Zhi-Zheng Yang

Hierarchical TiO2 spheres as highly efficient polysulfide host for lithium-sulfur batteries

Hierarchical TiO2 micron spheres assembled by nano-plates were prepared through a facile hydrothermal route. Chemical tuning of the TiO2 through hydrogen reduction (H-TiO2) is shown to increase oxygen-vacancy density and thereby modifies the electronic properties. H-TiO2 spheres with a polar surface serve as the surface-bound intermediates for strong polysulfides binding. Under the restricting and recapturing effect, the sulfur cathode could deliver a high reversible capacity of 928.1 mA h g−1 after 50 charge-discharge cycles at a current density of 200 mA g−1. The H-TiO2 additive developed here is practical for restricting and recapturing the polysulfide from the electrolyte.

Facile shape design and fabrication of ZnFe2O4 as an anode material for Li-ion batteries

Functional materials with exposed highly reactive planes have attracted considerable attention with respect to their enhanced electrochemical energy storage. However, highly active facets of ZnFe2O4 nanocrystals usually have high surface energy, and thus are hard to prepare in the equilibrium state or via traditional methods. In this regard, we propose a novel strategy to fabricate cubic, truncated octahedral, and octahedral ZnFe2O4 by a convenient hydrothermal method and subsequent thermal treatment for the first time. The ZnFe2O4 octahedra exhibit a reversible capacity of 450 mA h g−1 at a current density of 60 mA g−1 after 50 cycles, while the reversible capacity of ZnFe2O4 cubes is 367 mA h g−1. The electrochemical performance of the three types of ZnFe2O4 can be ranked as “octahedron > truncated octahedron > cube”. The exposed planes, which are filled with a high density of atoms, lead to the better electrochemical performance.

Assembling sulfur spheres on carbon fiber with graphene coated hybrid bulk electrodes for lithium sulfur batteries

A hybrid bulk electrode coated with sulfur spheres and graphene has been assembled via a two-step electrochemical deposition for the first time. Close-packed and layer-by-layer sulfur spheres were successfully deposited on the carbon fiber paper. The flexible bulk electrode is based on carbon fiber paper that is highly conductive and robust toward electrochemical cycling. When evaluated as a potential cathode for lithium sulfur (Li–S) batteries, such electrodes exhibit fine lithium storage capabilities by virtue of their advantageous structural features.

Refinement and modification of primary Mg2Si in an Al–20Mg2Si alloy by a combined addition of yttrium and antimony

The complex modification of primary Mg2Si in an Al–20Mg2Si alloy by simultaneous addition of yttrium (Y) and antimony (Sb) was investigated in the present work. It was found that the combined addition of 0.5 wt% Y–Sb had a more significant modifying effect than the single addition of an equivalent amount of Y or Sb. After modification, coarse Mg2Si dendrites were changed into fine polyhedra with an average size decreasing from 86 to less than 18 μm. It has been demonstrated that Sb participated in the formation of Mg3Sb2, which acts as a heterogeneous nucleus of primary Mg2Si and hence significantly refines their sizes. Meanwhile, Y could obviously adsorb and poison the preferred growth along the directions of primary Mg2Si crystals, and hence not only changed their final morphology to a truncated octahedron but also reduced their sizes. Furthermore, the skeleton-type growth process of the truncated octahedral primary Mg2Si in the Al–20Mg2Si alloy co-modified by 0.5 wt% Y–Sb was also revealed. Our study provides a new insight into the design of more efficient modifiers by the combined addition of a refiner (e.g. Sb) and a growth inhibitor (e.g. Y), which is critical to tailor new light-weight alloys with high strength and toughness.

One-step in situ growth of ZnS nanoparticles on reduced graphene oxides and their improved lithium storage performance using sodium carboxymethyl cellulose binder

ZnS nanoparticles are in situ grown on reduced graphene oxides (rGO) via a simplified one-step hydrothermal method. Sodium carboxymethyl cellulose (CMC) is firstly applied as the binder for ZnS based anodes and shows a more advantageous binding effect than PVDF. To simplify the synthesis procedure, L-cysteine is added as the sulfur source for ZnS and simultaneously as the reducing agent for rGO. The average diameter of ZnS nanoparticles is measured to be 13.4 nm, and they uniformly disperse on the rGO sheets without any obvious aggregation. As anode materials, the CMC bound ZnS–rGO nanocomposites can maintain a high discharge capacity of 705 mA h g−1 at a current density of 500 mA g−1 for 150 cycles. The significantly improved electrochemical performance mainly derives from the combined effects of the small and uniformly dispersed ZnS nanoparticles, the high conductivity and structural flexibility of rGO and the strong binding ability of CMC.

Ultrahigh-performance mesoporous ZnMn 2 O 4 microspheres as anode materials for lithium-ion batteries and their in situ Raman investigation

Currently, lithium-ion batteries play a key role in energy storage; however, their applications are limited by their low energy density. Here, we design a facile method to prepare mesoporous ZnMn2O4 microspheres with ultrahigh rate performance and ultralong cycling properties by finely tuning the solution viscosity during synthesis. When the current density is raised to 2 A·g−1, the discharge capacity is maintained at 879 mA·h·g−1 after 500 cycles. The electrochemical properties of mesoporous ZnMn2O4 microspheres are better than that for most reported ZnMn2O4. To understand the electrochemical processes on the mesoporous ZnMn2O4 microspheres, in situ Raman spectroscopy is used to investigate the electrode surface. The results show that mesoporous ZnMn2O4 microspheres have a great potential as an alternative to commercial carbon anode materials.

Microstructure and Tensile Properties of AZ61 Alloy Sheets Processed by High-Ratio Extrusion with Subsequent Direct Aging Treatment

A high extrusion ratio of 166:1 was applied to commercial AZ61 alloy in one step with an extrusion speed of 2.1 m·min−1. The effects of DA (direct aging) treatment on the microstructure and tensile properties of extruded alloy were investigated. The extruded alloy exhibits fine DRXed grains and the average grain size is ~11 μm. After DA treatment at 170 °C, the tensile strength and 0.2% offset yield strength is enhanced from 314 to 336 MPa and from 169 to 191 MPa respectively, sacrificing elongation from 26.5% to 23.3%. The grain size and texture distribution of extruded AZ61 scarcely evolve during the post aging treatment. However, the enhanced strength in peak-aged alloy is mainly caused by the high-density elliptical Mg17Al12 precipitates distributing uniformly along the grain boundaries or within the grains, by precipitation and dispersion hardening. Furthermore, the nano-sized precipitates effectively inhibit grains from coarsening by triggering pinning effects along the grain boundaries at elevated temperature. As a result, the peak-aged alloy exhibits a better superplasticity of 306.5% compared with that of 231.8% of extruded sample. This work provides a practical one-step method for mass-producing Mg alloy sheets with excellent tensile strength and ductility compared with those fabricated by conventional extrusion methods.