Controllable-Synthesis,Chemical-Exfoliation And Electrochemical Properties Of New-type Niobium-based Materials

Layered Nb-based materials have been widely studied in the fields of photocatalysis and nonlinear optics due to their rich structural properties and physicochemical properties.However,the applications of Nb-based materials in electrochemical fields such as lithium-ion batteries and electrocatalytic oxygen evolution are very limited due to their inherent defects such as poor electronic conductivity.Although Nb-based materials represented by K₄Nb₆O₁₇ and Nb₂O₅ have been proved to be promising cathode materials for the next generation of fast charging lithium-ion batteries,their reversible capacity still needs to be further improved.However,there are few reports on the electrochemical oxygen evolution of Nb-based materials.In this thesis,a typical layered Nb-based material K₄Nb₆O₁₇ was studied in terms of controllable synthesis,chemical exfoliation and interlayer modification.The phase transition from K₄Nb₆O₁₇ to Nb₂O₅ was used to improve its application potential in lithium-ion batteries.The transition from exfoliated K₄Nb₆O₁₇ nanosheets to nanotubes was realized through non-topological transformation.Finally,active cations were introduced to improve the electrocatalytic oxygen evolution activity.（1）The layered K₄Nb₆O₁₇ was synthesized by one-step alkaline hydrothermal method and modified with dopamine hydrochloride（PDA）as carbon source to enhance electronic conductivity.The whole process was accompanied by chemical exfoliation and phase transformation,resulting in the final Nb₂O₅@C composite.For lithium-ion batteries（LIBs）application,Nb₂O₅@C shows excellent rate and cycle performance,with discharge capacity up to 406 m Ah g^–1 at the current density of 0.1A g^–1.It can still maintain a high value of 232 m Ah g^–1 at a high current density of2 A g^–1,and the capacity decay rate is only about 0.02%after 1000 cycles.（2）Large-size layered K₄Nb₆O₁₇ was synthesized by high temperature solid state reaction.It can be exfoliated into ultra-thin two-dimensional nanosheets using tetrabutylammonium hydroxide（TBA）,and then transformed into more characteristic one-dimensional nanotubes through non-topological transformation.The nanotube has a large specific surface area and is negatively charged.It can combine with active cations such as Ni²⁺and Fe³⁺,and thus has the ability of electrocatalytic oxygen evolution.In particular,the prepared Ni&Fe-[Nb₆O₁₇]nanotubes show the best electrocatalytic performance,with a tafel slope of 35 m V dec^–1 and an overpotential of 320 m V at the current density of 10 m A cm^–2.The controllable preparation of K₄Nb₆O₁₇ with different morphology and size can be achieved by changing the synthesis method.The electronic conductivity of K₄Nb₆O₁₇ can be improved by exfoliation and modification,so as to explore its electrochemical application potential,which provides a feasible strategy for the application of other related Nb-based materials in LIBs and electrocatalysis.