| The development of green energy has become the research hotspot in recent years with theemergency of environmental crisis and energy crisis. The electric power is a kind of green energythat can be obtained by hydropower, wind power, and solar energy. This requires a storageequipment and the applications of electric power need storage and conversion styles. In many ofthe energy storage technology, the battery is a kind of high efficient and simple system. Lithiumion batteries (LIBs) have high working voltage, great energy and power density, long cycle life,flexible design, and no memory effect, etc., thus they have become the research hotspot. It can beapplied for a variety of mobile applications, including cell phones, digital cameras, and laptops.And LIBs have attracted more and more attentions in recent years. One of the bottlenecks in theapplication of lithium ion battery is the capacity, rate capability, and costs of cathode materials.The cathode material costs account for around40%of the LIBs total costs. And the safety andelectrochemical performance of LIBs largely depends on the cathode materials. So the research oncathode materials of LIBs is especially important.Polyanion type LiFePO4cathode material is the one the most promising alternative cathodematerial, because of its reasonable theoretical capacity, low cost, high safety, and environmentalfriendliness. However, the inherently poor electronic conductivity and sluggish diffusion oflithium ions are the major barriers for the application of LiFePO4. In order to overcome these drawbacks, considerable efforts have been made focusing on the following three parts:1.Preparation nanomaterial or special morphology material, in order to shorten the distance oflithium ions and electrons from the internal transmission to the interface of material, and increasethe contact area of material and electrolyte.2. Improve the electronic conductivity of the materialor formation of conductive network in the material.3. Increase the lithium ions diffusion of thematerial or try to use the fast ion channels in the crystal structure of natural material. In this study,the research works are focus on the above three parts. The LiFePO4-based composite cathodenanomaterials have been prepared using electrospun webs treated with thermal treatment processe.The electrospinning is a kind of directly method to obtain continuous nanofibers. At the same time,this method can prepare the nanomateial webs with network structure and high porosity. Theobtained nanomaterials have been forming the special conductive network after thermal treatment.In addition, compared with the conventional method, this web can be used directly as a batteryelectrode without adding polymer binder. It will avoid the polymer block a portion of activematerial surfaces, which could affect the lithium-ion diffusion to/from the bulk material. And itcould improve the diffusion of lithium ions.The main research contents including the following aspects:1. The thermal decompositionbehavior of the electrospun LiFePO4precursor-polyacrylonitrile (PAN) nanofiber composites andthe reaction of the LiFePO4precursors during thermal treatment. The effect of spinningparameters and the PAN concentrations in the electrospinning solution on the microstrucuture ofelectrospun composites. The effect of polymer percentage and thermal treatment on the structureand properties of composite cathode materials.2. The effect of non-lattice doped withmulti-walled carbon nanotubes (MWCNTs) on the thermal decomposition behavior of electrospunmaterials and the structure and properties of LiFePO4-CNF composites.3. The effect of latticedoped with Ti4+on the thermal decomposition behavior of electrospun materials and the crystalstructure stability, structure, and properties of LiFePO4-CNF composites. The specific contents areas follows:1. The thermal decomposition behavior of the electrospun LiFePO4precursor-polyacrylonitrile (PAN) nanofiber composites and the reaction of the LiFePO4precursors duringthermal treatment. The effect of spinning parameters and the PAN concentrations in theelectrospinning solution on the microstrucuture of electrospun composites. The effect of polymerpercentage and thermal treatment on the structure and properties of composite cathode materials.The thermal decomposition mechanism of composites and the formation reaction of LiFePO4werestudied. The electrostatic spinning parameters were studied by SEM, when the weight ratio ofPAN in the electrospinning solution is4%,6%, and8%, respectively. The results show that: A variable high voltage power supply was used to provide a potential difference of23kV. Thecollector plate was made of an aluminum foil and the tip-to-collector distance was15cm. Theflow rate of the solution was1.2mL h1used for the PAN concentration of4wt.%. The flow rateof the solution was1.1mL h1used for the PAN concentration of6wt.%. The flow rate of thesolution was1mL h1used for the PAN concentration of8wt.%. In addition, the concentration ofPAN in the electrospnning solution will affect the carbon content, fiber structure, and materialflexible of LiFePO4-CNF composite materials. So the concentration of PAN will affect theelectrochemical properties of LiFePO4-CNF composite materials. Compared with the electrospunLiFePO4precursor-PAN nanofiber composite materials prepared with different electrospnningsolutions, the microstructure of materials will be better with the weight ratio of PAN increase andthe diameter of fiber increase. In addition, the thermal treatment was critical for the formation ofan optimal composite materials structure and carbon contents, ensuring better cycling performanceand a higher rate capability. The effect of the heating rate, temperature, and duration ofstabilization and carbonization processes on the properties of the LiFePO4-CNF compositecathode materials was investigated. Based on experiments, the results show that, when the weightratio of PAN is8%, among all composites, cathode made from an optimal thermal treatmentprocess, namely stabilized at280°C for4h with a heating rate of2°C min1in air and carbonizedat800°C for14h with a heating rate of2°C min1in argon, showed the best electrochemicalproperties in terms of higher first discharge capacity (146.3mA h g-1,0.5C), more stable cyclingperformance, and better rate capabilities. The cylic voltammetry result also shows that thereasonable voltage charging and discharging platform. Based on the study of the physical,chemical, and electrochemical properties, the material formation model, mechanism of chargetransfer, and lithium ion diffusion mechanism were proposed.2. The effect of non-lattice doped with MWCNTs on the thermal decomposition behavior ofelectrospun materials before and after pre-oxidation, and the structure and properties ofLiFePO4-CNF composites. The non-lattice doped could improve the apparent conductivity. TheCNTs, as a very promising conductive additive, can either provide more routes or accelerate thespeed for transportation of electrons and lithium ions in cathodes by producing highly conductivenetwork in the cathodes, due to its fibrous morphology, high contact efficiency, excellent electricalconductivity, and high surface area. And the mixed MWCNTs did not affect the thermaldecomposition. The synthesized LiFePO4-CNF-MWCNTs composite materials exhibits better rateperformance and more stable cycle performance compared to the LiFePO4-CNF compositematerials, which is due to the increase of electron transfer and lithium ion diffusion within thecomposites by mixed MWCNTs. MWCNTs could serve as nucleation centers and thus lower the energy involved in the nucleation process, while the growth of LiFePO4crystals may not beinfluenced by MWCNTs. The LiFePO4-CNF-MWCNTs composites containing0.15wt.%ofMWCNTs, prepared without additional conducting agent and polymer binder, delivers an initialdischarge capacity of156.7mA h g1at the0.5C rate, and stable charge-discharge cycle ability(>95%capacity retention after100charge-discharge cycles), based on the weight of LiFePO4.3. The effect of lattice doped with Ti4+on the thermal decomposition behavior of electrospunmaterials and the crystal structure stability, structure, and properties of LiFePO4-CNF composites.Lattice doping could induce crystal lattice distortion, yield more space for intercalation anddeintercalation of lithium-ion, improving the intrinsic conductivity and promoting the redoxpotential, and thus enhance the electrochemical properties of the materials. The Ti4+doping didnot affect the macrostructure, microstructure and thermal treatment of electrospun materials, andthe crystal of active material. The Ti4+doping could improve the structural stability of the activematerials, leading to better cycling performance and high rate capability, as indicated by its lowercharge transfer resistance. The LiFePO4-CNF composites containing2%of Ti4+were shown thebest electrochemical performance in all composites. In addition, the discharge capacity of thecomposites with the Ti4+doped at the Fe site seemed to be slightly better than that of the samplewith Ti4+doped at the Li site. The discharge capacity of obtained nominal LiFe0.96Ti0.02PO4-CNFcomposites was153.5mA h g-1with0.5C. The electrochemical impedance spectroscopy resultshows that the Ti4+doping could improve the electrochemical performance of materials. |
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