Fabrication Of One-Dimensional Electrode Materials For Electrochemical Energy Storage Devices

With the ever-increasing demands for the smaller and lighter portable electronics, efficient energy storage systems with high energy and power are experiencing a huge rise in recent years. Among various energy storage techniques, electrochemical devices attract lots of attention due to their unique advantages, including low cost, long life span, high energy density and good reversibility. At present, electrode materials are still the key limits of performances and applications for electrochemical devices. This thesis studied and discussed one dimensional (ID) electrode materials for electrochemical energy storage devices where emphasis lies in the fabrication, electrochemical performances and outlook in the field of lithium ion batteries and supercapacitors. The main contents and results are as follows:1. Conformal carbon-coated Li4TisO12(LTO) fibers are easily synthesized through a simple electrospinning route combined with subsequent thermal treatment. The as-formed fibers comprise interconnected LTO nanoparticles and an external coating layer of carbon. When evaluated as an anode material for lithium-ion batteries, the resulting LTO/C fibers exhibit excellent high-rate lithium-storage performances. The influence of the carbon content in the composite LTO/C fibers on the electrochemical properties is explored in detail. The enhanced lithium-storage performances are attributed to the synergetic features of interconnected LTO nanocrystals and a highly electronically conductive coating layer of carbon, which reduce the Li+diffusion distance and improve the electronic conductivity. This work presents a facile and effective synthetic strategy, which is potentially competitive for scaling-up the industrial production of high-rate LTO anode materials for lithium-ionbatteries.2. An efficient and scalable strategy was developed to fabricate highly porous LTO/C nanofibers (denoted as PLTO/C) by an electrospinning route followed by a two-step annealing process. The as-formed LTO/C fibers were thermally treated in air, during which the carbon matrix was partially burnt off and numerous nanopores were formed simultaneously due to the release of gases. The carbonization and the formation of pores can be achieved at the same time. Such a unique nanostructured LTO/C nanocomposite exhibits outstanding electrochemical performances with ultra-high rate capability and excellent cycling stability, when evaluated as electrode materials for LIBs and supercapacitors.3.1D Porous LiNbaO8nanofibers were fabricated through a facile electrospinning method combined with post-annealing treatment. The porous nanofibers are composed of interconnected nanocrystals (-20nm) and numerous nanopores (-18nm). Benefting from these advantageous structural features, the electrodes made of the as-formed porous LiNb3O8nanofibers exhibit high capacity, good rate capability, and excellent capacity retention upon cycling, when used as anodes for LIBs. The initial discharge capacities at the current densities of0.1C is241.1mA h g-1and the capacity can still retain119.2mA h g-1even after100cycles. A possible lithium-storage mechanism based on the in situ XRD studies was proposed. Results show that LiNbaO8experiences a complete phase change after uptake of3.6Li, and then the newly formed products undergo an intercalation-reaction for the following lithiation/delithiation processes. As further evidenced, a significant improvement in electrochemical properties for the nanocomposite LiNb3Os@C electrode with an external carbon layer has been realized. It is expected that the present route can be extended to prepare other electrode materials, and provide the candidates for exploring their structure-related properties.4A flexible asymmetric supercapacitor (ASC) with high energy density is designed and fabricated using flower-like Bi2O3and MnO2grown on carbon nanofiber (CNF) paper as the negative and positive electrode, respectively. The lightweight (1.5mg cm-2), porous network, conductive and flexible features make CNF paper an ideal supporter for guest active materials, which permit a large areal mass of9mg cm-2for Bi2O3(-85%of the mass of the entire electrode). Thus, the optimal device with an operation voltage of1.8V can deliver a high energy density of43.4μWh cm-2(11.3W h kg-1, based on the total electrodes) and a maximum power density of12.9mW cm"2(3370W kg-1). This work provides an example of large areal mass and flexible electrode for asymmetric supercapacitors with high areal capacitance and high energy density, holding great promise for future flexible electronic devices. 5. A novel all-solid-state, coaxial, fiber-shaped asymmetric supercapacitor has been fabricated by wrapping a conducting carbon paper on a MnCh-modified nanoporous gold wire. This energy wire exhibits high capacitance of12mF cm-2and energy density of5.4μWh cm2with excellent cycling stability. The NPG wire possesses both high flexibility and satisfying tensile strength. It was used as the backbone to support MnO2nanoflowers, which not only enables full utilization of MnO2and fast electronic/ionic transfer through the electrode but also avoids using a binder. The use of hierarchical nanostructures and design of the coaxial structure facilitate effective contacts between the two core@sheath electrodes and active layers with high flexibility and high performances. This work provides the first example of coaxial fiber-shaped asymmetric supercapacitors with an operation voltage of1.8V, and holds great potential for future flexible electronic devices.