The Preparation Of Graphene-based Composite Materials And Their Application In Lithium-sulfur Battery Cathode Materials

Lithium-sulfur batteries are widely concerned because of the advantage of high theoretical specific capacity,high theoretical energy density,low cost and environmental friendliness.However,lithium-sulfur batteries inevitably face some challenges in the development process,such as low battery life and low sulfur loading rate.The reasons are mainly the poor conductivity of elemental sulfur and the formation of polysulfide dissolved in the electrolyte to cause a"shuttle effect".In the process of charge and discharge,the lithium-sulfur battery is easy to expand due to the density of charge and discharge products.The series of problems seriously restrict the development of lithium-sulfur batteries.To solve the problem,this work focuses on the cathode material for lithium-sulfur battery and introduced the semiconductor material by modifying the sulfur cathode to improve the ability of adsorb polysulfide.Then,the good electron transfer media-―reduction of graphene oxide（rGO）‖was used to further improve its electrochemical performance.The main reserch contents are shown as follows:1.The carbon nitride/redox graphene（CN/rGO）hydrogel was synthesized in situ by hydrothermal method,and the three-dimensional porous structural composite was finally constructed by means of freeze-drying technology.It can be confirmed by SEM and BET that the introduction of carbon nitride effectively increases the porosity and specific surface area of graphene,provides a large number of nucleation sites for sulfur nanoparticles.The results of electrochemical tests showed that the cycle performance and rate performance of CN/rGO-S were greatly improved.Benefiting from the unique pore structure and the loading of carbon nitride induced strong polysulfide adsorption ability,lithium-sulfur battery cells using 3D CN/rGO as sulfur host achieved a high capacity of～1317 mAh/g at the first cycle during activation.The cells also demonstrated that an ultralong cycle life exceeding 500 cycles and an extremely low capacity-decay rate（0.1%per cycle）.2.Rutile TiO₂ nanospheres with diameter of².2μm and TiO₂ nanospheres/graphene oxide composite were synthesized through a simple surfactant free solvent-thermal method.Titanium dioxide particles with different morphologies could be obtained by adjusting the concentration of hydrochloric acid and the types of alcohols.SEM images at different synthesis stages disclosed that the TiO₂ nanospheres initialized from radically assembled TiO₂ nanorods clusters and grew up through the nanorod rootfusion due to the lateral growth of the nanorods.The results of electrochemical test showed that the reversible specific capacity of titanium dioxide/rGO-S was still 887mAh/g after 100 cycles at 0.2 C（1 C=1 675 mAh/g）and the capacity decay rate of each cycle was 0.39%.3.Using cetyltrimethylammonium bromide（CTAB）as surfactant,MnO₂nanomaterials with regular cubic morphology were synthesized by microemulsion combined with oxidation etching method.Manganese dioxide hollow nanocube exhibited long cycling stability over 150 cycles and high capacity for high-performance lithium-sulfur batteries.The introduction of rGO had constructed a conductive network system for MnO₂ nanomaterials,which greatly enhanced the electron transport efficiency of the material and reduced the internal resistance.Electrochemical test results indicated that S@MnO₂ and S@MnO₂/rGO semi-batteries showed excellent"sulfur host"performance.Lithium-sulfur battery cells using 3D CN/rGO as sulfur host achieved a high capacity of～1214 mAh/g at the first cycle during activation.The S@MnO₂/rGO/Li also could deliver high specific discharge capacities at high rates,that is,～1137 mAh/g at 0.1C,～1037 mAh/g at 0.2 C,～888 mAh/g at 0.5 C,～697 mAh/g at 1 C,～594 mAh/g at 2 C,and～497 mAh/g at 3 C.The above research results indicated that some semiconductors not only have photocatalytic properties,but also have excellent lithium storage and persistent sulfur effect.By using modified semiconductors for lithium-sulfur batteries,which will serve as a guiding role in the development of energy storage materials.