Preparation And Application Of Modified Carbon Nitride Materials As Host Materials In Lithium-sulfur Batteries

At present,the energy demand of the new electronic devices,such as electric cars,mobile devices and so on,is difficult to meet by lithium-ion batteries,although its technology is very mature.As a result,the new generation of batteries with high energy density has been explored by research fellows.In the new generation of energy storage systems,lithium-sulfur batteries（LSBs）have been attracted the gaze of research fellows because of its unique advantages.Put in simple terms,first,lithium-sulfur batteries have3-5 times the theoretical capacity(1675 mAh·g^-1)of lithium-ion batteries on the market.Besides,as one of the most abundant elements in the Earth’s crust,sulfur has great advantages in terms of economy and environmental protection.However,some urgent problems,such as poor conductivity of sulfur and its compounds,dissolution and diffusion of intermediates in organic electrolytes,volume expansion of sulfur,have become obstacles to commercial development.In the existing improvement strategies,many researchers have applied graphite phase carbon nitride（g-C₃N₄）in lithium-sulfur batteries to relieve the problems mentioned.It has great potential in the field of energy storage due to its unique planar structure,good optical,physical and chemical properties,and low cost.In this paper,urea was used as the precursor of g-C₃N₄,N-N dimethylformamide（DMF）and citric acid as additive materials to modify the morphology and elemental state of g-C₃N₄.Modified carbon nitride has been improved the shortcomings of traditional carbon nitride with a small specific surface area and low electron transfer rate.Compared with the traditional g-C₃N₄,the modified g-C₃N₄ can significantly improve the properties of lithium-sulfur batteries.The main research contents of thesis are as follows:（1）N vacancy porous g-C₃N₄（NDCN）has been prepared by condensation of urea and N-N dimethylformamide（DMF）with a proper proportion at high temperature.In the heating process,DMF will partially decompose to form carbon monoxide and dimethylamine,affecting the formation process of g-C₃N₄,so that the N vacancy will be introduced into g-C₃N₄,which improving the adsorption performance of polysulfide.At the same time,the evaporated gas will produce more pores with the increase of temperature,which reduces the stacking of g-C₃N₄4 in the formation process,changing the structure of the formed g-C₃N₄,and making the prepared g-C₃N₄ with an obvious lamellar porous structure.The porous g-C₃N₄ with a layered structure not only has a large specific surface area for sulfur fixation,but also increases the contact with the active substance for faster ions transmission.The crystal structure,morphology,chemical composition,and other information of synthetic materials can be obtained by XPS,XRD,TEM.It can be concluded that the NDCN has a high specific surface area which up to 83.6 m²·g^-1,and many mesoporous structures with a diameter of about 20-50 nm that can serve as a buffer space to accommodate the volume expansion of sulfur during charge and discharge.In addition,due to the strong binding ability of N vacancy to polysulfide,the polysulfide adsorption test showed that polysulfide was adsorbed well by the prepared material.The sample prepared has a higher ion transfer rate,which can reduce the degree of polarization and improve the performance of the lithium-sulfur batteries.In electrochemical tests,as expected,the lithium-sulfur batteries with NDCN as the host material significantly improved the cycle stability and capacity of the batteries.Under the current density condition of 1 C,a high capacity of 620 mAh·g^-1was obtained after 300 cycles,and the average decay rate per turn was only 0.045%.（2）Porous g-C₃N₄ with graphite carbon（PCN）was prepared by using urea as raw material and citric acid as auxiliary material through the co-heating process.In the process of high temperature,the CO₂ and H₂O were released due to the thermal decomposition of citric acid,which can modify the morphology of the original g-C₃N₄and form a thin sheet porous structure.At the same time,some activated carbon obtained from the decomposition of citric acid will be doped into the lattice of g-C₃N₄ in an appropriate way to form graphite phase carbon,which can enhance the electrical conductivity of g-C₃N₄.The above theory was also proved by TEM,SEM and other characterization methods.The specific surface area of the prepared PCN is 79.9 m²·g^-1,which is much higher than ordinary carbon nitride of 7.1 m²·g^-1.The higher specific surface area can be used as a good load of sulfur and provide a sufficient reaction site for catalytic oxidation of sulfur.XRD,XPS,and adsorption tests showed that the lattice structure of the material was changed after the incorporation of C atom,which effected the chemical valence state of N element and enhanced the adsorption capacity of the material to polysulfide.Meanwhile,the interaction between the added C atom and N resulted in n-π*electron transition,that improving the electronic conductivity and charge transfer,and improving the utilization of sulfur element.At the same time,the addition of C atoms improved the conductive ability of the material and the utilization rate of sulfur.Due to these merits,the PCN/S composite cathode exhibits a remarkably excellent cycling performance,maintaining 700 mAh·g^-1 after 150 cycles at a current density of 1 C and 484 mAh·g^-1 after 350 cycles at a current density of 2 C.