The Energy Storage Mechanism Investigation Of Sulfide And Sulfur/carbon Composites For Alkali Metal Batteries

Among all kinds of energy storage devices,lithium-ion batteries have been popularized and applied in portable electronic devices and electric vehicles due to their high energy densities,long cycle life and environmental friendliness.However,its future large-scale development is seriously hindered because of the scarcity and high price of lithium salts.In recent years,researchers have gradually turned their attentions on sodium/potassium ion batteries which are chemically similar to lithium ion batteries.Besides,the sodium/potassium salts are much cheaper and more abundant compared to lithium salts.The development of high-performance electrode materials is the key to improve the electrochemical performance of sodium/potassium ion batteries and promote their commercial application.Sulfur-based electrode materials based on multi-electron transfer reactions usually have high theoretical capacity,but the poor electronic conductivity and large volume expansion effects greatly reduce the cycle stability of the batteries.Especially for the sulfur cathode material,the poor electrochemical performances are mainly caused by the severe dissolution and shuttle of active polysulfides during the charging and discharging process.Therefore,rational structural design of electrode materials is extremely necessary.In this thesis,we mainly focused on the key problems of sulfur-based electrode materials,including transition metal sulfides and sulfur cathode.By controlling and optimizing the structure of materials,we study the relationship between structure and performance of the electrode material via the systematic in-situ and ex-situ characterizations and reveal the electrochemical reaction mechanism of electrode materials.Our works are beneficial to develop high-performance electrode materials for alkali metal ion batteries and provide scientific basis and theoretical guidance for the future practical application.In first chapter,the background significance,research progress and operating principle of various alkaline metal ion batteries and room temperature sodium-sulfur batteries are introduced in detail.In second chapter,the experimental drugs,experimental instruments,testing and characterization methods in the article are detailedly introduced.In third chapter,we has designed a template method to synthesize a 3D porous interconnected architecture with ultrathin Sn S nanosheets grown on the inside and outside of the hollow mesoporous carbon spheres crosslinked r GO conductive networks.The ultrathin Sn S nanosheets can expose more active sites and contribute more capacity.The hollow mesoporous carbon spheres and r GO not only improved the conductivity of the electrode material,but they suppressed the volume expansion of Sn S nanosheets,thereby exhibiting excellent lithium and sodium storage capabilities.In forth chapter,a porous nanocomposite consisting of the heterostructured Ni S/Fe S embedded in porous carbon cube interconnected with CNTs framework has been rationally synthesized through the simple co-precipitation and post-heat treatment vulcanization process.It can provide a stable interconnected conductive network and greatly shorten the diffusion path of alkali metal ions.And the bimetallic sulfides could produce more redox reaction with the higher specific capacity.Then,the bimetallic sulfide nanocomposites exhibit excellent electrochemical performance.Finally,the electrochemical mechanisms are revealed by combining ex-situ XRD and ex-situ TEM characterizations.In fifth chapter,we have developed the methods of sol-gel method and ammonia water etching to construct the nanocage structure with uniform morphology and size.And an universal applicable vulcanization technology with one-step was found to obtain a variety of bimetallic sulfides called the Ni₃S₂/Co₉S₈,Ni_0.96S/Fe S and Fe S/Co S.Then,the bimetallic sulfides are coated by graphene.Open-ended nanocage structure with large specific surface area can increase the diffusion rate of ions,increase the contact area between the material and electrolyte,and inhibit the volume effect caused by bimetallic sulfide.Therefore,the electrode material exhibits excellent rate performance and cycle stability in potassium ion batteries.By matching the bimetallic sulfide anode with the K₂Fe Fe(CN)₆cathode material,the obtained potassium ion full battery can still display excellent electrochemical performance.In sixth chapter,the modification measures of sulfur cathode for the room temperature sodium-sulfur batteries are analyzed.we have designed the spherical Mo₂N-W₂N@PC superstructure by homogeneous hollow carbon nanospheres with the Mo₂N-W₂N heterostructure.The numerous micro-mesopores of the nanocomposites could confine S into the micro-mesopores and accommodate volume expansion during the cycling process.And the same time,the Mo₂N-W₂N heterostructure could not only promote the adsorption to sodium polysulfide,but catalyze the rapid conversion process from sodium polysulfide to Na₂S,thus realizing high-performance room temperature sodium-sulfur batteries.Meanwhile,the charge-discharge mechanisms of the room temperature sodium-sulfur batteries are revealed by UV-vis spectroscopy,precipitation experiments,in-situ XRD and ex-situ XPS.In eighth chapter,we summarize the conclusions of the whole thesis,put forward the innovations and shortcomings of our works and makes plans and prospects for the future work.