Modification And Electrochemical Performances Of Lithium Batteries With Small Molecule Organic Cathodes

The traditional transition metal oxide cathode materials for lithium ion batteries have many problems,such as severe shortage of resources,high energy consumption in the preparation process and difficulty in recycling.Organic electrode materials have become the promising candidates for the next generation of high specific energy battery materials due to their advantages of high specific energy,environmental friendliness,designability and flexibility.However,some chanllenges need to be solved before the practical application such as easily soluble in electrolyte,poor electrical conductivity and complex synthesis route.Herein,the mechanisms of improving the electrochemical performance of four types of small molecule organic electrode materials were systematically investiaged,including in-situ catalytic polymerization,hydrogen bond catalytic regulation,collector selection and composite seperator optimization.Meanwhile,the mechanisms of catalysis and energy storage in the reaction processes were also elucidated.This study aims to provide theoretical basis and experimental guidance for the development of organic cathodes with high specific energy,low cost and practical application potential.The main research contents and conclusions are as follows:（1）The energy storage mechanism of Cu²⁺ion induced in-situ self-polymerization to improve the electrochemical performance of copper phthalocyanine（Cu Pc）.The electrochemical performance,catalytic polymerization mechanism and energy storage mechanism of Cu Pc as a cathode material for lithium battery were systematically studied.The results show that when charging to a high voltage of 4.4 V,the benzene rings between the Cu Pc molecules undergo self-polymerization reactions that are catalysed by the copper（II）ions in Cu Pc.The in-situ self-polymerization of Cu Pc leads to a closer contact between the active materials and the conductive agent,thereby improving the cycling stability and rate capability of the Cu Pc cathode.Even after 1000 cycles at 20C,capacity retention of Cu Pc cathode can be remained as 67%.The density functional theory calculations and experiments reveal that the energy storage and release process of Cu Pc is accompanied by two PF₆^-doping/dedoping reactions,and the two PF₆^-ions are adsorbed on both sides of the Cu Pc structural plane.（2）Mechanism of nitrate regulating hydrogen bond to improve electrochemical performance of indanthrone（IDT）.As a cathode material for lithium ion battery,IDT has a high theoretical specific capacity.However,the utilization rate of carbonyl group is only 50%due to the strong interference of hydrogen bond on carbonyl group.The mechanism of regulating hydrogen bond by nitrate to improve the electrochemical performance of IDT was systematically studied.The results show that nitrate ion can oxidize IDT to lose H in-NH,thus promoting the nucleophilic reaction of N position and carbonyl group,weakening the effect of hydrogen bond on carbonyl group,and improving the utilization ratio of carbonyl group.On the other hand,the molecular structure deformation in the oxidized IDT caused the microexfoliation of closely stacked IDT bulk,leading to high-rate performance.At a current density of400 m A g^-1（approximately 2C）,the specific discharge capacity of IDT is approximately 175 m A g^-1,and IDT can maintain a capacity of 125 m A g^-1 after 1000cycles.（3）Study on improving the electrochemical properties of benzoquinone-based cathode by using porous carbon as collector.1,3,5-tris（4-benzoquinone phenyl）benzene（TBPB）was designed and synthesized based on active benzoquinone unit.The compatibility between different electrolytes and electrochemical properties of TBPB,the mechanism of improving cyclic stability and rate performance of TBPB by using the three-dimensional porous carbon paper as collector were systematically investigated,as well as the electrochemical reaction mechanism of TBPB.The results show that TBPB undergoes a 6-electron redox reaction and the initial discharge capacity of 220 m Ah g^-1（0.1C）can be obtained.The adsorption of three-dimensional porous carbon to TBPB inhibits its dissolution in the electrolyte,thus improving the utilization ratio of active materials and the high-rate performace.The capacity retention of TBPB is 93.3%after 200 cycles at 0.1C,and the capacity at 5C is 87.8%of that at 0.1C.（4）Study on separator modification to improve the electrochemical properties of the cathode with ion coupling.The small molecule organic electrode material（5-methylphenazin-5-ium）anthraquinone-2,6-disulfonate（AQ-PMS₂）with ion coupling features was synthesized by one step reaction.Meanwhile,the AQ-PMS₂/3D porous carbon composite cathode materials were fabricated.AQ-PMS₂ has a theoretical capacity of～150 m Ah g^-1 and undergoes the redox of 4 electrons.The solubility of AQ-PMS₂ in different electrolytes was systematically studied,as well as the mechanism of the modified composite separator and 3D porous carbon on inhibiting the diffusion of active materials and improving the electrochemical performance of AQ-PMS₂.The energy storage mechanism of AQ-PMS₂ was also proposed.The composite separator modified with carbon nanotubes and 3D porous carbon can inhibit the diffusion of AQ-PMS₂ and act as a collector for the active material adsorbed in the matrix,thus improving the rate performance and cycling stability of AQ-PMS₂.The specific discharge capacity of 110 mAh g^-1 can be obtained after 800cycles at a rate of 2C.