The Effective Three-layer Collector Free Cathode Design And Its Electrochemical Performance Of Lithium Sulfur Batteries

Due to the ever-increasing of social economy,serious shortage of traditional fossil fuels,environmental pollution increasing,developing a new energy is imminent.Lithium ion battery with high power density,safety,stable,green,high efficiency and no memory effect,etc.have become a new generation of high-energy green energy storage materials.Increasing the energy densities of batteries is essential to meet the requirement of electrical vehicles,electronic devices and stationary energy storage,etc.However,the commercialization of lithium-ion battery cathode material of theoretical specific capacit y is too low.Lithium-sulfur?Li-S?batteries,with a remarkably theoretical specific capacity and high theoretical energy density have attracted intensive interest.Furthermore,the abundance and non-toxicity of sulfur provide Li-S batteries with improved energy economy and environmental friendliness.However,the commerciality of Li-S battery is still hampered by several tough issues:?1?the electrically insulation of sulfur and its various discharge products?Li₂S_x,x=1-8?,which resulting in the large polarization;?2?large volume expansion?over 80%from S to Li₂S?of sulfur cathode during lithiation;?3?the formation of insoluble insulation layer on the electrode surface due to the shuttle effect of dissolved long-chain polysulfides?Li₂S_n,4?n?8?,which seriously impedes the full active material utilization.The shuttle effect is the key factor to affect the performance of Li-S batteries.In this regard,the insertion of barrier layer between sulfur cathode and separators,has been proved to be an effective and direct method to intercept the migrating polysulfides and reuse the trapped active materials,resulting in a significant suppression of shuttle effect.In the initial stage,the interlayer is always pure carbon material or metal oxides.Although inserting a function interlayer can improve the performance,but the extra interlayer will unavoidably increase the weight of cell and decrease the overall energy density.This issue is usually ignored in those publications because where the capacit ies of batteries were calculated only based on the sulfur content.When based on the total weight of the cathode,the specific capacity and energy density may be uncompetitive and undesirable.Herein,in this thesis,three-layered cathode structure was des igned to minimize the shuttle effect of polysulfides for adequate active material utilization.The self-standing TiO₂/C film is comprised of interconnected one dimensional nanofibers with TiO₂ nanocrystals embedded into porous carbon fabricated by electrospun.The dual interlayers composed of the TiO₂/C interlayer adhered on thin acetylene black coating layer on the separator were employed to promise the impenetrable immobilization for polysulfides.The active materials of sulfur and carbon?CB-S?composite slurry was directly dipped onto the barrier layers to form the self-standing three-layered cathode structure supported by separator,this design is free of Al current collector.The three-layered configuration with the Al foil current collector free can e ffectively suppress the migration of polysulfides to anode and increase the utilization of sulfur,while the conventional cell still suffers from the severe shuttle loss.As a result,the battery exhibited superior reversible capacity of 965 mAh g^-1 with an excellent Coulombic efficiency of 99.9%after 100 cycles at 0.1 C.With cycling at 0.1,0.2,0.5,1 and 2 C,the cells showed high rate capacities of 1203,1145,1035,934 and 820 mAh g^-1,respectively.Even after 300 cycles,a high reversible capacity o f 799 mAh g^-1 was still maintained at the high current density of 0.5C.The light mass of the three-layered configuration guarantees that the energy density(603W h kg^-1 after 100 cycles)is effectively improved,considering the overall mass of the cathode.Therefore,it is believed that the synergistic design for the three-layered cathode structure,which combines the mass-produced layer-by-layer structure,provides a novel protocol to the practical application of lithium-sulfur batteries.