Construction Of Hollow Structures Of Transition Metal Nickel-cobalt Compounds And Their Application In Lithium-sulfur Batterie

Lithium-sulfur batteries（LSBs）with high capacity and energy density have attracted much attention due to their abundant storage of elemental sulfur,low cost,and environmental friendliness.However,elemental sulfur and Li₂S₂/Li₂S have poor conductivity and adverse volume changes during oxidation-reduction reaction and the"shuttle effect"of polysulfide lithium（Li PSs）greatly limits their properties.In order to solve this problem,this thesis mainly focuses on the chelation between the nickel-cobalt compounds and poly-sulfide to enhance the reversible reaction of Li PSs.Based on the chemical adsorption of Li₂S_x on Lewis acid site,nickel-cobalt compounds are designed with superior adsorption performance as LSBs cathode materials as follows:（1）Ni Co-LDH with hollow dodecahedron structure was prepared by ZIF-67template to be positive material for LSBs.Adsorption tests of Li₂S₆ electrolyte showed that the material had good adsorption and catalytic conversion effects on Li PSs.The Ni Co-LDH/S battery delivers an initial discharge specific capacity of 805 m Ah g^-1 at0.2 C,and the capacity retention rate is 47.2%after 100 cycles.In 1 C high current density,the reversible specific capacity is 334 m Ah g^-1 after 300 cycles.Combined with the electrochemical impedance spectroscopy（EIS）and the coulomb efficiency of galvanostatic charge-discharge（GCD）test,it is proposed that the low electrochemical reaction efficiency of Ni Co-LDH may be caused by poor electrical conductivity.（2）Nickel-cobalt bimetallic sulfides with hollow dodecahedral structure were synthesized by hydrothermal sulfidation of Ni Co-LDH hollow dodecahedral precursors.The electrochemical properties of the Nickel-cobalt sulfide hollow dodecahedra were regulated by the Ni and Co ratio in the Ni Co-LDH precursor.The Electrochemical tests showed that the discharged specific capacity of Ni Co S₄-1/S（Ni:Co atomic ratio of 1:2）at a current density of 0.2 C decreased from 909 m Ah g^-1 to 395 m Ah g^-1 after 100cycles with a capacity retention rate of 43%.The Ni Co S₄-1/S delivers an initial specific capacity of 800 m Ah g^-1 at 1 C with a first-cycle Coulomb efficiency of 97%and a reversible capacity of 246 m Ah g^-1 after 300 cycles with a capacity retention of 31%.The Ni Co S₄-1 electrode retains excellent chemisorption properties for polysulfides and improves the catalytic conversion and electrical conductivity of polysulfides,which facilitates faster electrochemical reaction kinetics.However,the as-prepared Ni Co S₄-1with hollow dodecahedral structure shows extensive breakage or even skeleton collapse by scanning electron microscopy.Considering the physical properties of the nickel-cobalt sulfides and the inherent volume change of LSBs,the performance of nickel-cobalt sulifdes can be further enhanced by strengthening their mechanical stability.（3）The hollow dodecahedral nickel-cobalt sulfides（Ni Co₂S₄@NC）with a nitrogen-doped carbon-coated shell layer were synthesized by hydrothermal sulfidation and high temperature recrystallization/carbonization of dopamine hydrochloride coated Ni Co-LDH.The initial specific capacity of Ni Co₂S₄@NC/S（72%sulfur content）is1170 m Ah g^-1 at 0.2 C.At 1 C high current density,the capacity retention rate is 61%of Ni Co₂S₄@NC/S after 300 cycles,whose reversible specific capacity after 500 cycles is 420 m Ah g^-1,which is a great improvement in electrochemical performance compared to the nickel-cobalt sulfides.