Study On Preparation Nano Cobalt Boride Material And Its Application In Lithium Sulfur Battery

Lithium-sulfur(Li-S)batteres have attracted widespresd attention,owing to their high theoretical specific capacity,environmental benignancy,and cost effectiveness.However,the commercialization of Li-S batteries still faces many problems and challenges,especially for the"shuttle effect"and the sluggish reaction kinetics.In order to solve these obatacles,this thesis introduces three types of cobalt boride composite materials with different nanosturctures in lithium sulfur batteries.The mechanism of battery performance improvement is studied by using a variety of characterization techniques and theoretical calculations.Firstly,the Co₂B@rGO composite material is prepared by chemical reduction method.The experiments and theoretical calculations show that the p-empty orbital of B in Co₂B can accept the electrons of polysulfide ions and form B-S bond,which can restrain the“shuttle effect”.Due to the co-adsorption effect of Co and B for polysulfide,Co₂B has excellent adsorption capacity.The adsorption capacity of Co₂B with polysulfide is as high as 11.67mg/m²,which is more than twice that of the traditional polar adsorption materials.The cell of Co₂B@rGO shows a specific capacity of 1487mAh/g at 0.1C.When cycled at the rate of 1C and after 450th cycles,the specific discharge capacity of Co₂B@rGO cathode also remains at 870mAh/g and the average capacity decay rate is0.029%per cycle.Secondly,the Co₂B@MXene composite material is synthesized by chemical reduction method.Through experimental and theoretical calculations,it is found that the interfacial interaction between Co₂B and MXene will form the metal Co-Ti bonds.And it accelerates the electron transfer inside the interface.The theoretical calculations also reveal that the underlying mechanism for the electron transfer origins from different Fermi energy levels between two materials.This phenomenon leads to the decrease of electron cloud density on the surface of Co₂B,which makes polysulfides adsorbed on the surface of Co₂B more prone to bond breaking,enhancing the catalytic ability and favoring fast redox kinetics of the polysulfides.Benefiting from excellent adsorption-catalytic capacity of Co₂B@MXene,the cell with Co₂B@MXene exhibits a high initial capacity of 1577mAh/g at 0.1C and even at 5.1mg/cm² of sulfur loading,the cell with Co₂B@MXene delivers 5.2mAh/cm² at 0.2C.Next,the Co₂B@CNT composite material is prepared by template method.The co-adsorption effect of Co and B with polysulfide can effectively anchor the polysulfide on the cathode side of the separator and restrain the“shuttle effect”of polysulfide,and the CNT can improve the electronic conductivity,promote the transmission of electrons,reduce the internal resistance of the cell and improve the utilization of active materials.Meanwhile,the Co₂B@CNT modified separator has good affinity to the electrolyte.The contact angle with the electrolyte is 5.2°,the lithium ion transfer number of Co₂B@CNT modified separator is 0.93?and the Li⁺conductivity is 0.467mScm^-1.The Co₂B@CNT not only remits the"shuttle effect"of polysulfide,but also ameliorates the overall internal resistance and ion transport in the battery.The cell with Co₂B@CNT shows the initial discharge specific capacity of 1650mAh/g at 0.1C,and when the cell with Co₂B@CNT is cycled at 5C after 3000th cycles,the average capacity decay rate per cycle of the cell with Co₂B@CNT is 0.0072%.