| The over consumption of fossil fuels causes energy crisis and environmental pollution problems.The development of electric vehicles is of great significance for alleviating the energy crisis,reducing environmental pollution,and achieving carbon neutrality,but it also puts forward new requirements for the energy density of secondary batteries.Lithium-sulfur（Li-S）batteries are known as the most promising next-generation secondary batteries because of high energy density and environmentally friendliness.However,the poor conductivity of the sulfur cathode and the dissolution and loss of the lithium polysulfides hinder the commercial application of Li-S batteries.The addition of functional materials can inhibit the dissolution and loss of polysulfides and improve the electrochemical performance of Li-S batteries.Metal organic frameworks（MOFs）derived materials have the advantages of large specific surface area,high porosity and tunable structure,and have a wide range of application prospects in the field of Li-S batteries.Herein,MOFs derived porous functional materials were designed to address the key challenges of Li-S batteries,thus improving the electrochemical performance of Li-S batteries.In addition,the mechanism on the conversion and immobilization of polysulfides was revealed with various structure and chemical characteristics of functional materials.Life cycle assessment（LCA）was also carried out to evaluate the environmental impact of the Li-S batteries with different modification strategies.The main researches are as follows:1.To address the poor conductivity of sulfur,nitrogen-doped porous carbon fiber material was prepared to modify sulfur cathode,improving the reactivity of sulfur.The low conductivity of sulfur reduces the actual capacity of Li-S batteries.Therefore,nitrogen-doped porous carbon fiber was prepared using Zeolitic imidazolate framework-8（ZIF-8）as sacrificed template to regulate sulfur cathode（CF-9/S）,which significantly improved the reactivity of sulfur.The CF-9 fiber with high mechanical strength and three-dimensional conductive network can promote the transport of electron/ions,improving the utilization rate of active material and rate performance of batteries.In addition,the porous structure and doped nitrogen heterocyclic atoms derived from the decomposition of ZIF-8 can reduce the loss rate of polysulfides through physical adsorption and chemical bonding effects,significantly improving the cycling stability of the batteries.The Li-S battery with CF-9/S cathode has an initial discharge capacity of 1524 m Ah g-1 at 0.1 C rate,with a high utilization rate of 91%for active material.And the discharge capacity of Li-S battery with CF-9/S cathode is 706 m Ah g-1 after 200 cycles at 0.5 C rate,and the capacity retention rate reaches 83%.2.To further address the dissolution and loss of lithium polysulfides,carbon coated Co O/Ni O heterostructure cages modified separator was prepared to improve the cycling stability of batteries,the“adsorption-electrocatalysis”synergistic effect of Co O/Ni O heterostructure on polysulfides was also elucidated.The dissolution and loss of polysulfides during discharge/charge process will reduce the cyclic life of lithium sulfur batteries.N-doped porous carbon materials shows relative weak immobilization ability to lithium polysulfides.In this thesis,carbon coated Co O nanosheets assembled cages derived from ZIF-67 was firstly prepared to modify the separators of Li-S batteries,which significantly improved the electrochemical performance of batteries.By regulating the types of metal cations in the etching process of ZIF-67,the carbon coated Co O/Ni O heterostructure nanosheets assembled cages（Co O/Ni O@C-NC）were also synthesized,which showed higher immobilization ability and catalytic activity to polysulfides.Co O/Ni O heterostructure nanosheets assembled cages can enhance the physical adsorption and chemical bonding ability of Co O/Ni O@C-NC to polysulfides.Co O promotes the nucleation and deposition of Li2S,and Ni O accelerates the liquid-liquid conversion of polysulfides.The synergistic"adsorption-electrocatalytic"effect of Co O/Ni O@C-NC modified separator facilitates the redox kinetics and suppresses the dissolution and loss of lithium polysulfides,thus improving the rate performance and cyclic life of batteries.The discharge capacity of the battery with Co O/Ni O@C-NC modified separator is 738 m Ah g-1 at3 C rate,which is 617 m Ah g-1 higher than that of Li-S battery with unmodified separator.The capacity decay rates is 0.075%for 500 cycles at 1 C rate.When the sulfur loading reaches5.87 mg cm-2,an areal capacity of 5.8 m Ah cm-2 is obtained for the Li-S battery with Co O/Ni O@C-NC modified separator.3.Life cycle assessment of Li-S batteries with different modifying strategies.The complex material synthesis methods and battery manufacturing processes increase environmental impacts and energy consumption of Li-S batteries.In order to evaluate the environemal impacts of Li-S batteries with different modifying strategies,LCA model was developed for comprehensive environmental impact assessment of the Li-S battery with N-doped porous carbon fiber/S cathode（noted as cathode modified Li-S battery）,the Li-S battery with carbon/S cathode and Co O/Ni O heterostructure nansheets assembled cages modified separator（noted as separator modifed Li-S battery）,Li-S battery with Super P/S cathode and the lithium-ion battery with Li Ni1/3Co1/3Mn1/3O2cathode.The environmental impact assessment illustrates that cathode modified Li-S battery and separator modified Li-S battery are more environmentally friendly than Super P/S based Li-S battery and lithium-ion battery with Li Ni1/3Co1/3Mn1/3O2cathode.In addition,the fossil fuel consumption potential,global warming potential,freshwater eutrophication potential,freshwater ecotoxicity potential and human toxicity（cancer）potential of cathode modified Li-S battery and separator modified Li-S battery are all lower than those of Super P/S based Li-S battery and lithium-ion battery.However,the human toxicity（cancer）potential of cathode modified Li-S battery is higher than that of separator modified Li-S battery.This research method provides a reference for the environmental impact evaluation of the electrode materials and structures of greener lithium-sulfur batteries. |
