Synthesis And Modification Of Iron-based Fluoride Cathode Materials For Lithium-ion Batteries

Iron fluoride with high theoretical capacity and high theoretical energy densities have been extensive researched,and it are considered as the next-generation cathode materials for LIBs.However,iron fluoride face a challenge that its intrinsically low electron conductivity,leading to poor electrochemical performance and prevents the application.In order to improve the electrochemical performance of FeF₃ and FeF₂,we did a lot of modified methods,then characterized and researched it structure,morphology,charge and diacharge performances,cyclic performances and rate performances of the modified materials by XRD,SEM,TEM,EDS,TG and electrochemical performance testing.FeF₂/carbon composite nanoparticle was synthesized by a one-pot thermal reaction.The effects of different sintering temperatrue and material ratio on the material properties were studied.It is indicated that the optimum conditions flowing that:a)calcination temperature is 550?;b)weight ratio of[FeC₂O₄]:[PTFE]is 10:4.17.The composite exhibits a discharge capacity of 503.394 mA h g^-1 during the first charge/discharge process at a current density of 60 mA g^-1.And after 20 times the cycle performance is 183.328 mA h g^-1.Precursor FeF₃·3H₂O was obtained by a liquid phase reaction method,then selecting the sintering temperature of the FeF₃·0.33H₂O and Fe F₃ in the 180?and 400?by the TG analysis.The electrochemical results showed that the discharge capacity of FeF₃·0.33H₂O is 180.335 mA h g^-1 during the first charge/discharge process at a current density of 50 mA g^-1 and capacity retention is 57.7%after 40 times.However,the Fe F₃ is210.505 mA h g^-1 and capacity retention is 26.1%after 40 times.To improve the electrochemical performance of the Fe F₃,three modifications methods have been researched:1)The FeF₃ was modified by adding CNTS as conductig agents,both the nucleation and crystallization of precursor nanoparticles occurred on the wall of CNTs.The results showed that the structure of the FeF₃/CNTs had no change after modification.Its discharge capacity is 307.011 mA h g^-1 during the first charge/discharge process at a current density of 50 mA g^-1 and capacity retention is 56.4%after 40 times.The first discharge capacity and capacity retention is 96.506 mA h g^-1 and 30.3%higher than the FeF₃;2)The FeF₃/C composity,it was prepared by mechanical ball milling the mixture of conductive acetylene black and Fe F₃.The crystal structure of FeF₃ did not change when acetylene black was added.The electrochemical results showed that the discharge capacity of Fe F₃/C is 346.25 mA h g^-1 during the first charge/discharge process and capacity retention is 58.2%after 40 times.The first discharge capacity and capacity retention is 135.745 mA h g^-1 and 32.1%higher than the pure FeF₃;3)Cr-doped Fe F₃ was synthesized successfully and it can detect the peak of the Cr element?1.85 wt%?by EDS.Cr-doding does not chage the basic structrue of raw materials.The electrochemical results showed that the discharge capacity of Cr-doped FeF₃ is 390.109 mA h g^-1 when the battery was first working,it is 179.604 mA h g^-1 higher than the pure FeF₃.After 40 cycles,the capacity retention was keep at 57.2%,It is 31.1%higher than the pure FeF₃.Based on the analysis of the electrochemical performance obtained above,it is showed that electrochemical performance can be improved through the modifications.