Preparation,Modification And Electrochemical Properties Of Niobium-Base Oxide Anode Material For Sodium-Ion Batteries

In the past decades,lithium-ion batteries have been widely studied for their high energy density.However,the scarcity of lithium resources has limited the large-scale application of lithium-ion batteries.In order to solve this problem,many researchers have devoted their attentions to the study of sodium-ion batteries.The key technology for the development of sodium ion battery is the development of high specific energy sodium storage materials.The sodium storage anode materials with high capacity and excellent cycling performance are one of the important components.The crystal face（001）of Nb₂O₅ anode materials has a large lattice spacing,which facilitates the rapid diffusion of sodium ions.However,the Nb₂O₅ material has a low electronic conductivity,which hinders the development of this material.Therefore,in this paper,HNb₃O₈ nanosheets were synthesized by a simple hydrothermal method,and then the Nb₂O₅ anode material was obtained by calcination.The electrochemical performance of Nb₂O₅ anode material was further optimized by metal ion doping(W⁶⁺and Mo⁵⁺)and conductive substance C composite.Its sodium storage performance was studied and the results are as follows:（1）The HNb₃O₈nanosheet materials were prepared by hydrothermal method.The Nb₂O₅ anode material was obtained through calcining the HNb₃O₈nanosheet materials at300°C,500°C and 700°C,which was denoted as Nb₂O₅-300,Nb₂O₅-500 and Nb₂O₅-700.At the calcination temperature of 300°C,the XRD pattern of Nb₂O₅-300 is consistent with that of the HNb₃O₈ nanosheets,indicating that the HNb₃O₈ nanosheets cannot convert into Nb₂O₅ at the condition of 300°C calcination.When the calcination temperature was 500°C,Nb₂O₅-500 was pseudo-hexagonal crystalline Nb₂O₅（TT-Nb₂O₅）,and the crystallinity of the Nb₂O₅-500 material was poor.When the temperature was increased from 500°C to 700°C,the crystal structure of the material changed from TT-Nb₂O₅ to T-Nb₂O₅,and the intensity of the diffraction peaks increased,indicating that the crystallinity of the Nb₂O₅material increased with the calcination temperature increase.The constant current charge/discharge test showed that the Nb₂O₅-500 nanosheet material has good cycling stability and multiplicative performance.The Nb₂O₅-500 nanosheet material was also found to have a high sodium ion diffusion coefficient accroding to the EIS test.It shows that the Nb₂O₅material has better electrochemical properties when the calcination temperature is 500°C.（2）The electronic conductivity of Nb₂O₅ nanosheet materials was improved by tungsten-doped and carbon composited,thus improving the electrochemical properties of Nb₂O₅ materials.W-Nb₂O₅/C nanocomposites were synthesized by a secondary hydrothermal method.The experimental results demonstrated that the 5%W-Nb₂O₅/C-9composites had the best electrochemical properties.A discharge specific capacity of the material reached 191.0 m Ah g^-1 at a current density of 100 m A g^-1 in the voltage range of0.01-3V and its capacity retention rate reached 70.2%after 100 cycles.At a current density of 500 m A g^-1,the discharge specific capacity remains 130.7 m Ah g^-1 after 500 cycles.Compared with the unmodified Nb₂O₅ material,W⁶⁺doping can enhance the specific capacity of the electrode material,and carbon composite can enhance the electronic conductivity of the Nb₂O₅nanosheet material,resulting in good cycling performance and excellent rate performance.（3）The electronic conductivity of Nb₂O₅ nanosheet materials was improved by molybdenum-doped and carbon composite,thus improving the electrochemical properties of Nb₂O₅ materials.Mo-Nb₂O₅/C nanosheet composites were synthesized using a secondary hydrothermal method.The experimental results demonstrated that the 3%Mo-Nb₂O₅/C-8composites had the best electrochemical properties.The discharge specific capacity was172.8 m Ah g^-1 with a capacity retention rate of 84.9%after 100 cycles at a current density of100 m A g^-1 and a voltage range of 0.01-3.0 V.At a high current density of 500 m A g^-1,a discharge specific capacity of 119.2 m Ah g^-1 was achieved after 500 cycles,with a capacity retention rate of 76.0%.