Research On Controllable Synthesis And Modification Of Iron Fluoride Cathode Materials

In recent years,iron-based fluoride is considered as a promising cathode material due to their excellent theoretical capacity.However,the volume effect and poor electrical conductivity hinder their application development.In this paper,FeF₃·0.33H₂O,FeF₃-FeF₂and FeF₃-FeF₂/rGO cathode materials were prepared by assisted solvothermal method,high temperature calcination method and composite method with conductive agent for the purpose of reducing volume effect and improving conductivity.The properties of various materials were characterized and analyzed.（1）FeF₃·0.33H₂O submicron spheres with controllable morphology and size were prepared by polyethylene glycol（PEG-400）assisted solvothermal method.The growth mechanism of FeF₃·0.33H₂O submicron spheres were studied by molecular dynamics simulation and experiment.The initial discharge capacity of FeF₃·0.33H₂O（P-20）cathode is 345 mAh·g^-1,and the capacity retention rate is as high as 95.3%after 60 cycles at different rates to the original rate.Its excellent performance is mainly attributed to unique porous structure and small particle size of FeF₃·0.33H₂O submicron spheres,which is conducive to improving its specific surface area and shortening the diffusion distance of ions.（2）FeF₃-FeF₂ cathode material was prepared by high temperature calcination method with FeF₃·0.33H₂O as precursor.The results show that the initial decomposition temperature of FeF₃-FeF₂ single cell is 850℃,showing the excellent thermal stability.The no-load voltage of FeF₃-FeF₂ single cell is 3.22 V,which is 52.6%higher than that of FeS₂of 2.11 V.However,due to its poor conductivity,the discharge voltage of FeF₃-FeF₂ single cell is only 1.57 V when the current density is 100 mA·cm^-2.（3）FeF₃-FeF₂/rGO（～200nm）composite cathode material was prepared from reduced graphene oxide（rGO）by high temperature calcination with graphene oxide（GO）as conductive agent and FeF₃·0.33H₂O/GO as precursor.The results show that the no-load voltage of FeF₃-FeF₂/rGO single cell is 3.25 V,and the discharge voltages are 2.78 V and2.21 V when the current density is 100 mA·cm^-2 and 500 mA·cm^-2,respectively.FeF₃-FeF₂/rGO single cell can charge for 230 s at the current density of 200 mA·cm^-2 when the cut-off voltage reaches 2 V.It can be seen that the smaller particle size and the doping of graphene greatly improve the discharge performance of the cathode material.