Zhaoxiang Wang

Porous Li4Ti5O12 Coated with N-Doped Carbon from Ionic Liquids for Li-Ion Batteries

A uniform nitrogen-doped carbon coating layer is formed on Li4Ti5O12 particles by mixing porous Li4Ti5O12 powder with an ionic liquid and then treating the mixture at moderate temperature. Uniformly coated Li4Ti5O12 is shown to have significantly improved rate capability and cycling performance for Li-ion batteries. This relatively simple approach is versatile and can be extended to modify other electrode materials for electrochemical devices.

Graphene-Encapsulated Fe3O4 Nanoparticles with 3D Laminated Structure as Superior Anode in Lithium Ion Batteries

Fe(3)O(4)-graphene composites with three-dimensional laminated structures have been synthesised by a simple in situ hydrothermal method. From field-emission and transmission electron microscopy results, the Fe(3)O(4) nanoparticles, around 3-15 nm in size, are highly encapsulated in a graphene nanosheet matrix. The reversible Li-cycling properties of Fe(3)O(4)-graphene have been evaluated by galvanostatic discharge-charge cycling, cyclic voltammetry and impedance spectroscopy. Results show that the Fe(3)O(4)-graphene nanocomposite with a graphene content of 38.0 wt % exhibits a stable capacity of about 650 mAh  g(-1) with no noticeable fading for up to 100 cycles in the voltage range of 0.0-3.0 V. The superior performance of Fe(3)O(4)-graphene is clearly established by comparison of the results with those from bare Fe(3)O(4). The graphene nanosheets in the composite materials could act not only as lithium storage active materials, but also as an electronically conductive matrix to improve the electrochemical performance of Fe(3)O(4).

Lithium storage in nitrogen-rich mesoporous carbon materials

Nitrogen-rich mesoporous carbon materials were obtained by pyrolyzing gelatin between 700 and 900 °C with a nano-CaCO3 template. The mesoporous structure and the high nitrogen content endowed these materials with reversible capacities up to ca. 1200 mA h g−1. The high specific surface area and the nitrogen doping are responsible for the capacity loss in the initial cycle. FTIR and XPS studies indicate that the nitrogen in the material exists in the form of pyridinic, pyrrolic/pyridonic and graphitic nitrogen. The Raman spectroscopic analysis indicates that the structure of the mesoporous carbon becomes more disordered during discharge and is restored during recharge, a behavior similar to that in nitrogen-free hard carbon materials. The reversible structural variation of these carbon materials ensures their high cyclic reversibility.

Crystal Habit-Tuned Nanoplate Material of Li[Li1/3-2x/3NixMn2/3-x/3]O-2 for High-Rate Performance Lithium-Ion Batteries

A cathode for high-rate performance lithium-ion batteries (LIBs) has been developed from a crystal habit-tuned nanoplate Li(Li(0.17)Ni(0.25)Mn(0.58))O₂ material, in which the proportion of (010) nanoplates (see figure) has been significantly increased. The results demonstrate that the fraction of the surface that is electrochemically active for Li(+) transportation is a key criterion for evaluating the different nanostructures of potential LIB materials.

Controlled synthesis of CeO2 nanorods by a solvothermal method

Pure phase CeO2 nanorods (about 40?50?nm in diameter and 0.3?2??m in length) were synthesized through a solvothermal synthesis method. The addition of ethylenediamine is critical to obtain CeO2 nanorods. Other experimental conditions, such as the solvent composition, surfactant and the cerium source precursor were of importance in the final product morphology. The reaction temperature and reaction time also had significant influence on the yield of CeO2 nanorods. A possible formation mechanism of CeO2 nanorods was discussed mainly based on the dependences of controlling parameters on the final morphologies. In addition, the optical properties of CeO2 nanorods were investigated. The UV?visible adsorption spectrum and photoluminescence spectrum of the CeO2 nanorods showed unusual red-shift and enhanced light emission, respectively.

Structural and electrochemical characterizations of surface-modified LiCoO2 cathode materials for Li-ion batteries

Commercial LiCoO2 has been coated with metal oxides including MgO, Al2O3 and SnO2. The morphology and structure of the coating layer have been characterized with scanning electron microscope (SEM) and high-revolution transmission electron microscope (HRTEM). It is found that the coating layer is amorphous and rather compact. LiCoO2 coated with different metal oxides demonstrate different electrochemical performances. MgO-coated LiCoO2 cathode shows very good electrochemical stability up to a charge cutoff voltage of 4.7 V while Al2O3-coated LiCoO2 is electrochemically stable up to 4.5 V. However, SnO2-modified LiCoO2 is stable only at low-charge cutoff voltages. Cyclic voltammetry (CV) and charge–discharge cycling show that surface modification does not influence the phase transitions below 4.5 V but the phase transition at about 4.58 V is drastically suppressed. These improvements are attributed to the modifications to the surface property of the particles by coating and to the lattice structure of LiCoO2 during cycling.

Lithium Storage in Li4Ti5O12 Spinel: The Full Static Picture from Electron Microscopy

The full static picture of Li storage in Li(4)Ti(5)O(12) is derived using the latest spherical aberration-corrected scanning transmission electron microscopy and first-principles calculations. The accommodation of the additional Li(+) is directly visualized and the distribution of electrons introduced by lithium insertion deduced. Moreover, Li(4)Ti(5)O(12) is found to transform into Li(7)Ti(5)O(12) on lithiation by developing a dislocation-free coherent hetero-interface.

Electrochemical Evaluation and Structural Characterization of Commercial LiCoO2 Surfaces Modified with MgO for Lithium-Ion Batteries

Commercial cathode material LiCoO2 was modified by coating its surface with a thin layer of amorphous magnesium oxide (MgO). The surface morphology, crystalline structure, and electrochemical performance of the modified cathode material were characterized and compared with that of pristine LiCoO2. It is found that surface modification can improve the structural stability of LiCoO2 without decreasing its available specific capacity. Specific capacities of 145, 175, and 210 mAh/g were obtained in test cells composed of MgO-coated LiCoO2 cathode material when charged to 4.3, 4.5, and 4.7 V (Li+/Li), respectively. This improvement is attributed to the pillaring effect of the Mg2+ ions in the interslab space of the lattice and the protective effect of the MgO film against the escape of Co4+ ions from the bulk of LiCoO2 particles

Studies of stannic oxide as an anode material for lithium-ion batteries

The structural evolution of SnO2 fine powder prepared by a sol-gel method upon heat-treatment was investigated by using x-ray diffraction. Electrochemical measurements showed that the reversible capacity of lithium ion reaction with the SnO2 electrode could be as high as 600 mAh/g. The mechanism of lithium ion reaction was studied by ex situ x-ray diffraction and Raman spectroscopy. Two processes were revealed: a substitution reaction, in which SnO2 is reduced and Sn is formed, followed by LiySn alloy formation.

Atomic-Scale Clarification of Structural Transition of MoS2 upon Sodium Intercalation

Two-dimensional (2D) transition-metal dichalcogenides hold enormous potential for applications in electronic and optoelectronic devices. Their distinctive electronic and chemical properties are closely related to the structure and intercalation chemistry. Herein, the controversial phase transition from semiconductive 2H to metallic 1T phase and occupancy of the intercalated sodium (Na) upon electrochemical Na intercalation into MoS2 are clarified at the atomic scale by aberration-corrected scanning transmission electron microscope. In addition, a series of other complicated phase transitions along with lattice distortion, structural modulation, and even irreversible structural decomposition are recognized in MoS2 depending on the content of Na ion intercalation. It is shown that x = 1.5 in Na(x)MoS2 is a critical point for the reversibility of the structural evolution. Our findings enrich the understanding of the phase transitions and intercalation chemistry of the MoS2 and shed light on future material design and applications.