The Design And Preparation Of Carbon Nitride And Its Composites Applied In Lithium-sulfur Batteries

With the increasing demands for energy,it is urgent to develop efficient energy storage technology for renewable resources.Lithium-sulfur batteries are considered to be the promising next-generation electrochemical energy storage systems owing to their high theoretical specific capacity(1675 mA h g^-1)and high energy density(2600 Wh kg^-1).In recent years,carbon nitride as an emerging ordered polymer material has been widely used in the research of lithium sulfur batteries because of its rich polar functional sites.In this thesis,carbon nitride was used as a building block to design and prepare carbon nitride-based composites,which was used as the host material to solve the problems of lithium-sulfur batteries,such as “shuttle effect”,volume expansion,and so on.Through the structure characterization and electrochemical performance test of carbon nitride-based composites,the electrochemical behavior and lithium storage performance were analyzed,and it is mainly divided into the following three parts:1.CNFs,TiO₂/CNFs and TiO₂/C₃N₄/CNFs nanofiber membranes were prepared by electrospinning and placed between the electrode and the separator to test the electrochemical performance of lithium-sulfur batteries.With the sulfur loading of 70%,the electrochemical performance used TiO₂/C₃N₄/CNFs as the intermediate layer is better than those of TiO₂/CNFs and CNFs.The battery which used TiO₂/C₃N₄/CNFs has the specific discharge capacity reach to 1596.5 mA h g^-1in the first cycle at the high current density of 0.5 C while it can reach to 941.1 mA h g^-1after 300 cycles,and the capacity retention is 58.9%.Additionally,the battery used TiO₂/C₃N₄/CNFs as the intermediate layer delivers the capacity of 1250.9,1113,993.2,875.5,734,and 313.9 mA h g^-1when tested at 0.1,0.2,0.5,1.0,2.0,and 5.0 C.The high-efficiency effect of this material for lithium-sulfur batteries is mainly due to the polar effect of TiO₂,which accelerates the redox kinetics of elemental sulfur on the positive electrode side as while as the rich nitrogen active sites provided by carbon nitride further enhance the chemical adsorption,finally effectively limit the "shuttle effect" and improve the cycle stability of lithium-sulfur batteries.2.CNTs/C₃N₄-X composites were consisted of CNTs and C₃N₄,which were prepared by hydrothermal synthesis in different quality ratios.CNTs/C₃N₄-10 was synthesized by CNTs and C₃N4 complexed at a mass ratio of 9:1.The battery which used CNTs/C₃N₄-10 as the host material with the sulfur loading of 70% showed the best performance.It is concluded that CNTs/C₃N₄-10/S exhibited excellent cycle performance and rate performance.At the low current density of 0.1 C,CNTs/C₃N₄-10/S has the high discharge capacity of 1074.3 mA h g^-1in the first cycle while it can still remain 830.3 mA h g^-1after 100 cycles.Even at a high current density of 0.5 C,the initial discharge capacity can reach to 616.1 mA h g^-1,and preserves the discharge capacity of 431.4 mA h g^-1after 600 cycles,the capacity retention rate is 70%.At current density of 0.1,0.2,0.5,1.0,and 2.0 C,CNTs/C₃N₄-10/S still has the discharge specific capacity of 1295.4,1060.3,952.5,878.4,and 819.4 mA h g^-1.When the current density returns to a low current density of 0.2 C,the capacity still reach to 1007.3 mA h g^-1,close to the initial value at 0.2 C.Such excellent cycle stability is mainly owing to the synergetic effect of CNTs and C₃N₄,which limit the dissolution of lithium polysulfide through physical restriction and chemical adsorption,respectively,thereby greatly promote the cycle stability of lithium-sulfur batteries.3.Porous carbon nitride nanotubes（PCNNT）were designed via hard template method.Melamine sponge was fully absorbed saturated urea solution,then freeze-dried and calcined to form PCNNT.When used as host material with sulfur loading of 78.1%,the PCNNT/S at the current density of 0.5 C delivers the specific discharge capacity of 704.8 mA h g^-1in the first cycle while it delivers the specific discharge capacity of 585.9 mA h g^-1after 800 cycles,the capacity retention rate is 83.1%.The rate capacity of the battery was estimated by varying current densities from 0.1 to 2 C,the discharge capacity based PCNNT/S delivers an initial discharge capacity of 1472.2 mA h g^-1at 0.1 C,subsequently stabilized at 1001.4 mA h g^-1at 0.2 C,869.3 mA h g^-1at 0.5 C,480.1 mA h g^-1at 1 C,and 298.5 mA h g^-1at 2 C rate,followed by a reversible capacity of 779.8 mA h g^-1at 0.5 C.Such excellent cycle stability may be attributed to the reasonable structural design of C₃N₄,which makes PCNNT/S have the stable ultra-long cycle performance.