Fe-based Compounds As Advanced Electrode Materials For Electrochemical Energy Storage

Rechargeable Li/Na-ion batteries have attracted extensive attention,due to wide applications in portable electronics,electric vehicles?EVs?,stationary power station,and so on.However,the rapidly increasing demands on high power,high energy,low cost and ultra-long lifespan,stimulate the exploration of new electrode materials.In numerous candidates,Fe-based compounds present the features of high capacity,low cost and easy preparation,suggesting the great potentials.This thesis focuses on two Fe-based compounds,FeS_2-xSe_x and FeF₂,which are explored as electrode materials of Li/Na-ion batteries.The main results about them are summarized as follows.?1?Transition metal sulfides/selenides are explored as advanced electrode materials for non-aqueous sodium-ion capacitors,using FeS_2-xSe_x as an example.Solid solution of S/Se in FeS_2-xSe_x allows it to combine the high capacity of FeS2 and the good diffusion kinetics of FeSe2 together,thereby exhibiting excellent cycle stability(?220 mAh g^-1 after 6000 cycles at 2 A g^-1)and superior rate capability(?210 mAh g^-1 at 40 A g^-1)within 0.8-3.0 V.These results are much better than those of FeS2 and FeSe2,confirming the advantages of S/Se solid solution as supported by EIS spectra,DFT calculations and electron conductivity.As FeS_2-xSe_x is paired with activated carbon as Na-ion capacitors,this device is also better than sodium-ion batteries of FeS_2-xSe_x//Na3V2?PO4?3 and sodium-ion capacitors of metal oxides//AC,particularly at high rates.These results open a new door for the applications of sulfides/selenides in another device of electrochemical energy storage.?2?FeF₂/rGO composite in which FeF₂ nanorods are uniformly deposited on rGO,was synthesized by a simple solvothermal reaction,without high-temperature annealing and sophisticated equipment.To our knowledge,the wet-chemical synthesis of FeF₂ has not been reported before.This composite exhibited excellent performances as a cathode material in lithium ion batteries.It delivers a high capacity of?340 mAh g^-1 at 40 mA g^-1,which is much higher than conventional intercalation-type cathodes.Keywords:iron-based compounds,cathode materials,Li/Na-ion batteries,Na-ion capacitors...