Structure And Interface Optimization Of Metal Oxide/Carbon-based Materials Composites And Research Of Their Lithium Storage Properties

In order to realize the wide application of lithium-ion batteries in electric vehicles and large-scale energy storage devices,the development of electrode materials with high energy density,high power density,and long cycle life is a key research topic in recent years.Metal oxides have attracted much attention due to their high theoretical specific capacities.However,the slow Li-ion/electron transport kinetics and bulky effects limit their development in practical applications.In view of the above shortcomings,the preparation of nano-structured metal oxide composite materials,rational design of material structure and enhanced electrical conductivity can effectively improve the lithium storage performance of lithium-ion batteries.The physical and chemical properties of the interface of the nanocomposite have a great influence on the electrochemical performance of the electrode material.In-depth study of the heterointerface of the composite material is beneficial to improve the lithium storage performance of the electrode.By rationally optimizing the material interface,improving the surface reactivity and electron/ion transport,the electrochemical energy storage performance of lithium-ion electrode materials can be improved.In this paper,lithium-ion batteries anode materials with high-performance are constructed based on metal oxides with high theory capacity and carbon-based materials with high conductivity.Constructing high-performance lithium ion anode materials through nanostructure design and interface optimization,we have prepared porous nanostructured MnO₂nanowires/carbon（graphdiyne,fluorinated graphene）composites,fluorine-doped Mn₃O₄/fluorinated graphene composites,fluorine-doped GeO₂nanoparticles/fluorinatedcarboncomposites,andZn₃V₃O₈nanoflower@Nitrogen-doped graphene（ZnVO@NG）composites.The main research contents are as follows:（1）The porous nanostructured MnO₂nanowire/carbon-based composite material was designed and synthesized,and it was applied to the anode material of lithium ion battery to study the lithium storage performance,electrochemical kinetics and interfacial lithium storage mechanism of the composite electrode material.（a）MnO₂nanowire/graphdiyne anode material（MnO₂/GDY）was prepared by hydrothermal method.This design utilizes the high conductivity of graphdiyne to form a three-dimensional conductive network,which can alleviate the large volume change effect of MnO₂;the unique microstructure of graphyne is beneficial for lithium ion storage,and it can improve lithium ion diffusion capacity.Electrochemical results show that the MnO₂/GDY electrode exhibits significantly enhanced rate capability,with a reversible specific capacity of 450 m Ah g^-1at a current density of 1 A g^-1after200 cycles.The charge storage mechanism at the MnO₂/GDY interface was investigated by density functional theory（DFT）,and the increased interfacial storage contribution can be attributed to the“job sharing”mechanism in the interface.The built-in electric field at the interface accelerates the transport of lithium ions.Additional Li⁺and electrons are separated and stored at the interface,enhancing the lithium storage performance of the electrode.（b）The highly fluorinated graphene was prepared by the liquid phase exfoliation method,and then the MnO₂nanowire/fluorinated graphene（MnO₂/FG）nanocomposite was obtained by the hydrothermal method.The presence of C-F semi-ionic bonds in FG endows the material with good electrical conductivity,and the constructed highly conductive network enables the composite material to have faster lithium ion/electron transport kinetics.The C-F₂bonds on the interface of the composite material reflect a certain amount of carbon vacancies in the fluorinated graphene,which can provide active sites for the intercalation of lithium ions.Besides,the covalent C-F bond and the C-F₂bond can protect and maintain the structural integrity of the electrode after long cycling.For the MnO₂/FG heterostructure,the built-in electric field constructed at the interface can accelerate the transport of Li ions.Electrochemical results show that the MnO₂/FG electrode has a reversible specific capacity of 663 m Ah g^-1after 1200cycles at 3 A g^-1.The reasonably optimized surface/interface of the material has improved surface reactivity,which improves the specific capacity of the composite material.In addition,the high electron/ion transport conductance of the composite material improves the high-rate performance of the composite material.（2）The intrinsic conductivity of metal oxides is improved by F^-ion doping and the introduction of a large number of oxygen vacancies,and the electrochemical performance of metal oxide/carbon matrix composites is improved by multiple strategies.（a）Fluorine-doped Mn₃O₄/fluorinated graphene composite（F-Mn₃O₄@FG）with abundant oxygen vacancies was prepared,and the coated fluorinated graphene can alleviate the large volume change of Mn₃O₄nanowires during the electrode redox process.The experimental results show that the F^-doping and oxygen vacancies of the F-Mn₃O₄@FG composite can improve the intrinsic conductivity of Mn₃O₄and provide more active sites for lithium intercalation.The electrochemical results show that the rate capacity of F-Mn₃O₄@FG electrode reached 400 m Ah g^-1at 10 A g^-1.The performance is higher than those of most reported Mn₃O₄-based anode materials,which is attributed to the microstructure and high electrical conductivity of F-Mn₃O₄@FG.（b）Fluorine-doped GeO₂nanoparticles/fluorinated carbon composite（F-GeO₂@C）was prepared by one-step annealing method.Through experimental characterization,it is found that F^-replaces O^2-in GeO₂,which improves the intrinsic conductivity of the GeO₂material.In addition,a large number of oxygen vacancies in F-GeO₂@C further improve the conductivity of the material.and provide more lithium intercalation sites.Density functional theory（DFT）calculations find that F-GeO₂has a non-zero electron density at the Fermi level,which confirms that F-doping greatly improve the intrinsic conductivity of GeO₂.Besides,there is a stronger interaction between GeO₂nanoparticles and fluorinated carbon,which is beneficial to ensure the stability of the electrode structure.The electrochemical results show that the F-GeO₂@C anode material exhibits excellent electrochemical performance.The specific capacity of F-GeO₂@C reaches 935 m Ah g^-1at 5 A g^-1,and its rate performance is higher than those of most GeO₂-based lithium-ion anode.（3）The Zn₃V₃O₈nanoflower@Nitrogen-doped graphene（ZnVO@NG）composite was designed and synthesized,and it was applied in the lithium ion anode material.It is found that the nanoflower-like Zn₃V₃O₈shortens the diffusion time of lithium ions,and the introduced NG has excellent electrical conductivity,which not only provides a conductive network for Zn₃V₃O₈,but also provides an elastic buffer space for it.ZnVO@NG exhibits fast lithium ion/electron transport kinetics.Meanwhile,density functional theory calculations show that the interface between ZnVO and NG provides active sites for the adsorption of Li⁺,and charge transfer occurs at the interface of the composite material,which conduces to a constructed internal electric field accelerating the transport of lithium ions/electrons.Compared with most of the previously reported Zn_xV_yO_z-based anode materials,the ZnVO@NG electrode exhibits better high-rate performance and long cycling stability.The reversible specific capacity remains 601 m Ah g^-1after 1200 cycles at 4 A g^-1.The rate capacity reaches 597 m Ah g^-1at 5 A g^-1.The excellent electrochemical performance of ZnVO@NG is attributed to the fast Li-ion/electron transport kinetics,high surface pseudocapacitive capacitive behavior,and interfacial storage of the composite.