Application Of Biomass Carbon/Transition Metal Sulfide As Cathode Materials For Li-S Batteries

With the continuous development of the new energy era,electrochemical energy storage and conversion has become the focus of building a low-carbon harmonious society.The depletion of fossil fuels and the development of smart grids and portable electronic devices require storage devices with high energy density,high stability and high electromotive force.Lithium-sulfur（Li-S）batteries stand out among many energy storage devices due to theirs high theoretical specific capacity（1675 m Ah/g）and energy density（2600 Wh/kg）,which have attracted the attention of researchers.In addition,sulfur,as an active substance,has the advantages of low cost,safety and environmental protection,which can well meet the needs of sustainable energy development.However,due to the poor conductivity of the active substance sulfur and the solid product of the reaction（Li₂S_n,n=1-3）,the volume shrinkage and expansion caused by the positive electrode of the battery during the reaction,and the"shuttle effect"caused by the polylithium sulfide（Li PSs）,the battery has a low utilization rate of sulfur,low cycle stability,and poor rate performance.To solve the above problems,the effective way to improve the electrochemical performance of Li-S batteries are to fix Li PSs effectively.Based on the conductivity of biomass carbon materials and the high catalytic activity of transition metal sulfide positive electrode composites were designed and prepared.The main contents are as follows:（1）The high conductivity tubular biomass carbon（HPOC）was obtained from the chamellae wool by activation of H₃PO₄and carbonization at high temperature in tubular furnace,which was used as the substrate for the growth of sheet 1T-WS₂.Using WCl6 as tungsten source,1T-WS₂@HPOC composite material was prepared by one-step solvothermal method by growing 1T-WS₂sheet vertically on HPOC,and1T-WS₂@HPOC/S cathode material was obtained by loading sulfur（S₈）.The product was characterized by SEM,EDX,XRD and XPS,and it was found that oxygen-containing functional groups in carbon substrate could provide directional growth sites for 1T-WS₂.1T-WS₂crystals were grown in a directional arrangement.1T-WS₂@HPOC/S,as the cathode material of lithium sulfur battery,has an initial specific capacity of 1272 m Ah/g at0.1C.After 500 cycles of 0.5C current density,the specific capacity attenuates from the initial 922 m Ah/g to 695 m Ah/g,and the coulomb efficiency is above 95%.The capacity decay rate per turn is about 0.056%.（2）Then,using cheap and easily available grass flower as raw material,the biomass material was activated by alkaline reagent（KOH）and carbonized at high temperature to prepare three-dimensional porous biomass carbon material（KMC）,which provided active sites for the directed growth of terylene transition metal sulfide（Mo S₂-Ni S₂）nanosheets.The KMC/Mo S₂-Ni S₂composite was prepared by solvothermal method,and the KMC/Mo S₂-Ni S₂/S electrode material was obtained by loading S₈by thermal melting method.The composite was characterized by SEM,XRD,TEM and EDS,and it was found that the addition of porous carbon material（KMC）inhibited the agglomeration of sulfide nanosheets and made them uniformly anchored to KMC.By CV,EIS and battery charging and discharging tests,it is found that the constructed KMC/Mo S₂-Ni S₂/S electrode material has very low electrochemical polarization and electrochemical impedance,and has an initial specific capacity of 1292 m Ah/g at 0.1 C current density.After 400 cycles at 0.5 C,The specific capacity of KMC/Mo S₂-Ni S₂/S attenuates from an initial 1007 m Ah/g to 814m Ah/g with a coulomb efficiency greater than 96%and a capacity attenuation rate of about0.054%per turn.It still has a specific capacity of 586 mAh/g after 450 cycles at 1 C.