Sodium/Potassium Storage Mechanism And Properties Study Of High-Performance Carbon Composite Metal Oxide/Sulfide Materials

Under the guidance of the sustainable development strategy and the goal of achieving peak carbon dioxide emission and carbon neutrality,the requirements of the new power generation systems with renewable energy as the main body for energy storage equipment continue to increase.However,the scarcity of lithium resources and the increasing cost make chemical energy storage devices based on lithium-ion batteries unable to meet the current rapidly growing demand for energy storage.Due to the similar working mechanism to lithium ion batteries,abundant natural resources,and low cost,sodium/potassium ion batteries can be used as the supplement to current energy storage systems,and are expected to become the leading role of alkali metal ion batteries in the future.However,the large ionic radius of sodium/potassium ions makes the materials show poor performance in terms of reaction kinetics and cycling stability.Therefore,the development of anode materials with long cycle life,high rate performance,and high specific capacity is the prerequisite for the practical application of sodium/potassium ion batteries.In this thesis,transition metal oxygen/sulfur compounds were focused on the improvement in their electrochemical performance structural design,combination with carbon-based materials,heterostructure construction,and introduction of defects strategies.Meanwhile,in situ/ex situ characterization and Density Functional Theory（DFT）calculations were used to reveal the sodium/potassium storage mechanism and the relationship between the crystal structure and properties of the composites.The main research contents of this paper are as follows,1.Nitrogen-doped carbon encapsulated sea urchin-like Nb₂O₅ nanospheres（Nb₂O₅@C）were successfully prepared by hydrothermal and polydopamine pyrolysis processes.The conductive carbon layer can effectively suppress the volume expansion of the composite during charging and discharging process,and the three-dimensional densely aligned nanowire structure could shorten the migration path of ions/electrons.In addition,DFT calculation results indicated that the heterojunction interface formed between the carbon layer and Nb₂O₅ can not only serve as active sites for sodium/potassium ion（Na⁺/K⁺）storage,but also enable accelerate ion/electron transfer.Therefore,the Nb₂O₅@C exhibited excellent electrochemical performance.In the sodium ion batteries system,it delivered a high capacity of 160 m A h g^-1 at a current density of 10 A g^-1 over 1000 cycles.And in potassium ion batteries system,the composite still delivered a specific capacity of 116 m A h g^-1 after 1600 cycles at 3.5 A g^-1.At the same time,the sodium/potassium storage mechanism of the composite was studied by in-situ X-ray powder diffraction（in-situ XRD）and ex-situ X-ray photoelectron spectroscopy（ex-situ XPS）,revealing the intercalation reaction mechanism accompanied with the redox of Nb⁵⁺/Nb⁴⁺during the reaction.2.A hollow nanosphere-structured Fe OOH@C composite was prepared by solvothermal and polydopamine pyrolysis methods.The weak atomic interactions in the amorphous Fe OOH phase lead to the rapid Na⁺/K⁺diffusion.Furthermore,the exterior carbon layer and cavity within Fe OOH can effectively alleviate the volume change of the composite during the charge/discharge process and improve the electrical conductivity.Therefore,Fe OOH@C exhibited excellent cycling performance and rate capability as an anode material in sodium ion batteries.The composite maintained a high specific capacity of 234 m A h g^-1 after 400 cycles at 2 A g^-1.In potassium ion batteries system,it delivered a specific capacity of 196 m A h g^-1 after 300 cycles at 0.2A g^-1.In addition,the reaction kinetics were explored by cyclic voltammetry（CV）,galvanostatic intermittent titration technique（GITT）and electrochemical impedance（EIS）.And the high structural stability of Fe OOH was verified by ex-situ transmission electron microscopy（ex-situ TEM）,providing a basis for the subsequent development of stable amorphous structured sodium/potassium anode materials.3.A flower-like Bi₂S₃-Cu S/C composite with sulfur lattice defects was successfully synthesized by sulfurization and ion exchange process method using Zn O-Bi₂O₃ nanospheres as precursors.The obtained composite possessed high chemical activity and theoretical capacity.And DFT calculations indicated that Bi₂S₃-Cu S heterointerface can serve as active sites for sodium/potassium ion storage and facilitate electron/ion migration.In addition,the carbon coating in the composite not only enhanced the electrical conductivity but also suppresses the volume variation of metal sulfide during the reactions.Therefore,the Bi₂S₃-Cu S/C composite displayed excellent electrochemical properties as the anode material of sodium/potassium ion batteries.The composite delivered specific capacities of 540 m A h g^-1after 1300 cycles at 15 A g^-1 in sodium ion batteries and 407 m A h g^-1 after 210 cycles at 2 A g^-1 in potassium ion batteries.In addition,the sodium storage mechanism of Bi₂S₃-Cu S/C was investigated by ex-situ XRD and ex-situ high-resolution transmission electron microscopy（ex-situ HR-TEM）.4.A composite of N,S co-doped carbon-coated Co Ni S（Co Ni S@C）was synthesized through methionine coating and sulfidation using a Co-Ni Prussian blue analog as the precursor.The hollow structure of the composite can not only make the material fully infiltrated by the electrolyte,but also buffer the volume expansion/contraction of the composite during charging and discharging process.Furthermore,the N,S co-doped carbon shell can maintain the structural stability,promote the electron transfer,and improve the electrical conductivity.Therefore,the Co Ni S@C composite exhibited excellent cycling and rate performance as an anode material in sodium ion batteries.The composite maintained a high specific capacity of464 m A h g^-1 after 400 cycles at 10 A g^-1.In addition,the sodium storage mechanism of the composite was explored through EIS,GITT test,differential capacity analysis,and ex-situ XRD/XPS/TEM.For the phenomenon of its specific capacity firstly decreasing and then increasing,and tending to stable,the relation between the capacity change and the phase transformation/crystalline evolution was preliminarily elucidated.