| The continuous development of science and emerging technologies including electric vehicles（EVs）and smart grids urgently boosts the need for electrochemical power sources with high energy densities.Nowadays,Li-ion batteries cannot meet the requirements of high energy density batteries any more due to the limitation of their theoretical capacities.The theoretical specific capacity of lithium-sulfur battery is much higher than that of traditional Li-ion batteries,and it is considered to be the most promising secondary lithium battery system.However,a series of problems,such as Li dendrite and polysulfide shuttling,still hinders its practical application.In this thesis,new electrolyte systems are developed to suppress the growth of lithium dendrite and improve the interfacial properties as well as the electrochemical performance of lithium-sulfur cells.Meanwhile,S@pPAN composite cathode material is adopted to avoid the shuttle effect.Thus,the cell cycle performance is greatly improved.The relevant research results are as follows:1.A new gel-like electrolyte（LiPF6/EC-DMC+8 wt.%Al2O3）based on fumed alumina nanoparticles has been developed.The proposed electrolyte not only maintains high ionic conductivity,but also creates a favorable SEI layer containing LiAlF4 on both the Li anode and the S@pPAN cathode,providing lower impedance and better protection.Due to the good compatibility between the gel-like electrolyte and Li anode,the Li|Li symmetric cell could stably cycle for 1400 hours at a current density of 0.25mA cm-2 with dendrite-free Li deposit.What’s more,the cycle performance and rate performance of the Li-S@pPAN cells are remarkably enhanced.the capacity retention maintains 92.2%even after 350 cycles with the high average coulombic efficiency（ca.99.8%）.In addition,the anti-flatulence performance of the soft pack cells also demonstrates the potential advantage in terms of safety.2.The feasibility of applying 4,5-dicyano-2-trifluoromethyl lithium（LiTDI）to Li-S@pPAN cells has been examined.Due to the special structure of the TDI-,the proposed electrolyte shows high lithium transference number(tLi+=0.62).Under the synergistic effect of FEC and VC,a denser and stable interface is formed and the overall impedance is reduced.Besides,the cycle performance of lithium metal is greatly improved with high deposition-dissolution coulombic efficiency（93%）.As a result,the cycle performance of Li-S@pPAN cells is remarkably enhanced with 99.6%average coulombic efficiency after 450 cycles. |
