Study On Modification And Electrochemical Performances Of FeF₃ As Cathode Material For Lithium Battery

Iron fluoride?FeF₃?has attracted extensive attention due to its high mass and volumetric energy density and is considered to be a candidate as cathode material candidate for Lithium battery.However,the poor conductivity caused by the strong ionicity of Fe-F bond severely limits the lithium storage capacity;and the volume expansion of FeF₃ grains during the insertion process of Li⁺would lead to the electrode structure collapse and result in poor cyclic stability.In order to ameliorate these defects of FeF₃,strategys that surface modification and construction of buffer space are adopted.Herein,the conductivity and the electrochemical performance of FeF₃ were significatly enhanced by a simple etching process and carbon material coating.Various of characterization and performance testings were conducted to analyze the etching and coating process.Moreover,the modification mechanism to enhance the electrochemical performance of FeF₃ was revealed.The conductivity of FeF₃ was improved by decorating with oxygen compound,which produced by air etching.The content and composition of the compounds could be regulated through controlling the exposure time.The results show that air etch method was an effective measure to achieve surface modification.The FeOF would be rapidly converted into Fe₂O₃ with prolongation of etching time.And trace amount of oxygen compounds can enhance the electrical conductivity,which reducing the charge transfer resistance by 25.8%.Therefore,the modified electrodes deliverd initial capacity of¹09.3 mAh·g^-1 at the current density of 2C.However,the countless FeOF/Fe₂O₃interface can weaken the modification ability of FeOF and Fe₂O₃,and increase the charge transfer resistance at the interface between FeF₃ and electrolyte.H₂O is generally used to perform mild etch of materials.Accordingly,crystalline water in FeF₃·3H₂O is utilized as oxygen source,and the surface layer of FeF₃ would be in situ oxidized to the FeOF compound to realizing modification during dehydration.The etching reaction is intensified with the increase of dehydration temperature,which causes the surface of FeF₃ grains to become rough and increase the content of FeOF.Furthermore,the high dehydration temperature can trigger crystal collapse and promote the conversion of FeOF to Fe₂O₃.Benefit from semiconductor feature and the optimum content of FeOF,the charge transfer resistance of FeF₃ electrodes can be reduced from 126.6 to 36.9?,which exhibiting better modification ability than oxygen etch;and the reversible specific capacity of the electrode can deliver 106.7 mAh·g^-1at 2C.The deficiencies that FeOF could not stably exist during cycling because it participated in the electrochemical reactions and the problem of volume expansion of FeF₃ do not be solved.The FeF₃/C nanocomposites were successfully prepared by fluorinating the Fe₃O₄/C precursor and dehydrating at low temperature under vacuum,which was prepared by using Fe₂O₃ and stearic acid as raw materials.The carbon matrix can not only limit the agglomeration of FeF₃ particles,increase the contact area between the electrode and the electrolyte(the specific surface is 51.9 m²·g^-1);but also significantly improve the conductivity and reduce the interface transfer resistance.Furthermore,it provided a buffer space for the volume expansion of FeF₃ grains to achieve a better cycle stability.The composite electrode delivered an initial discharge capacity of about 165.4 mAh·g^-1 at a low current?0.084C?and the capacity retention of 76.4%after 100 cycles.However,the rate performance of the electrode was insufficient,and the specific discharge capacities at the currents of 1C and 2C were only 104.1 and 83.11 mAh·g^-1,respectively.N doping can improve the conductivity of carbon and provide large amounts of active sites.So,the lithium storage ability of FeF₃ can be further improved by enhancing the electrochemical capacity of the carbon matrix.The conductive carbon matrix can not only anchor the FeF₃ grains to stabilize the electrode structure,but also reduce the interface charge transfer resistance.Meanwhile,the doping site can limit Li⁺to the vicinity of the surface of FeF₃ to shorten its migration time in the electrolyte and promote chemical reaction between them.The modified electrode exhibited the specific capacity of 115.9mAh·g^-1 at 2C.And it persisted the capacity of about 84.9 mAh·g^-1 with the fading rate of only 0.13%during 200 cycles process.