Preparation And Electrochemical Properties Of Graphene/Metal Compound Composites

Graphene is considered to have great development prospects in the field of alkali metal-ion batteries due to its high electrical conductivity,large specific surface area,and excellent mechanical properties.To solve the agglomeration problem of the graphene electrode in the application process,it is usually compounded with other active materials to improve the capacity and maintain the cycle stability.In this thesis,SnS2 was selected as the second component,and the hydrothermal method was used to synthesis SnS2-rGO composites with different morphologies.The obtained materials were tested as anode for lithium(sodium)-ion batteries,and the main research contents are as follows:(1)Using the GO prepared by the modified Hummers method as the carbon source and PEG400 as the structure director,a flower-like SnS2-rGO electrode was fabricated by a two-step hydrothermal process.When the GO concentration is 1.5 mg mL-1,SnS2-rGO-1.5 has a capacity of 648.7 mAh g-1 after cycling 120 times at 0.1 A g-1 in lithium-ion batteries.In sodium-ion batteries,after cycling 150 times at a current density of 50 mA g-1,the reversible specific capacity is 84.2 mAh g-1.In order to further improve the reversibility of the conversion reaction of SnS2,a flower-like SnS2/C-rGO ternary composite was synthesized based on the low-temperature carbon deposition technology.In lithium-ion batteries,SnS2/C-rGO has the same capacity activation process as SnS2-rGO-1.5 anode.The reversible discharge specific capacity of SnS2/C-rGO reached 952.8 mAh g-1(0.1 A g-1)after 90 cycles,which is mainly caused by the synergistic effect of carbon layer and graphene that improving the reversibility of conversion reaction and electrochemical performance.In sodium-ion batteries,after cycling 100 times at a current density of 50 mA g-1,the specific capacity of SnS2/C-rGO is 198.4 mAh g-1,and the corresponding capacity retention rate is 46.88%.Although the capacity is improved compared to SnS2-rGO-1.5,it still cannot be commercialized,which is mainly caused by the larger volume change when sodium ion insertion,resulting in the destruction of the material structure.(2)In order to mitigate the volume effect of SnS2,nano-particle SnS2-GNS composite and SnS2 and GNS contrast samples were synthesized by a simple one-step solvothermal method.During preparation,the introduction of graphene can induce the generation of SnS2 nanoparticles,forming a three-dimensional network structure where SnS2 nanoparticles anchored on the graphene nanosheets.In lithium-ion batteries,0.8-SnS2-GNS displays a capacity of 1250.8 mAh g-1 when cycled at 0.1 A g-1 for 150 times,and the coulombic efficiency is close to 100%,which is significantly better than other reported SnS2-based anode materials.In sodium-ion batteries,0.8-SnS2-GNS owns a capacity of 510.2 mAh g-1 after 100 cycles at a current density of 50 mA g-1.Furthermore,by analyzing the CV images of 0.8-SnS2-GNS at different scanning rates,it is found that the pseudocapacitance plays a significant role in the electrochemical performance.(3)In order to further improve the rate performance,the electrical conductivity of the anode is improved by constructing a heterostructure.Cubic-like SnS2/Co3S4-rGO composite and SnS2/Co3S4,SnS2,SnS2-rGO contrast samples were synthesized by the coprecipitation method followed by a simple solvothermal method.In lithium-ion batteries,40-SnS2/Co3S4-rGO displayed a reversible charge specific capacity of 1315.5 mAh g-1 when cycled 140 times at 0.1 A g-1.The excellent performance can be ascribed to the energy storage of graphene and the heterostructure of SnS2/Co3S4-rGO.The electric field existed in the heterostructure can accelerate the transmission of ions,and the heterogeneous interface can effectively fix the intermediate products that produced during the discharge process and prevents sulfides from reacting with the electrolyte,thereby improving the reversibility of SnS2/Co3S4-rGO.In sodium-ion batteries,40-SnS2/Co3S4-rGO displays 984.3 mAh g-1 after 100 cycles when tested at 0.1 A g-1,with a capacity retention of 75.19%.Furthermore,the composite also exhibits an ultrafast sodium storage capability where 392.9 mAh g-1 can be obtained at 10 A g-1 and the charging time is less than 3 minutes which is beneficial to the commercial application of sodium-ion batteries.