Study On Preparation And Energy Storage Of Diperovskite-type（NH₄）₃FeF₆/Graphene Composites

Transition metal fluorides(TMFs)have attracted much attention as electrode materials for sodium-ion batteries(SIBs)due to their abundance,low cost,and high specific capacity.However,intrinsic low electronic conductivity and high resistance to Na ion diffusion impede the applications of TMFs in SIBs.Herein,diperovskite-type(NH4)3FeF6/graphene nanosheet((NH4)3FeF6/GNS)composites were successfully synthesized using a facile co-pyrolysis approach from ferric acetylacetonate(Fe(acac)3)and ammonium fluoride(NH4F)with the addition of GNS.The pure diperovskite phase of(NH4)3FeF6 was generated at reaction temperatures ranging from 150-300℃using a high NH4F/Fe(acac)3 molar ratio.The stable diperovskite structure,bearing an open three-dimensional framework,was favorable for the fast storage of large-sized Na ions.Therefore,the(NH4)3FeF6/GNS displayed excellent electrochemical properties as electrode materials for Na-ion storage.The(NH4)3FeF6/GNS composites exhibited a high capacity of 487.8 mA h g-1 at 0.1 A g-1 and excellent rate performances as anode materials for SIBs.The specific capacities of the(NH4)3FeF6/GNS composites were up to 257.8,219.4,and 180.6 mA h g-1 at 2.0,5.0,and 10.0 A g-1,respectively.Even at 20.0 A g-1(approx.58 C),the(NH4)3FeF6/GNS composites still delivered a specific capacity of 149.5 mA h g-1 and exhibited a long lifespan up to 3000 cycles.Moreover,when the(NH4)3FeF6/GNS composites used as cathode materials for SIBs,the(NH4)3FeF6/GNS composites exhibited outstanding rate performance.At 0.1 A g-1.0,2.0 and 10.0 A g-1,the(NH4)3FeF6/GNS delivered the specific capacities of 121.9,99.3,94.8 and 90.7 mA h g-1,respectively.Therefore,the(NH4)3FeF6/GNS composites are potential eletrode materials for SIBs.The(NH4)3FeF6/GNS delivered a high specific capacity of 44.3 mA h g-1,energy density of 75.7 Wh kg-1 and power density of 8750 W kg-1 for symmetrical Na-ion capacitor(NNSNC).In addition,the diperovskite-type(NH4)3FeF6/GNS take NH4+as cavities filler of crystal structure.The NH4+show highly reversibility in the electrochemical process.At 0.05 A g-1,the initial reversible capacities of(NH4)3FeF6/GNS is 41.1 mA h g-1 for symmetric ammonium ion capacitor.Moreover,when the energy density is 75.7 Wh kg-1,the power density of(NH4)3FeF6/GNS as high as 87.5 W kg-1.