| Compared to traditional secondary batteries,lithium-sulfur（Li-S）batteries are one of the most promising commercial energy storage systems due their its high theoretical specific capacity（1675 m Ah/g）,high energy density（2600 Wh/kg）,and lower cost.However,there are also many obstacles in Li-S batteries,such as the"shuttle effect"caused by the dissolution of polysulfides（Li PS）in the cathode,the slow redox reaction kinetics,and the lithium dendrite growth.These issues result in decreased battery capacity,rate performance,and cycle stability,which will severely restrict the practical application of Li-S batteries.Based on the above problems,micro-nano structural cobalt-based compounds and Co-N-C materials were designed in this thesis and applied to modify the commercial separator to build a functional separator.The synergistic strategy of"adsorption-catalyzing"functional separators was used to trap soluble polysulfides and effectively inhibit the shuttling effect,and also accelerate the catalytic conversion of polysulfides.Benefiting from the special micro-nano structure of modified materials,ion transport was accelerated and uniform deposition of Li at the anode was also promoted.Finally,the overall performance of Li-S batteries has been greatly enhanced.The main contents are as follows:（1）Defect-rich Fe-Co3O4 materials with a 3D nanorod-assembled urchin-like architecture were rationally designed by a facile microwave hydrothermal method.The morphology and surface defect density of materials were finely regulated by altering the Fe doping content.It was demonstrated that the existence of Fe doping and oxygen defects can improve the conductivity,and the 3D nanorod-assembled urchin-like architecture of Fe-Co3O4 facilitated the electron/ion transport.Moreover,the surface defect endowed Fe（0.25）-Co3O4 with strong chemical immobilization and fast redox reaction kinetics.Therefore,the assembled Li-S cell with Fe-Co3O4 modified separator exhibited long cycle life（a low capacity decay rate of 0.04%per cycle at 1.0 C after 800 cycles）and excellent rate capability(528 m Ah g-1 at 5.0 C).（2）In this section,by combining the microwave hydrothermal method with the solid sulfuration process,Fe S2-Co S2heterostructure was successfully synthesized with a spherical cluster morphology composed of nanoparticles.Due to the enhanced electron interaction between the interface of Fe S2 and Co S2,the electrical conductivity of Fe S2-Co S2was greatly improved.Meanwhile,with strong chemical immobilization of soluble polysulfides,the Fe S2-Co S2 heterostructure improved the utilization of S and effectively mitigated the shuttling effect.The assembled Li-S batteries with Fe S2-Co S2heterostructure modified separator exhibited excellent rate performance（623 m Ah/g at 3C）.Even at a high S loading of 3.1 mg/cm2 at 0.1 C over 100 cycles,the capacity still remained at 708 m Ah/g.In addition,the application of this functional separator also promoted uniform Li deposition at the anode,as confirmed by Li||Li symmetric cell tests..（3）Leaf-like cobalt-embedded nitrogen-doped carbon material（Co-N-C）was successfully synthesized by pyrolysis of Zn-Co-ZIF composites,and then coated on the commercial separator to construct a multifunctional separator.The Co nanoparticles were uniformly dispersed in the prepared Co-N-C.With a large surface area and an abundance of micropores,Co-N-C materials could provide more active sites for the redox reaction.Furthermore,the rich micropores were beneficial for storing the electrolyte and will accelerate fast ion transport.Benefiting from the integrated"adsorption-catalyzing"effect,the Li-S cell constructed with a Co-N-C separator exhibited a high capacity of 1408m Ah/g at 0.1 C,enhanced cycling stability（a low capacity fading rate of 0.05%per cycle over 400 cycles at 1 C rate）,and outstanding rate performance（551 m Ah/g at 5 C）. |
PDF Full Text Request