Preparation Modification And Electrochemical Properities Of T-Nb₂O₅ Anode Marerials For Sodium Ion Batteries

The lithium ion batteries have been used into the portable electronics and electric vehicles with the advantages of the high energy density,low self-discharge,long service life,no memory effect,and so on.However,the limited content and uniform distribution confine the wide application of the lithium ion batteries.The sodium ion batteries can replace the lithium ion batteries,because the sodium element is abundant in the earth,and the physical and chemical properties of the sodium element is similar to the lithium element.Therefore,the sodium ion batteries are an ideal replacement for lithium ion batteries.Unfortunately,we cannot copy anode materials used to lithium ion batteries into sodium ion batteries,because the radius of sodium ion is 1.02?,which is larger than lithium ion of 0.76?.Therefore,it is necessary for us to develop appropriate anode materials to promote the development of the sodium ion batteries.Compared to the alloy-type and conversion-type materials,intercalation-type anode materials are particularly promising in mitigating this challenge,because they can reversibly host additional Na⁺ions without structural disruption.Therefore,intercalation-based materials can be expected to realize commercialization.In this thesis,T-Nb₂O₅@CNTs composites are synthesized through the two-step method.The carbon nanotubes play essential roles in improving the electrical conductivity and reducing the size of the T-Nb₂O₅,which is favor for shortening the diffusion distance of the ions.All of those mentioned above can promote the electrochemical performance of the T-Nb₂O₅@CNTs composites.The specific capacity can be up to 205.2 mAh g^-1,110.6 mAh g^-1 at 0.05 A g^-1,3.0 A g^-1,respectively,which is superior to the electrochemical performance of the pure T-Nb₂O₅.Meanwhile,T-Nb₂O₅@CNTs composites exhibit an excellent cycling stability,and the specific capacity can still be up to 98 mAh g^-1 after cycling 500 cycles with the capacity retention of 57%.In conclusion,the excellent electrochemical performance of T-Nb₂O₅@CNTs composites can be contributed to outstanding electrical conductivity of CNTs and 2D tunnels in the T-Nb₂O₅,which can be favor to the ion's transportation.Based on the chapter mentioned above,we further synthesized the PNb₉O₂₅@CNTs composites with the red phosphorus served as the phosphorus source.The structure of the PNb₉O₂₅ is built from the NbO₆ octahedra edge-connected and a fraction of the cavities occupied by NbO₆ octahedra/PO₄ tetrahedra.Moreover,the structure of the PNb₉O₂₅ exists amounts of the type-III and type-IV cavities,which is favor to the transportation of the ions.The PNb₉O₂₅@CNTs composites regarded as the anode materials of the sodium ion batteries,exhibit excellent electrochemical performance.The coulombic efficiency of the PNb₉O₂₅@CNTs composites is 83%at 0.05 A g^-1,and the specific capacity is 283.1 mAh g^-1,252.4 mAh g^-1,125.4 mAh g^-1,111.2 mAh g^-1 at 0.05 A g^-1,0.1 A g^-1,3.0 A g^-1,5.0 A g^-1,respectively.When the current changes from the large current density of 5.0 A g^-1 to 0.05 A g^-1,the specific capacity still be obtained of 220.9 mAh g^-1with the capacity retention of78%.In addition,the PNb₉O₂₅@CNTs composites also exhibits the excellent cycling stability with the specific capacity of 85 mAh g^-1after cycling 500 cycles which is superior to the pure PNb₉O₂₅ with 53.5 mAh g^-1.