Preparation And Sodium Storage Performance Study Of NASICON Type NaTi₂（PO₄）₃

Lithium-ion batteries have been widely used in electric vehicle and portable electronic devices due to their light weight,high energy density and long cycle life.However,there is less lithium in the earth’s crust and uneven distribution,the cost of lithium-ion battery is rising and the safety of lithium-ion batteries have severely limited the practical application of lithium-ion batteries in the field of large-scale energy storage.Therefore,there is an urgent need to develop a new generation of secondary batteries that are low-cost and replace lithium-ion batteries.Because sodium and lithium belong to the same main group and have similar physicochemical properties,and sodium is the sixth element of crustal reserves,it is widely distributed.Compared with lithium ion batteries,sodium has the advantages of abundant resources and low cost.Therefore,sodium ion batteries are considered to have great application potential in the field of large-scale energy storage.By looking for suitable sodium ion battery electrode materials,the sodium ion battery system can be pushed to practical applications.NASICON type sodium titanium phosphate material as a negative electrode material for sodium ion battery has low cost,environmental friendly and no pollution.The open three-dimensional frame structure facilitates reversible deintercalation of sodium ions.Suitable potential can work in water-based electrolyte system,greatly improving the safety of sodium ion battery and has a good research prospect.However,its inherently low electronic conductivity greatly affects its electrochemical performance,and the results are unsatisfactory.This thesis aims to obtain sodium titanium phosphate material with excellent electrochemical performance through simple and controllable preparation method.The sodium titanate material is optimized by various modification methods to explore the structure-activity relationship between material structure and electrochemical performance.And it is applied in the field of sodium ion battery organic electrolyte system,mixed sodium ion capacitors,etc.,in order to expand its application in the water electrolyte system.The specific content and results of this thesis are as follows:1.The uniformly shaped titanium phosphate sodium cube can be prepared controllably by a simple and large-scale hydrothermal method,and the conductive polymer polypyrrole is uniformly coated on the surface of the sodium titanium phosphate by chemical polymerization for the first time,by controlling the pyrrole monomer.The amount of NTP@PPy series with different polypyrrole coating content was obtained,and it was studied by structural morphological characterization and electrochemical test.The results showed that a series of NTP@PPy samples were obtained by in-situ coating of polypyrrole on the surface of sodium titanate.The morphology of the pyrrole was successfully polymerized on the surface of sodium titanium phosphate,and it was uniformly coated as sodium ion.The electrode material of the battery is found to improve the electrical conductivity of the sodium titanium phosphate material after the surface polypyrrole coating,so that the rate performance and the cycle performance are improved.Among them,NTP@PPy-2 composite exhibits the most excellent electrochemical performance due to the proper coating amount,releasing a discharge specific capacity of100.4 mAh g^-1 at a current density of 0.2 C,and releasing 94 mAh g^-1 at a current density of 1 C.After 300 cycles,the discharge specific capacity still maintains 94%of the initial capacity.2.Sodium titanium phosphate due to inherently slow charge transfer kinetics and low electronic conductivity resulting in low capacity release,short cycle life and poor rate performance,we introduced fluoride ion doping to modify the material lattice,A series of different fluorine doping amounts of NTP-F_x/C（x=0,0.02,0.05 and 0.10）were synthesized by typical sol-gel method as anode materials for sodium ion batteries,and the electrochemical properties,morphology and structure of the synthesized samples were studied.Electrochemical analysis and theoretical calculations showed that fluorine doping increased the intrinsic electronic conductivity and sodium ion diffusion coefficient,thus greatly improving the material’s rate and cycle performance.Among all fluorine-doped samples,NTP-F_0.05/C exhibited the best electrochemical performance:from 0.2 C to 20 C charge and discharge,the specific capacity ranged from 121 mAh g^-1to 85.8 mAh g^-1.Even at 30 C,the specific capacity of 62.5 mAh g^-1 can be released,showing superior rate performance.NTP-F_0.05/C also released a reversible specific capacity of 102.5 mAh g^-1 at 10 C current density,and the capacity retention rate was as high as 90%after 1000 cycles,showing superior cycle performance and structural stability.These test results and excellent electrochemical performance prove that the anion doping is effective in optimizing the NASICON electrode material,which can be used as a stable and high-rate anode material in sodium ion batteries,and further reveals that anion doping causes rapid sodium ion dynamics.Anion doping is beneficial to improve the electrochemical performance of electrode materials.3.In order to improve the electronic conductance and inter-particle electron conduction of sodium titanium phosphate particles,we successfully synthesized one-dimensional porous sodium titanium phosphate/carbon nanofibers by electrospinning combined with subsequent carbonization process,and proved that we prepared by structure and morphology analysis.The porous NTP/C-NFs samples have good crystallinity and uniform carbon coating.The electrochemical performance test also proves that the NTP/C-NFs samples have excellent cycle stability and large rate performance,and uniform carbon coating improves.The electronic conductance of the material,the nano-size shortens the sodium ion transport path and the mesoporous structure facilitates the full wetting of the electrolyte,allowing the NTP/C-NFs sample to release a reversible specific capacity of 120 mAh g^-1 at a current density of 0.2 C,at 2 C The current capacity is 700 cycles and the capacity retention rate is as high as 93%.At the same time,we have successfully assembled sodium ion full cells and mixed sodium ion capacitors,which also show superior electrochemical performance.