Studies Of Electrochemical Performance For FeF₂ Cathode In Solid State Sodium Ion Batteries

Sodium ion battery is expected to replace lithium ion battery as an important choice in large-scale energy storage technology.In order to solve the problem of low energy density of sodium ion battery,a phase transition cathode material metal fluoride ferrous fluoride with high theoretical specific capacity was discovered and applied to polymer electrolyte,showing good electrochemical cycle performance and rate performance.However,FeF₂ cathode has some problems,such as low conductivity,iron solubility,interface side reaction and voltage lag,which seriously restrict its commercialization.In this study,the electrochemical performance of FeF₂ in solid state sodium ion battery was studied.For the first time,the electrochemical performance of polyepoxide ethane（PEO）electrolyte modified by different sodium salts was systematically explored,and the electrochemical performance of FeF₂ composite carbon fiber positive electrode in electrolyte was further studied.Due to the low conductivity of FeF₂ and the solubility of iron,the structure of the positive electrode was optimized to obtain a solid sodium ion battery with excellent electrochemical performance,and the interface evolution between the positive electrode of FeF₂ and PEO based electrolyte was discussed in detail.It provides basic theoretical research for the development of FeF₂in sodium ion battery.The main work of this study is as follows:（1）The electrochemical and physical properties of PEO-based electrolytes modified by NaTFSI,NaFSI,NaClO₄ and NaPF₆ were studied.By comparing the electrochemical properties of electrolytes,such as electrical conductivity,it was found that the electrochemical properties of electrolytes modified by NaTFSI,NaFSI and NaPF₆ were superior.It was found that NaClO₄ could improve the crystallinity of PEO by comparing the physical and chemical properties of electrolytes.The polymer electrolytes modified by NaTFSI,NaFSI and NaClO₄ have good ductility.PEO-based electrolytes modified by different sodium salts have different properties.（2）The electrochemical properties of polymer electrolytes modified by different sodium salts were investigated using ball-milled carbon fiber composite ferrous fluoride as positive electrode.By comparing the long cycle performance,rate performance and charge-discharge curve,it is found that FeF₂ positive electrode in different sodium modified PEO based electrolyte has obvious difference in the performance curve:FeF₂ positive electrode in NaFSI modified PEO based electrolyte has better rate performance;The initial discharge curve of FeF₂ positive electrode in NaClO₄-modified PEO electrolyte was unstable,and it was speculated that iron dissolved by SEM analysis.The long cycle performance of the battery with NaPF₆-modified PEO electrolyte modified by FeF₂ cathode is poor,because the mechanical properties of the corresponding electrolyte need to be improved.It is found that the performance of FeF₂ in NaTFSI modified PEO based electrolyte is better by comparing its comprehensive properties such as long cycle performance and rate performance.（3）To study the electrochemical performance and interface evolution of the polymer Na-electron of the composite f FeF₂ three-dimensional structure cathode based on carbon fiber film.The cathode synthesized by the"infiltration method"provides a stable channel for the transport of sodium ions,and the design of the cathode with a three-dimensional structure is beneficial to improve the dispersibility and conductivity of FeF₂.The polymer sodium electrocatalyst with FeF₂ as the cathode achieves excellent electrochemical performance at high temperature.