Design And Synthesis Of High-Efficiency Organic Cathode Materials Based On Carbonyl Compounds And Phenazine Derivatives And Their Application In Sodium-Ion Batteries

Today,lithium-ion batteries（LIBs）play an important role in electronic devices,new energy vehicles,and large-scale smart grids.However,due to the lack of lithium resources,there is a need to find alternatives for LIBs in future.Sodium-Ion Batteries（SIBs）offer a new solution for energy storage due to the abundant reserves of sodium in the earth,low price and similar working mechanism to LIBs.Currently,research on cathode materials for SIBs has focused on inorganic materials,such as transition metal oxides and polyanionic compounds,but the large radius of Na⁺tends to lead to the collapse of inorganic materials during cycling.Organic materials are usually composed of Van der Waals forces and thus consist of enough space to store large radius sodium ions.In addition,organic electrode materials have the advantages of low cost,wide source and designable versatile structure which are expected to be complementary to inorganic electrode materials.Nevertheless,organic electrode materials still have a series of issues,such as unsatisfactory electronic conductivity,severe dissolution in the electrolyte and low working voltage,resulting in low energy density and poor cycling stability,which limit the practical application of organic electrode materials.Considering the advantage that the molecular structure of organic materials can be easily regulated,the above problems are expected to be optimized by rational molecular design and synthesis to achieve high-performance of SIBs.In this thesis,a series of low-solubility organic small molecules and polymers were designed and synthesized as cathode materials for SIBs.The molecular structures were characterized by NMR,FT-IR and elemental analysis.The obtained electrode materials were finally assembled into coin cells,and their electrochemical properties and the electrochemical mechanisms in SIBs were further investigated.The specific results of the study are as follows:（1）A new organic small molecule using furan as a bridging group was designed and synthesized for SIBs cathode materials:2,5-bis-（p-benzoquinone）furan（named QFQ）was synthesized by bridging two p-benzoquinone units with an electron-rich furan.The introduction of the furan core provides a near-planar configuration and effectively extends theπ-conjugation of the molecule,avoiding the dissolution of QFQ in the electrolyte and thus enhancing the cycling stability of the cell.In addition,while p-benzoquinone stores Na⁺,furan can also act as p-type cathode to store anion（PF₆^-）at high potential to provide additional capacity.In half cells,QFQ provides a reversible specific capacity of 223 mAh g^-1 at a current density of 100 mA g^-1,with 97.7%capacity retention after 200 cycles.The stable cycling performance is attributed to the planar conformation of QFQ that inhibits its dissolution in the electrolytes.In addition,the storage mechanism of QFQ in which Na⁺and PF₆^-jointly provide capacity during cycling was verified by FT-IR combined with XPS.（2）Two phenazine-pyridine polymers were designed and synthesized as p-type cathode materials for sodium-ion batteries:two D-A polymers were synthesized using electron-deficient pyridine and electron-rich 5,10-dihydrophenoxazine（Pz）,the polymerization of which inhibits the dissolution of the materials in the electrolyte,and the conjugated structure of D-A enhances the intermolecular charge transfer.The difference between BPy Pz and TPy Pz lies in the different pyridine substitution sites of the two compounds,and the active site on Pz is affected by the pyridine nitrogen position and thus exhibits different electrochemical properties.The relationship between substitution sites and electrochemical performance was explored through density functional theory calculations combined with cyclic voltammetry,galvanostatic charge-discharge and other tests.Among them,BPy Pz as the cathode exhibits a reversible specific capacity of 205 mAh g^-1 at a current density of 0.5 C,and can maintain 87%of the capacity after 1000 cycles at a current density of 10 C.It provides an output voltage of 3.0 V and a specific capacity of 162 mAh g^-1 in full cells with Bi as the anode.（3）A D-A polymer ABPZ was designed as a bipolar cathode in sodium-ion batteries by the polymerization of 4,4-dibromoazobenzene and 5,10-dihydrophenoxazine.The azo group as an n-type material can provide capacity in the active cell,and the electron acceptor properties reduce E_HOMO to raise the redox potential in the phenazine.ABPZ undergoes a 4-electron redox at 1.0-4.0 V,with Na⁺and Cl O₄^-simultaneously acting as charge carriers for energy storage,provides a high specific capacity of 283 mAh g^-1 and a high energy density of 792 Wh kg^-1 in half cells.The polymerization provides a stable material with a capacity retention of 91.7%after300 cycles at a current density of 0.5 C.Based on the bipolar molecular structure of the symmetric battery,providing 66 mAh g^-1 at a current density of 100 mA g^-1 and an energy density of 112.2 Wh kg^-1.