Preparation Of V₂C MXene-based Electrode Materials And Their Applicatins In Eletrochemical Energy Storage

Excessive consumption of fossil energy has caused increasingly serious energy crises and environmental problems.The search for renewable resources such as solar and wind energy,the development of environmentally friendly new energy materials,and the improvement of energy conversion and utilization efficiency are hot issues of global concern.Electrochemical energy conversion and storage technology is regarded as one of the core links in the development and utilization of new energy.Supercapacitors and lithium-ion batteries have attracted the attention of researchers due to their high power density and energy density,respectively,and their performances mainly depends on the electrode materials.MXenes is a new type of two-dimensional materials.Their unique physical and chemical properties make them widely used in the field of energy storage.As a member of MXenes,V₂C MXene(V₂CT_x)has high conductivity and low ion transport barrier,and pseudocapacitive properties ascribed to the multi-oxidation states of vanadium.Therefore,it is considered to be a potential electrochemical electrode material.This dissertation takes V₂CT_x as the research object.In view of the unique structure and performance advantages of V₂CT_x and the problems of low actual lithium storage capacity and poor cycle stability in current researches,it is proposed to use nitrogen doping and cation intercalation to modify V₂CT_x.The modification aims to adjust its interlayer and surface structure,in order to improve the performance as an electrode material for supercapacitors and lithium-ion batteries,and promote the application of V₂CT_x in the field of energy storage.The main findings are as follows:(1)Using sodium fluoride and hydrochloric acid as the etchant,two-dimensional layered V₂CT_x with higher purity and regular structure was successfully prepared.The supercapacitive performance of V₂CT_x in aqueous electrolyte is significantly affected by the type of electrolytes.In Na₂SO₄ and Li Cl electrolytes,the charge storage of V₂CT_x is dominated by electric double layers;while in the H₂SO₄ electrolyte,V₂CT_x stores charge mainly through intercalation pseudocapacitance,showing excellent supercapacitor performance.V₂CT_x has a specific capacitance of 363.8 F g^-1 at a scan rate of 2 m V s^-1 in 1 M H₂SO₄.After 5000 cycles at a high current density of 10 A g^-1,the specific capacitance of V₂CT_x can still reach 289.5 F g^-1,and the capacitance retention rate is 98.87%,which is better than other MXenes materials that have been reported.(2)Using urea as the nitrogen source,V₂CT_x with different nitrogen doping contents was successfully prepared by a one-step hydrothermal method.The research results show that the hydrothermal temperature has important effets on the doping contents of nitrogen in V₂CT_x and the performances of supercapacitors.When the hydrothermal temperature is 160?,the specific capacitance of N-V₂C-160 reaches592.9 F g^-1 at a scan rate of 2 m V s^-1.After 5,000 cycles at a high current density of 10A g^-1,the specific capacitance of N-V₂C-160 is 464.4 F g^-1,and the capacitance retention rate is 90.13%,showing excellent supercapacitor performance.It is mainly attributed to higher content of quaternary nitrogen(N-Q),abundant oxygen functional groups and lower fluorine functional groups in the material.(3)V₂CT_x intercalated with different metal ions(Fe²⁺,Ni²⁺and Co²⁺)was successfully prepared through alkali treatment and cation exchange,which significantly improved the lithium storage performance of V₂CT_x.V₂C@Fe-0.05 shows the best lithium storage performance,with a specific capacity of 565.9 m Ah g^-1 after 100 cycles at 0.1 A g^-1,and a specific capacity of 301.9 m Ah g^-1 after 500 cycles at 1 A g^-1,which is 1.69 times and 1.86 times that of pure V₂CT_x,respectively.The improvement of its performance is mainly attributed to the abundant oxygen functional groups on the surface of V₂C@Fe-0.05,less fluorine functional groups,more high-valence V⁴⁺and the pseudocapacitance contributed by interlayer iron.