Regulation Of Electrode Surface And The Morphology Of Lithium Sulfides Deposition In Lithium-sulfur Batteries

Rechargeable lithium-sulfur(Li-S)batteries have attracted academic and industrial attention with high energy density,low cost and environmental benignity.However,the notorious shuttle effect causes severe active materials loss and low coulombic efficiency(CE),resulting in cell degradation.Additionally,the poor electronic conductivity of S(?=5×10-30 S cm-1)and Li2S(?=10-13 S cm-1)gives rise to sluggish reaction kinetics.The uncontrollable Li2S deposition rapidly covers the conductive surface,leading to the passivation of the electrode.Lithium dendrite causes short-circuit in the battery,resulting in severe safety hazard.To realize the practical application of Li-S batteries,effective strategies are highly desirable.Surface regulation is a promising strategy to address above issues in Li-S batteries.Chemical and electrochemical behavior of polysulfides can be well-controlled by rational design of electrode surface,promoting polysulfides transformation accordingly.The main contents in this thesis are as follows:In chapter 1,the principles and the advantages of Li-S batteries are provided.In addition,the intrinsic challenges and the corresponding strategies are reviewed in detail.At the end of this chapter,we comprehensively summarize the research progress of practical Li-S batteries with high sulfur loading and lean electrolyte.The materials,experiments methods and the instruments employed to investigate Li-S batteries are provided in-chapter 2.In chapter 3,we regulate surface chemistry of electrode by temperature mediation.The morphology of Li2S deposits is dense and continuous at 25?,which rapidly covers the conductive surface,inhibiting further reduction of polysulfides.In contrast,Li2S deposits are particulate at 60?,which is attributed to a faster Ostwald ripening rate at high temperature.3-dimensional Li2S formation at 60? mitigates the passivation of conductive surface,enhancing the utilization of active materials.This temperature effect remains valid for Li-S batteries with high S loading(3.6 mg cm-2)and lean electrolyte(E/S=3 ?L mg-1).The cells operated at 60? can deliver a high specific capacity of 1228 mAh g-1.In chapter 4,we investigate the correlations between Li2S deposition and wetting properties of electrode surface.In distinct contrast with previous report,improved cell performance is achieved with hydrophobic carbon paper.Particulate Li2S is formed with hydrophobic carbon paper,which mitigates the passivation of conductive surface.By comparison,the insulating Li2S film cover the conductive surface with hydrophilic carbon paper,leading to passivation of the electrode surface.In chapters 5,we synthesize the conductive MOFs and COFs materials,which are applied to control the behaviour of polysulfides in Li-S batteries.In chapter 6,a typical conductive MOF(Ni-HHTP)are applied to regulate the surface of self-supported carbon paper.In contrast with non-conductive MOF(Ni-BTC),Ni-HHTP not only have strong adsorption towards polysulfides,but also possess superb electronic conductivity,therefore accelerating the reaction kinetics of Li2S deposition.In this work,we demonstrate that synergizing strong polysulfides adsorption and high electronic conductivity is critical for rational design of electrode materials in Li-S batteries.In chapter 7,we modify the surface of routine separator with LiF to improve the utilization of active materials in Li-S batteries.The functional separator not only suppress shuttle effect and accelerate the reaction kinetics in Li-S batteries,but also inhibit the formation of lithium dendrite,avoiding the short-circuit in Li-S batteries.In this work,we provide a facile and effective strategy to resolve the challenges from both cathode and anode in Li-S batteries.