The Investigation Of Niobium-based Oxides As Lithium-ion Battery Anode Material

While lithium-ion batteries are already making our lives healthier,smarter and cleaner by powering wearable devices,electric vehicles and grid energy storage,the need for significant improvements in battery performance is growing as electronic devices continue to evolve.The development of new anode materials is one of the most promising ways to meet these requirements and solve the problems existing in graphite anode（low safety performance and poor rate capacity）.Recently,niobium-based oxides anode materials have received widespread attention due to their high specific capacity,high safety,superior cycle stability,and significant pseudocapacitance behavior.However,niobium-based oxides still have the electrochemical kinetics problem of poor electronic/ionic conductivity,which greatly limits their practical applications.Therefore,micro-nano structured niobium-based oxides were synthesized through morphology energineering to improve the dynamics problem.The main work is as follows:1.The microspheres V₄Nb₁₈O₅₅with porous micro-nano structure(V₄Nb₁₈O₅₅-P)were synthesized by adjusting V:Nb atomic ratio to 2:9.V₄Nb₁₈O₅₅-P exhibits porous microspheres which is composed of numerous primary nanoparticles with sizes of60～100 nm.The micro-nano structure of the material exhibits a larger specific surface area and provides a high pseudocapacity contribution rate of 89.1%,which helps to improve the rapid charge/discharge characteristics.Meanwhile,the nano-size of the primary particles greatly reduces Li-ion diffusion length,which can enhance the rapid ion transport during the lithiation/delithiation process.The lithium-intercalation mechanism of V₄Nb₁₈O₅₅-P and the small volume change during the cycle ensure the cycle stability of the battery.Based on the above structural advantages,V₄Nb₁₈O₅₅-P electrode provides the reversible capacity of 186.6 m Ah g^-1at 10C and the reversible capacity of 76.3%was retained after 500 cycles.Moreover,V₄Nb₁₈O₅₅-P shows an extremely high rate performance.When the current density increases to 80C,V₄Nb₁₈O₅₅-P electrode still retains a reversible specific capacity of 144.0 m Ah g^-1,which is higher than the rate performance of most niobium-based oxides reported at present.2.Porous microspheres VNb₉O₂₅with micro-nano structure(VNb₉O₂₅-30h)were prepared by adjusting the V:Nb atomic ratio to 1:9.In addition,by optimizing the annealing process,abundant oxygen vacancies were introduced,and the prepared VNb₉O₂₅-30h obtain excellent LIB performance.Here,the VNb₉O₂₅-30h composites showed advanced rate capability with reversible capacity of 122.2 m Ah g^-1at 4 A g^-1,and delivered excellent capacity retention of～100%after 2000 cycles.Meanwhile,VNb₉O₂₅-30h provides unexpected long-cycle life(i.e.,reversible capacity of 165.7 m Ah g^-1at 1 A g^-1with a high capacity retention of 85.6%even after 8000 cycles).Additionally,coupled with the Li Fe PO₄cathode,the Li Fe PO₄//VNb₉O₂₅-30h full cell delivers superior LIB properties with high reversible capacities of 91.6 m Ah g^-1at 5C for1000 cycles.The superior performance of VNb₉O₂₅-30h is attributed to the hierarchical structure,which can enhance the contact of active material/electrolyte and shorten the Li⁺diffusion distance,while the abundant pores provide sufficient reserved space to buffer volume changes during the cycle.In addition,the introduction of oxygen vacancies can increase the reaction sites and the intrinsic conductivity of VNb₉O₂₅-30h.3.Due to the existence of three free electrons in the 3d orbital of Cr³⁺,Cr Nb₄₉O₁₂₄has a higher conductivity than other niobium-based oxides.Cr Nb₄₉O₁₂₄microrods(Cr Nb₄₉O₁₂₄-M)and Cr Nb₄₉O₁₂₄porous microspheres(Cr Nb₄₉O₁₂₄-P)were prepared by sol-gel method and hydrothermal method,respectively.Compared with Cr Nb₄₉O₁₂₄-M,porous microspheres Cr Nb₄₉O₁₂₄-P have larger specific surface area,shorter lithium ion diffusion distance and larger active substance/electrolyte contact area.Cr Nb₄₉O₁₂₄-P electrode provides initial discharge/charge capacity of 287.9/265.7 m Ah g^-1at 0.1A g^-1,with super high initial Coulomb efficiency of 92.3%.In addition,Cr Nb₄₉O₁₂₄-P electrode provides a reversible specific capacity of 218.2 m Ah g^-1at 1A g^-1,which reaches to 152.2m Ah g^-1after 1000 cycles,showing a good electrochemical performance.