Controllable Synthesis And Application Of Hollow Nano-structured Metal Sulfides In Lithium-sulfur Batteries

With the development of society and the advancement of technology,traditional lithium-ion batteries?LIBs?based on intercalation reactions can not satisfy the ever-growing demands of large-scale energy storage systems and electric vehicles owing to their limited theoretical energy density and high cost.Lithium-sulfur?Li-S?batteries,based on multielectron redox conversion,are regarded as one of the most promising next-generation energy storage devices due to their high theoretical specific capacity(1675 mAh g^-1),energy density(2500 Wh kg^-1),as well as the natural abundance,low cost and environmental benignity of S.However,there are some limitations for the commercialization of Li-S batteries.The design of hosts is a common method to improve electrochemical performance of Li-S batteries.Among various sulfur hosts,the hollow nano-structures with large internal void space can not only provide high sulfur loading but also effectively accommodate the volume expansion during lithiation,the shells also can physically confine the soluble polysulfides;polar transition metal sulfides have strong chemical adsorption for polysulfides and can further catalyze conversion.Therefore,application of both two as hosts is becoming a research hotspot.In this thesis,the controllable synthesis of the nano-hollow cubic boxes?Ni-Co-MoS₂-HCBs?assembled by ultra-thin MoS₂ nanosheets with dopant Ni and Co atoms?Ni-Co-MoS₂-HCBs?was investigated.Followed by infusion of sulfur into the hollow cubic boxes using the melt-diffusion strategy at 155 ^oC,S/Ni-Co-MoS₂ was prepared and further used as a cathod to study its effects on electrochemical performanceof Li-S battery.Secondly,a facile metal-organic framework?MOF?-derived self-template approach was employed to prepare hollow cobalt sulfide polyhedralwith vertically aligned MoS₂ nanosheets?denoted as CoS@MoS₂-HPDs?as efficient hosts for Li-S batteries.Both two hosts are synthesized by a simple solvent thermal reaction without further energy-and time-consuming annealing procedures.The main contents and conclusions of this work are summarized as follows:?1?Prussian blue analogue?Ni-Co-PBA?nanocubes were used as a hard template to synthesize nickel and cobalt incorporated MoS₂ hollow cubic nano-boxes?Ni-Co-MoS₂-HCBs?.As a control group,MoS₂-NSs were prepared without Ni-Co-PBA templates.S/Ni-Co-MoS₂ was prepared by melting diffusion at 155 ^oC and then heating to 200^oC to remove the redundant sulfur on the surface,which was used as cathodes for lithium-sulfur batteries.It is shown that Ni-Co-MoS₂-HCBs exhibits stronger adsorption capability for polysulfides than MoS₂-NSs.Moreover,the capacity of S/Ni-Co-MoS₂electrode is higher than S/MoS₂ with the nearly same S loading.From the reults of discharge/charge and cyclic voltammetry?CV?,it can be demonstrated that Ni-Co-MoS₂-HCBs is able to boost the electrochemical reaction kinetics during the discharge/charge processes in Li-S batteries due to its improved discharge capacity and reductive polarization,which is mainly attributed to the the synergistic effect of polar MoS₂with dopant Ni and Co atoms can effectively anchor polysulfides and nano-hollow cubic boxes physically confine the soluble polysulfides.?2?A self-template approach was used with ZIF-67 as the template to in situ prepare hollow cobalt sulfide polyhedral with vertically aligned MoS₂ nanosheets?CoS@MoS₂-HPDs?.Then,different methods of sulfur loading were carrird out to explore the best condition for S/CoS@MoS_2.It is found that combination the advantages of melt diffusion and vapour phase infusion method,the best S/CoS@MoS₂ maintaining the original polyhedral shape with smooth surfaces can be obtained,which was furtherused as cathode for Li-S batteries.A initial discharge capacity of 995.1 mAh g^-1 at 0.1 C is reached with coulombic efficiency above 99%.The intial discharge specific capacity at 0.5 C is 567.9 mAh g^-1and the discharge capacity of 392.2 mAh g^-11 is maintained after 200 cycles.During the cycle tests,the capacity retention rate was 69%with a low decayrate of 0.15%per cycle and the coulombic efficiency is up to 100%.It was indicated that the polar CoS@MoS₂-HPDs effectively anchor polysulfides due to the synergistic effect of bilayer polar sulfide?interior cobalt sulfide and exterior MoS₂?and the unique hollow structure which can physically block the outward diffusion pathways.