Preparation,modification And Electrochemical Performance Of Lithium Sulfide-graphene Free-standing Cathodes

Lithium sulfide has been considered as one of the promising cathode materials for lithium batteries due to its high specific capacity,high lithium content and matching with non-metallic anodes.However,suffering from the low conductivity of lithium sulfide and the dissolution issue of intermediate products,the specific capacity and cycling performance of the cathode are largely compromised,limiting the practical application of the lithium sulfide positive electrode.In this thesis,through the structural design,controllable preparation and lithium storage mechanism of lithium sulfide cathode,a high-performance lithium sulfide-graphene free-standing cathode was obtained,which provides the theoretical and technical basis for material selection and application of lithium sulfide cathode.According to solid-liquid diffusion theory and combining the changing regulation of adsorption energy of graphene to lithium polysulfide,multidimensional lithium sulfide-graphene free-standing cathode with the structure of r GO coating lithium sulfide was designed,in which lithium sulfide was acted as the active material,r GO was performed as the conductive substrate coating on lithium sulfide.The coating structure can inhibit the dissolution of lithium polysulfide and then improve the cycling performance of cathode.Two-dimensional film-like Li2S/r GO,three-dimensional spongy Li2S-r GO,and three-dimensional nano-bundle Li2S@r GO free-standing cathodes were prepared via vacuum distillation,vacuum impregnation and freeze-drying methods,respectively.The influences of raw materials ratio and temperature on the structure and electrochemical performance of the cathode were investigated.The preparation parameters of the cathode were optimized.Among three kinds of the cathodes,the three-dimensional nano-bundle Li2S@r GO free-standing cathode shows the relatively high capacity.When lithium sulfide content is 58 wt.%,Li2S@r GO freestanding cathode shows the specific capacity of 679.5 m Ah g-1Li2 S after 100 cycles at 0.2 C and 95.0 m Ah g-1Li2 S at 10 C.The reason is that Li2S@r GO free-standing cathode exhibits a conductive network structure with lithium sulfide coated by graphene.r GO coating limits the dissolution of lithium polysulfide.The conductive network structure provides a fast-electronic transmission channel.Thus,the capacity of Li2S@r GO free-standing cathode was improved.In order to improve the high-rate discharge performance,Li2S@r GO freestanding cathode was modified by carbon nanotubes through the vacuum freezedrying method.A free-standing cathode with a nano-bundle array structure was obtained.The influences of raw materials ratio and carbon nanotubes types on the structure and performance of the cathode were investigated.Among them,the cathode modified by FWNTs presents a higher specific capacity.When the mass ratio of FWNTs to GO is 1:4 and the content of lithium sulfide is 58 wt.%,the discharge specific capacity of Li2S/FWNTs@r GO free-standing cathode is 861.9 m Ah g-1Li2 S after 100 cycles after 0.2 C and 433.0 m Ah g-1Li2 S at 10 C.The improvement of high-rate discharge performance of the cathode is because r GO scrolls around FWNTs after introducing FWNTs,which makes lithium sulfide was embedded between r GO and FWNTs.Consequently,the electronic transmission among lithium sulfide,r GO and FWNTs is accelerated.The conductivity of the cathode is increased by 53.0%,and the dissolution of lithium polysulfide is limited.To further improve the cycling performance,Li2S/FWNTs@r GO free-standing cathode was modified via atomic layer deposition technique.Al2O3 modified Li2S/FWNTs@r GO free-standing cathode was obtained.The modification mechanism of Al2O3 was studied by the first principles.The specific capacity and capacity retention of Al2O3 modified Li2S/FWNTs@r GO free-standing cathode is improved to 887.4 m Ah g-1Li2 S and 89.6% respectively,after 100 cycles at 0.2 C.The main reason is that the formation of Li-O bond and Al-S bond between Al2O3 and lithium polysulfide,which effectively adsorbs lithium polysulfide and limits the dissolution.Thus,the dissolution amount of lithium polysulfide is reduced by 2.1%,and the cycling performance of the cathode is improved.The lithium storage mechanism of Al2O3 modified Li2S/FWNTs@r GO freestanding cathode was discussed through the thermodynamic calculation and the analysis of the variation of active material composition during lithium storage.In the process of lithium storage,the cathode undergoes t he multi-step reactions.The long-chain lithium polysulfide gradually stores lithium,and forms Li 2S6 and Li2S4 at 2.3 V and 2.1 V,respectively.Each sulfur atom stores two lithium at the end of the lithium storage process with the production of Li2 S,which indicates the sulfur in the cathode reacts sufficiently and endows the cathode a relatively high specific capacity.