Controllable Preparation And Modification Of Biomass Porous Carbon/sulfur Composite Cathode Materials For Lithium-sulfur Batterie

In recent years,lithium-sulfur batteries have received much attention because of their high theoretical specific capacity and energy density,economic and extensive sulfur resources.They are the best alternative to lithium-ion batteries and have great development prospects.However,the commercialization of lithium-sulfur batteries is severely hindered by their inherent drawbacks:the poor conductivity of sulfur and its reduction products leads to a poor electrochemical performance and slow down the reaction kinetics;polysulfides are easily soluble in liquid electrolytes,leading to shuttle effect and resulting in self discharge and capacity attenuation;the huge volume expansion of sulfur during charging process will cause irreversible structural damage of the electrode and rapid capacity decay;the polysulfide that reaches the anode surface will corrode the lithium surface.Therefore,the rational design and preparation of composite cathode is worth studying.In this thesis,we aim to prepare a porous carbon/sulfur composite cathode with lower price,facile composite method and excellent electrochemical performance.An appreciate carbon skeleton was prepared using biomass materials with special microstructure.The composite method was determined under the guidance of first principles study.The electrochemical performance of the battery was improved by doping and modification.The results of the study are as follows.1.Porous carbon skeleton suitable for lithium sulfur battery was prepared,and then composite cathode were synthesized.Porous carbon was prepared by using natural pipe shape corn silk as carbon source under the experimental conditions of different activators,different amounts of activators and different temperatures.The surface morphology of the prepared porous carbon were compared to determine the most appropriate manufacture conditions:K₂CO₃was used as the activator with a3:1 ratio at 800°C.The first principle was used to study the sulfur adsorption capacity of carbon surface.The result indicated that the S₈molecule tends to be adsorbed onto the carbon porous surface rather than agglomerate themselves.So a facile and effective dissolution-crystallization strategy of composite carbon to sulfur was selected.The influence of different sulfur content on the electrochemical performance of carbon/sulfur composite cathode was investigated to assure the 70%sulfur content.The initial discharge capacity can reach 980 m Ah g^-1at 0.1C.2.The influence of sulfur crystal morphology on the properties of composites was studied.The synthesis of sulfur carbon composites under different stirring speeds,temperatures and vacuum circumstance was experimented to clarify that sulfur crystals loaded at stirring speeds of 600 rpm,synthesis temperatures of 40°C,and vacuum environments can yield nanometer-sized particles and be distributed uniformly on the surface of the carbon.The composite cathode material exhibits higher electrochemical performance.3.The electrochemical properties of the composites were improved by doping the carbon skeleton with Fe,N and Fe-N co-doping.The mechanism of doping’s effect on material properties was examined by using the first principle study.The findings demonstrate that nitrogen doping can enhance the ability of carbon materials to adsorb Li₂S₆of 0.548 ev,and iron doping can reduce the decomposition energy barrier of Li₂S of 5 ev and alter the diffusion path of Li⁺.The amount of doping was determined by comparing the physical phase,morphology and electrochemical properties.The capacity of the composite cathode rose by 187 m Ah g^-1and 115 m Ah g^-1at 0.1C when doped with half melamine and one third FeCl₃·6H₂O.4.The separator was modified to further improve the electrochemical performance of the battery.The commercial separator was modified with corn silk-based porous carbon.The shuttle effect of Li PSs was prevented by establishing a physical barrier,and the specific capacity of the battery were improved to 1286m Ah g^-1.Another porous carbon modified separator was prepared by replacing part of the conductive agent with graphene oxide,and the composite cathode expressed the specific capacity of 1497 m Ah g^-1because of the further chemical adsorption of Li PSs based on the physical barrier.The utilization of sulfur reached 90%.