Controllable Preparation And Electrochemical Properties Of Electrospun Flexible Carbon Nanofibers

For today's society to develop,energy and environmental issues are two key issues that must be settled.Developing new environmental and efficient energy storage technologies is a significant way to solve the problem.Among them,the development of various new advanced composite materials has become the core of achieving this important goal.Carbon nanocomposite materials,that have the inherent characteristics of carbon materials,simultaneously have the advantages of rich sources and diverse forms,have become one of the hot materials in current research,which are widely used in machinery,transportation,biomedical,aerospace and other fields.Carbon nanocomposite fibers have great potential for application in electrode materials,multifunctional composite materials,hydrogen storage materials and other fields,because of excellent characteristics,such as thermal conductivity,light weight and high electrical conductivity.Thus,two multifunctional new carbon-based composite nanofiber materials have been fabricated by designing the components and structure of the basic carbon material in this topic,the LLZO/C flexible mixed ionic and electronic conductor carbon nanofiber membrane and B-F-N triply doped CNT composite porous carbon nanofibers?CNT-PCNFs?.In addition,their electrochemical energy storage in lithium-sulfur batteries have been investigated.More,the properties of CNT-PCNFs in supercapacitors was also explored.To sum up,the work in this thesis mainly includes as following three parts:?1?Developing mixed ionic and electronic conductor?MIEC?as sulfur cathodes is significant for solid-state lithium-sulfur battery technology.We fabricated LLZO/C MIEC nanofiber membranes with uniformly distributed fast ion conductor particles on the fiber surface adopting molten salt method,electrospinning technique and inert gas carbonization method with polyacrylonitrile?PAN?as carbon source and Li₇La₃Zr₂O₁₂?LLZO?as additives for liquid lithium-sulfur batteries.S-cathode of LLZO/C mixed conductor formed continuous and stable conductive network inside,without any binder and current collector,which greatly reduced battery quality and increased the energy density.As S-cathode,it possessed the discharge capacity of 1026 mAh·g^-1 at the first circle under 0.1 C discharge density,demonstrating good electrochemical stability.Mixed conductor cathode materials offer a promising direction for the development and construction of cathode materials for solid state lithium-sulfur batteries,and are of great significance for promoting the industrialization and practicality of solid-state lithium-sulfur batteries.?2?Flexible porous carbon nanofibers?PCNFs?as electrode scaffold materials have received extensive attention and application in the field of lithium-sulfur batteries and supercapacitors,and the construction of finer morphology and conductivity-enhancing networks inside PCNFs is an effective strategy to improve their application performance.We fabricated B-F-N triply doped CNT-PCNFs with continuous and uniformly distributed hierarchical microstructure by dispersing CNT with excellent electrical conductivity and mechanical properties into the stable sol of poly?tetrafluoroethylene??PTFE?,boric acid?BA?and poly?vinyl alcohol??PVA?,via electrostatic spinning,followed by carbonization.In addition,CNTs tended to paralleled to the NF axis tend to be parallel to the fiber axis.The formation mechanism and mechanism of flexibility of CNT-PCNFs prepared by electrospinning were explored.?3?Based on the research in?2?,the electrochemical performance of CNT-PCNFs electrode materials in lithium-sulfur batteries and electric double-layer supercapacitors was further explored.The first-cycle discharge capacity of CNT-PCNFs as S-cathode reached1026 mAh·g^-1,at a current density of 0.2 A·g^-1 and the coulomb efficiency reached 99.04%after 150 cycles,showing good cycle stability;Fabricated symmetrical supercapacitors with the PCNF cages exhibited high gravimetric capacitance of 164 F·g^-1 at 20 mV·S^-1 and 92.5%capacity retention after 20000 cycles at 2 A·g^-1.The reported approach allows green synthesis of a new PCNF scaffold material with properties appealing for applications.The electrode scaffold materials prepared by this design strategy possess micro-open hierarchical pore structure in a high precision range,which is beneficial to the penetration and transport of electrolyte and paves the way for the electrode support of high performance energy storage devices.