Controllable Preparation Of C₃N₄ Doped PVDF Nanofiber Separator And Its Application In Lithium-Sulfur Batteries

Non-renewable fossil fuels are being rapidly consumed.The problem of the effective use of energy seriously restricts social development,making energy storage and conversion highly concerned.Compared with current commercial lithium-ion batteries with low energy density,the advantages of lithium-sulfur batteries are very obvious.The theoretical energy density can reach 2500 Wh·kg^-1.It is a more advanced energy storage and conversion device that can meet people's need for a quality life.However,due to the constraints of its own factors,the industrialization process of lithium-sulfur batteries has been hindered.These factors include the shuttle effect,electrochemical inertness,volume expansion,lithium dendrites,lithium loss,and self-discharge.As one of the important components of lithium-sulfur batteries,the performance of the separator greatly affects the basic performance of the battery.This thesis mainly started from the perspective of the separator.By building a new type of separator with a nanoporous three-dimensional network structure,the shortcomings of commercial separator were made up,such as low porosity,poor thermal stability,and insufficient electrolyte wettability.New PVDF?PVDF-b-PTFE?obtained by graft modification of PVDF and PTFE.And this and commercial PVDF were used as raw materials.New PVDF fiber separators and commercial PVDF fiber separators were respectively prepared by electrospinning and hot-pressing processes.In this process,the electrostatic fuzzing problem faced by the separators was solved.At the same time,the rule is explored between the spinning process parameters and the separator structure forming.The fiber membrane spun under the condition of 8%mass fraction concentration and15 kV voltage was excellent in fiber morphology,tensile state and spatial structure.So,it was selected as the research object for further investigation.Then,it was studied that the related physical and chemical properties of the separator and the electrochemical performance of the battery by assembling the button cell.The results showed that the new PVDF was easier to electrospinning than the commercial PVDF.The new PVDF fiber separators were superior to commercial PVDF fiber separators in terms of mechanical properties,pore structure,electrolyte absorption capacity,and electrochemical performance.On the other hand,in order to slow down the shuttle effect of the lithium-sulfur battery to improve its performance,this thesis further functionally modified the new PVDF fiber separator,making the separator had excellent ion adsorption capacity.Using melamine as the raw material,C₃N₄ was synthesized by calcination and ball milling.Then a new PVDF fiber membrane doped with C₃N₄ was prepared by electrospinning and hot pressing.First,the synthesis conditions of C₃N₄ were explored,and the morphology and structure of C₃N₄ and its adsorption performance for polysulfide ions were characterized and analyzed in detail.Subsequently,it was analyzed that the morphological structure and mechanical properties of the PVDF fiber separator with this new composite structure.The electrochemical performance of the lithium-sulfur battery fabricated with this separator was studied.Experiments showed that the optimal synthesis temperature of C₃N₄ was 540?,and it had a strong ability to adsorb polysulfides,which could alleviate the shuttle effect in lithium-sulfur batteries and improved the heat resistance of the separator.In addition,the C₃N₄ doped new PVDF fiber separator had a liquid absorption rate of up to 473%and a tensile strength of 30.62 MPa.In the electrochemical test,the initial discharge capacity of the battery using this new composite structure PVDF fiber separator was753 mAh·g^-1 at 0.2 C rate.After 80 cycles,the discharge capacity was 611 mAh·g^-1,and the capacity retention rate was 81.14%,and the average capacity attenuation per lap was 0.23%.Compared with the unmodified new PVDF fiber separator,it had a greater improvement in physical and chemical properties and electrochemical performance.In terms of practical applications,this thesis applied the C₃N₄ doped new PVDF fiber separator to the field of flexible electronics.And flexible ultra-thin lithium-sulfur soft pack battery was successfully prepared.This provided the possibility for the manufacture of safety,high energy density and flexible electronic products that were urgently needed by the market.