Fabrication Of One-dimensional Transitional Metal Oxides Based On Electrospinning And Their Lithium-Storage Properties

With the rapid growth of population and economy, global energy consumption increases strongly, which has stimulated intense research on renewable energy conversion and storage systems with high efficiency, low cost and environmental friendliness. In particular, rechargeable Lithium-ion batteries (LIBs), as the heart of key renewable energy conversion and storage technologies, become increasingly imperative with the increasing need of portable electronic devices and the upcoming electric vehicles. However, graphite, the conventional anode material in LIBs, only has a theoretical capacity of372mAh/g, which cannot meet the growing need for LIBs with higher capacity and power.Recently, transition metal oxides (TMOs) had been widely investigated since Tarascon et al first report that electrodes made of TMOs exhibit perfect reversible capacity, cyclic life and rate performance as alternative anode materials for LIBs. However, the TMOs are reduced in a conversion reaction to small metal clusters with the oxygen reacting with Li ion to form Li2O, leading to large volume expansion and destruction of the structure upon electrochemical cycling, and thus resulting in a rapid capacity fading and limiting their wide applications. Considerable approaches have been carried out to tackle this issue, including the use of nanostructure, porous structure, and the introduction of various carbon additives. These studies have brought significantly improvements of the lithium-storage performance of TMOs-based anodes.The past decade has witnessed significant growth in one dimensional (1D) nanomaterials, such as nanofibers, nanorods, nanotubes and nanowires, for high-performance LIBs by their high surface-to-volume ratios and excellent electrical conductivity along the lateral direction. Electrospinning is a simple and versatile method that provides direct and controllable fabrication strategies to construct well-defined1D nanostructure. Inspired by this, a series of1D nanostructured TMOs have been fabricated by electro spinning combining with post-heating. Furthermore, the lithium-storage properties of these1D nanostructured TMOs have been attempted to be integrated in this study.Molybdenum dioxide (MoO2) has recently received much attention as host substances for lithium-storage, owing to the low electrical resistivity, high stability, and high density (6.5g/cm). By employing a facile route based on single-nozzle electrospinning, air stabilization, and reduction/carbonization processes,1D carbon-coated MoO2nanofibers were prepared. The as-obtained1D carbon-coated MoO2nanofibers comprise hierarchically assembled MoO2nanocrystals of～20nm that are encapsulated within a thin carbon shell. This method does not require a complex coaxial-nozzle electrospinning device or a specialized immiscible solution-based precursor that is usually indispensable in an electrospinning process to form1D core-shell composites. The electrode made of the as-formed MoO2@C nanofibers exhibits excellent cyclability and a reversible capacity as high as762.7mAh/g at50mA/g after100cycles.Moreover, Titanium dioxide (TiO2) has also been fabricated as promising alternative anodes to graphite in LIBs because of their superior safety, low cost, chemical stability, and non-toxicity. Nevertheless, the main weakness of TiO2lies in the intrinsically slow kinetics of lithium ions diffusion and low electronic conductivity, resulting in the deterioration of reversible capacity and rate capability. To overcome this issue, an economical route based on electrospinning and layer-by-layer (LBL) self-assembly processes has been developed to synthesize unique MoO2-modified TiO2nanofibers, comprising a core of TiO2nanofibers and a thin metal-like MoO2nanolayer. The thickness of the MoO2nanolayer can be tuned by altering the precursor concentration or the LBL cycles. When evaluated for their lithium-storage properties, the MoO2modified TiO2nanofibers exhibit a high discharge capacity of514.5mAh/g at0.2C over50cycles and excellent rate capability, demonstrating that enhanced physical and/or chemical properties can be achieved through proper surface modification.Lastly, we have successfully fabricated porous ZnCo2O4nanotubes by an electrospinning method followed by annealing procedures. The formation mechanism of the unique porous nanotubular structure mainly relies on the different rate of mass transfer and the removal of polymer during the heating process. Zn(NO3)2and Co(NO3)2near the surface of the as-spun Zn(NO3)2/Co(NO3)2/PVP nanofibers decompose oxidatively to generate metal oxide nanocrystals during the thermal treatment initially. Meanwhile, the included metal nitrates in the core region do not react promptly due to lack of oxygen, but are unstable and tend to diffuse to the surface, driven by the concentration gradient. In addition, there exists another concentration gradient of metal-oxide nanocrystals from the surface to the interior of the nanofibers. Furthermore, PVP chains are highly flexible when the temperature is above Tg of PVP, facilitating the diffuse of metal nitrates. When the heating temperature is further enhanced, PVP decomposes completely. Therefore, the as-spun fibers have a great tendency to form hollow tubes eventually under annealing in air, owing to the Kirkendall effect related to the higher diffusion speed of melting metal nitrates in comparison to the newly-formed metal oxides. When evaluated for their lithium-storage properties, the porous ZnCo2O4nanotubes exhibit not only a high specific capacity of1454mAh/at100mA/g, but also a perfect rate capability of794mAh/g at a current density as high as2000mA/g, indicating a promising anode candidate for lithium-ion batteries.