Design And Preparation Of Heteroatom-doped Carbon Nitride For Separator Modification Of Lithium Sulfur Batteries

Lithium-sulfur（Li-S）batteries are highly attractive as one of the most promising energy storage systems owing to their superior theoretical capacity,low cost and environmental compatibility.Nevertheless,the low utilization of active ma terials and detrimental shuttle reactions severely inhibit the practical application of Li-S batteries.This paper aims to suppress the active material loss and rapid capacity decay caused by the free migration of polysulfides in lithium sulfur systems.A series of boron-doped carbon nitrides were designed and prepared to minimize the shuttle effect by strongly chemical adsorption between the polysulfides and doped carbon nitrides.The main research work is shown as follows:（1）A lightweight 2D boron doped g-C₃N₄ nanosheets（BCN）with abundant active sites prepared by a facile route is introduced onto commercial separators to achieve high performance Li-S batteries.The boron doping not only can increase surface area and improve electrical conductivity,b ut also chemically anchor more polysulfides due to the formed B-N bond,which can effectively minimize the shuttling of polysulfides through the synergistic effect of physical yield and chemical confinement.As a result,the assembled Li-S batteries employing BCN separators with multifunction display large discharge capacity of 1197.0 m Ah g^-1,high sulfur utilization and outstanding durability with a capacity decay rate of 0.09%per cycle at1 C after 500 cycles,which are also supported by the density fun ctional theory simulation.Additionally,the alleviated self-discharge behavior and good areal capacity(up to 6.4 m Ah cm^-2)at a high sulfur loading of the cell are also demonstrated.The exploration of BCN modified separator furnishes a viable way to construct high energy density and long lifespan Li-S batteries.（2）Boron doped g-C₃N₄ and reduced graphene oxide composite（BCNG）was prepared by directly pyrolyzing dicyandiamide and graphene oxide at 550℃under an inert atmosphere.The effect of doping temperature on Li-S battery performance was studied.The as-prepared two-dimensional BCNG was then directly filtered on the separator as modified layer for electrochemical testing.The lithium sulfur battery with BCNG₄₀₀ modified layer delivers a large speci fic capacity of 737.7 m Ah g^-1 at 3C and also shows good rate and cycling performance.（3）Boron-doped carbon nitride nanodots and nitrogen-doped reduced graphene oxide composites（BCND/N-r GO）were prepared by using N-doped reduced graphene oxide（N-r GO）as a cooperative adsorption conductive carrier under a one-step hydrothermal reaction.The as prepared BCND/N-r GO was directly filtered on one side of the separator,which was then used separator to assemble lithium sulfur batteries.The Li-S battery with BCND/N-r GO modified separator shows good cycle stability and delivers a large specific capacity of 858.3 m Ah g^-1 after 200 cycles at 0.5C.