High-Rate Ti-based Anode Materials In Sodium Ion Batteries

In recent years,sodium-ion batteries have gradually become one of the most promising energy storage systems due to the advantages of abundant sodium resources and similar working principles to lithium-ion batteries.The development of anode materials with high rate,long life,high tap density and low cost is of great significance for the development of sodium ion batteries.Among them,Ti-based materials show excellent electrochemical performances and great application potential,because of their good stability,moderate working voltage,and small volume change.This dissertation focuses on the application of high-rate Ti-based oxides in sodium-ion batteries.The main contents of the dissertation are as follows:(1)N and F co-doped TiO2 microspheres(NF-TiO2/C)were successfully synthesized by a solvothermal process followed by high-temperature doping.Meanwhile,the carbon layer was formed by the pyrolysis of organic molecules.NFTiO2/C has a high tap density of 1.51 g cm-3,benefiting a high volume capacity of the electrode.Theoretical calculations show that the doping of N and F simultaneously narrows the bandgaps of TiO2 and carbon,and improves the electronic conductivity.At the same time,the doping of N and F enhances the capacity of TiO2 and carbon and accelerates the diffusion of sodium ions in NF-TiO2/C.Therefore,NF-TiO2/C can maintain a specific capacity of 125.9 mAh g-1 and a volumetric capacity of 190 mAh cm-3 at a current density of 100 C.Meanwhile,NF-TiO2/C can cycle stably for 10,000 cycles at a current density of 10 C,maintaining a capacity retention of 80.2%.When the mass loading of NF-TiO2/C reaches to 21.4 mg cm-2,the areal capacity goes up to 4.8 mAh cm-2.In full cells,the material also exhibits excellent cycling stability and ultrahigh power density of 25.2 kW kg-1.(2)Carbon coated titanium vanadate(TVO@C)was prepared by a simple coprecipitation method followed by chemical vapor deposition.The thickness and content of the carbon layer can be controlled by the flow rate of acetylene and the calcination time.TVO@C as an anode material of sodium ion batteries can deliver a specific capacity of 85 mAh g-1 at a current density of 100 A g-1.It runs～10,000 cycles at 1 A g-1 without capacity decay.Even at a high current density of 5 A g-1 for 2000 cycles,the anode still has a specific capacity of～215 mAh g-1.The underlying mechanism of sodium storage was explored via ex-situ characterizations.