Study On Two-dimentional Transition Metal Compound@graphene Composites Applied In Sulfur Host And Modified Separators

The high specific capacity(1673 m A h g^-1)and energy density(2,600 W h kg^-1)of lithium-sulfur（Li-S）batteries make them the most promising next-generation energy storage batteries.Although,Li-S batteries have the potential to be very useful,the shuttle effect,low conductivity of sulfur,and the lithium dendrite issue continue to make them difficult to use in real-world applications.In this thesis,a series of two-dimensional transition metal compounds@graphene composites were designed and prepared.With the help of electrostatic,coordination,and molecular self-assembly techniques,two-dimensional defective transition metal sulfide/selenide@graphene and Fe（OH）₃@graphene composites with the unique structure were controllably constructed.The relationship between doped metal and two-dimensional defect transition metal sulfide/selenide structure was discussed,and the effects of experimental conditions on the composition and structure of graphene surface defect transition metal ultra-thin materials were systematically studied.On the one hand,such functional composites as transition metal sulfide/selenide and hydroxide compounds have strong adsorption and catalysis,which can effectively promote the conversion of elemental sulfur and lithium polysulfide.On the other hand,the presence of graphene makes the composites have good conductivity,which enhances the electrochemical performance of the lithium-sulfur battery.The internal relationship between their composition,structure,and electrochemical behavior is revealed through the comprehensive and systematic investigation of the electrochemical properties of transition metal compounds@graphene composites such as Fe-Mo Se₂@r GO,Ni-WS₂@r GO,and Fe（OH）₃@GO in lithium-sulfur batteries,and the essential mechanism of energy storage behavior of such composites is clarified.The following works mainly have been carried out:1.Design and synthesis bifunctional electrocatalyst（Fe-Mo Se₂@r GO）and apply for a high-sulfur-loading cathode in Li-S batteries.A bifunctional electrocatalyst（Fe-Mo Se₂@r GO）is fabricated（by adjusting cation iron doping in Mo Se₂ and then in situ hybridized with conductive r GO nanosheets）to meet the practical requirement of Li-S batteries.The introduction of the iron atom into Mo Se₂ would result in more metal defects and expose more selenium edges sites,which has been verified on TEM images,XRD patterns,and Raman spectra.The Fe-Mo Se₂@r GO nanohybrid not only exhibits the extraordinary adsorption effect on Li₂S₆ solution,leading to a strong chemical interaction between Fe-Mo Se₂@r GO and lithium polysulfides confirmed by XPS spectra but also shows the excellent catalytic performance on lithium polysulfides measured from the CV results in symmetric cells.At the same time,the nanohybrid presents a faster response time and higher response current than the Mo Se₂@r GO in Li₂S potentiostatic deposited experiment as well as in Li₂S potentiostatic dissolved experiment,which displays the significant enhancement in Li₂S formation/dissolution kinetics on Fe-Mo Se₂@r GO nanohybrid.The functional plane,which is made from the Fe-Mo Se₂@r GO nanohybrid with plenty of defects,has been designed and applied in Li-S batteries to develop the functional separator and the multi-layer sulfur cathode.The cell with the Fe-Mo Se₂@r GO decorated functional separator exhibits the superior anti-self discharge behavior and the excellent electrocatalytic ability,which results in the retention capacity of 462 m A h g^-1 after the 1000^th at 0.5 C,and 516 m A h g^-1 after the 600^th at 0.3 C.Even at low electrolyte conditions and high sulfur loadings,7.0μL mg_sulfur^-1 and 3.46 mg cm^-2,15μL mg_sulfur^-1 and 3.73 mg cm^-2,it can still present high reversible discharge capacities 679 and 762 m A h g^-1 after 70 cycles,respectively.Moreover,towards the way that paves for developing high electrochemical behavior of Li-S batteries at a high-sulfur-loading cathode,the cell assembled with a unique multi-layer cathode technique（as a novel strategy）has been introduced to explore the electrocatalytic effect of Fe-Mo Se₂@r GO further.Although their sulfur loadings are up to 8.26 mg cm^-2 and 5.2 mg cm^-2,the Li-S Cells assembled with the bi-layer sulfur cathode and the tri-layer sulfur cathode give good electrochemical performances with the reversible discharge capacities of 3.3 m A h cm^-2 after the100^th cycle and 3.0 m A h cm^-2 after the 120^th cycle,respectively.This study not only proves that the Fe-Mo Se₂@r GO functional plane is successfully designed and applied in Li-S batteries with excellent electrochemical performances but also puts forward a novel strategy to construct a unique multi-layer sulfur cathode for developing advanced Li-S batteries with high-sulfur-loading and lean E/S ratio.2.Synthesis and application of ultrathin nickel-doped tungsten sulfide（Ni-WS₂@r GO）as a modified separators in Li-S batteries.The conventional separators in Li-S batteries,such as PE and PP,could not prevent the migration of lithium polysulfides（Li PSs）to the Li anode due to their open-porous structure.Hence,the modified separators are introduced in Li-S cells to hinder the shuttle effect of Li PSs and promote cell performance.In this work,the ultrathin nickel-doped tungsten sulfide anchored on reduced graphene oxide（Ni-WS₂@r GO）is developed as a new modified separator in the Li-S battery.The surface engineering of Ni-WS₂@r GO could enhance the electrocatalysis performance and afford abundant chemical anchoring sites for lithium polysulfides（Li PSs）adsorption,which is convinced by the high adsorption energy and the elongate S-W-S bond given using density functional theory（DFT）calculation.Concurrently,the Ni-WS₂@r GO as a modified separator could effectively catalyze the conversion of Li PSs during the charging/discharging process.The Li-S cell with Ni-WS₂@r GO modified separator achieves a high initial capacity of 1160.8 m A h g^-1 at the current density of 0.2 C with a high sulfur-content cathode up to 80 wt%,and a retained capacity of 450.7 m A h g^-1 over 500 cycles at 1.0 C,showing an efficient preventing polysulfides shuttle to the anode while having no influence on Li⁺ion transference across the decorating separator.The strategy adopted in this work would afford an effective pathway to construct an advanced functional separator for practical high-energy-density Li–S batteries.3.Fe（OH）₃@GO nanocomposite synthesized and developed as a sulfur host for Li-S batteries.The Fe（OH）₃@GO nanocomposite,10–20 nm particles anchored on GO surface,as a novel sulfur host has been developed for Li-S batteries,where the construction of Fe（OH）₃@GO nanoarchitecture proceeds via electrostatic self-assembly process between the negatively charged GO sheets and the positively charged Fe（OH）₃ colloid particles.The wealthy hydroxyl groups of Fe（OH）₃ would appeal to intermediate polysulfides more strongly than Fe₂O₃,resulting in the surface chemical bonds between Fe（OH）₃and polysulfides.On the other hand,GO often contains different functional groups capable of entrapping polysulfides via forming chemical interactions.These advantages of both Fe（OH）₃ and GO give the S/Fe（OH）₃@GO cathode high specific capacity(1569.8 m A h g^-1 at 0.5 C),good rate performance up to 5.0 C,and long cycling stability over 500 cycles under high sulfur loading.Compared to Fe₂O₃@r GO nanocomposites,the enhanced electrochemical performance for the Fe（OH）₃@GO nanocomposite could be attributed to both sufficient surface binding interaction and fast charge-transfer kinetics.