Shuwei Wang

In Situ AFM Imaging of Solid Electrolyte Interfaces on HOPG with Ethylene Carbonate and Fluoroethylene Carbonate-Based Electrolytes.

Chemical and morphological structure of solid electrolyte interphase (SEI) plays a vital role in lithium-ion battery (LIB), especially for its cyclability and safety. To date, research on SEI is quite limited because of the complexity of SEI and lack of effective in situ characterization techniques. Here, we present real-time views of SEI morphological evolution using electrochemical atomic force microscopy (EC-AFM). Complemented by an ex situ XPS analysis, fundamental differences of SEI formation from ethylene carbonate (EC) and fluoroethylene carbonate (FEC)-based electrolytes during first lithiation/delithiation cycle on HOPG electrode surface were revealed.

Synthesis and Electrochemical Properties of Two-Dimensional Hafnium Carbide

We demonstrate fabrication of a two-dimensional Hf-containing MXene, Hf3C2Tz, by selective etching of a layered parent Hf3[Al(Si)]4C6 compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm-3 for lithium and sodium ions batteries, respectively, at a current density of 200 mAg-1 after 200 cycles. Thus, Hf3C2Tz MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.

Bismuth and chromium co-doped strontium titanates and their photocatalytic properties under visible light irradiation

Modification of prototype perovskite compound SrTiO3 by introducing foreign elements has been an appealing means to endow this wide band gap semiconductor with visible light responses. Here we systematically investigated a series of Sr1-xBixTi1-xCrxO3 solid solution compounds prepared by two different synthetic routes, namely, solid state reactions and the hydrothermal method. Their crystal structures as well as other physicochemical properties were explored. Our results showed that a number of important factors such as microstructures, crystallinity, light absorbance and surface compositions etc. are all strongly correlated with the synthetic methods used. The hydrothermal method is generally helpful for morphology controls as well as avoiding Cr(6+) defects and Sr segregation at the surface, thereby contributing to a high photocatalytic activity. Better performance normally occurs in samples with a high crystallinity and free of defects like Bi(5+). Theoretical calculations suggest that Cr plays an important role in band gap reduction and photocatalytic reactions, while Bi only acts as a constituent cation for the perovskite structure and does not significantly alter the electronic structures near the Fermi level. Our findings have revealed how synthetic routes are relevant to the final photocatalytic properties of a compound, and therefore comparisons among various photocatalysts have to include concerns about their preparation history.

ZnO-based ultraviolet avalanche photodetectors

By virtue of the carrier avalanche multiplication caused by an impact ionization process occurring in MgO insulation layer, zinc oxide (ZnO)-based ultraviolet (UV) avalanche photodetectors (APDs) have been fabricated from Au/MgO/ZnO/MgO/Au structures. The responsivity of APDs can reach 1.7 x 10(4) AW(-1), and the avalanche gain of the photodetectors is about 294 at 73 V. Considering that no previous report on ZnO APDs can be found, the results reported in this paper may promise a route to high-performance ZnO UV photodetectors.

Volumetric variation confinement: surface protective structure for high cyclic stability of lithium metal electrodes

A surface protective structure to efficiently improve the cyclic stability and lifetime of the lithium metal electrode is investigated. By volumetrically confining plated lithium metal in the inter-space of a ceramic porous layer and isolating the confined lithium via a reinforced skin-layer from attack by electrolyte solvents, the coulombic efficiency of the protected lithium metal electrode reaches very high values of ∼97–99%.

Direct visualization of solid electrolyte interphase on Li4Ti5O12 by in situ AFM

Whether Li4Ti5O12 has a solid electrolyte interphase (SEI) layer on the electrode surface has been the subject of controversy for a long time due to the delicate nature of this SEI layer and the lack of reliable characterization tools. In this paper, we report direct visualization of SEI layer formation on an LTO electrode surface by in situ atomic force microscopy under potential control. Our results showed that no SEI layer formed from EC/DMC based electrolyte in the potential range of 2.5–1.0 V. However, by extending the reduction potential down to zero, it is possible to grow a SEI layer on the LTO surface. The strategy of forming an SEI layer by discharging an LTO anode down to 0 V in the first cycle and then operating the battery in the normal range of 2.5–1.0 V might be a facile method to improve LTO battery performance. Various additives such as Vinylene Carbonate (VC), ethylene sulfate (ES) and fluoroethylene carbonate (FEC) were used as additives to evaluate their effect on SEI layer formation.

Improving the cyclability performance of lithium-ion batteries by introducing lithium difluorophosphate (LiPO2F2) additive

The cyclability of lithium-ion batteries (LIBs) is often affected by the components of the solid electrolyte interphase (SEI) layer which is generated from electrochemical decomposition of electrolyte. Here, lithium difluorophosphate (LiPO2F2) is studied in this work. When 1.6 wt% LiPO2F2 additive is incorporated into the reference electrolyte, the capacity retention of graphite/Li half-cell is increased from 82.53% to 98.04% and the capacity retention of LiCoO2/Li half-cell is increased from 89.60% to 97.53% after 160 cycles. Electrochemical impedance spectroscopy (EIS) indicates that the SEI layer containing LiPO2F2 can decrease the surface impedance of cells in the last stage cycle. In situ atomic force microscopy (AFM), DFT calculations and X-ray photoelectron spectroscopy (XPS) results show that LiPO2F2 is deposited on the surface of both LiCoO2 and graphite electrodes, which effectively protects the graphite anode and suppresses the degradation of the cathode during the long-term cycling of LIBs.

Ultraviolet emissions excited by accelerated electrons.

By employing an insulating zinc oxide (i-ZnO) as an electron accelerating layer, and an n-type ZnO as an active layer, ultraviolet (UV) emissions at 385 nm caused by the excitation of the n-ZnO layer by the accelerated electrons from the i-ZnO layer have been realized. By replacing the active layer with larger bandgap Mg0.39Zn0.61O and properly optimizing the structure, shorter wavelength emissions at around 328 nm have been obtained. Considering that the p-type doping of wide bandgap semiconductors is still a challenging issue, the results reported in this Letter may provide a promising alternative route to UV emissions.

Degenerate layer at ZnO/sapphire interface

Zinc oxide (ZnO) films have been prepared on sapphire substrates by molecular beam epitaxy. It is found that the electron concentration of the films decreases, while the mobility increases with increasing the film thickness. Temperature-dependent Hall measurement reveals the existence of a degenerate layer at the ZnO/sapphire interface, which will increase the electron concentration and decrease the mobility in the ZnO film. By using a two-layer conduction model, the electron concentration and mobility of the film excluding the influence of the degenerate layer have been determined. A fitting to the corrected electron concentration of the ZnO film yields an activation energy of about 31 meV for the residual donors.

Two-dimensional nanosheets associated with one-dimensional single-crystalline nanorods self-assembled into three-dimensional flower-like Mn₃O₄ hierarchical architectures.

Three-dimensional (3D) flower-like hausmannite architectures of Mn3O4 with uniform morphology have been successfully synthesized by a novel chemical reaction route using cetyltrimethylammonium bromide as the template. Micro/nanostructures of the as-synthesized 3D flower-like Mn3O4 architectures were investigated in detail by a series of analytical techniques. The geometrical shape of 3D flower-like Mn3O4 architectures is structurally perfect, and they are produced with diameters in the range of several hundred nanometers to a few micrometers. Experimental results indicate that two-dimensional nanosheets associated with one-dimensional single-crystalline nanorods self-assembled into three-dimensional flower-like Mn3O4 architectures. The single-crystalline Mn3O4 nanorods are a few hundred nanometers long and several tens of nanometers wide. Different dimensional systems, such as two-dimensional nanosheets, one-dimensional nanorods, and three-dimensional nanoflowers, could provide different building blocks to constitute nanostructured materials. These specific building blocks, which constituted the complex hierarchical architectures with nanostructural features, may offer exciting opportunities for both fundamental research and technological applications.