Preparation And Sodium Storage Performance Of Micro-Nano Structured Carbon Based Multi-Metal Sulfides

With the rapid increase of energy shortage and environmental pollution,the exploration and usage of clean energy are of great significance.To realize the continuous use of the clean energy,large-scale energy storage system plays an important role on the construction of smart grids.Sodium-ion batteries（SIBs）have certain advantages in the large-scale energy storage fields due to its high abundance and wide distribution of the sodium.However,the limited rate capability and poor cycling stability still restrict their practical application.And the anodic materials play a vital role in the improvent of SIBs.Metal sulfides have been considered as attractive candidates for high-performance SIBs attributed to its high electrochemical reaction activity.However,based on the conversion reactions,the serious volume expansion and the product loss will occur during the charge/discharge process,which will lead to capacitive decay.To address these problems,in this thesis we will study the design of carbon-based multi-metal sulfides,structure properties,electrochemical performance and sodium storage mechanism,through their structure and component control.Based on the advantages of the bridging between metal ions and ligands and rich functional groups,metal-organic frameworks（MOFs）are considered as designable self-templates,to derive carbon-based metal sulfides.The in-situ carbon coating,multi-metal substitution,and precise transformation of hierarchical structure can be realized in one-step.Considering this advantage,this thesis realized the design of multi-component-based MOFs,including multi-MOFs,MOFs-on-MOFs,and MOFs-oxide hybrids,through ligand competitive mechanism and interface modification.First,ternary MOFs（Mn ZnCo-TATB）are designed using 2,4,6-tris（4-carboxyphenyl）-1,3,5-triazine（H₃TATB）as ligands.After high temperature sulfidation,ternary metal sulfides（MnS-（ZnCo）S/N-C）with hollow polyhedral structure is obtained.As anode material of SIBs,the hollow structure,carbon coating,and the synergistic effects among the multi-components will efficiently improve the electrochemical performance.Thus,at current of 1.0 A g^-1,MnS-（ZnCo）S/N-C anode can maintain a high capacity of 353 m Ah g^-1 after 400cycles.Compared with the corresponding oxides,MnS-（ZnCo）S/N-C exhibits superior electrochemical activitty.To further investigate the effect of the molar ratio of metal ions on the structure and electrochemical performance of sulfide anodes.A series of Ni-Co-MOFs with various metal molar ratios have been synthesized.It is found that binary-MOFs possess high thermal-stability and the metal ratios and sulfur weight ratio will affect the morphology,units in the hierarchical structure,micro electric structure and the hollow structure formation.As anode material,the sulfides will exhibit the structure-depend electrochemical performance.The as prepared(Co_0.5Ni_0.5)₉S₈/N-C exhibits excellent electrochemical reversibility,including high electrochemical reversibility cycling stability,specific capacities,cycling stability and rate performance.The ex-situ XRD for the charging-discharging electrodes reveals the insertion-extraction reaction occurs at the high potential and the conversion reaction occurs at the low potential.Considering the rich functional groups,MOFs-based assembled structures are also considered to be ideal precursors for the carbon-based multi-metal sulfides.First,ZIF-8 and nano-sized Co₃O₄ are assembled into Co₃O₄/ZIF-8 composite with spherical structure,which is further in-situ transformed into Co₄S₃/ZnS/N-C.It is found that the size of ZIF-8 and the ratio between Co₃O₄ and ZIF-8 can affect the assembly structure and the thermal-stability during the heat treatment.The optimal Co₄S₃/ZnS/N-C can integrate the high activity of Co₄S₃ and the cycling stability of ZnS/N-C,exhibiting a superior sodium storage performance.The assemble strategy is further adopted to design MOFs-on-MOFs structure.Through controlling the synthesis condition,MIL-53,ZIF-8 and polymeric resorcinol-formaldehyde（RF）are assembled into MOFs-on-MOFs（MIL-53@ZIF-8@RF）with a novel core-double-shelled structure.It can be precisely derived into Fe₇S₈/C@ZnS/N-C@C with core-double-shelled structure.The effects of core-double-shell structure on the electrochemical performance and the the conversion reaction properties during the discharge/charge process,are studied in detail.It is observed that the abundant interfaces can provide sufficient area for the contacting between the electrode and electrolyte,and the interspace among the core-double-shell structure and the carbon matrix can accelerate the reaction kinetics.Combine the analysis of the peaks in the cyclic voltammetry and the phases transformation in the ex-situ XRD patterns,the reaction occurs from the inner core of Fe₇S₈/C to the shells of ZnS/N-C and C is obtained.This sodium storage mechanism can efficiently alleviate the stress originated from the volume change,and utilize the space in the core-double-shelled structure,and facilitate the structural stability of the anode during cycling.Finally,in order to study the usability of templating method,hard template-freeze drying-high temperature sulfidation method is adopted to design carbon-based multi-metal sulfides with 3D porous hierarchical structure.2-methylimidazole is selected as carbon sources,and Na Cl is used as hard template.We mainly studied the resulted phases and structure affected by the ratio of Fe/Ni.It is found that Ni can promote the formation of nanosheets on carbon skeleton surface,and the Ni,Fe binary-metals are in favor of the formation of 3D porous structure.As a result,the optimal Fe₃Ni₆S₈/N-C can synergistically take the advantages of the various components in the composite,exhibiting excellent sodium storage performance,including high rate capability and cycling stability.