Strategies To Improve The Cyclability Of Organic Anode Materials In Sodium-ion Batteries

The development of portable electronic devices,large-scale smart grids and renewable energy has created new opportunities and challenges for energy storage technology.Due to the advantages of abundant resources and low cost of sodium,the sodium ion battery provides a new scheme for energy storage.However,due to the large ion radius of Na⁺,the structure of the electrode material will collapse and crush when it is disintercalated.Therefore,people have turned their attention to flexible organic electrode materials.However,the poor cycle stability of soluble organic compounds during the charge/discharge process limited further electrochemical performance research and practical applications.To solve this problem,from the perspective of molecular design,we used polymerization and organic-inorganic hybridization to inhibit the dissolution of the known electrochemically active organic small molecules,and improve their cyclic stability in the charge/discharge process.The main contents of this paper are as follows:(1)The benzothiadiazole small molecule contains two conjugated C=N bonds,which can reversibly gain and lose electrons during the charge/discharge process,and has electrochemical activity.However,benzothiadiazole is easily soluble in organic solvents.Therefore,we used 4,7-dibromo-2,1,3-benzothiadiazole as the raw material,and obtained the poly(benzothiadiazole)by the coupling dehalogenation polymerization method of aromatic halogen compound and Ni(0)complex.The extended conjugated system was beneficial for maintaining the structural stability of the electrode material during the charge/discharge process and improved the cycle stability.The electrochemical test results showed that the poly(benzothiadiazole)electrode exhibited 91%capacity retention rate after 100 cycles at a current density of 0.1 A g^-1.and had good rate performance.However,the coulombic efficiency was not stable during the long cycles.(2)Tetrathiafulvalene(TTF)small molecules can be used as electrode materials because of their reversibly redox ability.However,TTF is easily soluble in organic solvents,causing rapid capacity decay.In order to solve this problem,we used organic-inorganic hybridization strategy to synthesise the?-d conjugated coordination polymer(tetrathiafulvalene-tetrathio nickel)by one-pot method,with1,3,4,6-tetrathio-cyclopentadiene-2,5-dione as starting material.Compared with the organic polymerization method,after organic-inorganic hybridization,not only the solubility of the material was inhibited but also the electrical conductivity was improved,which was due to the?-d interaction between the organic/inorganic components.The electrochemical test results showed that the tetrathiofulvalene tetrathionide nickel electrode exhibited 90%capacity retention ratio after 100 cycles at the current density of0.1 A g^-1,and maintains a coulombic efficiency of 99%for 500 cycles at the current density of 1 A g^-1,with almost no fluctuation.The cycle stability and rate performance were better than the previous work.