Constructing Catalysts For Lithium-Sulfur Batteries Based On Crystal Facet/Interface Regulation Strategy And Their Electrochemical Performance Research

Lithium-sulfur（Li-S）batteries have attracted widespread attention due to their high theoretical energy density,low price,and environmental friendliness.Li-S batteries are recognized as one of the most promising battery systems in the new generation of high-energy energy storage systems.However,Li-S batteries are facing many problems,such as low utilization of active sulfur,severe lithium polysulfide shuttle,and sluggish reaction kinetics.As a result,the actual energy density and cycle stability of Li-S batteries still do not meet the requirements of commercial applications.Applying catalytic materials with good adsorption capacity and high catalytic activity to inhibit the shuttle of lithium polysulfides（Li PSs）and enhance the reaction kinetics of the interconversion of sulfur species is an effective strategy to improve the practical energy density and cycle stability of the batteries,which has become a research hotspot in the field of Li-S batteries.In this thesis,the surface/interface structures of metal oxide-based catalytic materials are regulated by strategies such as crystal facet engineering and interface engineering,and the improvements and related mechanisms of various regulation strategies on the performance of Li-S batteries are studied through experiments and theoretical simulations.The composite materials（C-Fe₂O₃-G）of concave Fe₂O₃ nanocubes supported on reduced graphene oxide were prepared by hydrothermal synthesis.Experiments and theoretical simulations show that C-Fe₂O₃-G have strong adsorption capacity and high catalytic activity due to the abundant unsaturated coordinated Fe active sites on the high index Fe₂O₃{13-44}and{12-38}crystal facets exposed on the surface of the concave Fe₂O₃ nanocubes.Therefore,C-Fe₂O₃-G catalysts can not only efficiently chemically anchor Li PSs to inhibit the shuttle effect,but also significantly accelerate the reaction kinetics of Li PSs conversion and reduce the energy barrier of lithium sulfide（Li₂S）decomposition,thus significantly improving the discharge capacity and cycle stability of Li-S batteries.The batteries maintain a specific capacity of 491 mAh g^-1 after 1600charge-discharge cycles at 2 C,and the corresponding single-cycle capacity decay rate is only 0.025%.In order to test the wide applicability of adjusting the electrochemical performance of Li-S batteries by means of the crystal facet effect,the p-block metal oxide SnO₂,which has better adsorption performance than the d-block metal oxide Fe₂O₃,is used as a model to study the mechanism of crystal facet effect in Li-S electrochemistry.Two kinds of SnO₂ nanocrystals/reduced graphene oxide composites（SnO₂{332}-G and SnO₂{111}-G）were synthesized with the assistance of the structure-directing agents.The two kinds of SnO₂ nanocrystals are SnO₂ nano-octahedrons with{332}and{111}facets,respectively.Compared with the{111}facets,the SnO₂{332}facets with more coordination-unsaturated Sn sites not only have a stronger binding ability with Li PSs,but also reduce the decomposition energy barrier of Li₂S more effectively.Therefore,SnO₂{332}-G not only inhibit the shuttle of Li PSs more effectively,but also catalyze more efficiently the reaction kinetics of sulfur species conversion,thereby significantly improving the electrochemical performance of Li-S batteries.The batteries achieve2000 charge-discharge cycles at 2 C,and the average capacity decay rate is as low as0.021%per cycle.On the basis of the research works in the previous two chapters,the influence of the exposed crystal facets of the carriers on the interface structures formed by heterogeneous material loading growth is further explored,and the impact of catalytic materials with different heterointerfaces on the electrochemical performance of Li-S batteries was investigated.The Fe₂O₃-CeO₂ composite nanoparticles with high-energy interfaces were prepared by growing CeO₂ nanocrystals on the surface of Fe₂O₃octadecahedrons with high-index{113}and{104}crystal facets via a two-step growth method.The results show strong interfacial electron transfer at the high-energy Fe₂O₃-CeO₂ interfaces.Benefiting from the formation of abundant high-energy heterointerfaces with strong interfacial interactions,the Fe₂O₃-CeO₂ composite nanoparticles exhibit strong adsorbability for Li PSs and high catalytic performance for the conversion of sulfur species.The batteries still have a reversible capacity of 546mAh g^-1 after 2000 charge-discharge cycles at 2 C,and the average capacity attenuation rate of a single cycle is only 0.016%.Even with a high sulfur loading of 10.17 mg cm^-2,the batteries still exhibit an areal capacity of 8.65 mAh cm^-2 after 200 cycles.In compared with crystal facet engineering and interface engineering,strain engineering can achieve efficient and accurate control on material surface characteristics by finely adjusting the spacings of atoms on the surface of materials.FeOOH nanorods are fabricated via hydrothermal synthesis reaction,and then thermally treated at different temperatures to obtain FeOOH catalysts with different compressive strains.Experiments and theoretical calculations validate that the lattice compression strain can weaken the adsorption capability of the catalytic material for Li PSs,and the appropriate compressive strain makes FeOOH catalysts exhibit the moderate adsorption capability for Li PSs,which is conducive to optimize their catalytic effects.The FeOOH catalytic material with 4.2%lattice compressive strain effectively anchor Li PSs and catalyze the interconversion of sulfur species.The assembled Li-S batteries accomplish3000 charge-discharge cycles at 2 C,and the average capacity decay rate of each cycle is as low as 0.013%.Under the condition of lean electrolyte with E/S ratio of 4.6μL mg^-1,the batteries with sulfur loading of 9.17 mg cm^-2 retain an areal capacity of 7.17mAh cm^-2 after 70 cycles.