Flexible Sulfur Cathode Materials And Separators For Lithium-Sulfur Batteries

Lithium sulfur?Li-S?batteries are considered to be a promising candidate for the next novel energy storage system,which possesses many merits of high theoretical specific capacity,high energy density,environmentally friendly and low cost.With the rapidly developing of flexible wearable electronics,ever-increasing attention has been attracted on developing flexible energy storage devices with high energy density.To tackle the two key problems of sulfur cathode of low sulfur utilization and lithium polysulfides?LiPSs?shuttling,this dissertation started the researches from two aspects of sulfur cathode and separator.In the aspect of sulfur cathode,a high flexible,free-standing hierarchical porous carbon nanofiber?G/NPCFs?was designed as sulfur carrier.Then,ultrafine polar titanium dioxide?TiO₂?particles were further used to optimize the carbon nanofiber,which results in improving of the electronic conductivity and LiPSs chemical adsorption ability of the sulfur cathode.In the aspect of separator,two modified layers of intercalation-structured Fe-N-doped porous carbon nanofiber/G?Fe-N-C/G?and close-packed structured manganese dioxide?MnO₂?were designed via physical or chemical modified method,respectively.These measurements aim to suppress the LiPSs shuttling and enhance the chemical reaction kinetics.Finally,the electrochemical performance of Li-S batteries was improved.1.A high flexible,free-standing hierarchical porous carbon nanofiber/sulfur composite?S/G/NPCFs?was designed via electrospinning.To enhance the structural stability and the total pore volume of the nanofiber,the silica?SiO₂?nanoparticles and graphene oxide?GO?nanosheets were adopted to modify the nanofiber.Thereby,the sulfur loading capability and the mechanical tensile strength of the nanofiber were enhanced.Furthermore,the SiO₂ can promote the conversion of amorphous carbon to graphitic carbon during carbonization of polymer fiber,which enhances the electronic conductivity of the carbon nanofiber.As a result,the S/G/NPCFs exhibited a specific discharge capacity of 540 mAh g^-1 at 5 C.A decay rate of0.047%per cycle was achieved at 5 C after 500 cycles.2.Based on the above research,polar TiO₂ was employed to modified the carbon nanofiber,and a TiO₂ and G co-modified porous carbon nanofiber/sulfur composite was obtained?S/TiO₂/G/NPCFs?.The sulfur dispersion and storage of the nanofiber were improved by controlling the structure and composition of the nanofiber,which leads to enhance the utilization and mechanical strength of the sulfur cathode.The obtained S/TiO₂/G/NPCFs possesses high electrical conductivity and strong LiPSs chemical adsorption ability.As a result,S/TiO₂/G/NPCFs exhibited a high specific capacity of 1501 mAh g^-1 at 0.1C and 668 mAh g^-1 at high current rate of 5 C.A decay rate of 0.074%per cycle was achieved at 1 C after 500 cycles.3.A porous Fe-N-C nanofiber was designed via electrospinning.The Fe-N-C nanofiber and G were used to design a intercalation-structured modified layer?Fe-N-C/G?for Li-S battery separator via vacuum filtration.The Fe-N-C/G modified layer can not only suppress the LiPSs shuttling,but also improve the electrolyte wettability and electrical conductivity of the interface between sulfur cathode and separator,which results in fast Li ion transport and high sulfur utilization.As a result,the cell configured with Fe-N-C/G@PP separator can maintain a high specific capacity of 601.9 mAh g^-1 at 0.5 C after 500 cycles,and the corresponding decay rate is 0.053%per cycle.Moreover,a high specific capacity of 847.9mAh g^-1 can be reserved at high current rate of 2 C.4.A chemical,self-assembled method was designed to fabricate ultra-stability and ultra-light modified separator for Li-S battery.The strong oxidizing property and self-decomposition characteristic of potassium permanganate?KMnO₄?were used to activate PE separator.A MnO₂ modified PE separator?MnO₂@PE?was fabricated.The thickness of the MnO₂ modified layer is only 380 nm,and the areal mass loading is 0.014 mg cm^-2.The MnO₂ modified layer can not only improve the electrolyte wettability and electrical conductivity of the interface between sulfur cathode and separator,but also catalytically promote the conversion of soluble LiPSs to insoluble polythionate complex,which can efficiently suppress the LiPSs shuttling and improve the electrochemical performance of Li-S battery.As a result,the cell configured with MnO₂@PE separator can maintain a capacity of603 mAh g^-1 at 0.5 C after 500 cycles,and the corresponding decay rate is 0.059%per cycle.Moreover,a high specific capacity of 689 mAh g^-1 can be reserved at high current rate of 3 C.