Electrochemical Performance Of Porphyrin-Based Organic Electrode Materials As Cathode In Secondary Batteries

Although lithium-ion batteries have been widely used in all aspects of daily life,people have never stopped exploring low cost,environmental-friendly and resource-abundant electrode materials.The commercialized inorganic electrode materials,such as polyanionic compounds and layered metal oxides,are impeded by limited energy density,scarcity of resources,high price and other factors.The exploration of eco-friendly electrochemical energy storage materials with good comprehensive performance is considered as one of the main topics.Organic electrode materials have attracted great attention because of its rich resources,recyclability and structural designability.However,the low electronic conductivity of organic electrode materials and its high solubility in the electrolyte system result in low utilization of active materials and poor cycle stability,which are the main bottlenecks that plague the development of organic batteries.This thesis aims to develop high performance organic cathode based on molecular design using modified porphyrin molecules as active materials.Utilizing the strategies of changing environmental factors to induce in-situ polymerization of organic molecules,we developed a universal organic cathode with stable structures in organic battery systems.In this thesis,two types of porphyrin materials have been prepared:The first type is[5,15-bis(ethynyl)-10,20-diphenylporphyrin]copper(II)(CuDEPP)and5,15-bis(acetylene)base-10,20-diphenylporphyrin(DEPP);the second type is5,10,15,20-tetra(ethynyl)porphyrin copper(II)(CuTEPP)and 5,10,15,20-tetra(ethynyl)porphyrin zinc(II)(ZnTEPP).The electrochemical energy storage performance of cathode and its redox mechanism are sysmatically studied.The main research contents of this paper are as follows:(1)Porphyrin-based CuDEPP andDEPP are synthesized as organic cathode materials for organic lithium batteries.The electrochemical performance at different temperatures is systematically studied,and different spectroscopical and microscopical techniques are used to carefully monitor the changes of porphyrin molecules to understand the charge storage mechanism of the porphyrin cathode at high temperature.The reversible capacity of CuDEPP andDEPP cathodes increases with the increase of working temperature and it shows excellent performance even at50°C.After 2000 cycles,the reversible capacity of the cathode is 272 m Ah g^-1and the average voltage is 2.8 V(vs.Li⁺/Li).The reason for the excellent electrochemical performance of the electrode is related to the in-situ electrochemical polymerization of the acetylene group in the initial cycle.The stability of the electrode after the first charge and discharge has been significantly improved.Copper ions can stabilize the molecular structure of porphyrin and it can participate in redox reactions during the cycle,CuDEPP shows a higher reversible capacity thanDEPP.The result indicates that the charge storage mechanism of CuDEPP andDEPP is mainly due to the pseudocapacitance behavior where both anions and cations react at the surface and near the surface of the porphyrin molecules.(2)The ethynyl functional groups stabilize the electrode during the initial cycle due to the self-polymerization and the electrochemical performance of the porphyrin material is significantly increased.In order to further investigate the influence of ethynyl groups,porphyrin materials of CuTEPP and ZnTEPP with four alkynyl functional groups are designed and synthesized.CuTEPP and ZnTEPP cathodes deliver reversible capacities of 125 and 74 m Ah g^-1after 300 cycles with capacity retention of 87.4%and 66.1%,respectively.The current density is 0.5 A g^-1.Different anions can react with the metal porphyrin cathode.According to theoretical calculations,it indicates that the PF₆^-and TFSI^-anions and Na⁺cations interact with nitrogen atoms of porphyrin ring on the opposing position when they are in the charged or discharged states.X-ray photoelectron spectroscopy also further confirmed that the chemical environment of nitrogen has changed significantly after charging and discharging.To further understand the porphyrin cathode,a full battery with hard carbon anode and a CuTEPP cathode is designed.It also exhibits good electrochemical performance with a discharge capacity of 138 m Ah g^-1in the first cycle and a coulombic efficiency of 98.5%.