Synthesis Of Double-shelled Structure Composites For Advanced Lithium-Sulfur Batteries

With the increasingly rigorous environmental problems and the rapid development of electronic equipment,it is high desire to design and develop efficient energy storage equipment with high energy density and long-term cycling stability.Lithium-sulfur（Li-S）batteries,as a promising candidate for next generation energy storage devices,have received widely attentions in terms of their attractive theoretical energy density,high theoretical capacity,abound resources together with the low price and environment benignity of the sulfur cathode.Nevertheless,the commercial development of Li-S batteries is impeded by several obstacles.Briefly,the intrinsic insulating property of both sulfur(5×10^-30 S cm^-1)and the discharged products,a significant volume expansion（⁸0%）occurs during the cathode discharge process as well as the serious of"shuttle effect",resulting in the poor utilization of sulfur,fast decay capacity and poor rate performance.Based on this,it has important theoretical and practical significance to design and develop composite materials with special structure and composition to overcome the aforementioned problems,improving their application performance.Double-shelled structure nanocomposites will be capable of solving those difficulties of Li-S batteries.Primary,the hollow structure not only provides sufficient space for sulfur loading,but also accommodates volumetric effect of sulfur upon cycling.Next,the carbon layer improved the electrical conductivity for electron transport during the charging-discharging process.Finally,the interaction between metal oxides/sulfides and lithium polysulfides is able to inhibit the diffusion of polysulfide,alleviate the shuttle effect and improve the reaction kinetics.Inspired by this,this paper aims to construct two kinds of double-shelled structure nanocomposites as sulfur hosts for advanced Li-S batteries.Sulfur was introduced into those nanocomposites through a melt-diffusion method.The S/MoS₂@HCS composites and S/NiO-NiCo₂O₄@carbon composites have been synthesized respectively.Besides,the preparation conditions and microstructure as well as the electrochemical properties of those materials were studied,the results are as follows:1.Characterization of S/MoS₂@HCS composite:A well defined Ni-MOF precursor was first fabricated by a facile hydrothermal method.After a simple combination of carbonization and Fe³⁺treatment,the Ni-MOFs was then converted into Hierarchical carbon spheres（HCS）,which constructed from building blocks of hollow carbon nanobubbles.Next,a layer of MoS₂ nanosheets was coated onto the surface of the HCSs through a solvothermal strategy.Finally,sulfur was steamed into the MoS₂@HCS host by a melting-diffusion process to form the S/MoS₂@HCS composite.The TGA analysis shows that the mass loading of sulfur is⁷0 wt%in the MoS₂@HCS composite.Theoretical studies show a strong absorption of the MoS₂ nanoshell to polysulfides.In addition,we first calculated the elastic constants of MoS₂ during cycling,indicating that the excellent mechanical performance of the MoS₂ nanosheets after the adsorption of polysulfides.When tested as the cathode of Li-S batteries,the S/MoS₂@HCS composite electrode shows excellent electrochemical properties with high capacity of 1048 mAh g^-1 at 0.2 C,attributing to the component and unique double-shelled structure.Hollow carbon bubbles can not only reduce the resistance of electron and ion transport during the charge-discharge process but also accommodate large volumetric expansion of sulfur upon lithiation.The polar MoS₂ nanosheets throughout the hierarchical carbon spheres effectively adsorb polysulfide,suppressing the polysulfide shuttling effect and promoting the electrochemical redox kinetics in Li-S batteries.2.Characterization of S/NiO-NiCo₂O₄@C composite:Uniform Ni-Co prussian blue analogue（PBA）nanocube precursor was first prepared by a facile co-precipitation strategy.Afterwards,double-shelled NiO-NiCo₂O₄ heterostructure@C nanocages were obtained through a facile calcination treatment coupled with a simple hydrothermal carbon coating process.After a melt-diffusion process,sulfur was successfully introduced into the NiO-NiCo₂O₄ heterostructure@C nanocages.The mass proportion of sulfur in the S/NiO-NiCo₂O₄@C composite was determined by TGA as 73wt%.In addition,we analyzed the interaction between NiO-NiCo₂O₄ heterostructure and lithium polysulfide.Meanwhile,we also verified the catalytic role of NiO-NiCo₂O₄in Li-S battery.As a result,the prepared S/NiO-NiCo₂O₄@C electrode exhibits stable cycling life up to 500 cycles at 0.5 C with high coulombic efficiency of over 98%and a very low capacity decay rate of only⁰.059%per cycle,which is superior to that of many other metal oxides electrodes.Benefiting from the structural and compositional superiorities,this electrode exhibits remarkable electrochemical performance with high specific capacity,excellent cycle life,and good coulombic efficiency in Li-S batteries.In summary,we have designed and synthesized two kinds of double-shelled structure nanocomposites as the host of sulfur,exhibiting brilliant electrochemical performance.The structure can not only improve the sulfur loading,but also provide sufficient space for sulfur to effectively cope with the volume expansion during the lithiation process.In terms of composition,the good conductivity of carbon can promote electron conduction,thus improving the conductivity of the cathode.Besides,the polarity of metal oxides/sulfides as well as the catalytic activity in Li-S battery can inhibit the diffusion of lithium polysulfides and improve the kinetics of the conversion reaction.We believe that it provides new ideas for the preparation of high capacity cathode composite materials for Li-S batteries.