Synthesis And Optimization Of Titanium-based Anode Material For Sodium Ion Batteries

With the increasing use of lithium-ion batteries,the price of lithium sources has risen sharply,which severely limits the future application of lithium-ion batteries.The physical and chemical properties of sodium are similar to lithium,and sodium source is rich in resources and low in cost.Therefore,sodium-ion batteries are considered to be the most likely secondary batteries to replace lithium-ion batteries,and will be widely used in large-scale energy storage.In sodium ion batteries,titanium-based anode materials have broad application prospects with the advantage of suitable working potential,small lattice strain,excellent cycle life,low cost and no toxicity.At present,the formation of nano-morphology is a common method to increase the diffusion rate of sodium ions and electrochemical activity in titanium-based materials.However,the inevitable introduction of water molecules into the product during the synthesis process adversely affects the structure and properties of the material.Therefore,it is an important task to analyze the structure,morphology and electrochemical properties of aqueous titanium-based materials and optimize the materials.The initial coulombic efficiency is an important parameter to measure the performance of the anode material.The electrochemical performance of full-cell are reduced due to that the initial coulombic efficiency of most titanium-based anode materials is less than 50%.At present,the pre-sodiation method for improving the initial coulombic efficiency has the disadvantages of complicated process and low safety.In order to solve the serious problems of titanium-based anode materials which affecting the electrochemical performance,we carry out the following work:(1)We synthesize an aqueous sodium titanate material by hydrothermal method,and test the content of water molecules in the material.The molecular formula of the hydrothermal product is considered to be NaTi₃O₆(OH)·2H₂O.The water molecules were gradually removed by heat treatment,and the structure and morphology of the water molecules were characterized.It indicates that the structure of material has changed from layered to tunnel-like,and the particles of the material are changed from nanowires to nanorods which are thicker and shorter.The rate performance and cycle stability of the materials were significantly improved after structural transformation.It shows that the stable tunnel-like structure and the removal of water molecules in the material promote the transport of sodium ions and improve the electrochemical properties of the materials.The material of the layered structure and the tunnel-like structure is separately immersed in water.The layered material undergoes phase change under the influence of water molecules in the environment,while the initial coulombic efficiency is reduced.The material of the tunnel structure exhibits higher stability to water molecules in the environment,avoiding the damage of water molecules to the structure and electrochemical properties of the material.(2)For the low initial coulombic efficiency of the titanium-based anode material and the defects of the current pre-sodiation method,the pre-sodiation is completed by quantitatively adding the sodium foil to the electrode.The initial coulombic efficiency of sodium titanate was improved,and the relationship between the amount of sodium foil and the initial coulombic efficiency was explored.Furthermore,in order to ameliorate the poor fabricability and safety of the sodium foil,we convert the solid sodium metal into a solution of sodium-naphthalene.The pre-sodiation of titanium-based material is completed by immersing the electrode sheet in the sodium-naphthalene solution for a moment.The electrochemical performance and the structure of titanium-based material after pre-sodiumation was tested.There is a positive relationship between the concentration of sodium-naphthalene solution and the promotion of initial coulombic efficiency.The performance of the full-cell was improved after pre-sodiation.The safety advantages of sodium-naphthalene solution is verified by the react with water comparing to sodium foil in air atmosphere.Beyond that,the simplification of the pre-sodiation process is analyzed.It indicates that sodium-naphthalene solution promotes the development and application of titanium-based anode materials.