Study On Electrochemical Properties Of Iron Difluoride Cathode In PEO-based Solid Electrolyte

As a conversion cathode material,compared with the traditional intercalation cathode material,iron-based metal fluoride has become one of the preferred cathode materials for the next generation of lithiμm-ion batteries because of its high energy density,environmental protection and low price.However,there are many problems in this material,such as low electronic conductivity,large voltage hysteresis,poor electrochemical performance at high temperature,dissolution of metal ions and serious side reactions with liquid electrolyte,which hinder its commercial development.In this paper,the modification of cathode materials and the selection of PEO based solid electrolyte are used to solve these problems.Systematically studied different FeF₂@C composite morphology of cathode materials and electrochemical properties in PEO based solid electrolyte.the electrochemical performance in PEO based solid.The problems of low electronic conductivity and metal ion dissolution of FeF₂cathode are improved,realized the excellent electrochemical performance at high temperature（60℃）.（1）Aiming at the problems of metal ion dissolution and incompatibility with liquid electrolyte in the cycle of FeF₂cathode,the FeF₂-PEO（Li TFSI）-Li battery system was designed.At the same time,the PEO based electrolyte was doped with LLZTO ceramic particles,it was found that 10 wt%LLZTO was the best doping ratio.The high temperature performance of FeF₂cathode in PEO based solid electrolyte system is improved.When charged and discharged at 0.2 C and 60℃,its specific discharge capacity can be increased by～30 m Ah/g.Meanwhile,it is found that PEO based solid electrolyte also effectively hinders the shuttle of metal ions.（2）In order to improve the conductivity of FeF₂material and the wettability of PEO electrolyte,a new ball milling strategy of different carbon materials and FeF₂was proposed.By controlling the ball milling time and speed,the composites of FeF₂and carbon fiber,carbon nanotube,Cochin clack,graphite and graphene were obtained respectively.It was found that the composites formed C-F bond,indicating that FeF₂particles were closely attached to the carbon material,The conductivity is effectively improved.The composite obtained with carbon fiber FeF₂@CF It has a higher specific surface area,which is more conducive to the infiltration of PEO.The first discharge specific capacity is～550 m Ah/g at 0.2 C and 60℃.After 300 cycles,the discharge specific capacity is～210 mah/g.（3）In order to further improve the cycle performance of FeF₂in PEO based electrolyte and study the CEI composition after cycled.A polymer film cathode material was designed.FeF₂@CF material was mixed with PEO（Li TFSI）and conductive carbon to produce a film for the cathode.The structure in 60℃all-solid polymer battery,0.2 C charge and discharge cycle 400 cycles,discharge capacity～300 m Ah/g;0.4 C charge and discharge cycle 1500 cycles,discharge specific capacity～200 m Ah/g,showing excellent cycle performance and rate performance.At the same time,it is found that some iron oxides are enriched on the surface of FeF₂particles after circulation,a～5 nm CEI layer is formed in the outermost layer of FeF₂particles,which consists of Li F,Li₃N,Li₂S,Li₂CO₃,ROLi and other substances.A double-layer protection mechanism of"external flexibility and internal rigidity"was found in FeF₂ cathode.