Preparation And Modification Of Fe₂F₅·H₂O As Cathode Materials For Sodium Ion Batteries

Recently, sodium ion batteries（SIBs） have become a research hotspot due to its abundant and cheap resources. Iron based fluoride as a promising cathode material attracts the attention of researcher owing to their high operating voltages, high energy density, low price, as well as a variety of stoichiometries which lead to a versatility of structures and physicochemical properties. According to the characteristics of the iron based fluoride, Fe₂F₅·H₂O and its modified compound were prepared in this work by an ionic-liquid-assisted approach. And various test methods are applied to characterize the physicochemical properties and electrochemical behaviors of the samples. The main research works are as follow:（1） A pyrochlore Fe₂F₅·H₂O with the sphere-like morphology was firstly synthesized by ionic liquid（IL） based precipitation route and used as the cathode material of SIBs, in which IL 1-butyl-2, 3-dimethylimidazolium tetrafluoroborate（BMMimBF₄） acting as a fluoride source, soft template and solvent. The results show that the Fe₂F₅·H₂O has a larger cell volume（¹.12490 nm3） and the interconnected open 3D channel which is beneficial for transporting Na+. The results of XPS prove that the Fe₂F₅·H₂O sample exists with mixed valence states（+2, +3） of iron ions. Besides, electrochemical tests show that the sample possesses a high initial discharge capacity of 253.7 mAh g-1 and the reversible capacity of 161.2 mAh g-1 at 20 mA g-1 in the voltage of 1.0-4.0 V.（2） A reduced graphene oxide loading iron-based fluoride（abbreviated as Fe₂F₅·H₂O/rGO） as a cathode material for SIBs had been successfully prepared by an ionic-liquid-assisted route. The results show that numerous Fe₂F₅·H₂O nanoparticles are firmly adhered on the surface of the rGO and the introduction of rGO doesn’t influence the original crystal structure of the material. Besides, electrochemical tests show that the Fe₂F₅·H₂O/rGO can deliver a high initial discharge capacity of 248.7 mAh g-1 at 20 mA g-1 in the voltage of 1.0-4.0 V, and a discharge capacity of 164.2 mAh g-1 can be achieved after 100 th cycle.（3）The multi-wall carbon nanotubes wired Fe₂F₅·H₂O composites（MWCNTsFe₂F₅·H₂O） was successfully prepared in this work via an ionic-liquid-assisted approach. The results show that the MWCNTs are encased in the interior of the Fe₂F₅·H₂O particles and the nanoparticles are closely wrapped by the fluffy MWCNTs producing a chestnut-like structure. Furthermore, the MWCNTs-Fe₂F₅·H₂O sample shows a high initial discharge capacity of 251.2 mAh g-1 at 20 mA g-1 in the voltage of 1.0-4.0 V, and the corresponding reversible discharge capacity is 197.4 mAh g-1. When the current density increased to 100 mA g-1, the discharge capacity of the MWCNTs-Fe₂F₅·H₂O particles still can maintain about 115.0 mAh g-1 after 50 cycles and the corresponding capacity retention reaches 90.2%.