Study On Designing Cathodes For Lithium-Sulfur Batteries And Their Seepage And Energy Storage Characteristics

The development of modern society depends on energy supply to a large extent.Lithium-sulfur（Li-S）batteries,based on multi-electron redox reactions,have been regarded as a promising next-generation energy storage technology for their ultrahigh theoretical energy density compared with those of the traditional lithium-ion batteries(2,600 Wh kg^-1,five times larger than lithium ion bateries).In addition,Li-S batteries own many advantages including low cost,natural abundance,and environmental friendliness.However,the commercial implementation of Li-S batteries has been seriously hindered by some shortcomings,particularly the severe shuttling effect and the sluggish redox reaction kinetics,so developing high-performance cathode host materials is efficient to achieve high-performance Li-S batteries.The improvement of the electrochemical performance of Li-S batteries essentially relies on the interaction between the host material and polysulfides at the microscopic level.Therefore,the improvement of the effective utilization of host materials is crucial for improving the energy storage properties of Li-S batteries.The analytical approach of numerical simulations provided a theoretical direction for the experimental research in this thesis.The main conclusions of this thesis are as follows:（1）For the wettability of the active material to the electrode reduces the energy storage performance,the Shan-Chen multiphase model has been applied to investigate the flow behavior of liquid active materials during the preparation of Li-S batteries.The results show that the porosity of the electrode and the viscosity of the active material have a great influence on the wetting effect of the electrode.The wettability of the active material to the electrode is insufficient,and the air in the large pore space in the electrode will form bubbles that hinder the electrochemical reaction,resulting in uneven electrochemical reaction.The agglomeration of sulfur element in the electrode leads to the decrease of the porosity inside the electrode,which hinders the wettability of the electrolyte to the electrode.The Li₂S₆solution,which is more uniformly distributed in the electrode and does not agglomerate,is a better choice as an active material for lithium-sulfur batteries.（2）In order to improve the energy storage characteristics of Li-S batteries,Fe WO₄nanorods,a marigold-like In₂S₃/r GO composite(In₂S_3-x/r GO)rich in sulfur vacancies,carbon-doped tungsten nitride（C-WN）and N,S co-doped carbon-coated In₂O₃-In2S3heterostructure（In₂O₃-In₂S₃@NSC）composites are used as cathode materials for Li-S batteries.The highly thiophilic Fe WO₄ nanorods can both facilitate ion transport and accelerate the transformation kinetics of polysulfides.The In₂S_3-x/r GO composite not only promotes the formation of thiosulfate and polythionate which can provides stronger adsorption capacity than ordinary sulfides,and the vacancies effectively tune the surface or interface electronic structure of In₂S₃ to promote fast electrochemical reactions.The carbon atoms are doped into the WN lattice,thus exposing new multi-site catalyst surface,whereby the polysulfide conversion is effectively accelerated.The In₂O₃-In₂S₃ heterostructure interface can reduce the activation energy of the reaction,promote the generation of rate-limiting step intermediates,and accelerate the reaction rate.Li-S batteries based on the above cathode additives exhibit excellent discharge capacity,rate capability and cycle stability.（3）The deterioration of cycle performance of Li-S batteries caused by heterogeneous electrochemical reactions during cathode charge and discharge.A low tortuosity Li-S batteries cathode is constructed using a salt template strategy,and the effect of low tortuosity on the electrochemical performance is investigated.The low tortuosity cathode in Li-S batteries reduces the concentration polarization inside the battery and reduces the occurrence of inhomogeneous reactions in the Li-S batteries.The low tortuosity electrodes reduce cell polarization and accelerate ion transport,resulting in excellent rate performance for Li-S batteries.By reducing the tortuosity of the electrode,the utilization rate of active materials can be effectively improved,the accumulation of active materials can be prevented,and the energy storage characteristics of Li-S batteries can be improved.The uniform electrochemical reaction of the electrode with low tortuosity is beneficial to obtain excellent cyclability.