Application Of Nano-Carbon Functional Materials For High Performance Lithium-Sulfur Batteries

To solve the ever-increasing problems in resources and environment,developing the battery energy storage systems has become an important part of harnessing renewable clean energy.Li-S batteries are ones of the most attractive and promising next generation secondary battery systems to meet the demands for EVs and large-scale energy-storage systems.However,Li-S batteries are still facing some problems such as the insulating nature of elemental sulfur and the highly solubility of lithium polysulfides in organic electrolyte.To solve the problems mentioned above,by applying the highly conductive nano-carbon functional materials and organic polymer materials with special functions on restricting the shuttle of polysulfides,we successfully improve the overall electrochemical performances of the Li-S batteries.The main research contents are summarized as follows:?1?High performance sulfur electrode material has been prepared by sequentially coating polyelectrolyte multilayers and graphene sheets on the surface of hollow carbon spheres/sulfur composite with a fast and robust electrostatic layer-by-layer self-assembly strategy.The layer-by-layer assembled polyelectrolytes and the graphene sheets display superior synergistic effect on electrochemical improvement of the electrode.The possible reasons for the prominent impact are as follows.Firstly,the polyelectrolyte multilayers with high density of ionized SO₃^-groups can act as a blocking shield to confine the S_n^2-anions within the hollow carbon by electrostatic repulsion.Secondly,the graphene sheets with high surface area can greatly increase the conduction efficiency of the electrons among the carbon/sulfur particles.Thirdly,the highly flexible polyelectrolyte multilayers and graphene sheets can work together to accommodate the volume expansion of the electrode and maintain its compatibility.Based on the multifunctional layer-by-layer film,the prepared sulfur cathode exhibits very stable cycling stability for over 200 cycles at 1 A g^-1,along with a high average coulombic efficiency of over 99%.?2?Coral-like,nitrogen and sulfur co-doped mesoporous carbon has been prepared by a facile hydrothermal-nanocasting method to house sulfur for Li-S batteries.The lagre numbers of doped species enable this carbon matrix to suppress the diffusion of polysulfides,while the interconnected mesoporous carbon network is favourable for rapid transportation of both electrons and lithium ions in the electrode.Based on the synergistic effect of N,S co-doping and the mesoporous conductive pathway,the as-fabricated C/S cathode yields excellent cycling stability at a current rate of 4 C with only 0.085%capacity decay per cycle for over 250 cycles and ultra-high rate capability(693 mAh g^-1)at 10 C rate.?3?Polydopamine and polyethylene imine enhanced graphene film is prepared by a vacuum filtration method and applied as a supporting material for Li-S battery separator.The organic polymer materials can introduce large numbers of functional groups with specific adsorption ability into the film,and therefore the film with high adsorption surface area can effectively restrict the shuttle of the polysulfides.Moreover,the high conductive film can also act as a second current collector and increase the conduction ability of the sulfur cathode.The test battery based on a pure sulfur electrode and polymers enhanced graphene interlayer shows good electromecial performance.The maximum discharge capacity can reach 1103 mAh g^-1 at a discharge rate of 0.1 C and capacity of 786 mAh g^-1can be retained after 100 cycles.