Research On Preparation And Electrochemical Performance Of Key Materials For Lithium-sulfur Batteries

Lithium-sulfur?Li-S?batteries has regarded as one of the most promising next-generation energy-storage systems due to their exceptional theoretical specific capacity(1672 mAh g^-1)and energy density(2600 Wh kg^-1).Moreover,sulfur has the advantages of natural abundance,low cost,and environmental friendliness.However,the large-scale commercialization of Li-S batteries has been plagued by several obstacles,and the most severe ones are“shuttle effect”caused by the high solubility of polysulfides in the electrolyte and serious safety risks caused by unstable solid electrolyte interphase?SEI?and uncontrollable Li dendrites growth on Li metal anode.Aiming to address the above issues,this dissertation have conducted four aspects researches,including the structural design of sulfur cathode,electrolyte additive,functional separator and the protection of lithium anode.The major research results and discussions are listed as follows:?1?Porous carbon nanosheets?PC?and 3D interconnected porous carbon nanosheets/carbon nanotubes?PC/CNT?were synthesized by a simple one-pot pyrolysis method as sulfur host,and the sulfur-carbon composites with different sulfur contents were also prepared.The as-prepared PC and PC/CNT possess a hierarchical pore structure with micropores and macropores,which not only provides a good conductive network and pore structure for sulfur cathode,but also exhibits strong adsorption and anchoring polysulfides ability,thus effectively suppress the"shuttle effect"of polysulfides.The results demonstrated that the sulfur-carbon composite with low sulfur content exhibits better cycle stability and rate performance.In addition,3D interconnected hierarchical porous structures and conductive networks of the S-PC/CNT composite provide more pathways for rapid electrical/ionic transport and accelerate electrolyte infiltration,thus the S-PC/CNT electrode exhibits superior electrochemical performance.?2?Hierarchical nitrogen-doped carbon nanotubes/ultrathin molybdenum disulfide?MoS₂?nanosheets with a core-shell architecture was designed and prepared as sulfur host.The core-shell structure could avoid the agglomeration of MoS₂ nanosheets,and MoS₂ has strong chemical adsorption capacity and catalytic properties of polysulfides,which can improve the electrochemical performance.Both experimental investigations and theoretical studies reveal that MoS₂ nanosheets can chemically immobilize lithium polysulfides and catalyze the conversion of polysulfides.Moreover,this unique core-shell architecture could facilitate rapid electrical transport and favorable electrolyte infiltration.The results demonstrated that the S-NC@MoS₂ cathodes exhibit excellent rate capability and superior cycle stability.When the sulfur loading increased to 3.6 mg cm^-2,it still maintains high rate capability and stable cycling performance over 300 cycles.?3?A novel sandwiched gel polymer electrolyte?GPE?was designed and prepared by a facile method as separator for Li-S batteries.The good ionic conductivity and physical barrier of GPE functional separator can effectively suppress“shuttle effect”.The PVDF layers could absorb electrolyte thus enhance Li⁺transfer,and the PMMA layer can be utilized to trap polysulfides.The results demonstrated that the sandwiched GPE separator can improve the utilization of sulfur and cycle stability.?4?Pyrrole has been investigated as an electrolyte additive to trap polysulfides.When pyrrole is added into electrolyte,a surface protective layer of polypyrrole can be formed on the sulfur cathode,which not only acts as a conductive agent to to improve the utilization of sulfur but also acts as an absorbing agent and barrier layer to suppresses the"shuttle effect"of polysulfides,and further improves the electrochemical performance of Li-S batteries.The results demonstrated that an appropriate amount of pyrrole?5 wt%?added into the electrolyte leads to excellent cycling stability and rate capability.?5?The metallic Li was treated with a DMSO solution containing lithium nitrate and ammonium persulfate to form an artificial protective SEI on the surface of Li metal.Firstly,the optimal treatment time was 5 min and the optimal solution concentration was 1wt%.Secondly,the artificial protective SEI was mainly composed of Li_xSO_y and Li_xNO_y,which could effectively stabilize the interphase between electrolyte and lithium metal anode and restrain the“shuttle effect”of polysulfides.By employing the pre-modified lithium metal as anodes for Li-S batteries,the cells exhibit excellent cycle stability and outstanding rate capability.