Cathodes Construction And Separator Modification For High-performance Lithium Sulfur Batteries

Due to the high theoretical specific capacity（1675 mAh/g）and high energy density（2600 Wh/kg）,lithium sulfur battery is expected to be one of the new generation energy storage systems with high efficiency.However,the inherent shortcomings of lithium-sulfur batteries limit its commercialization,such as polysulfides dissolution,Li anode degradation and electrolyte decomposition during cycling.In this paper,a series of strategies which modified cathode materials and separators of lithium sulfur batteries are constructed by cooperating with physical limitation,chemical adsorption and catalytic conversion to solve the above problems,and their electrochemical properties and mechanism are studied to obtain high-performance lithium sulfur batteries.1.To solve the shuttle effect of lithium polysulfide,a facile and scalable synthetic route is presented to suppress the polysulfide shuttle effect through anchoring inherently polar polymer Triton X-100 on the surface of carbon materials.Based on the analysis of experimental results and first-principles calculations,it has been found that the oxygen containing functional groups（hydroxyl and ether group）of Triton X-100are able to carry out effective trapping of lithium polysulfides by strong Li-O interactions,demonstrating a feasible strategy to alleviate the shuttle effect and further improve the overall performance of lithium sulfur batteries.2.A simple and environment synthetic route is reported by ionic surfactants modified hierarchical porous carbon derived from bio-waste lotus seedpod shells to suppress the polysulfides shuttle effect.Based on the analysis of experimental results and first-principles calculations,it is found that lithium polysulfides（LiS and Li₂S）can bind strongly to the O or N functional groups in the surfactants to alleviate the shuttle effect and further improve the overall performance of lithium sulfur batteries.3.Honeycomb-like nitrogen and sulfur dual-doped hierarchical porous biomass-derived carbon/sulfur composites are successfully fabricated for high energy density lithium sulfur battery.The results show that N,S dual-doping not only introduces strong chemical adsorption and provides more active sites but also significantly enhances the electronic conductivity and hydrophilic properties of hierarchical porous biomass-derived carbon,thereby significantly enhancing the utilization of sulfur and immobilizing the notorious polysulfide shuttle effect.4.N,B,S tri-doped ACNTs is designed to enhance the adsorption ability of polysulfides.Based on the analysis of experimental results and first-principles calculations,it is found that the strong adsorption behavior of lithium polysulfides on the surface of N,B,S tri-doped ACNTs.The results show that N,B,S tri-doped active carbon nanotubes with large specific surface and high pore volume enable fast Li⁺transmittal and provide strong polysulfide adsorption ability.Accordingly,reasonable design for the heteroatom doping element in carbon material will be distinctly vital for enhancing the electrochemical performance of the lithium sulfur battery.5.To solve the problem of poor cycle stability of lithium sulfur battery,based on a highly effective sulfur host,namely manganese oxide nanosheets grown on both sides of the N-doped hollow porous carbon nanospheres,a rational physical and chemical dual-encapsulation strategy is presented for the application of advanced lithium sulfur batteries.The multifunctional,integrated and hollow hybrid nanospheres can provide efficient electron-modified interface,hold much more active material,and importantly face-to-face effectively prevent polysulfide dissolution and diffusion via the synergistic restriction.The cycle life of lithium sulfur battery is greatly improved.6.Aiming at the problem of low discharge capacity in lithium sulfur battery,a multifunctional synergistic composite enables ultrahigh sulfur content for advanced lithium sulfur battery,which comprises the sulfur particle encapsulated with an ion-selective polymer with conductive carbon nanotubes and dispersed around Magne?li phase Ti₄O₇ by the bottom-up method.The ion-selective polymer provides a physical shield and electrostatic repulsion against the shuttling of polysulfides with negative charge,whereas it can permit the transmission of lithium ion through the polymer membrane,and the carbon nanotubes twined around the sulfur promote electronic conductivity and sulfur utilization as well as strong chemical adsorption of lithium polysulfide by means of Ti₄O₇.The design of high sulfur content cathodes is a viable approach for boosting practical application of lithium sulfur battery.7.To solve the issue of poor rate performance in lithium sulfur batteries,through the coordination of physical limitation and chemical adsorption as well as catalytic conversion of polysulfide,a flexible multifunctional composite electrode is designed and prepared based on nitrogen,sulfur co-doped carbon cloth,which is supported by flower-like MoS₂ decorated with BaMn_0.9Mg_0.1O₃ perovskite particle and carbon nanotubes.The flexible composite material realizes the smooth"adsorption-diffusion-conversion"process of lithium polysulfide,which can greatly inhibit the shuttle effect of lithium polysulfide even under the high sulfur load,which significantly improves the sulfur utilization,cyclic stability and rate performance as well as reduced polarization.8.To solve the problem of high electrolyte consumption in lithium sulfur battery,the quick ion conductor La_0.56Li_0.33TiO₃（LLTO）with a perovskite structure is used for the first time as an absorbent and catalysts for polysulfides.It has been found that LLTO exhibits a high ionic conductivity and plays a significant electrocatalytic role in the redox reaction of polysulfides in lithium sulfur battery under lean electrolyte conditions.Therefore,lithium sulfur battery containing LLTO can readily reduce the electrolyte/active material ratio,accelerate the reaction kinetics,suppress the polysulfide shuttling and significantly reduce the overpotential for the charge/discharge process.This consequently results in the improved electrochemical properties of lithium sulfur batteries in terms of rate capability and cycling stability.9.To suppress the shuttle effect of polysulfides and promote polysulfides conversion kinetics,a modification strategy of functional separator is presented by coating transition metal coordinated graphitic carbon nitride with crystalline carbons（M-C₃N₄/C）on the surface of a commercial separator.Based on the analysis of experimental results and first-principles calculations,it has been found that the M-C₃N₄/C can strongly bind polysulfides and expedite polysulfides conversion kinetics as well as promote electrons/ion transfer,which can consequently facilitate the utilization of active sulfur species and enhance the performance of lithium sulfur even at lean electrolyte and high sulfur content.