Constructing Sulfur/Selenium Based Cathode Materials For High Energy Density Rechargeable Batteries

The development of cathode materials with high energy density has attracted more and more attention,and it is also an important way to solve energy crisis and realize efficient utilization of clean energy.Sulfur based electrode materials have attracted extensive attention because of their ultrahigh theoretical energy density.However,it is great challenge to apply them for practicle application due to their poor conductivity,easily soluble intermediates and material volume expansion during the cycling process,leading to low specific capacity and poor cycle life of batteries.Selenium shows similar chemical properties with sulfur,and possesses enhanced electrical conductivity compared with sulfur.As the cathode material,selenium could provide similar high theoretical volumetric capacity with that of sulfur.However,Se cathode also suffers from short cycle life and low Coulombic efficiency derived from the dissolution of intermediates polyselenides and large volume expansion of materias during the charge/discharge process.Therefore,the rational design and optimization of sulfur/selenium electrode structure is the key to realize the high energy density of batteries.In this thesis,aiming at the existing problems of high energy density sulfur/selenium cathode materials,we carry out rational electrode structure design to improve electrochemical performance,reveal the reaction mechanism of electrode materials,and further clarify the correlation between structure and property.The main contents of this thesis as presented as follows:In the first chapter,the research progress of sulfur/selenium-based cathode materials for rechargeable batteries are reviewed in detail.In the second chapter,the chemicals,instruments,preparation methods and characterization methods used in our experiment are introduced in detail.In the third chapter,one-step synthesis procedure is developed to prepare flexible S0.6Se0.4@CNFs electrode by co-heating S0.6Se0.4 powder with electrospun PAN nanofiber papers at 600?.The obtained S0.6Se0.4@CNFs film displays superior electrochemical performance in Li-S batteries that attributed toits rational structure design including the chemical bonding between Se and S,the chemical bonding between S0.6Se0.4 and CNFs matrix and the 3D CNFs network.In the fourth chapter,we design a dual polysulfide confinement strategy by confinement of sulfur in polydopamine-coated MXene nanosheets(S@Mxe@PDA)that performs as a high-performance cathode for Li-S cells owing to their inherently high underlying metallic conductivity,chemical bonding and strong chemical adsorption to lithium polysulfides.Thus,the S@Mxe@PDA cathode delivers outstanding electrochemical properties even with high areal sulfur loadings.In the fifth chapter,we design a dual functional free-standing flexible conductive carbon scaffold embedded with TiN-VN heterostructures nanoparticles(TiN-VN@CNFs)as advanced host simultaneously for both S and Li electrodes.As the cathode host,the TiN-VN@CNFs combines the merits of strong anchoring ability for polysulfides and catalyzing their fast transformation,thereby greatly suppressing the polysulfides shuttling,promoting the redox kinetics.When used for the Li host,the TiN-VN@CNFs matrix with lithiophilic feature can reduce local electron/ion flux,thus significantly realize uniform Li deposition with markedly suppressed dendrite growth.Benefitting from the synergistic advantages,the coupled Li-S full-battery exhibits superior rate performance and ultralong cycling life with nearly 100%Coulombic efficiency.In the sixth chapter,a flexible freestanding Se/carbon composite film is prepared by encapsulation of Se into the nanotubes(CNTs)interwoven N,O dual-doped porous carbon nanosheets(Se@NOPC-CNT).The 3D interconnected CNT uniformly wrapped on the N,O dual-doped porous carbon skeletons improves the flexibility and offers an interconnected conductive pathway for rapid ionic/electronic transport.In addition,the N,O dual-doping significantly enhances the chemical affinity for polyselenides.Therefore,the Se@NOPC-CNT displays excellent electrochemical performance in both Na-Se batteries and K-Se batteries.In the seventh chapter,we develop an advanced K-SeS2 battery by encapsulating SeS2 in free-standing N-doped porous carbon nanofiber film(SeS2@NCNFs).The NCNFs provide enough pores to host large amounts of SeS2 with enhanced energy density,accommodate the volume change during cycling and facilitate the electrolyte to penetration.Combined with these merits,the as-prepared SeS2@NCNFs exhibits good electrochemical performance in the low-cost carbonate electrolyte in terms of high reversible capacity,long cycle life and high energy density.In the eighth chapter,we present the summary of the main innovative work and deficiencies,outlook and perspective of this thesis.Finally,some research work about structural design and optimization of Na3V2(PO4)3 cathode material for sodium ion batteries and all solid-state sodium batteries are involved in this thesis,which are described in the appendix A and appendix B.