Uncovering The Electrochemical Mechanism Of Transition Metal Chalcogenides Based Electrodes For Rechargeable Alkali Cations Storage

In this dissertation,a variety of transition metal chalcogenide materials were chosen to reveal the real function of ether-based electrolyte in sodium ion batteries.The reaction mechanism and failure mode of each material have been clarified via multiple test methods.And many strategies have been proposed to improve the performance of each material.The main achievements are as follow:(1)For the first time Fe₃S₄ was employed as a promising host material for sodium storage.The Fe₃S₄ anode delivered a high reversible capacity of 548 mAhg^-1,corresponding to 6 Na⁺storage per formula.The reaction mechanism of the as-prepared Fe₃S₄ was systematically investigated and established.The conversion mechanism with 6 Na⁺storage per formula has been proved by ex-situ⁵⁷Fe-M?ssbauer spectra and TEM.The Fe₃S₄ particles were pulverized to FeS_xcompounds composed of FeS,FeS₂ and Fe₃S₄ quantum dots during the sodiation/desodiation processes.These ultrafine size of FeS_xquantum dots is comparable to that of the Fe diffusion distance during cation exchange process,overcoming the kinetic and thermodynamic constraints of electrochemical conversion reactions.Benefiting from its novel mechanism,the Fe₃S₄ delivered remarkable high capacity,high rate capability.(2)Four typical TMCs(FeS₂,FeSe₂,CoSe₂ and NiS₂)were chosen to study their electrochemical properties.The results demonstrated that the superior performance of TMCs actually resulted from one common factor:the copper current collector.In a very unique and interesting process,Cu gradually replaced the transition-metal elements in TMCs to be the active materials during cycling.Therefore,the analyses and assumed mechanisms based on the electrochemical properties reported in some previous studies are incorrect.In-situ XRD technique has been used to analyze the actual chemical/physical process of this unique Cu-driven conversion mechanism,which demonstrated that the transition metals in TMCs were replaced by Cu during the charge process.Based on this new discovery,a new type of sulfur-based battery was designed in which copper foil acts simultaneously as the current collector and as part of the active material.This intelligent design greatly improves the utilization efficiency of sulfur.(3)The fundamental problem of conversion reaction type materials is the volume expansion caused by their reactions.To address this issue,confined NiS_x@C yolk-shell microboxes and yolk-shell microsphere(M-CoS@C)are constructed to address volume changes and confine the active material in the internal void space.Having benefited from the yolk-shell structure design,the prepared electrodes displayed excellent electrochemical performance in lithium-ion batteries.The one-step conversion Li-storage mechanism has been demonstrated via in-situ synchrotron HEXRD.Moreover,three electrode in-situ EIS reveals that the steady SEI layer and stable electrochemical reaction environment are the key factors in improving the performance of conversion type metal sulfide anodes.However,the yolk-shell structure is not the best structure for conversion-type materials.Therefore,the ultrafine confined structure has been proposed in this thesis.