Research On The Morphology And Structure Regulation Of SnS₂ And Its Application In Energy Storage

Two-dimensional layered tin disulfide（SnS₂）has attracted wide attention in the field of electrochemical energy storage due to its advantages of low cost,high specific capacity,and environmental friendliness.However,as an anode material for lithium/sodium ion batteries,some problems such as poor conductivity and large volume expansion during charging/discharging easily cause sluggish electron transport and electrode cracking,thereby lead to decreased cycle life and poor rate performance.In this dissertation,structure control,compounding with carbon material and heterojunction construction were considered to solve these two problems.The morphology and structure of the materials were controlled by the synthesis conditions,and the structure-activity relationship of the materials was analyzed and discussed by various characterization methods.At the same time,according to the strong polarity,adsorption and catalytic effect of SnS₂,it was also used to modify the PP separator in Li-S batteries,and the inhibitory effect for the polysulfide shuttle was studied.The main research contents and conclusions are as follows:（1）SnS₂-cys with a rosette-like structure formed by interspersed nanosheets was hydrothermally synthesized with thiourea and SnCl₄·5H₂O as raw materials,and L-cysteine as template.At the same time,SnS₂-F127 with a denser flower spherical structure assembled by bending and winding nanosheets was also prepared with thioacetamide（TAA）and SnCl₄·5H₂O as raw materials,and polyether F127 as a template agent.The influence of the synthesis parameters such as reaction temperature and reaction time on the morphology and structure of the material was investigated,and the growth process of SnS₂ with different morphologies was discussed.Combined with the constant current charge/discharge test,CV,EIS and other electrochemical tests,the performance and mechanism was studied in LIBs.As a result,it was found that SnS₂-F127 with a 3D flower spherical appearance exhibited more stable cycling and better rate performance.This is mainly due to the fact that there are certain gaps and holes between the SnS₂-F127 curved nanosheet layers,which are beneficial to ion transport and charge transfer on the one hand,and on the other hand,it also helps to relieve the volumetric strain during charging/discharging process.However,SnS₂-cys is sparsely interleaved randomly,and the bonding between the layers is not tight enough,it is easy to crack and break due to the volume expansion during cycling,which deteriorates its cycle stability.This work provides a research foundation for the controllable preparation of SnS₂ with a special three-dimensional（3D）morphology structure.（2）Using ZIF-67 as the precursor,the 3D porous carbon matrix was obtained through carbonization and etching processes.Then,3D C@SnS₂micro-nano follower with uniformly dispersed SnS₂ nanosheets was synthesized with TAA,SnCl₄·5H₂O,and 3D porous carbon matrix by a one-step solvothermal method.The3D C@SnS₂ shows unique advantages in structure and composition:Firstly,many vacancies appeared due to the etching process and a large number of carbon active sites distributed around it,which is conducive to inducing the uniform and orderly growth of SnS₂ nanosheets without obviously agglomeration.The space between the SnS₂ nanosheets is beneficial to obtain a larger specific surface area,provide more active sites,and thus increase the specific capacity of the material.Secondly,the space between the evenly dispersed SnS₂nanosheets in C@SnS₂ can alleviate the volume change during the cycle and improve the cycle stability of the cells.Thirdly,the internal porous C matrix has strong electrical conductivity,which improves the kinetics of ions/electrons transmission,and enhances the rate performance of C@SnS₂.Therefore,the material exhibited relatively stable cycle performance and higher rate performance in LIBs with a specific capacity of658 m Ah g^-1 after 100 cycles at 100 m A g^-1 and 651 m Ah g^-1 at 3 A g^-1.（3）In order to further improve the rate and cycle stability of the material,another sulfide-Mo S₂ was combined with SnS₂ to construct SnS₂/Mo S₂ heterostructure by a one-step hydrothermal method.The morphology,structure and electrochemical performance of LIBs/SIBs were studied.The results show that obvious pseudocapacitor phenomenon during the process of Li⁺/Na⁺storage due to its unique structure plays an important role in maintaining the cycle stability and improving the rate performance.Finally,it shows excellent electrochemical performance in LIBs:the specific capacity of 601 m Ah g^-1after 200 cycles at 0.5A g^-1 and 451 m Ah g^-1 at 3 A g^-1 can be obtained.In addition,the electrochemical performance in SIBs is more excellent:the specific capacity is basically unchanged after 100 cycles at 0.1 A g^-1,it still has a specific capacity of 331 m Ah g^-1 after 900 cycles at 0.5 A g^-1 with～100%coulombic retention.The excellent electrochemical performance can be attributed to:1)The 1T metal phase in Mo S₂ shows higher conductivity,and the formation of an electric field in the heterojunction interface is beneficial to accelerate the interface charge transfer and ion diffusion process,thus enhance the reaction dynamics.2)The difference in the redox platform of SnS₂ and Mo S₂ in Li⁺/Na⁺storage causes the asynchrony of the electrochemical reaction,which is beneficial to alleviate the material damage caused by the volume strain in the cycling,maintains the stability of the material structure,and promotes the cycle performance.（4）Li-S batteries have attracted the attention of researchers due to their higher theoretical specific capacity and cost advantages.However,there are still many problems in the current development,the shuttle effect of lithium polysulfide is one of the important factors restricting its development and application.The prepared 3D C@SnS₂ material was applied as the PP separator modification in Li-S batteries and the electrochemical performance was significantly improved.A specific capacity of 646 m Ah g^-1is exhibited after 500 cycles at the current density of 1 C,corresponding to the retention rate of 76.5%and loss rate of0.062%per cycle,and 789 m Ah g^-1 can be obtained at 2 C.In addition,when the C@SnS₂ modified separator is matched with the S/C cathode at a high sulfur loading of 5 mg cm^-2,the electrode can still maintain 825m Ah g^-1 at 0.1 C after 50 cycles.The excellent electrochemical performance benefits from the synergistic effect of the 3D carbon matrix and SnS₂:1)The vacancies and active sites generated in the 3D carbon matrix during etching process are conducive to the uniform dispersion of SnS₂ and enhance the kinetic process of Li⁺transport.2)The polar SnS₂ has strong chemical adsorption capacity with Li PSs and constrain it near the cathode side.More importantly,the SnS₂ also has the ability of further catalytic-conversion of Li PSs,thus the utilization of sulfur can be enhanced.This research work proves the outstanding adsorption and catalytic-conversion effects of the 3D C@SnS₂ material on Li PSs,which further broadens the application fields of C@SnS₂ and provides a certain reference basis for the following in-depth study of metal sulfides in Li-S batteries.In this dissertation,the layered metal sulfide SnS₂anode material with the advantages of low cost,high specific capacity and environmental friendliness was taken as the research object.Aiming at the problems of large volume expansion effect and poor electrical conductivity in lithium/sodium ion batteries,modification methods such as controlling structural,compounding with carbon materials and constructing heterojunction were employed to improve the electrochemical performance effectively.Additionally,the prepared 3D C@SnS₂ was also applied in Li-S batteries as a PP separator modification,which significantly inhibited the shuttle effect of Li PSs,improved the utilization of active sulfur,and enhanced the kinetic performance,thus excellent cycle stability and rate performance are exhibited.The thesis has played an important guiding significance for the study of metal sulfide materials in lithium/sodium ion batteries and Li-S batteries.