Preparation And Lithium Storage Research Of High Performance Mirco/nano Structure Iron Fluride Electrode

In recent years,transition metal fluorides?Mx Fy,M = Fe,Mn,Co,Cu,etc.?gradually attract particular attention because of high specific capacity.Iron fluoride becomes a favorable cathode material because of the merits of high-capacity,high-voltage,and low-cost.A high discharge plateau of 2.73 V and specific capacity of 237 mAhg^-1 for iron fluoride can reach benefit from the high ionic bonding characteristics and small molecular weight.Among various crystal forms of iron fluoride,Fe F₃·0.33H₂O has unique advantages.Hexagonal-tungsten-bronze-type Fe F₃·0.33H₂O has unusual tunnel structure and better electrochemical activity compare with the common Re O3-type Fe F3 without crystal water.Despite many advantages,there are still some problems for practical application of Fe F₃·0.33H₂O,such as poor conductivity and limited electrode reversibility.In this thesis,different nanostructured iron fluorides were prepared by various means,and they were combined with different carbon materials to construct high-performance micro-nano materials.The synthesis process and preparation mechanism of the materials were discussed.At the same time,the lithium storage performance and influence factors were studied.The main research contents are as follows:N,O-double-doped porous carbon?3DPC?was prepared as a carbon matrix by using biomass nori as raw material,then Fe F₃·0.33H₂O@3DPC composite?FF@3DPC?was constructed by solution reaction and fluorination process.The Fe F₃·0.33H₂O nanocrystals in the FF@3DPC have very small size?20 nm?,which greatly shortens the migration distance of Li+,so the lithium intercalation reaction can be carried out quickly and thoroughly.On the other hand,the highly conductive 3DPC forms a high-speed electron transfer channel,the synergistic effect of nano Fe F₃·0.33H₂O and 3DPC results in excellent high-rate performance of the composite.The composite can deliver a capacity of 170 mAhg^-1 at 1 C and a capacity of 104 mAhg^-1 can be obtained at 20 C.The prepared composite material also has excellent cycle stability.The capacity retention rate is as high as 91% after 200 cycles at 1 C rate.Even after cycling 500 times at 5 C,the specific capacity also maintain at 101 mAhg^-1,and the attenuation is only 0.046% per cycle.A highly graphitized carbon layer coated Fe F₃·0.33H₂O composite?GCF?is prepared by high temperature carbonization of iron-containing metal framework MIL-53 template and HF vapor fluorination.The highly graphitized porous carbon skeleton is origins from in-situ catalysis of Fe element in MIL-53 during high temperature carbonization and Fe F₃·0.33H₂O nanoparticles with average size of 6.14 nm are uniformly distributed in it after fluorination.The highly conductive porous carbon skeleton and ultra-small Fe F₃·0.33H₂O particles endow the composite with excellent lithium storage properties.The composite can deliver a capacity of 86 m Ah at 20 C.After 50 cycles at 1 C rate the capacity decreased to 162 mAhg^-1,the capacity retention rate is 95%.After 300 times charge/discharge at 5 C,the capacity can be maintained 89% of the initial value,capacity attenuation rate is 0.036%/per cycle.Carbon nanotubes and porous graphene are used as conductive carbon and fluorine-containing ionic liquid is used as fluorine source to synthesize porous graphene/Carbon nanotubes/Fe F₃·0.33H₂O microspheres?GCFF?by solvothermal method.The incorporation of one-dimensional carbon nanotubes into a two-dimensional graphene interlayer forming a three-dimensional conductive network while the micro-nano multi-level structure Fe F₃·0.33H₂O particles embedded in the surface of the carbon nanotubes and the graphene layers have fast ion and electron transfer channels.GCFF material possess excellent rate performance and cycle performance.A capacity of 165 mAhg^-1 at 1 C and a capacity of 70 mAhg^-1 at 20 C can reach for GCFF at mass loading of 1 mg cm-2.The specific capacity of 128 mAhg^-1 can be maintained after 100 cycles of charge/discharge at 1 C rate.When the mass loading rise to 5 mg cm-2,the composite still have a capacity of 153 mAhg^-1 at 1 C rate.The Fe F₃·0.33H₂O vertical nanosheet array electrode is synthesized by solvothermal method using fluorine-containing ionic liquid as the fluorine source,and graphene quantum dots is used to modificate the surface to construct graphene quantum dot modified Fe F₃·0.33H₂O array electrode?GQDs@FFNA?.The array electrode avoids the using of conductive agent and binder,and can be directly used as cathode.Graphene quantum dots modification can significantly reduce the charge transfer resistance and can avoid the directly contact between the nanosheets and the electrolyte to reduce the occurrence of side reactions.Based on these characteristics,GQDs@FFNA exhibits good high rate performance and long cycle performance.The prepared array electrode have a high capacity of 113 mAhg^-1 at 20 C,and can maintain a capacity of 96 mAhg^-1 after 1000 cycles at 2 C rate.The decay rate of the electrode is only 0.03% per cycle.