Design And Fabrication Of High-performance Cobalt Based Anode Materials And Study On Lithium/Sodium Storage Properties

Lithium-ion batteries（LIBs）are widely used in various electronic devices due to the high working voltage,high energy density and long cycle life.However,the energy density of LIBs has reached to its upper limit due to the limitation theoretical capacity of commercial graphite anode.Compared with LIBs,sodium-ion batteries（SIBs）have greater cost advantages in the field of large-scale energy storage devices owing to its rich resources and low price.But the research on anode materials for SIBs is also trapped in the identical dilemma as LIBs.Therefore,the development of an high efficient anode materials to solve the dilemma of LIBs and SIBs has become the focus of current research.As a typical conversion anode material,cobalt based compounds are considered as a potential anode material due to the high theoretical capacity.Unfortunately,the practical applications of cobalt based compounds are largely hampered because of the sluggish ion diffusion kinetics and large volume deformation during charge/discharge.Some modification strategies can endow materials with unique properties,thereby improving their electrochemical performance.In this thesis,cobalt based compounds are selected as research object.Through limiting the material size,element doping,constructing composite and designing unique nanostructures to improve the lithium/sodium storage capability.In addition,the reasons for performance improvement are explored in depth via density functional theory calculation.Hierarchical porous structure Co₃O₄ nanosheets（P-Co₃O₄）were fabricated by solvothermal method.Then,based on the fact that Co₃O₄ is a p-type semiconductor and Ti O₂ is an n-type semiconductor.A layer of Ti O₂ is introduced on the surface of P-Co₃O₄ to construct p-n junction.The construction of p-n junction can induce a built-in electric field to accelerate the ion migration and enhance the rate performance.In addition,the volume expansion of Co₃O₄ was well alleviated by Ti O₂ layer and porous structure,which improves the cycling stability.The discharge capacities of P-Co₃O₄/Ti O₂,P-Co₃O₄ and Co₃O₄ were 1065,619 and 135 m Ah g^-1 after 100 cycles at a current density of 0.2 A g^-1,respectively.Moreover,the reversible capacity of P-Co₃O₄/Ti O₂ can still be maintained at 801 m Ah g^-1 after 1600 cycles at a current density of 2 A g^-1.Cu²⁺-doped Co₃O₄ hollow spheres were prepared by a solvothermal method.Then,a nitrogen doped carbon（NC）layer was coated outside the Co₃O₄ using Si O₂as template.And a void structure was constructed between Co₃O₄ and NC layer.The effect of Cu²⁺-doping on the morphology and performance of Co₃O₄ was investigated.Optimal amount of Cu²⁺-doping was determined by comparative experiments.Theoretical calculation,XRD and XPS confirm the position of Cu²⁺in the Co₃O₄.In addition,the effect of void structure and Cu-doping on the Li⁺/Na⁺storage performance of Co₃O₄ were systematically investigated.Experiment results show that void structure can alleviate the volume deformation of Co₃O₄,improving the cycling stability.Theoretical calculation confirms that Cu²⁺-doping can promote the electronic conductivity of Co₃O₄ enhancing the rate performance.The composites deliver a Li⁺storage capacity of 544 m Ah g^-1 after 1000 cycles at a current density 5A g^-1.The Na⁺storage capacity is maintained at 254 m Ah g^-1 after 500 cycles at a current density 2 A g^-1.Three dimensional structure Se/N co-doped carbon framework encapsulated CoSe₂ nanocrystals（CoSe₂@3DSNC）were prepared by metal nitrate induced polymer blowing-bubble strategy.Se/N co-doping was confirmed by the XRD,TEM and XPS characterizations.The interlayer distance of carbon layer was successfully expanded from 0.348 nm to 0.432 nm due to the Se-doping.Theoretical calculation show that the expanded interlayer distance can effectively reduce the migration energy barrier of Na⁺and enhance the rate performance.Additionally,the encapsulation effect of carbon framework buffers the volume deformation of CoSe₂,improving the cycling performance.CoSe₂@3DSNC delivers excellent Li⁺/Na⁺storage performance.The sodium storage capacity was up to 338 m Ah g^-1 at a current density of 10 A g^-1.ZnSe/CoSe₂@NC-NT/NRs anode material was synthesized by metal organic frameworks derived selenization strategy.ZnSe/CoSe₂@NC-NT/NRs show hierarchical branched architecture composed of carbon nanorods and carbon nanotubes.ZnSe/CoSe₂ nanoparticles were encapsulated into the inner of carbon nanorods and the end of carbon nanotubes.Hierarchical branched architecture carbon framework could not only inhibit the volume expansion,but also provide numerous paths for ion migration.The purpose of selecting ZnSe and CoSe₂ as integrated components to construct heterojunction is that there is a large band gap difference between ZnSe and CoSe₂,which can induce a strong built-in electric field to promote the migration of ions and electrons.Theoretical calculation shows that the ionic migration energy barriers at the heterointerfaces is smaller than that in ZnSe and CoSe₂ bulk phase.ZnSe/CoSe₂@NC-NT/NRs deliver better lithium/sodium storage capability than most reported CoSe₂ based anode materials.