Preparation Of TiO₂ And ZIF-8 Nanomaterials With Adjustable Defect Structure And Study On The Performance Of Secondary Battery

Up to now,the anode materials of lithium-ion batteries and sodium-ion batteries can be segmented into three genres on the basis of the ion-storage mechanisms:intercalation/de-intercalation,alloying/dealloying,and conversion reaction.The intercalation/de-intercalation anode material has steady cycling ability but numbered active sites,which leads to its low theoretical specific capacity,and alloying and conversion reactive anode materials have high theoretical specific capacity,but their cycling ability is poor due to its structure be damaged during the cycle.Therefore,it is needful to develop a kind of anode material with superb properties,but it also faces huge challenges.In recent years,a large number of studies have shown that the introduction of defects into the anode electrode material has a certain impact on the chemical environment around the material,which in turn affects its electrochemical properties.This dissertation mainly explores the influence of the cation vacancies constructed in TiO₂and the number of intrinsic defects in carbon materials on its electrochemical properties as the starting point for experiments.(1)Inspired by the reservoir,this work introduces a large number of Ti vacancies on the TiO₂nanosheets to explore whether it can be used as active sites for lithium ion storage to improve its performance.The results show that its charge and discharge capacity and cycle stability have been optimized.Compared with the stoichiometric TiO₂(137.7 mA h g^-1)nanosheets,they do show a higher reversible capacity(332.1 mA h g^-1)in the lithium ion half-cell,and they have more stable cycle properties(the capacity is 296 mA h g^-1)better than the stoichiometric TiO₂nanosheets(99 mA h g^-1)after 100 cycles.In summary,it shows that the introduction of Ti vacancies significantly optimizes the electrochemical performance of TiO₂nanosheets.Therefore,we believe that the introduction of cation vacancies in electrode materials is an effective measure to optimize electrochemical performance.(2)In this work,we also tested the sodium electrical properties of Ti_1-xO₂nanosheets,and the results showed that the introduction of Ti vacancies also optimized its sodium electrical properties.At 0.1A g^-1,the discharge capacity is 195.7 mA h g^-1,which is significantly higher than the stoichiometric TiO₂nanosheets(111 mA hg^-1);its cycle performance and rate performance are also significantly better than the stoichiometry without vacancies TiO₂nanosheets.Electrochemical analysis and theoretical calculations show that we have introduced a new Li^+/Na⁺adsorption/desorption storage path,which can be used as Li⁺/Na⁺storage sites by constructing Ti vacancies in Ti_1-xO₂nanosheets.This result highlights the advantages of cation vacancies.The proposed adsorption/desorption mechanism provides a new idea for the design of advanced electrode materials.(3)Polyhedral carbons from zeolitic imidazolate frameworks(ZIFs)have been researched as anode materials for LIBs,because their topological feature enabled ideal research objects.However,the differences and similarities of lithium ion diffusion behavior between intrinsic defect ZIFS polyhedral carbons have not been systematically studied.Here we synthesized four kinds of ZIF-8 polyhedral carbons,including cubes(ZIF-8/C-C),small rhombicuboctahedrons(ZIF-8/C-R1),large rhombicuboctahedrons(ZIF-8/C-R2)and dodecahedrons(ZIF-8/C-D),and well-studied their lithium-ion diffusion behavior.The as-made ZIF-8/C-R2 with 24 vertexes and 48 edges exhibited and the most intrinsic defects superior electrochemical performance(657 mA h g^-1at 0.1 A g^-1)compared to other ZIF-8/C polyhedrons with less vertexes and edges.In situ TEM was employed to observe the lithiation process of various ZIF-8/C polyhedrons,showing that the solid electrolyte interface films were initiated to form at the vertex sites followed by the edges,which indicates that it is reasonable to design advanced polyhedral electrode materials with abundant vertexes,edges and intrinsic defects by applying Euler's formula.