Design Of Separators And Solid Electrolytes And Investigation Of Ion Transfer Characteristic For Lithium Sulfur Batteries

As a new secondary battery device,Lithium–sulfur?Li-S?batteries have the advantages of high energy density and low cost,showing great application potential in energy storage field.The multi-electron reaction characteristic endows Li-S batteries with high theoretical capacity,while causes the complexity of reaction system.In Li-S batteries,the shuttle of dissolved polysulfides leads to rapid capacity fading and poor cycling life.All-solid electrolytes without polysulfide shuttle are expected to completely solve these problems.However,due to the low ionic conductivity and high interfacial impedance,current solid electrolytes cannot satisfy the requirements of solid Li-S batteries.Based on the key issues,especially on the polysulfide shuttle in liquid electrolytes and the poor Li⁺conductivity of solid electrolytes,this study mainly aims at optimizing the ion transport properties in Li-S battery system and reveals the relationships between structure design and transport properties.Some new strategies are proposed,which are expected to control the ionic transfer process and support the design of high-performance Li-S batteries.Main conclusions are as follow:?1?Design of ion selective separator for liquid Li-S batteries.Based on electrospinning,a new modified separator with unique fiber-network structure and abundant anion group was prepared by lithiation of Nafion/poly?acrylic acid?fiber mat on a commercial separator.In physical structure,after a simple lithiation treatment,the fibers are fused with each other to form a relatively compact network with semifused pores,which effectively inhibits polysulfide shuttle,while retaining the low-barrier Li⁺transfer channels.In chemical environment,based on electrostatic interactions,abundant electronegative groups on the separator strongly repel anionic polysulfides,and simultaneously act as transfer sites for Li⁺.Therefore,the modified separator highly inhibits polysulfide shuttle and enhances Li⁺conduction on the synergistic effect of a physical structure and chemical interaction.The assembled Li-S battery achieves excellent cycle performance.The capacity retention is as high as 77%after 100 cycle at 1 C,and the corresponding capacity decay is 0.023%per cycle.?2?Intensification of Li⁺transfer process in composite solid electrolyte.Based on breath figures method,the polyethylene oxide/Li_0.34La_0.56Ti O₃?PEO/LLTO?composite solid electrolyte was synthesized by calcinating a honeycomb polystyrene template with LLTO precursor followed by a PEO matrix casting.As a perovskite-type Li⁺fast ionic conductor material,honeycombed LLTO framework is designed to build a continuous,low-barrier,vertical Li⁺transfer channel.In addition,the continuous interface between PEO matrix and LLTO matrix contributes to Li⁺transfer.Therefore,the PEO/LLTO composite electrolyte reaches a superior ionic conductivity of1.05×10^-3 S cm^-1 and a high Li⁺transference number of 0.57 at 60 ^oC.Besides,the honeycombed LLTO framework is beneficial to Li⁺uniform deposition during charging,which avoids the growth of lithium dendrites and significantly improves the stability of lithium cathode.The Li-S battery with PEO/LLTO electrolyte obtains decent electrochemical performances and runs stably for more than 100 cycles with a coulombic efficiency above 99%at 60 ^oC and 0.05 C.The initial and reserved discharge capacities reach 1234.6 m Ah g^-1 and 907.6 m Ah g^-1,respectively.