Synthesis And Electrochemical Characterization Of Organosulfur Polymer Composite Materials For High Performance

With the rapid development of the global economy,urgent demands for energy storage devices have caused people to pay more attention to the development of the rechargeable batteries with higher energy density and lower cost.In addition to the high capacity(1675 m Ah g^-1),sulfur cathode also has the advantages of high natural abundance and low cost,well meeting the future demands for next-generation battery.Therefore,Li-S batteries and room temperature sodium-sulfur batteries(RT Na-S batteries)have been considered as the most promising next-generation battery systems.However,sulfur cathode suffers from the poor conductivity of element sulfur and its discharge products(Li₂S),the high solubility of the intermediate polysulfides and the large volume change during charge-discharge process,which lead to low active material utilization and poor cycling stability.The organosulfur materials can suppress the dissolution of polysulfide and boost the electrochemical conversion of elemental S on the molecular,providing great research thinking of addressing the above problems.Therefore,this thesis focuses on the design of high-performance organosulfur cathodes.Combined with the modification of electrolyte,two kinds of organosulfur cathode materials are synthesized.The two composites are used to construct the cathodes for RT Na-S battery and Li-S battery,respectively.The main contents and results in this work are as follows:(1)The sulfur-polyacrylonitrile(SPAN)composite is used as the cathode for RT Na-S battery with ether-based electrolyte with fluoroethylene carbonate(FEC)as an additive.During the initial discharge,the nucleophilic reactions between polysulfides and FEC molecule in-situ form a protective cathode electrolyte interphase(CEI)film on the surface of SPAN electrode,which can effectively eliminate the further dissolution of soluble polysulfides,thereby realizing a“quasi-solid”conversion of sulfur.The cycle stability of SPAN cathode in the ether-based electrolyte is greatly improved:it cycles stably for over 200 cycles with a capacity decay as low as 0.23%per cycle at 0.2 A g^-1and a high coulombic efficiency close to 100%.(2)Due to the low sulfur content of SPAN composite limited by its molecular structure,a new organosulfur composite with triallyl isocyanurate(TAIC)as precursor is synthesized.The resulted product(STI)can achieve a high sulfur content of 94.1 wt%.To further improve the rate capability,carbon nanotubes(CNT)are introduced to construct the composite with STI.The three-dimensional conductive network prepared by CNT improves the ionic and electronic conductivity.Thus,with 5 wt%addition of CNT,the STI composite cathode can achieve good cycle stability and rate capability.The resulting STI/CNT-5 composite reveals an initial reversible capacity of 980.9 m Ah g^-1 at 0.2 C.After 300 cycles,the capacity still remains 513.7 m Ah g^-1 with a capacity decay as low as 0.16%per cycle.The synthesis method is simple and the raw materials are low cost,which has great potential for mass production and displaying high performance in terms of high sulfur loading.