Study On The High Performance Metal-based Cathode Materials Of Lithium Sulfur Battery Based On The Catalytic Conversion Of Lithium Polysulfide

Li-S battery with high theoretical capacity and energy density has drawn much attention in recent years as a possible replacement for current Li-ion battery technologies.A major drawback to Li-S battery is the severe capacitance fading effect which,to a large extent,stems from the dissolution and diffusion of lithium polysulfides(Li PS)formed during both charge and discharge cycles.Severe migration of Li PS leads to the shuttle effect that may break down a Li-S battery.Meanwhile,the sluggish kinetics of Li PS transformation is also the obstacle of the commercialization of Li-S battery.In order to improve the existed physical confinement approach showing weak interactions with Li PS and leading to unsatisfied stability in long-term cycling,we propose a new strategy which combines physical confinement with chemically catalytical conversion of Li PS.Several porous micro-/nano-structured materials with high activity in Li PS transformation were developed and their physical/chemical structure-electrochemical performance correlations have been systematically stuided.The main results of this thesis are listed below:(1)The preparation and electrochemical performance of hollow-structured porous metal oxidesTo solve the insufficient confinement capability and low activity in Li PS conversion of the reported metal oxides,hollow structured metal oxide(Co₃O₄,Mn₂O₃,and Ni O)submicro-spheres are prepared by a novel method and employed as efficient Li PS immobilizers.The Li-S batteries,based on the developed metal oxide spheres,possess outstanding rate capability and cycling stability.The best performing S/C/Co₃O₄ electrode delivers excellent cycling stability with only a 0.066%capacity decay per cycle during 550 cycles,lower tha that of S/C/Mn₂O₃,S/C/Ni O and S/C(with decay rate of 0.071%,0.109%and 0.129%per cycle).Moreover,its discharge capacity is as high as 428 m Ah g^-1 at a 3 C rate which is far superior to that of bare S/C(115m Ah g^-1).The fast kinetics of the electrocatalytic conversion of Li PS on the developed Co₃O₄ electrode and its unique hollow structure are the key factors to its outstanding performance as a Li-S battery cathode material.(2)The preparation and electrochemical performance of yolk/shell structured metal sulfide@nitrogen-doped carbonConsidering the low conductivity and the still unsatisfied capacity and rate performance of metal oxides in Li-S battery,we further study the metal sulfide which is believed to have higher conductivity than oxides.A yolk/shell structured Mn S@N-C was prepared and employed as the sulfur host,providing both the physical confinement of sulfur species and strong binding capability of Li PS.Thanks to the high electrocatalytic activity in Li PS transformation,the S/Mn S@N-C delivers a better rate performance in comparison to S/N-C and long cycle life with a low capacity decay rate of 0.09%per cycle after 330 cycles at 0.5 C.(3)The structure engineering and optimization of electrochemical performance of molybdenum phosphideIn order to further improve the catalytic activity of the cathodes in Li PS transformation,we optimize the structure of the catalyst from the aspects of conductivity,particle size,dispersion,and elemental type of the nanomaterials.Given that metal phosphides generally show higher conductivity than metal oxides/sulfides and the stable Mo-S bonds in molybdoenzyme for biological catalysis,we employed molybdenum phosphide as a catalyst for Li PS transformation in this study.The prepared Ru-doped Mo₄P₃(Ru-Mo₄P₃)through Ru-doping induced phase transformation from Mo P to Mo₄P₃(Ru-Mo₄P₃)could significantly facilitate the electrocatalytic conversion of Li PS.When 5 nm Ru-Mo₄P₃ nanoparticles(NPs)are homogeneously decorated on hollow carbon spheres(HCS),the obtained HCS-Ru-Mo₄P₃delivers excellent activity in Li PS transformation.In Li-S battery,the S/HCS-Ru-Mo₄P₃ electrode shows high reversible capacities of 1178 m Ah g^-1 and 660 m Ah g^-1 at 0.5 C and 4 C,respectively.The rate performance of the developed S/HCS-Ru-Mo₄P₃ is among the best of the reported transition metal phosphide cathodes for Li-S batteries.When the S loading is as high as 6.6 mg cm^-2,S/HCS-Ru-Mo₄P₃ retains a reversible areal capacity of 5.6 m Ah cm^-2 after 50 cycles,higher than that of the commercial Li-ion battery(4 m Ah cm^-2).The excellent Li-S battery performance can be attributed to the intrinsically active Ru-Mo₄P₃ phase combining with hollow carbon structure,which significantly facilitates the electrocatalytic conversion and entrapment of Li PS.(4)Investigation of the atomically dispersed Mo-N-C catalyst and its mechanism for Li PS catalytic conversionIn order to solve the relatively low utilization of Mo atoms in the case of Ru-Mo₄P₃NPs,the atomically dispersed Mo-N-C nanosheets are developed as a cathode to boost the Li PS conversion for Li-S battery.X-ray adsorption spectroscopy(XAS)indicates the Mo atoms are two-fold coordinated with edge N to form a unique Mo-N₂/C site.Thanks to its high intrinsic activity,the rate capability and cycling stability of S/Mo-N-C are greatly improved compared with S/N-C due to the accelerated kinetics and suppressed shuttle effect.The S/Mo-N-C delivers a high reversible capacity of 743.9m Ah g^-1 at 5 C rate and an extremely low capacity decay of 0.018%per cycle after 550cycles at 2 C rate,which outperforms most of the results in literatures.Density functional theory(DFT)calculations suggest that the Mo-N₂/C sites can alleviate the activation energy for Li₂S₄ to Li₂S conversion and the decomposition barrier of Li₂S,accounting for its inherently high activity towards Li PS transformation.