δ membranes, a lower oxygen permeation activation energy and higher oxygen permeability are obtained for the 2.5 at% Gd-doped material, which might be attributed to the expanded lattice by doping large Gd3+ cations and a limited negative effect from the strong Gd–O bond. A combination study of first principles calculation and experimental measurements was further conducted to advance the understanding of Gd effects on the oxygen migration behavior in BaFe1−xGdxO3−δ. These findings are expected to provide guidelines for material design of high performance MIECs.

(101) Plane-Oriented SnS2 Nanoplates with Carbon Coating: A High-Rate and Cycle-Stable Anode Material for Lithium Ion Batteries

Tin disulfide is considered to be a promising anode material for Li ion batteries because of its high theoretical capacity as well as its natural abundance of sulfur and tin. Practical implementation of tin disulfide is, however, strongly hindered by inferior rate performance and poor cycling stability of unoptimized material. In this work, carbon-encapsulated tin disulfide nanoplates with a (101) plane orientation are prepared via a facile hydrothermal method, using polyethylene glycol as a surfactant to guide the crystal growth orientation, followed by a low-temperature carbon-coating process. Fast lithium ion diffusion channels are abundant and well-exposed on the surface of such obtained tin disulfide nanoplates, while the designed microstructure allows the effective decrease of the Li ion diffusion length in the electrode material. In addition, the outer carbon layer enhances the microscopic electrical conductivity and buffers the volumetric changes of the active particles during cycling. The optimized, carbon coated tin disulfide (101) nanoplates deliver a very high reversible capacity (960 mAh g-1 at a current density of 0.1 A g-1), superior rate capability (796 mAh g-1 at a current density as high as 2 A g-1), and an excellent cycling stability of 0.5 A g-1 for 300 cycles, with only 0.05% capacity decay per cycle.

Effective calcium doping at the B-site of BaFeO3−δ perovskite: towards low-cost and high-performance oxygen permeation membranes

A cost-effective doping strategy was developed to enhance the oxygen permeability and structural stability of BaFeO3−δ. We demonstrated that the alkaline earth metal element Ca, which is usually considered an A-site dopant for perovskite oxides, can be successfully introduced into the B-site of BaFeO3−δ. The cubic perovskite structure of BaFe1−xCaxO3−δ was stabilized down to room temperature for the Ca-doping concentration range from 5 to 15 at%. First principles calculations not only proved the preference of Ca at the B-site with lower defect formation energies than the A-site, but also demonstrated that the migration of the oxygens located greater distances from the Ca position is characterized by lower barrier energies than those in the Ca vicinity and even lower than that for the undoped BaFeO3−δ. We found that these favourable, low energy barrier paths away from the Ca sites exert more pronounced effects on the oxygen migration at diluted dopant concentrations, and hence, the material with x = 0.05 level of substitution shows a higher oxygen permeability with a lower activation energy compared to the undoped or highly-doped BaFeO3−δ. The BaFe0.95Ca0.05O3−δ membrane is characterized by a high oxygen permeability of 1.30 mL cm−2 min−1 at 950 °C and good long-term stability at 800/900 °C, as obtained over 200 h. Therefore, the feasibility and applicability of Ca-doping at the B-site of the perovskite can be highlighted, which allows for the enhancement of the oxygen migration ability, originating from the appropriate tuning of the lattice structure.