Preparation And Electrochemical Properties Of Niobium-tungsten-titanium Oxide Anode Materials

For a successful transition from internal combustion engines to electric vehicles and from conventional power plants to renewable energy supply,battery technology plays a vital role.In the past few decades,great attention has been paid to the research and development of lithium-ion batteries,especially in battery materials,which has led to the great improvement of the electrochemical properties of lithium-ion batteries.However,at present,graphite is still the most commonly used negative electrode material.It is imperative to find a negative electrode material that can replace graphite because graphite cannot meet the current needs for rapid charging and discharging.Recently,the niobium tungsten oxide material has become one of the ideal alternative materials because of its high-rate performance and high stability.However,it still exhibits relatively low specific capacity.In this paper,the structure characteristics and synthesis method of Nb₁₄W₃O₄₄ are firstly summarized and analyzed,and then the synthesis conditions of Nb₁₄W₃O₄₄ material by hydrothermal method was studied in detail.Furthermore,in order to improve the capacity of Nb₁₄W₃O₄₄ material,Nb₁₄W₃Ti_xO_44+2x material was synthesized by solid phase method.The application of Nb₁₄W₃Ti_xO_44+2x material in the field of supercapacitors and lithium-ion batteries was also discussed.The main research contents and achievements are as follows:In the first part of the work,the precursors of niobium tungsten oxide were synthesized by hydrothermal method using niobium oxalate and ammonium tungstate as raw materials,and then calcined to obtain Nb₁₄W₃O₄₄ material.The particle sizes of Nb₁₄W₃O₄₄ material synthesized by hydrothermal method are reduced to 1-2?m compared with the 3-5?m of particles synthesized by the traditional solid phase method,but the its structure remains unchanged.At the same time,the hydrothermal method greatly reduced the calcination temperature and duration,thus effectively saving energy.Through the analysis of its electrochemical properties,the optimal conditions for the synthesis of Nb₁₄W₃O₄₄ was determined:hydrothermal reaction at 200°C for 24 h,pre-calcination at 700°C for 6 h and calcination at 1050°C for 4 h.Among them,hydrothermal conditions have an important influence on the particle size of the final product,while calcination temperature greatly affects the purity of the product and its cyclic specific capacity.In half-cell batteries,pure Nb₁₄W₃O₄₄phase has a high reversible specific capacity of 172 m Ah g^-1 at 0.5C,and can maintain a specific capacity of 30 m Ah g^-1 at 100 C.In addition,the capacity retention rate can reach 97.2%(106m Ah g^-1)after 1000 cycles at 10 C.In the second part of the work,in order to overcome the shortcomings of the low capacity of Nb₁₄W₃O₄₄ material,Ti elements were incorporated to Nb₁₄W₃O₄₄,and the obtained Nb₁₄W₃Ti_xO_44+2x material show higher specific capacity than Nb₁₄W₃O₄₄.In addition,the suitable content of Ti in Nb₁₄W₃Ti_xO_44+2x was explored.The results show that Nb₁₄W₃Ti₂O₄₈present the best electrochemical performance.The capacity of the Nb₁₄W₃Ti₂O₄₈material reaches 239.8 m Ah g^-1 0.5 C,and the initial coulombic efficiency is 95.3%.During 1000 cycles at 10 C,its capacity still maitains125.3 m Ah g^-1,and coulombic efficiency is still close to 100%.At a high rate of 100 C,it still has a capacity of 29.2 m Ah g^-1,indicating that it is a new type of high rate and high cycle anode material.This provides a basis for the subsequent research of micro-sized electrode materials.In the third part of the work,the application of Nb₁₄W₃Ti₂O₄₈ anode material for supercapacitors were explored.A button battery is assembled by using Nb₁₄W₃Ti₂O₄₈ as anode material and the activated carbon as cathode material.By exploring the effect of the negative/positive electrode(N/P)ratio on the electrochemical properties,the most appropriate ratio of N/P is 1:2.The Nb₁₄W₃Ti₂O₄₈//AC supercapacitor still has a capacity of 75 m Ah g^-1after 1000 cycles at 1 C,and the retention rate reaches 84%.This provides some insights for the application of niobium-tungsten oxide in hybrid capacitors.