Synthesis And Lithium Storage Performance Study Of Nano-Li₂S/Graphene Composites

In the past decades of years,energy shortage problems have become increasingly serious,thus development and utilization of various clean and economical power sources is hot spot in current.High energy-density battery is considered to be one of the most efficient energy storage ways to solve the problem.Now more recent interest has tended to focus more on the use of lithium sulfide?Li₂S?cathode materials as a way of eliminating the problems which existed in the Li-S battery.it represents quite a promising candidate for use as a cathode material in high-energy secondary lithium ion batteries.Unfortunately,Li₂S also has a characteristic of poor electrical and ionic conductivity.What's more,there is a large potential barrier?about 1.0 V?during the initial charge process to fully convert Li₂S into sulfur.This paper reviews the progress of research and new characteristics of Li₂S cathode materials,improving the cathode materials current defects as the purpose of this study,prepare new graphene/Li₂S composites materials,and research the microstructure and electrochemical performance.The details summarized as follows:?1?In this work,nano Li₂S was prepared by the reaction of elemental sulfur with lithium triethylborohydride in tetrahydrofuran?THF?,and then core-shell nano Li₂S@Li₃PS₄?denoted as NLPS?particles were prepared by the reaction of nano Li₂S and P₂S₅,after that,the as prepared NLPS coated 3D doped graphene aerogels as a self-supported cathode through a facile liquid infi ltration–evaporation coating method for high performance Li-S batteries.In the composite,nano-scaled Li₂S with high crystallinity means fast electrode kinetics,short ionic and electrical migration distance,and superior structural stability during cycling.The formation of superionic conducting Li₃PS₄ coating layer could significantly improve the ionic conductivity.Moreover,strong solvent adsorbency of GA guarantees the uniform distribution of NLPS nanoparticles inside the three-dimensional conductive network,which would create shortened ionic/electron transport pathways and then reduce the energy barrier effectively.With these inspiring merits,the NLPS/GA composite exhibits much smaller potential barrier?².45 V?during the initial charge process and lower overpotential at the following cycles than the micrometer-sized composite.Moreover,a high discharge capacity of 934.4 mAh·g^-1 can be obtained at 0.1C rate,and the value retains 485.5 mAh·g^-1 after 100 cycles,demonstrating its promising specific capacity and good cycle stability as cathode for Li–S batteries.?2?We use a one-step hydrothermal method synthesis of three-dimensional porous graphene aerogels/sulfur nanocrystals.And then LiEt₃BH/THF was droped into graphene aerogels/sulfur and form in-situ graphene aerogels/lithium sulfide.Through the characterization and analysis of composites microstructure,lithium sulfide contenting,bonding interaction and lithium sulfide and electrochemical performance to investigate the effect of in-situ method to electrochemical performance.The results show that the in-situ GA/Li₂S composites with high lithium sulfide contenting,which is attributed high sulfur content in GA/S.What's more,bonding interaction between GA and Li₂S is C-S bond,C-S bond could prohibit the polysulfide dissolution into the electrolyte.Meanwhile,the specific surface area of the in-situ GA/Li₂S composites is much higher than ex-situ GA/Li₂S,making the better contact between composites and electrolyte.Last but not the least,the ulsmall Li₂S and uniform distribution of Li₂S nanoparticles would create shortened ionic/electron transport pathways and then reduce the energy barrier effectively.?3?The mixture of graphene oxide/tetrahydrofuran and sulfur/carbon solution were reacted with lithium triethylborohydride/tetrahydrofuran to obtain graphene/lithium sulfide composite material by one-step solvothermal method.This is the first time to produce lithium sulfide composite materials by the solvothermal method.The Li₂S/GNS composites have a large specific surface area?223 m²/g?,which result in the composite material can have sufficient contact with the electrolyte.Secondly,the nanoscale Li₂S size obtained in this chapter is only 5⁸ nm,which is smaller than the size of the previous two chapters,which result in the first charge activation voltage only 2.4 V in the composite,the barrier is almost zero,far less than the potential barrier of the ex-situ GA/Li₂S and in-situ GA/Li₂S.And small particle size Li₂S can shorten the ion/electron transport path,making the material has the best rate performance.In addition,the Li₂S/GNS composites was prepared by the one-step solvothermal method,the graphene and Li₂S were bonded with the C-S bond,so that the material has a capacity retention rate of 54.7%after 100 cycles.Compared with the in-situ method GA/Li₂S obtained in Chapter 4,the Li₂S/GNS composites in this chapter have better electrical performance than those of Li₂S/GNS composites,which can be attributed to the one-step solvothermal method can effectively avoid the Li₂S active material?High loading?to ensure uniform dispersion of Li₂S on the surface of graphene,while promoting the reduction of graphene oxide and the high conductivity of the final product.