Preparation And Modification Of Iron Fluoride As Cathode Materials For Lithium/Sodium Ion Batteries

With high theoretical capacities and energy density obtained through reversible conversion reaction,iron fluorides have been regarded as very appealing and potential cathode candidates for lithium ion baterries?LIBs?and sodium ion batteries?SIBs?.However,fluoride generally has stronger polarity,wider band gap and lower electronic conductivity,which would lead to serious electrode polarization problems.In addition,its poor cycling stability and large irreversible capacity,caused by the obvious volume changes during the repeated charge-discharge processes,and the kinetic limitations of its low conductivity intrinsic nature,still strongly hinder its large-scale practical applications.Accordingly,the purpose of this thesis is to enhance the electrical conductivity and structural stability of iron fluorides electrodes through combining carbon materials?e.g.,carbon black and multi-walled carbon nanotubes?,fabricating various nanostructures and constructing hybrid composites so as to achieve highly desirable electrochemical properties.According to the characteristics of the iron fluoride,in this work,an ionic liquid?IL?based on precipitation route,one-pot chemical method and one-step solvothermal route were used to synthesize FeF_2.5·0.5H₂ O ? FeF₃·0.33H₂ O and those modified compounds.And various test methods are applied to characterize the physicochemical properties and electrochemical behaviors of the samples.?1?The multi-wall carbon nanotubes wired FeF_2.5·0.5H₂ O nanocomposites were successfully prepared in this work via an ionic-liquid-assisted approach.The BMMimBF4 is used as environmentally friendly fluorine source,appropriate binder.The results show that the addition of MWCNTs can not only increase the conductivity of the active material,but also provide more nucleation sites to reduce the particles size.Furthermore,the FeF_2.5·0.5H₂O-MWCNTs sample shows a high initial discharge capacity of 324.7 mAh·g^-1 mAh g^-1 at 40 mA·g^-1 in the voltage of 1.5-4.5 V,the discharge capacity of the FeF_2.5·0.5H₂O-MWCNTs particles still can maintain about175.2 mAh·g^-1 after 50 cycles.When the current density increased to 500 mA·g^-1,the discharge capacity of the FeF_2.5·0.5H₂O-MWCNTs particles still can maintain about136.0 mAh·g^-1.?2?The novel FeF₃·0.33H₂O/C nanocomposite with open mesoporous structure is successfully prepared using a one-pot chemical method and mechanical ball millingroute.The results show that the as-synthesized FeF₃·0.33H₂O/C nanocomposite exhibits a particle size of about 1-5 ?m,open mesoporous structure with average pore diameters of 16.5 nm.Although the particles size is large up-to micron scale,the sample displays excellent electrochemical performance.When FeF₃·0.33H₂O/C is used as cathode material of LIBs,it exhibits a specific capacity of 276.4 mAh·g^-1and excellent cyclic performance with a capacity of 193.5 mAh·g^-1 after 50 cycles at 1 C for LIBs.Moreover,it also exhibits a high reversible capacity of 104.8 mAh·g^-1after40 cycles at 1 C for SIBs.?3?Via a liquid-phase method and a ball milling process with acetylene black?AB?have been used to form Fe_1-xTi_xF₃·0.33H₂O/C?x=0,0.06,0.08,0.10?nanocomposite.The results show that Ti can effectively dope into the samples,and replace partially Fe3+ ions in the Fe F₃·0.33H₂ O crystal.Smaller particles size,faster lithium ion diffusion rate and less polarization confer the Ti-doped Fe F₃·0.33H₂O/C nanocomposite outstanding electrochemical performance.Especially,the Fe0.92Ti0.08F₃·0.33H₂O/C nanocomposite exhibited excellent electrochemical performance.It can achieves an initial capacity of 460.15 mAh·g^-1 and retains a discharge capacity of 294.86 mAh·g^-1after 40 cycles in the voltage range of 1.5-4.5 V.Besides,the as-prepared material shows excellent rate capability,it can deliver a discharge capacity of 146.06 mAh·g^-1 even at 2 C.?4?A facile,simple and one-step solvothermal approach been successfully used to fabricate mesoporous spherical FeF₃·0.33H₂O/MWCNTs nanocomposite.In our synthesis,complicated procedures and apparatus are avoided.The results show that the MWCNTs has entered to the interior of particles rather than simply coated on the surface of particles.Benefitting from the special mesoporous structural features,FeF₃·0.33H₂O/MWCNTs nanocomposite exhibits much better electrochemical performances in terms of a high initial discharge capacity(350.4 m Ah·g^-1)and cycle performance(a discharge capacity of 123.5 mAh·g^-1 after 50 cycles at 0.1 C range from 1.0 V to 4.0 V).The?10?NaD values of the FeF₃·0.33H₂O/MWCNTs?1.39×10-13 to2.55×10-9cm2s-1?cathode active material are slightly higher than FeF₃·0.33H₂ O in the discharge process of Na+.It can effectively shorten the Na+ diffusion pathway,leading to high rate capability.