Design And Preparation Of Tin-Based Composites And Their Applications For Sodium Ion Storage

Sodium ion batteries（SIBs）have shown great potentials for large-scale energy storage due to its abundant sodium resources and low cost.However,the practical applications of the SIBs still face many obstacles,such as poor cycle performance and low energy density.Exploiting the anode material with high specific capacity and excellent cycling stability is therefore of great significance.Tin-based materials have been well considered as promising anode materials for SIBs because of their high theoretic capacity and low working potential.These materials,however,suffer from severe problems,such as low conductivity and huge volume expansion during discharge,which hindered their wide applications as anode materials for SIBs.This thesis aims to tackle these problems through constructing the substoichiometric amorphous tin sulfide and SnO₂/Fe₂O₃ heterostructuring.Additionally,a strategy of electrically conductive material coating and carbon nanotubes supporting were also adopted to solve the rapid capacity decay and other problems caused by the volume expansion.The electrical conductivity and stability of the tin-based compound anode materials are obviously improved by these methods.The obtained tin-based compound anode materials showed excellent rate performance and cycle stability when used as the anode for the SIBs.Specifically,the main research contents of this thesis can be expressed as follows:1.Preparation of polypyrrole derived carbon coated and carbon nanotubes supported substoichiometric amorphous SnS_x composites（PPY-C@SnS_x/ACNTs）and their applications in the SIBs.A three-step procedure was used for the synthesis of PPY-C@SnS_x/ACNTs composites.Firstly,SnO₂ composites coated with polypyrrole and supported by carbon nanotubes（PPY@SnO₂/ACNTs）were synthesized by a solvothermal method and the subsequent polypyrrole coating.Secondly,the SnS₂ was obtained by solid-phase vulcanization of SnO₂ by the calcination of PPY@SnO₂/ACNTs in the presence of thiourea.Finally,PPY-C@SnS_x/ACNTs composites were obtained by high temperature thermal desulfurization.The results shows that the high temperature can partially desulfurize SnS₂under the presence of polypyrrole derived carbon（PPY-C）to form substoichiometric amorphous SnS_x,which is demonstrated to be important to improve the sodium storage performance and stability of PPY-C@SnS_x/ACNTs.The electrochemical test results indicates that the PPY-C@SnS_x/ACNTs has better cycling and rate performance than the high crystalline PPY-C@SnS₂/ACNTs.The capacity retention rate of PPY-C@SnS_x/ACNTs is close to 100%even after 8000 cycles at the current density of 10 A g^-1,indicating that PPY-C@SnS_x/ACNTs is an excellent anode material for SIBs.The results of DFT calculation shows that the substoichiometric structure is conducive to the diffusion of sodium ions in the tin sulfide material and can improve the conductivity of the tin sulfide material.The result of the work is of great interest since it provides a new idea for the construction of non-stoichiometric amorphous tin sulfide and the design and preparation of high-performance tin-based anode for sodium storage.2.Preparation of polypyrrole derived carbon coated and carbon nanotubes supported SnO₂/Fe₂O₃ heterostructure composites（PPY-C@SnO₂/Fe₂O₃@ACNTs）and their applications in the SIBs.SnO₂/Fe₂O₃ composites supported by carbon nanotubes（SnO₂/Fe₂O₃@ACNTs）were first prepared by solvothermal method and then PPY@SnO₂/Fe₂O₃@ACNTs was obtained through polypyrrole coating.The PPY-C@SnO₂/Fe₂O₃@ACNTs composite were finally obtained by high temperature heat treatment.The results show that the addition of Fe₂O₃ increase the reaction sites,and improve the reversibility of the reaction between SnO₂ and sodium ion.Polypyrrole derived carbon coating layer（PPY-C）can effectively prevent the aggregation of metal oxide particles and significantly improve the cycle life of the PPY-C@SnO₂/Fe₂O₃@ACNTs as the anode material for the SIBs.The PPY-C@SnO₂/Fe₂O₃@ACNTs showed good sodium storage performance(390 m Ah g^-1 at the current density of 100 m A g^-1,and the capacity retention of100%after 1000 cycles at the current density of 2 A g^-1).The kinetic analysis indicates the high Na⁺diffusivity in the active material and the large pseudocapacitance behavior in the electrochemical process,which are beneficial to improve the cycle life of the battery.This indicates that heterostructuring is an effective way to improve the sodium storage performance of tin-based compounds.In conclusion,construction of the substoichiometric amorphous structure and material heterostructuring are two effective ways to improve the sodium storage performance of tin-based compound anode materials.The amorphous structure can increase the cycling stability of the active material,promote the transfer of electrons and ions during the sodiation/desodiation process,and increase the diffusivity of Na⁺.The electrochemical performance of the tin-based materials can be correspondingly improved.Heterostructuring can improve the sodium storage properties of the active materials by providing more active reaction sites and improving their sodiation/desodiation reversibility.The results of this thesis are therefore of great significance to design the tin-based anodes with enhanced performance and promote the practical applications of the SIBs.