Novel Architectural Design And Study Of Negative Electrode Materials In High-Performance Lithium-Ion Batteries

In response to the increasing demands for energy conversion and storage worldwide, considerable efforts in this aspect have to be directly connected with a challenging search for new materials concepts as well as multidisciplinary approaches. It is generally accepted that breakthroughs in materials hold the key to new generations of lithium-ion batteries, not only for applications in portable electronics but especially for clean energy storage and use in hybrid electric vehicles. In this dissertation, we have reviewed the development in research of negative electrode materials of lithium-ion batteries. Pursuing these efforts towards improving the rate and cycling capability of inorganic fullerene-like materials, tin-based composite oxides and transition-metal sulphides (oxides), we have explored the feasibility of by using low-temperature hydrothermal techniques to design electrode materials with novel architectures, which were characterized by XRD, TEM, SEM and XPS measurements. Their electrochemical performances have been evaluated by means of various techniques including galvanostatic method, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The main content is the following:1. Inorganic fullerene-like materials such as WS₂ and MoS₂ have demonstrated the surprising large reactivity versus lithium based on a conversion process reaction. However, such excitement needs to be tempered because ensuring rapid and reversible conversion reactions is an unavoidable issue. To benefit from the large capacity gain advantages offered by the conversion reactions and to overcome poor kinetics, a coaxial MoS₂/CNT architecture is reported. By taking advantage of carbon nanotubes, the nanometer-sized multifunctional heterostructures offer enhanced properties through the cooperative contribution of each component. On the basis of our results, the coaxial MoS₂/CNT architecture has been confirmed to demonstrate highly reversible capacity (approaching 400 mAh/g) and excellent cyclability (less than 0.8 mAh/g per cycle), improved rate capability. These favorable results suggest that the key role of carbon nanotubes in increasing electric conductivity, maintaining thermodynamical/kinetical stability and decreasing charge-transfer resistance of the MoS₂.2. For lithium-ion batteries, tin-based composite oxides are among the most appealing and competitive candidates for new types of electrode materials. Unfortunately, the enormous volume changes generated during the alloying processes cause critical mechanical damage to the electrode, resulting in cracking and crumbling of the electrode and a remarked loss of capacity. Such problems might be addressed with the self-assembled microspheres with exciting nanosize effects. Accordingly, achieving the outstanding rate and cycling capability become possible. The porosity obtained via nanoplate assemblies within the Sn_1.0P_1.17O_4.72 microspheres not only help accommodate the mechanical stress resulting from the large volume change during alloying to maintain the structure integrity, but also maximize the rate capability benefits of shorter diffusion pathways at the nanoscale. For such microspheres, the reversible capacity was maintained at 514 mAh/g after 20 cycles and the discharging capacity retention was more than 87% at a 1C rate. Moreover, more than 70% of the reversible capacity could be delivered even at a 6 C rate. In addition, the electrochemical reactions toward lithium involved in TiO₂ self-assembled microspheres were investigated, and superior characteristics associated with unique three-dimensional architectures have been observed. We believe that this material could be employed as an anode material for next-generation high-safety lithium-ion batteries.3. Although the introduction of the conversion electrode materials into lithium-ion batteries might seem quite attractive, implementation is beset with formidable technical difficulties such as intrinsic poor electronic conductivity of the materials or to inadequate means of formulating/configuring the electrodes. In anticipation of such needs, we embarked on a study of electrode designs that could efficiently marry the current collectors with transition-metal sulphides so as to utilize the capacity gains associated with conversion reactions. Furthermore, to favor the electrochemical conversion performances of such carbon-free self-supported configuration and fight kinetic limitations, we significantly decrease the diffusion length while preserving an effective electronic and/or ionic percolation. A preliminary result shows that the electrode is found to achieve sustained reversible capacities (400 mAh/g) and more than 81 % of the reversible capacity was retained after 20 cycles at a 2C rate. Strikingly, the electrode can sustain high rates and it turns out that more than 43% of the reversible capacity could be delivered even at a 16C rate, which is the largest level specific capacity for transition metal sulphides at the high rates.4. Taking into account the proper potential window, high specific capacity, extended cycle life, fast rate capability and low safety concerns, one-dimensional nanostructured TiO₂ seems to be extremely attractive and should be a promising candidate for nonaqueous hybrid supercapacitor application. We for the first time fabricated a hybrid supercapacitor which consisted of a carbon nanotube cathode and a TiO₂ nanowire anode, and presented the preliminary results for such an energy storage device operating over a wide range of 0-2.8 V. Based on the total weight of both active materials, such hybrid supercapacitor delivered an energy density of 12.5 Wh/kg at 10 C rate, twice higher than the values of the carbon nanotube-based supercapacitor, while maintaining desirable cycling stability. All these profit from using the carbon nanotubes with large surface and proper pore distribution as the cathode, which ensure the TiO₂ nanowire anode precede with the faradic reaction in the applied potential range. The hybrid supercapacitor exhibits improved power and energy performances by the increased potential window, particularly in the larger current density.