Preparation And Electrochemical Properties Of Iron Fluoride Nanostructure As Cathode Materials

The capacity and energy density of lithium-ion batteries（LIBs）are generally confined by the cathode materials,so the exploration of novel cathode with multiple electron redox chemistry is an efficient approach to achieve high energy densities.Iron fluoride（FeF₃）has been proposed as an ideal candidate of LIBs cathode material because of the high theoretical energy density,good thermal stability and low cost.However,the low electronic conductivity and slow kinetics inhibit the practical application of FeF₃ as the high-capacity cathode of LIBs.In order to overcome these drawbacks,the fabrication of nanostructures and collaboration with conductive materials are employed to improve the electrochemical performances of FeF₃.In this paper,we controlled the crystal growth of（NH₄）₃FeF₆by varying the components of the precipitation solution and investigated the crystallinity,morphology and electrochemical performances of FeF₃ nanocrystals（NCs）derived from different（NH₄）₃FeF₆precursors.The thermal decomposition of（NH₄）₃FeF₆precursor precipitated rapidly generates FeF₃ NCs with low crystallinity,small size and porous structure.Consequently,the FeF₃ NC,derived from（NH₄）₃FeF₆precipitated from the solution with the ethanol/water volume ratio of 20,delivers the initial specific capacity of 217.6 m Ah g^-1 at 0.2 C,associated with excellent rate capability up to 20 C(93.8m Ah g^-1),and shows the capacity retention of 80.6%after 500 cycles at 20 C.A hybrid nanostructure of FeF₂ coated by carbon was synthesized through the pyrolysis of polyvinylidene fluoride.The results show that FeF₃ is reduced to FeF₂,and Fe₃C is partially formed due to the effect of carburization during the coating process.The coating of carbon increases the electrical conductivity of FeF₂/C nanocomposites,which is beneficial for the ionic and electronic transport.And the Fe₃C improves the stability of electrode materials.The electrochemical measurements show the obtained materials display a reversible discharge capacity of 246.7 m Ah g^-1 after 100 cycles at0.1 C,and the capacity retention is up to 93.6%compared with the discharge capacity at second cycle,which exhibits excellent cycling stability as the cathode materials for LIBs.