Design And Charge-discharge Mechanism Of Sulfur-carbon Electrocatalyst

Lithium-sulfur battery has become one of the energy storage systems with great potential due to its advantages of excellent theoretical specific energy and theoretical specific capacity and low cost of the positive sulfur electrode.However,its commercial application is hindered by shuttle effect and slow charging and discharging kinetics of lithium polysulfide.The basic reason lies in the electric insulation and slow redox kinetics of the conversion products of the positive sulfur electrode.In recent years,defect engineering has shown great potential in promoting the transformation of lithium polysulfide intermediates in lithium-sulfur batteries with high energy density.However,the research on its catalytic mechanism is still not clear.In this paper,electrocatalysts rich in defects were prepared by hydrothermal synthesis,hydrogenation reduction and in situ selenization,aiming at developing sulfur-carbon cathode materials for lithium sulfur batteries with stable structure and excellent charge-discharge kinetics.The role of defect engineering in improving the redox reaction kinetics and inhibiting the shuttle effect was studied by combining theoretical calculation and electrochemical measurement.The specific research contents are as follows:（1）Terminal sulfur atoms formation via defect engineering strategy to promote the conversion of lithium polysulfides.In order to solve the problem of unclear catalytic mechanism in defect engineering,oxygen-deficient Li₄Ti₅O_12-x hollow microspheres uniformly coated with N-doped carbon layer（OD-LTO@NC）were prepared by using hydrated Li₄Ti₅O₁₂ hollow microspheres as precursor through dopamine coating and hydrogen reduction,and their catalytic mechanism was studied.Theoretical simulation shows that the presence of oxygen deficiency enhances the adsorption capacity of spinel Li₄Ti₅O₁₂ for soluble lithium polysulfide.On the surface of Li₄Ti₅O₁₂ with oxygen defects,part of-S-S-of Li₂S₆ is broken by strong adsorption force,which converts inert bridging sulfur atoms into sensitive end sulfur atoms,and reduces the activation energy of polysulfide conversion to a certain extent.In addition,secondary hollow microspheres constructed from primary ultrathin nanosheets provide a large number of voids and active sites for sulfur storage,adsorption and transformation in the nitrogen-doped carbon layer.The sulfur anode thus designed exhibits a considerable rate performance of 547 m Ah g^-1 specific capacity at 4 C(1 C=1675 m A g^-1)and an excellent long cycle performance of 519 m Ah g^-1 specific capacity at 1000 cycles at 3 C.（2）Bidirectional catalyst with in-situ selenium defects promotes bidirectional redox conversion of sulfur species.In view of the problem that there are few researches on the charging process and oxidation kinetics of lithium sulfur battery by defect engineering,the Gibbs energy spectrum of the reduction and decomposition process of Li₂S on VSe_2-x was studied for the first time by theoretical simulation,and the adsorption effect of VSe_2-x on Li PSs and the bidirectional catalytic effect of S₈ to Li₂S were found from the perspective of energy.In order to verify the design idea,the layered nanosheet materials（NC@VO,NC@MVO-VSe,NC@VO-VSe,NC@VO-MVSe）with different oxide selenide ratio were cleverly designed.Because it contains enough VSe_2-x components,NC@VO-VSe has strong adsorption capacity for Li PSs,and has bidirectional catalytic effect on sulfur reduction and sulfur oxidation.The charge and discharge dynamics of Li-S cells with positive S/NC@VO-VSe electrodes are fundamentally improved(excellent rate performance at 2 C with 693.7 m Ah g^-1 and significant long-term cycle capacity of 400 cycles at 1 C with 2.8 mg cm^-2 sulfur).