Research On The Controllable Synthesis And Performance Of Ferrous Fluoride As Cathode Material For Lithium-ion Batteries

Iron fluoride(FeF2 or FeF3)is considered as one of the most promising conversion reaction cathode materials because of its theoretical specific capacity that is three to four times higher than the existing intercalation type cathode materials.However,the irreversible growth of the CEI layer,the agglomeration of active material conversion products,excessive resistance accumulation,and electrochemical annealing,which are extremely prone to occur during use,have severely restricted practical application of FeF2.In order to stabilize the interface layer of FeF2 material and improve its cycle stability and rate performance,aiming at the current problems of the synthesis method and structure of FeF2,this paper spreads the research of the mild synthesis method and the optimization of FeF2 property which lays the foundation for the development and commercial application of FeF2 cathode materials.The main contents are as follows:1.A one-step solvothermal method was used to prepare FeF2 materials with controllable morphology,and carbon nanotubes were used to improve their conductivity.Firstly,FeF2@x%G series materials with rich functional groups were successfully prepared by solvothermal method at mild temperature using FeF3·3H2O and glucose as raw materials and n-butanol as solvent.The synthesized FeF2 series materials have different morphologies,which are micro-cluster structure and nano-spherical structure composed of nanorods,and the growth mechanism of different morphologies is discussed.The addition of carbon nanotubes has not changed the material morphology.Subsequently,the electrochemical performances of FeF2@x%G and CNT-FeF2@x%G were compared,and special structure of the micro-structure FeF2 did not play a positive role in cycling performance.The nano-spherical FeF2 shows a good performance in the performance test.Among them,FeF2@100%G was maintained above 100 m A h g-1 after 100 cycles,which reach a specific capacity of 463 m A h g-1 at first with a current density of 100 mAg-1.On this basis,CNT-FeF2@100%G has further improved specific capacity and owns relatively excellent electrochemical performance due to its better conductivity.The above research shows that the FeF2 cathode material that we can synthesize under mild conditions has great potential in practical applications.2.In order to improve the cycle stability and rate performance of the material,PVDF-coated FeF2@x%G-y%P series materials were synthesized in one step by using solvothermal method with artificial SEI/CEI as the starting point.After the introduction of PVDF,the macroscopic morphology of the material is consistent with FeF2@100%G,that is,the regulation of morphology should be attributed to glucose,instead of PVDF.Secondly,FeF2@100%G-40%P has increased defects,laying a foundation for an effective path for lithium insertion and extraction.PVDF did not significantly increase the viscosity of the composite,indicating that it does not exist independently or in the form of a binder.Compared with FeF2@100%G,the specific surface area of FeF2@100%G-40%P is smaller,and the voids generated by nanomaterials are filled by PVDF.The artificial CEI layer prepared by equivalent PVDF can greatly stabilize the volume expansion of FeF2 and limit the loss of active particles.The artificial CEI layer plays an excellent role in maintaining structure and retaining active particles.The FeF2@100%G-40%P electrode exhibits a strong cycling performance of 314 m A h g-1 after 100 cycles under a current of 100 mAg-1,and maintaining a capacity of 186.59 m A h g-1 at a high rate current of 500 mAg-1.