Design,Preparation And Electrochemical Performance Of Nanostructured Sulfur Cathode

The theoretical specific capacity and energy density of lithium-sulfur?Li-S?batteries are1675 mAh g^-1and 2600 Wh kg^-1,respectively.Furthermore,the sulfur has the advantages of rich resources,low contamination,and low price.Therefore,lithium-sulfur batteries are considered to be the most promising secondary batteries.However,the low conductivity of sulfur,the shuttle effect of polysulfides,and poor rate performance have limited the commercialization and practical application of lithium sulfur batteries on a larger scale.The design of nanostructured electrode offers enhanced dynamical behavior,electron/ion transport,and electrochemical stability,thereby improving the overall performance of batteries.In this thesis,under consideration of lithium ion conductivity and aborption capability of polysulfides,three types of nanostructured sulfur-based materials were prepared to imporve the rate performance and cycle stability of Li-S batteries.The results are as follows:1.In order to solve the issue of lithium ion transport within the cathode at high current density,a flexible free-standing cathode was synthesized by using carbon materials and two-dimensional V₂C-Li MXene as a sulfur host.In the nanoarchitecture,reduced graphene oxide and carbon nanotubes are used as a flexible skeleton to improve the flexibility of the cathode.At the same time,the high conductivity of carbon materials can also effectively improve the utilization of sulfur.V₂C-Li MXene,as a sulfur host,can effectively inhibit the shuttle effect due to its strong chemisorption on polysulfides by its inherent polarity.The most important thing is that the expanded interlayer distance of V₂C-Li MXene embedded with lithium ions can ensure the fast diffusion of lithium ions,so that lithium-sulfur batteries can still work normally under high current density.Due to the synergistic effect of chemical adsorption and rapid lithium ion migration and exchange,the prepared S@V₂C-Li/C composite cathode exhibits excellent rate performance.At a current density of 0.5 C,the initial specific discharge capacity of the S@V₂C-Li/C composite cathode reaches 800 mAh g^-1,and the specific capacity of 600 mAh g^-11 is maintained after 500 cycles.When the current density is increased to 5 C,The reversible charge-discharge specific capacity of S@V₂C-Li/C composite cathode still reached 400 mAh g^-1.2.TiO₂/Fe₂TiO₅ heterostructure hollow spheres?TFHS?were prepared by the template method,and then cathode materials?TFHS@S?were obtained by the melting method.First,the hollow structure has physical interaction with polysulfides,reduces the specific capacity loss and improves the capacity retention rate of the lithium-sulfur batteries.Second,the polarity of the metal oxides TiO₂ and Fe₂TiO₅ synergistic adsorption of polysulfide greatly reduces the effect of shuttle effect on cycle stability.Finally,the internal electric field at the heterogeneous interface greatly improves the electron/lithium ion conductivity,which can significantly imporve the overall performance of lithium-sulfur batteries.The TFHS@S cathode maintains a capacity of 650 mAh g^-11 after 500 cycles at current density of 0.5 C,with a small capacity decay rate of 0.061%.When the cureent density is incresed to 5 C,the capacity is still up to 470 mAh g^-1.3.The water-soluble fast ion conductor Li₃InCl₆ and Li₃InCl₆@PC were prepared by dissolution and re-precipitation method,and then the S@Li₃InCl₆@PC cathode material was obtained by melting the sulfur method.The abundant hollow pores of porous carbon can accommodate a large amount of sulfur.Secondly,porous carbon can significantly increase the conductivity of the cathode to enhance the specific capacity of lithium sulfur batteries.Especially after the addtion of fast ion conductor Li₃InCl₆,the lithium ion conductivity of cathode is greatly enhanced,and the specific capacity at high current density is also improved.At current density of 0.5 C,the initial specific capacity of the S@Li₃InCl₆@PC cathode is around 900 mAh g^-1,and the capacity retention rate is 67%after 200 cycles.When the current density is increased to 5 C,the specific capacity can reach 420 mAh g^-1.