Design And Electrochemical Properties Of Nickel-cobalt Sulphides And Their Composite Systems

Lithium-ion batteries have the advantages of high specific capacity,high voltage and wide operating temperature range.However,due to the change of the crystal structure of electrode materials and the mismatch between the electrode materials and electrolyte during charge and discharge,the lithium-ion battery do not have good cycle stability while maintaining a high specific capacity.Therefore,it is necessary to design and develop anode materials with high specific capacity and good cycle life.Nickel-cobalt sulfide because of its excellent REDOX activity;Graphene oxide(GO)is characterized by high specific surface area,rich oxygen and defects.Metal-organic frameworks(MOFs)derived materials with ultra-high porosity,rich structural diversity,high specific surface area and topologically adjustable structure are potential anode materials for lithium-ion batteries.In this paper,the pure phase of nickel-cobalt sulfide was prepared by solvothermal method,and the effect of annealing temperature on material properties was investigated.Secondly,using MOFs as the matrix material,porous nickel-cobalt sulfide@sulfur doped carbon composites with different nickel-cobalt ratios were prepared by controlling different nickel-cobalt ratios,and the influence of nickel-cobalt ratios on their electrochemical performance was studied.Using the unique properties and structural characteristics of GO to improve the electrochemical properties of nickel-cobalt sulfide@sulphur-doped carbon composites.The main research contents and results are as follows:(1)In order to obtain the pure cobalt nickel sulfide phase,A kind of spherical nickel-cobalt sulfide was successfully prepared by solvothermal method and high temperature annealing vulcanization,respectively studied the influence of different curing temperature on the structure characteristics,the ratio cycle test,under the current density of 1 A g^-1,the initial specific capacity around 1000 m Ah g^-1,but the specific capacity rapid attenuation in the later test,at the current density of 5 A g^-1,the specific capacity is only less than 100 m Ah g^-1,which is attributed to the poor cycling stability caused by the destruction of the electrode material morphology in the process of charge and discharge,which needs to be improved in the later work.(2)In order to improve the structural stability of single nickel-cobalt sulfide,porous nickel-cobalt sulfide@sulfur doped carbon composites were prepared by high temperature curing annealing treatment with MOF-74 as precursor.The effects of different nickel-cobalt ratios on their structure and properties were investigated.The porous nickel-cobalt sulfide@sulfur doped carbon composites prepared with the same nickel-cobalt ratios provided the best reversible Li⁺storage capacity.The specific capacity of charging and discharging for three times reached 999.1 m Ah g^-1at the current density of 0.1 A g^-1.After 500 cycles,the capacity retention rate is 86.8%,and the cycle stability is good.(3)In order to improve the cyclic stability of the materials,GO was introduced to improve the specific surface area and defect rate of the prepared composites by using its two-dimensional structure characteristics.Using MOF-74/GO as the precursor,the three-dimensional porous nickel-cobalt sulfide@sulfur doped carbon/rGO composites were obtained by high temperature annealing treatment.In the test of lithium-ion battery,the charge-discharge specific capacity of 0.7 GO mg m L^-1concentration sample slowly increased to about 1000 m Ah g-1 in the first 600 cycles due to the influence of activation,at the high current density of 1 A g^-1.The specific capacity was maintained at about 1000 m Ah g^-1until the 1000 turn,with excellent cycling stability.At the same time,the energy density is 73.9 Wh kg-1 when the power density is 185W kg-1 for the anode of lithium-ion battery.