Synthesis And Lithium-ion Storage Properties Of Nitrogen-rich Conjugated Polymer Cathode Materials

Lithium-ion battery has become one of the greatest inventions in modern times due to its high operating voltage,high specific energy density,light weight,stable cycle performance,and has been widely used.The electrode materials（especially the cathode materials）determine the performance of the battery.Therefore,it is particularly important to develop advanced cathode materials with excellent performance.The development of the traditional transition metal-based cathode materials is limited due to their expensive,scarce resources and poor safety.Thus,the organic cathode materials,mainly containing carbon,hydrogen,oxygen and nitrogen elements,have attracted the attention because of their light weight,easy accessibility,low cost,safe and environmental friendness,and easy preparation.However,organic materials suffer from poor intrinsic conductivity and high solubility in organic electrolyte,leading to poor charge transfer,low actual capacity and poor cycling stability.To resolve these two problems,this thesis reports the design and synthesis of two kinds of nitrogen-rich conjugated polymer with rigid cores at the molecular level as cathode materials.These two kinds of materials have rich redox active sites and do not dissolve in organic electrolytes.The main research contents are classified into the two parts:（1）The HATN-CN monomer is synthesized by the reaction of cyclohexahexanone octahydrate and 3,4-diaminophenonitrile in dehydrated acetic acid.Then,a nitrogen-rich covalent triazine polymer（HATN-CTP）with hexaazatriphenylene as the rigid core was obtained from the polymerization of HATN-CN monomer in trifluoromethanesulfonic acid,which solved the problem of high solubility for HATN-based organic materials.The successful polymerization of HATN-CTP material was proved by various characterization methods.The material was used as the cathode materials in LIBs,and partially reduced graphene oxide（PRGO）was used as the conductive agent.After 50charging and discharging cycles at 50 mA g^-1,the specific capacity of the electrode maintained at 185 mAh g^-1,and the coulombic efficiency maintained at about 100%.More importantly,the capacity of the electrode still achieved155 mAh g^-1after 600 cycles when the current density increased to 200 mA g^-1,indicating that the material has good long-term cycle stability.The above excellent properties are attributed to the insolubility of HATN-CTP in the electrolyte and the high conductivity provided by partially reduced graphene oxide.（2）We designed a hexazotriphenyl-based nitrogen-rich organic conjugated polymer（HAT-COP）containing imine group C=N and carbonyl group C=O.The polymeric material was synthesized by hydrothermal polymerization of hexadecarboxylic monomer containing hexaazatriphenylene and tri（4-aminophenyl）amine.As a cathode for LIBs,the charge-discharge capacity of HAT-COP electrode could be kept at 147 mAh g^-1 after 200 cycles at a current density of 50 mA g^-1,and it delivered good cycling stability at higher current density.This work provides a method for solving the problem of the solubility of organic cathode materials without reducing the theoretical capacity of the electrode.