Design And Synthesis Of Organic Cathode Materials And The Applications In Sodium Ion Batteries And Lithium Sulfur Batteries

Rechargeable batteries play an important role in the development of mobile electronic products and new energy vehicles.Lithium-ion batteries（LIBs）have been rapidly popularized and used due to their excellent electrochemical performance.However,the shortage and uneven distribution of lithium resources limit their development,so it is necessary to develop low-cost,green,and high energy density battery systems.Sodium has similar properties with lithium,with rich reserves and low costs.Therefore,developing sodium-ion batteries（SIBs）to replace lithium-ion batteries is considered a feasible strategy.The improvement of the performance of SIBs largely depends on the characteristics of positive electrode materials.Widely used inorganic positive electrode materials generally have shortcomings such as limited capacity for improvement,high production process costs,and potential safety hazards.Compared to inorganic positive electrode materials,organic positive electrode materials have the advantages of high theoretical specific capacity,environmental friendliness,wide source of raw materials,strong molecular designability,and safer system,thus it is a kind of energy storage material with large-scale application prospects.However,the problems of poor intrinsic conductivity,easy dissolution in electrolyte,and low operating voltage of organic materials still need to be addressed.Based on the characteristics of organic materials,reasonable design of materials from the molecular level,and its properties can be regulated,which is expected to improve the conductivity of the material and prevent its dissolution in the electrolyte.Lithium sulfur batteries（LSBs）have attracted the attention of researchers due to their high energy density,abundant sulfur reserves,and low cost.However,some inherent characteristics of sulfur and polysulfides and the serious"shuttle effect"problem during charging and discharging seriously hinder their commercial application.Using porous carbon materials and covalent organic frameworks as composite carriers for sulfur positive electrodes is an effective way to suppress the shuttle effect.Therefore,in this paper,a thiophene-based covalent organic skeleton/carbon nanotube composite carrier material was prepared for sulfur cathode,its structure and morphology were characterized,and its adsorption and catalytic performance for polysulfide conversion were studied.The specific research content is as follows:1.Two small molecules,2,6-bis（p-benzoquinyl）pyridine（QPQ-1）and 2,5-bis（p-benzoquinyl）pyridine（QPQ-2）,were designed and synthesized and used as positive electrodes for SIBs.Using electron absorbing pyridine to connect two p-benzoquinone（BQ）units at different locations,the introduction of pyridine expands theπconjugation and improves planarity of the molecule,avoiding molecular dissolution in the electrolyte,which is beneficial to improving the cycle stability of the battery.In addition,electron absorbing pyridine unit can reduce the LUMO energy level of the molecule,which is beneficial to improving the working voltage of the electrode.The electrochemical test results show that the QPQ-2 electrode operates at a voltage of 2.71V/2.26 V,exhibiting a high reversible capacity of 214 m Ah g^-1.After 100 cycles at a rate of 0.1 C,it still retains 93%of its initial value,and exhibits a capacity of 160.3m Ah g^-1 in the first cycle.After 1000 cycles at a rate of 1 C,the capacity retention rate is 82.7%.Its oxygen reduction reaction mechanism has also been verified by XPS and FT-IR tests.2.Thienothiophene-based covalent organic skeleton/carbon nanotube composites were prepared by in-situ synthesis（TT-BOST@CNT）and used as a positive electrode carrier material for lithium sulfur batteries.Structurally,TT-BOST is uniformly wrapped on the outer surface of the CNT,and the interlaced composite materials facilitate the transmission of electrons.The cavity of the CNT provides ample space for the storage and volume expansion of sulfur,and acts as a physical constraint on polysulfides.Based on DFT calculations and experimental results,the introduction of rich heteroatoms B,O and S into the COF structure can not only adsorb Li⁺in polysulfides,but also has a strong adsorption effect on S_x^2-.In addition,TT-BOST as a catalyst promotes the conversion of long chain polysulfides to short chain polysulfides,effectively slowing down the shuttle effect.TT-BOST@CNT/S electrode exhibits excellent magnification performance and excellent reversible capacity of 1545 m Ah g^-1 at a current density of 0.2 C.More importantly,after 1000 cycles at a high current density of 1 C,the attenuation rate of each cycle is only 0.035%.