Zaiping Guo

Enhanced sodium-ion battery performance by structural phase transition from two-dimensional hexagonal-SnS2 to orthorhombic-SnS

Structural phase transitions can be used to alter the properties of a material without adding any additional elements and are therefore of significant technological value. It was found that the hexagonal-SnS2 phase can be transformed into the orthorhombic-SnS phase after an annealing step in an argon atmosphere, and the thus transformed SnS shows enhanced sodium-ion storage performance over that of the SnS2, which is attributed to its structural advantages. Here, we provide the first report on a SnS@graphene architecture for application as a sodium-ion battery anode, which is built from two-dimensional SnS and graphene nanosheets as complementary building blocks. The as-prepared SnS@graphene hybrid nanostructured composite delivers an excellent specific capacity of 940 mAh g(-1)and impressive rate capability of 492 and 308 mAh g(-1) after 250 cycles at the current densities of 810 and 7290 mA g(-1), respectively. The performance was found to be much better than those of most reported anode materials for Na-ion batteries. On the basis of combined ex situ Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and ex situ X-ray diffraction, the formation mechanism of SnS@graphene and the synergistic Na-storage reactions of SnS in the anode are discussed in detail. The SnS experienced a two-structural-phase transformation mechanism (orthorhombic-SnS to cubic-Sn to orthorhombic-Na3.75Sn), while the SnS2 experienced a three-structural-phase transformation mechanism (hexagonal-SnS2 to tetragonal-Sn to orthorhombic-Na3.75Sn) during the sodiation process. The lesser structural changes of SnS during the conversion are expected to lead to good structural stability and excellent cycling stability in its sodium-ion battery performance. These results demonstrate that the SnS@graphene architecture offers unique characteristics suitable for high-performance energy storage application.

Self‐Assembled Germanium/Carbon Nanostructures as High‐Power Anode Material for the Lithium‐Ion Battery

The automobile industry is currently shifting towards hybrid and electric vehicles which are powered by electrochemical energy storage systems. However, these “greener” alternatives still suffer from low mileage when compared to a full tank of gasoline. Therefore, it is important to develop batteries that have a high energy density, high power density, and long cycle life. Lithium-ion batteries (LIBs) have been widely used in the consumer market for portable electronic devices since their introduction in the 1990s. This battery system is a more suitable candidate for hybrid/electric vehicles compared to nickel metal hydride, alkaline, and lead–acid batteries because of its higher volumetric and gravimetric energy density. However, there is still room for improvement in the case of the energy and power densities of LIBs. One strategy to increase the performance of LIBs is to find alternative anode materials that satisfy both requirements. Germanium is an excellent candidate as anode material for LIBs when compared to other metallic anode materials that undergo lithium alloying reactions, such as tin and silicon. This is because of its high theoretical capacity (1600 mAh g , 4.4 Li ions per Ge atom), good lithium diffusivity (400 times faster than in silicon), and high electrical conductivity (104 times higher than silicon). Nevertheless, the price of germanium is the major drawback for the commercialization of this anode material. Furthermore, similar to silicon and tin, germanium suffers large volume changes during lithium alloying/de-alloying reactions. With prolonged cycling, the mechanical stress causes electrode pulverization from the current collector, and this leads to capacity fading. Various approaches have been reported to enhance the cycling stability of germanium. These include using morphologies that have better structural stability for accommodating volume changes (nanoparticles, nanowires, nanotubes, and porous and mesoporous structures), germaniumbased composites (tin-germanium, germanium/carbon nanotubes), germanium oxides, and carbon coating of germanium. However, rate capabilities of germanium anodes that have been reported need more improvement to satisfy the requirements of electric vehicles with high energy consumption. Herein, we report a facile synthesis method to produce germanium/carbon nanostructures by carbon coating and reduction of the oxide precursor. When the particle size of the germanium oxide precursor was varied, two different selfassembled germanium/carbon nanostructures could be obtained, namely, a cluster (C) nanostructure and a nonclustered (NC) structure. Hereafter, they are denoted as “CGe/C” and “NC-Ge/C”. Both germanium/carbon nanostructures displayed good cycling stability at the 0.2 C rate (0.32 Ag ) for over 50 cycles and at the 1 C rate (1.6 Ag ) for over 120 cycles. Surprisingly, the C-Ge/C structure shows an exceptionally high rate capability up to the 40 C rate (64 Ag ). The NC-Ge/C structure, however, showed very poor capacity retention at rates over 1 C. These two extremes demonstrate the advantages of the nanostructured cluster, which will be further discussed. A brief summary of the synthesis procedure is shown in Scheme 1. The germanium oxide nanoparticle precursors were prepared by hydrolysis of GeCl4 in either a water/ ethanol mixture (< 100 nm) or a reverse micelle water/ heptane system (around 700 nm). Acetylene gas was chosen

Catalytic role of ge in highly reversible GeO2/Ge/C nanocomposite anode material for lithium batteries

GeO2/Ge/C anode material synthesized using a simple method involving simultaneous carbon coating and reduction by acetylene gas is composed of nanosized GeO2/Ge particles coated by a thin layer of carbon, which is also interconnected between neighboring particles to form clusters of up to 30 μm. The GeO2/Ge/C composite shows a high capacity of up to 1860 mAh/g and 1680 mAh/g at 1 C (2.1 A/g) and 10 C rates, respectively. This good electrochemical performance is related to the fact that the elemental germanium nanoparticles present in the composite increases the reversibility of the conversion reaction of GeO2. These factors have been found through investigating and comparing GeO2/Ge/C, GeO2/C, nanosized GeO2, and bulk GeO2.

Boosted Charge Transfer in SnS/SnO2 Heterostructures: Toward High Rate Capability for Sodium‐Ion Batteries

Constructing heterostructures can endow materials with fascinating performance in high-speed electronics, optoelectronics, and other applications owing to the built-in charge-transfer driving force, which is of benefit to the specific charge-transfer kinetics. Rational design and controllable synthesis of nano-heterostructure anode materials with high-rate performance, however, still remains a great challenge. Herein, ultrafine SnS/SnO2 heterostructures were successfully fabricated and showed enhanced charge-transfer capability. The mobility enhancement is attributed to the interface effect of heterostructures, which induces an electric field within the nanocrystals, giving them much lower ion-diffusion resistance and facilitating interfacial electron transport.

Synthesis of MoS2–C One-Dimensional Nanostructures with Improved Lithium Storage Properties

Uniform one-dimensional (1D) MoS2-C composite nanostructures including nanorods and nanotubes have been produced through a sulfidation reaction in H2S flow using MoOx/polyaniline hybrid nanostructures as the precursor. These MoS2-C 1D nanostructures exhibit greatly enhanced electrochemical performance as anode materials for lithium-ion batteries. Typically, stable capacity retention of 776 mA h g(-1) can be achieved after 100 cycles for MoS2-C nanotubes. Even cycled at a high current density of 1000 mA g(-1), these structures can still deliver high capacities of 450-600 mA h g(-1). The unique 1D nanostructure and the extra carbon in the hybrid structure are beneficial to the greatly improved electrochemical performance of these MoS2-C nanocomposites.

Rapid microwave-assisted synthesis of Mn3O4-graphene nanocomposite and its lithium storage properties

A nanocomposite of Mn3O4 wrapped in graphene sheets (GSs) was successfully synthesized via a facile, effective, energy-saving, and scalable microwave hydrothermal technique. The morphology and microstructures of the fabricated GS–Mn3O4 nanocomposite were characterized using various techniques. The results indicate that the particle size of the Mn3O4 particles in the nanocomposite markedly decreased to nearly 20 nm, significantly smaller than that for the bare Mn3O4. Electrochemical measurements demonstrated a high specific capacity of more than 900 mA h g−1 at 40 mA g−1, and excellent cycling stability with no capacity decay can be observed up to 50 cycles. All of these properties are also interpreted by experimental studies and theoretical calculations.

Highly Reversible and Large Lithium Storage in Mesoporous Si/C Nanocomposite Anodes with Silicon Nanoparticles Embedded in a Carbon Framework

A magnesiothermic reduction approach is designed to synthesize mesoporous Si/C nanocomposites with ultrasmall, uniform silicon nanoparticles (ca. 3 nm) embedded in a rigid mesoporous carbon framework. The resultant mesoporous Si/C nanocomposites present excellent performance with high reversible capacity, good Coulombic efficiency and rate capability, and outstanding cycling stability in lithium-ion battery applications.

Synthesis of uniform TiO2@carbon composite nanofibers as anode for lithium ion batteries with enhanced electrochemical performance

Very large area, uniform TiO2@carbon composite nanofibers were easily prepared by thermal pyrolysis and oxidization of electrospun titanium(IV) isopropoxide/polyacrylonitrile (PAN) nanofibers in argon. The composite nanostructures exhibit the unique feature of having TiO2 nanocrystals encapsulated inside a porous carbon matrix. The unique orderly-bonded nanostructure, porous characteristics, and highly conductive carbon matrix favour excellent electrochemical performance of the TiO2@carbon nanofiber electrode. The TiO2@carbon hybrid nanofibers exhibited highly reversible capacity of 206 mAh g−1 up to 100 cycles at current density of 30 mA g−1 and excellent cycling stability, indicating that the composite is a promising anode candidate for Li-ion batteries.

Heterogeneous spin states in ultrathin nanosheets induce subtle lattice distortion to trigger efficient hydrogen evolution

The exploration of efficient nonprecious metal eletrocatalysis of the hydrogen evolution reaction (HER) is an extraordinary challenge for future applications in sustainable energy conversion. The family of first-row-transition-metal dichalcogenides has received a small amount of research, including the active site and dynamics, relative to their extraordinary potential. In response, we developed a strategy to achieve synergistically active sites and dynamic regulation in first-row-transition-metal dichalcogenides by the heterogeneous spin states incorporated in this work. Specifically, taking the metallic Mn-doped pyrite CoSe2 as a self-adaptived, subtle atomic arrangement distortion to provide additional active edge sites for HER will occur in the CoSe2 atomic layers with Mn incorporated into the primitive lattice, which is visually verified by HRTEM. Synergistically, the density functional theory simulation results reveal that the Mn incorporation lowers the kinetic energy barrier by promoting H-H bond formation on two adjacently adsorbed H atoms, benefiting H2 gas evolution. As a result, the Mn-doped CoSe2 ultrathin nanosheets possess useful HER properties with a low overpotential of 174 mV, an unexpectedly small Tafel slope of 36 mV/dec, and a larger exchange current density of 68.3 μA cm(-2). Moreover, the original concept of coordinated regulation presented in this work can broaden horizons and provide new dimensions in the design of newly highly efficient catalysts for hydrogen evolution.

Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art.

One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed.