The Regulation Of Electrochemical Reaction Of Lithium-sulfur Batteries And The Research Of Electrochemical Performance

With the environment pollution and gradual exhaustion of fossil energy around the world,various clean and renewable energy such as wind energy and solar energy have attracted much attention in recent years.However,these energy have a big defect namely instability and discontinuity.Therefore,to store these instable energy in the stable electrochemical storage system for the application of smart grids,electric vehicles and electronic equipment will be an effective develop pathway for clean energy.Bacause of high energy density of 2600 Wh kg^-1,lithium-sulfur batteries（LSBs）have become the most potential next-generation energy storage system to replace lithium-ion batteries.In the last twenty years,LSBs have attracted extensive research interest and great progress has been achieved.However,the actual capacity and cycle stability is still very inferior,which hinders the commercialization of LSBs.This is mainly attributed to some well-known defects of LSBs:poor ionic and electronic conductivity of sulfur and a series of discharge products;severe shuttle effect of intermediate polysulfides;large volume change of the electrode during discharging.Among these defects,the shuttle effect of polusilfides is the most important factor to impact the electrochemical reversibility of LSBs,which is caused by not only concentration-gradient diffusion in the electrolyte but also sluggish reaction kinetics of polysulfides.Therefore,the synergistic regulation of thermodynamic and kinetic diffusion of polysulfides will relieve the shuttle effect of polysulfides to the great extent and thus improve the capacity and cycle stability of LSBs.In this thesis,based on the regulation of electrochemical reaction as the core,the effective materials are selected and designed through simple and commercial synthetic strategies to well alleviate the thermodynamic and kinetic diffusion of polysulfides and improve the cycle performance and energy density of LSBs.These works provide effective ideas for the future development of LSBs.The main contents and research results are listed below:（1）Employing the physical adsorption of porous carbon materials to decrease the thermodynamic diffusion of polysulfides.With the simple and scalable Fe-based metal organometallic complex and large-sized graphene oxide as the precursor,two hierarchically porous carbon materials are acquired through simple one-step carbonization and served as the sulfur host.The high conductivity and porous structure of carbon materials ensure quick electron and ion transfer of sulfur electrode.Moreover,the hierarchical porosity not only provides enough space for uniform dispersion of sulfur but also physically adsorb polysulfides,resulting in the decreased thermodynamic diffusion of polysulfides.As a result,LSBs exhibit improved electrochemical performance.The simple preparation process and satisfactory electrochemical performance give effective pathway for the practical application of LSBs.（2）Selecting and designing effective chemical adsorbents to remit the thermodynamic diffusion of polysulfides and regulate the electrochemical reaction process.Compared with N-doped carbon materials to chemically adsorb polysulfides reported in many literatures,simple and scalable g-C₃N₄ with enriched pyridinic-N can act as the more effective polysulfides adsorbent.Theoretical calculation and spectrum analysis after cycles confirm the strong chemical interaction between g-C₃N₄ and polysulfides,which favors for effective immobilization of polysulfides in the cathode region.Next,nano-sized TiO₂ is selected as the polysulfides adsorbent.Through simple and commercial electrospinning method,ultrafine TiO₂ nanocrystals are uniformly embedded into N-doped porous carbon,which provides abundant chemical adsorption sites for polysulfides.Moreover,N-doped carbon increases the conductivity of TiO₂ and promotes quick electrochemical conversion of polysulfides.The synergistic effect can well regulate the electrochemical reaction,which endows the LSBs with excellent cycle and rate performance.（3）Combining the electrocatalysis and chemical adsorption to synergistically regulate the thermodynamic and kinetic diffusion of polysulfides.First,through multicomponent electrospinning and subsequently controllable carbonization,low-cost and catalytic metal Co nanoparticles are evenly attached on intertwined nanofibers,which are composed of TiO₂ and Co₃O₄ nanocrystal uniformly embedded into N-doped porous carbon.The co-existed two chemical adsorbents of TiO₂ and Co₃O₄ ensure abundant restriction for polysulfides by S-Ti-O bonding and Lewis acid-base interaction,largely decreasing the thermodynamic diffusion of polysulfides.Moreover,metal Co catalyzes the transformation of adsorbed polysulfides,which is favorable for decreased kinetic diffusion of polysulfides.Subsequently,enriched edge sites of layered metal selenides are direct employed to achieve the synergistic regulation of thermodynamic and kinetic diffusion of polysulfides.The edge sites can not only achieve preferentially chemical adsorption of polysulfides but also promote polysulfides conversion via the electron transfer mechanism.As a result,the shuttle effect of polysulfides is suppressed to the large extent and the excellent room-temperature and low-temperature performance is obtained,which extends the practical application of LSBs.