Preparation Of Carbon Coated Metal Chalcogenide Nanomaterials For Storage Of Sodium/Potassium

Sodium/potassium belong to the same family as lithium,they have similar chemical properties.However,natural resources of sodium and potassium are much richer,more widely distributed,and cheaper compared to lithium.Therefore,sodium/potassium ion batteries?SIBs and PIBs?would replaced lithium ion batteries?LIBs?in the field of energy storage.Exploring suitable electrode materials is always the key to realize the commercialization of SIBs and PIBs.Metal chalcogenides have received a great deal of attention for their high theoretical capacity and abundance of resources.However,the main obstacles faced by these materials as anodes are large volume expansion and poor conductivity.At present,the preparation of various of nanostructured metal chalcogenization combined with carbon materials is an effective strategy for improving the electrochemical performance of the battery,representing direction of development of battery materials.In this thesis,metal chalcogenides and carbon materials were chosen as research objects.We focused on exploring simple synthesis method,and constructing carbon-metal chalcogenide composites with different nanostructures by flexible and reasonable design.On the other hand,we also explored the carbon materials by heteroatom dropped to improve its capacity.It mainly included the following four works:?1?Graphene/porous FeS₂/carbon?rGO@p-FeS₂@C?composite.We used porous SiO₂ as the template to distribute FeS₂ nanoparticles within graphene oxide matrix and carbon layers with large void volume.Nanostructured FeS₂ nanoparticles were advantaged for shorting the diffusion distance of soudium/potassium ion and enhancing the numbers of active sites.The better conductivity of the graphene and carbon layer can improve the conductivity of the system and accelerate the transportation of the ions and electrons to achieve improved capacity and excellent rate performance.In addition,the void space in the composite can reserve a certain space for the expansion of FeS₂,and the carbon layer can prevent the aggregation of FeS₂.Therefore,the composite exhibited high capacity and superior rate performance as anode material for SIBs and PIBs.As anode material for SIBs,at the current density of 0.1 A g^-1,the capacity maintained at 598 mA h g^-1 after 100 cycles,and still had a high capacity 428 mA h g^-1at a high current density of 10 A g^-1.For PIBs,it also exhibited excellent rate performance,and the capacity was 298 mA h g^-1 at 2 A g^-1.?2?Nickel sulfide/carbon?Y-S NiS_x@C?composite.The Y-S NiS_x@C composite prepared by simple etching and sulfuration methods.Small sized NiS_x nanoparticle was encapsulated into two-dimensional nanosheets with sufficiently large space between the nanoparticle and the carbon shell.The composite showed extremely high capacity and ultra-long cycle performance as anode of PIBs.At a current density of 0.1 A g^-1,it can obtain a capacity 300 mA h g^-1.It also presented excellent rate performance,the capacity was 232mA h g^-1 at 2 A g^-1.The battery still maintained stability after 8000 cycles at 0.5 A g^-1,much superior than the most reported anode materials of PIBs.The outstanding performance was mainly due to its novel yolk-shell structure.The enough internal space can effectively buffer the volume expansion of the active material;the nanostructured NiS_x and thin carbon coating layer contributed to rapid diffusion of potassium ions;the carbon coating layer can also improve conductivity of the composite and prevent the active material from agglomerating.Thereby,structural stability and electrochemical reaction kinetics of the composite were enhanced.?3?Cobalt selenide/carbon/single-walled carbon nanotubes?CoSe₂@C@SWNTs?composite.In this section,we used ZIF-67 as a precursor to fabricate CoSe₂@C composite twined by SWNTs by pyrolysis,electrostatic adsorption self-assembly and selenization methods.As for this composite,CoSe₂ particles were embedded in the carbon matrix,which can buffer the volume expansion and contraction to prevent pulverization and agglomeration;the three-dimensional conductive network formed by SWNTs can enhance the conductivity of the system and accelerate the conduction of ions and electrons.Therefore,CoSe₂@C@SWNTs,as anode for PIBs presented excellent cycle and rate performance.At 0.1 A g^-1,the specific capacity was 247 mA h g^-1 after 100 cycles,and at 2A g^-1,the rate capacity was 133 mA h g^-1.?4?Sulfur-doped graphene/hollow carbon?S-G@HC?composite.On graphene substrate,we in-situ constructed hollow carbon structure which can suppress stacking of graphene sheets,besides,the conductivity of amorphous hollow carbon can also be improved.The composite possessed large specific surface area and pore volume,thereby capacity and rate performance were enhanced compared to a single carbon material.In addition,sulfur doping can increase the defects of the system,reduce the adsorption energy of potassium ions and improve the conductivity,to further boost electrochemical performance of the composite.As anode for PIBs,S-G@HC composite showed superior cycle stability and rate performance.The capacity remained stable at 306 mA h g^-11 after 300cycles at 0.1 A g^-1.The number of cycles can be 2000 at 2 A g^-1.In addition,at a high current density of 10 A g^-1,the rate capacity remained at 211 mA h g^-1.