Modifying Capacity Of Commercial FeS₂ And Invesitigating Mechanisms Of Promoted Capacity

Because of abundant resources and low cost,sodium-ion batteries(SIBs)were considered to work as large-scale energy storage system.FeS2 existed as pyrite ore in nature which meant sufficient resource and facile obtainment.As anode material for SIBs,it possessed high theoretical capacity up to 894 mAh g-1.However,direct utilization of commercial FeS2 ore as active material resulted in low capacity and short cycling life due to the huge volume expansion and "shttule effect" of active materials.To enhance sodium-ion storage capacity of FeS2,former researchers mostly focused on the structural design of FeS2 by reducing particle size or combining with carbonaceous materials.But the complex synthesis routines limited the large-scale production.In this work,a surficial modification of commercial FeS2 bulks via simple solid-phase selenization was carried out.Reverisible discharge capacity and cycling life was evidently elevated indicating bright commercial utilization and provided theoretical guidance for modifying electrode materials.Two main contents in this work were shown below:(1)The preparation of modified FeS2 composite and sodium-ion storage test.Commercial FeS2 bulks were directly modified via calcination under high temperature to introduce FeSe2 on surface.By altering the time calcination,a series of modified FeS2 were obtained.Through analyzing the contents in synthesized samples,it was found that ratio of FeSe2 was lifted which meant higher degree of selenization when calcination time ranged from 20 min to 4 h.In the modified FeS2 with calcination time of 4 h,the content of Fe7S8,which was the decomposition product of FeS2,was obviously promoted.Under the rate performance and cycling stability tests,the specific capacity of modified FeS2 increased during calcination time enhanced up to 3 h.However,the specific capacity decreased when calcination time was 4 h.It was because FeSe2 possessed lower theoretical capacity than FeS2 and excessive introduction of FeSe2 lowered the capacity of entire composite.Meanwhile,severe decomposition of FeS2 also weakened the content of high-capacity FeS2.But all the modified FeS2 could deliver better rate performance and cycling stability than commercial FeS2.The best sample was FeS2-3H with calcination time of 3 h.It kept a specific capacity of 623 mAh g-1 after 100 cycles at current density of 0.5 A g-1 and it still possessed 383 mAh g-1 after 6000 cycles at current density of 10 A g-1.(2)Investigation of mechanisms for promotd electrochemical performance of commercial FeS2.Cyclic voltammetry was utilized to discover the reaction mechanisms of both FeS2-3H and commercial FeS2.By comparing different signals,it was found that sodium-ion inserting into FeSe2 in FeS2-3H occurred before the reaction of main phase of FeS2.Through usting SEM,TEM,XPS to track the behavior of FeS2-3H in different reaction stages,after the insertion of sodium ion into FeSe2,a layer of solid electrolyte interface(SEI)was formed due to the decomposition of electrolyte.This SEI was attached close to the surface of active material and uniform in thickness.The existence of this SEI make following insertion of sodium ion homogeneous and rapid.Ultimately,stable and integrated SEI on surface was formed after only one cycle and initial coulombic efficiency was promoted.Even though bulk particles were divided,self-healing of SEI on fracture surface was found to be swift and it provide effective path for diffusion of sodium-ion which weakened the polarization of FeS2-3H.Favorable interface of FeS2-3H also could suppress the "shuttle effect" of active materials and simultaneously ensured fast transport of ions.Consequently,long cycling life and high rate performance were both obtained in FeS2-3H.