Synthesis And Performance Of Two-dimensional Nanomaterials For Lithium-sulfur Batteries

Lithium–sulfur battery（Li–S）is regarded as one of the most promising next-generation energy storage systems owing to its extremely high theoretical capacity,natural abundance of sulfur,low cost,and environmental benignity.However,the shuttle effect of intermediate lithium polysulfide renders the battery with low capacity and short cycle life,which limits its development.In order to pursue the excellent electrochemical performance of Li–S batteries,the research was devoted to solving the above problem by designing and synthesizing novel two-dimensional nanomaterials for separator modification.Besides,the anchoring effect and catalytic mechanism on polysulfides of two-dimensional nanomaterials with different properties were further explored.The main research contents and results are as follows.Siloxene nanosheets（Si OX）with rich oxygen-containing functional groups on the surface were combined with reduced graphene oxide（r GO）by electrostatic self-assembly to construct an ordered structure,which was used as the separator modification layer of Li–S battery.The unique and rich surface functional groups exposed by Si OX show strong adsorption ability for lithium polysulfides,the introduction of highly conductive r GO and the structural properties of the composites are beneficial to electron/ion migration.The shuttle effect of lithium polysulfide in batteries with Si OX/r GO modified separator was effectively inhibited,and the electrochemical conversion kinetics was improved.The discharge capacity of the first cycle can reach 1411.9 m Ah g^-1 at 0.1 C,the capacity can still remain at 621.3 m Ah g^-1after 500 cycles at 1 C,and the coulombic efficiency of the battery is always close to 100%.Considering the limited adsorption ability of surface functional groups and the interface contact issues of composites,the WS2 nanosheets with high content of 1T phase and 2H phase with basal plane S vacancies were obtained by lithium ion intercalation induced exfoliation method and high temperature phase transition method,respectively.The adsorption ability on lithium polysulfies of WS2 with phase,edge sites and S vacancies was investigated by DFT calculation.It is found that the phase plays a key role in the adsorption and catalytic conversion of polysulfides.Benefiting from the high conductivity,strong polysulfide trapping ability and abundant catalytic active sites of 1T-WS2,the reversible specific capacity of the battery can reach 1008.8 m Ah g^-1 at1 C,and the average capacity decline is only 0.039%after 1000 cycles.Under the sulfur load of 2.5 mg cm^-2,highly reversible capacity of 727 m Ah g^-1 is still remained after300 cycles at 1 C.Heterostructure and doped atoms were constructed to improve the kinetics of polysulfide conversion reaction,expanding the conductive interface and adsorption sites of two-dimensional nanomaterials.MXene with anchored Co nanoparticles was obtained by molten salt etching,and then the strongly coupled nitrogen-doped heterostructure was achieved by in-situ sulfidation.The electrochemical results show that the nitrogen-doped heterostructure reduces the polarization of lithium polysulfide redox reaction and promotes the redox reaction kinetics.Benefiting from the synergistic effect of adsorption-catalytic conversion,the battery achieved high initial specific capacity(1031 m Ah g^-1 at 1 C),excellent cycle stability(721.6 m Ah g^-1 after 400cycles at 2 C)and excellent rate performance(775 m Ah g^-1 at 4 C).