Application Of Molybdenum-Based Materials And Their Heterostructure To The Separator Of Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）battery is one of the most promising next-generation secondary batteries due to its high theoretical specific capacity(1672 m Ah g^-1)and high energy density(2600 m Ah g^-1),and has the advantages of abundant sulfur raw material reserves,low cost,non-toxic and environmentally friendly.However,if Li-S batteries want to be commercialized,there are still some problems that have not been solved.For example,the insulation properties of sulfur and its discharge product Li₂S,the huge change of sulfur volume（up to 80%）during the whole lithification/delithiation process,and the"shuttle effect"of soluble long-chain polysulfides（Li PSs）.At the beginning of the study,the above problems are mainly solved by non-polar carbon-based materials.However,due to the weak van der Waals force between non-polar carbon and polar polysulfide,the shuttle effect cannot be effectively suppressed.Due to its tunable crystal structure,adjustable composition,and variable molybdenum center valence,polar transition metal molybdenum-based materials can be used to suppress the"shuttle effect"through various mechanisms such as polar-polar adsorption,Lewis acid-base interaction,and catalytic conversion reaction.Therefore,in this work,molybdenum（Mo）matrix composites are prepared and applied to the separator as a modification layer to improve the diffusion rate of lithium-ions and achieve rapid conversion of Li PSs.Through the characterization test,the micromorphology of the molybdenum-based derived composites were observed,and the catalytic conversion mechanism was explained by means of CV curve of symmetrical battery,deposition and decomposition of Li₂S,and Galvanostatic Intermittent Titration Technique（GITT）,which proved that the molybdenum-based derived composites could effectively limit the"shuttle effect"and improve the overall cycle performance of the battery.The main research contents are as follows:（1）The rGO/MoS₂ composite was first prepared,and abundant sulfur vacancies were introduced into the composite material using an anion vacancy strategy to achieve high performance of Li-S batteries.The absorption and redox reaction kinetics of Li PSs can be adjusted by optimizing the sulfur vacancy to a higher concentration.In addition,the abundant sulfur defect vacancies in the composite can accelerate the separation of electrons and holes,easily reduce the activation energy barrier of the reaction process,facilitate the electron transfer and adsorption of Li PSs to promote the occurrence of catalytic reactions.Benefiting from these reasons,the Li-S battery based on the r GO/Mo S_2-x-500 modified separator exhibited a satisfactory specific discharge capacity of 1016 m Ah g^-1 at 1 C and a low capacity decay rate of 0.0618%over 900 cycles.（2）The lightweight nanoflake graphitic carbon nitride（g-C₃N₄）was compounded with nanosheet layered molybdenum trioxide（Mo O₃）.In this composite,the polar transition metal oxide Mo O₃ can form a chemical bond with Li PSs,which can effectively slow down the dissolution of Li PSs.Based on the"Goldilocks"principle,Li PSs can be oxidized to thiosulfate by Mo O₃,which promotes the rapid conversion of long-chain Li PSs and provides an interface for the adsorption of Li PSs.The introduction of g-C₃N₄can improve the conductivity and electron mobility,and the large specific surface area after ultrasonic treatment can also provide an interface for the deposition and decomposition of Li₂S,increasing the deposition and decomposition area of Li₂S.The initial capacity is 726 m Ah g^-1 at 1 C,and the capacity decay rate of 400 cycles is 0.0703%.In addition,initial capacity is 542 m Ah g^-1 at 4 C,and the capacity decay rate over 700cycles is 0.0053%.（3）Through the preparation of NiSe₂-CoSe₂ and MoSe₂-Co Se₂ heterostructure composites,the performance advantages of Co Se₂ electronic structure control were compared.Herein,the electronic structure of Co Se₂ can be adjusted by introducing Ni and Mo elements,thus the adsorption and catalytic ability of Li PSs can be enhanced.Since the electronegativity of Mo and Ni is greater than that of Co,this makes Mo and Ni attract electrons of Co,reducing the electron density around Co.At the same time,the introduction of Ni and Mo changes the position of the d band,which can improve the electron dissociation,expand the lattice spacing,partially fill the d orbital,increase the charge transfer rate,and promote the redox reaction of Li PSs.The specific discharge capacity of the first cycle at 0.2 C is 1646.27 m Ah g^-1 and 1521.96 m Ah g^-1,respectively.Even after 530 cycles at 1 C,it can still maintain 924.40 m Ah g^-1 and 755.42 m Ah g^-1,respectively,and their capacity decay rates are 0.053%and 0.076%,respectively.