Research On Preparation And Modification Of Iron Fluorides As Cathode Materials For Sodium-Ion Batteries

In recent years,sodium-ion batteries(SIBs)have become a research hotspot in the field of energy storage owing to their advantages such as abundant resources and low prices.However,its larger ionic radius and slower kinetic rate have become the main factors restricting the development of sodium storage materials.Therefore,developing the high-performance sodium storage cathode materials is the key to increase the specific energy of SIBs and promote their application.Among them,iron fluorides,as a cathode material for multi-electron conversion reactions,has many merits such as high working potential,large theoretical specific capacity,and low cost,and it has been widely studied and focused in recent years.Under this background,this dissertation takes it as the main object of study.According to the shortcomings of low electronic conductivity and poor cycle stability of the materials,different modification methods are used to modify its electrochemical performance.The specific research contents of the paper are as follows:(1)The sphere-like Fe(2-x)CrxF5·H2O(x=0,0.03,0.05,0.07)compounds with open framework are synthesized through an ionic-liquid-assisted precipitation approach.The results indicate that the Cr-doped materials not only reduce the crystalline size,but also remarkably enhance electronic conductivity.Meanwhile,the electrochemical tests show that Fe1.95Cr0.05Fs·H2O as the cathode active material of NIBs displays a prominent initial discharge capacity of 357 mAh g-1,and retains a discharge capacity of 171 mAh g-1 after 100 cycles at 0.1 C(1 C = 200 mAh g-1).Moreover,even at high rate of 1 C,it can still deliver a high discharge capacity of 147 mAh g-1 Compared with the Fe2F5·H2O sample,the Fe1.95Cr0.05F5·H2O sample shows higher discharge capacity,excellent cycle stability and better rate capability.(2)By rationally regulating and controlling the amount of highly viscous polyethylene glycol(PEG)in the solvent to limit the size growth of the crystal,a series of nanostructured FeF3·0.33H2O materials with different morphology and size were obtained via a simple PEG-assisted solvothermal method.The probable formation mechanism related to the role of the PEG is explored.Furthermore,the electrochemical properties of all cathode materials for SIBs are investigated.Among them,the spherical mesoporous FeF3·0.33H2O nanoparticles exhibit the best electrochemical performance,and it can deliver a noticeable initial discharge capacity of 551 mAh g-1 and retain 206 mAh g-1 after 100 cycles at 0.1 C within a potential range of 1.0-4.0 V.Especially,even at a high rate of 2.0 C,the material can still deliver a high discharge capacity of 181 mAh g-1.(3)A hollow porous FeF3·0.33H2O microsphere is successfully prepared via a solvothermal route and further modified with A1PO4.The results show that appropriate modification treatment can satisfactorily decrease charge-transfer resistance and enhance sodium diffusion rate.Compared with the pristine FeF3 0.33H2O,4 wt.%A1PO4-coated sample possesses a prominent initial discharge capacity(290 mAh g-1 within 1.2-4.0 V),outstanding cycling stability(211 mAh g-1 after 80 cycles)and excellent rate capability(167 mAh g-1 at 2.0 C).