Synthesis,Modification And Electrochemical Performance Of Transition Metal(Nb,Co)Oxides Nanosheets

Transition metal oxides are expected to become the next generation of lithium ion battery electrode materials due to their high abundance,low cost,high theoretical capacity and high electrochemical stability.However,the problems such as poor conductivity and large volume change during the cycles seriously hinder its practical application.In order to solve the above problems,this thesis successfully prepared Nb₂O₅ nanosheets and porous Co₃O₄ nanosheets,and carried out liquid phase reduction modification to study the effect of defect structure on electrochemical performance.Firstly,the Nb₃0₇F precursor was prepared by hydrothermal method.Then,the phase composition of Nb₂O₅ nanosheets was controlled by adjusting the calcination temperature.The results show that the three-phase composite Nb₂O₅ nanocomposites with orthogonal phase,tetragonal phase and monoclinic phase have excellent electrochemical properties.The reversible capacity is 270.4 mAh g-1 at a current density of 1 C for 200 cycles.The excellent electrochemical properties are attribute to the large number of interfaces and defects generated by the three-phase composite structure,which promote the transportation of lithium ions and increase the electrochemical active sites.The phase content of the three-phase composite Nb₂O₅ nanosheets was adjusted by adjusting the calcination time.After calcination at 750℃for 4 h,the samples obtained exhibited better electrochemical performance.And the reversible capacity after 200 cycles at current density 1 C is 320.9 mAh g-1,which even under the condition of 5C still maintains 205.6 mAh g-1 after 2000 cycles.Secondly,the Nb₂O₅ nanosheets were subjected to NaBH₄ liquid phase modification treatment.The morphology and phase composition of the modified samples did not change.Oxygen vacancies were formed on the surface,and the amount of oxygen vacancies increased with the increase of NaBH₄ concentration.However,when the concentration of NaBH₄ solution was 5 mM,it showed better electrochemical performance.By changing the concentration of NaBH₄ solution,the oxygen vacancies content can be well regulated.And the appropriate oxygen vacancy concentration is most beneficial to the improvement of electrochemical performance.Finally,Porous Co₃O₄ nanosheets were successfully obtained by hydrothermal method and calcination treatment.And Co₃O₄ nanosheets were chemically reduced with different amounts of N-₂H₄ solution.The morphology of the samples after treatment was not changed.Oxygen vacancies were generated on the surface of Co₃O₄ nanosheets,and increased with the increase of the amount of N-₂H₄ added.When N-₂H₄ was added in a large amount,Co（OH）₂ was produced on the surface.The Co（OH）₂ has a certain protective effect on the electrode and improves the cycle stability.By changing the amount of N-₂H₄ added,the oxygen vacancies content and the formation of Co（OH）₂ can be well controlled.Reasonable surface modification engineering can create unique microstructures which enhances electrochemical performance.