Synchrotron Radiation X-ray Specroscopic Study Of Two Dimensional Layered Electrochemical Energy Materials

Along with the sustainable development of modern society,people's acknowledgement and demand for clean energy has gradually increased and shown higher requirements for the storage and application of green energy.In particular,the energy storage devices with high energy density,high power density and long durability is becoming the focus of attention due to the global development of power electric vehicles.Two-dimensional layered materials show breaches for high effective storage of new energy sources by virtue of its prominent layered structure and excellent physicochemical properties.At the same time,synchrotron radiation sources and advanced characterization techniques have become the important experimental and research platforms for advanced basic science and national strategic core technologies.This dissertation focuses on the precise regulation and advanced characterization of two-dimensional layered energy materials,especially by regulating multi-dimensional nanomaterials and atomic covalent intercalation strategies.We also use static and dynamic spectroscopy characterizations based on synchrotron x-ray spectroscopy to study the structural evolution and energy storage mechanism of 2D materials.The related results provide experimental basis and reference for the rational design,structural regulation,fine characterization and future application of new electrochemical energy materials.The main research contents and results of this dissertation are summarized briefly as follows:1.Two-dimensional graphene(GO),one-dimensional multi-walled carbon nanotubes(MWCNT)and zero-dimensional carbon nano onions(OLC)were firstly assembled into three-dimensional all-carbon composites by hydrothermal method for electric double layer supercapacitors(EDLC).And we discussed the synergistic effect of the three low-dimensional carbon materials for the contribution of performance.Synchrotron-radiation-based X-ray absorption spectroscopy(XAS)confirmed the re-stacking of GO during the hydrothermal reaction.The introduction of proper proportions of MWCNT and OLC can significantly improve the structure of GO aerogel,making the all-carbon composites rich in pore structure and superior conductive network,ultimately showing excellent energy storage behavior:a high capacity about 329 F g-1 at a current density of 1 A g-1,and ultra-fast charge and discharge rate with current densities up to 200 A g-1 and long stability up to 15,000 cycles.This synthetic strategy provides a reference for ultra fast supercapacitors based on all-carbon materials and other energy storage fields.2.A membrane-assisted vacuum filtration method was developed to prepare flexible electrodes for symmetrical all-solid-state supercapacitors(ASSSCs)and microcapacitors(MSCs)based on the composite of sulfur-doped graphene,OLC and nickel sulfide(GCNi3S2).The transfer of electrons from carbon atoms to nickel atoms was revealed by X-ray absorption fine structure(XAFS),demonstrating the tight interaction between GO,OLC and Ni3S2 nanoparticles and contribution to pseudocapacitance performance.The assembled flexible symmetrical ASSSCs showed excellent performance with a high volumetric specific capacity of 55.3 F cm-3,a high energy density of 3.63 Wh cm-3 and a long cycle stability of 5000 cycles.The membrane-assisted vacuum filtration method and the synchrotron radiation spectroscopy can provide an experimental basis for the preparation of multifunctional flexible electrodes and the application of solid-state electrochemical energy storage devices.3.The HF etching and metal ion intercalation process were developed to prepare two-dimensional layered vanadium carbide(V2C MXene)and intercalation materials with adjustable interlayer spacing,and a new layerd MXene-based electrode with excellent lithium storage capacity was obtained.More importantly,the existence of cobalt ions in the interlayer was studied by XAFS.It was proved that the formation of V-O-Co bonding provided theoretical support for performance optimization(0.1 A g-1,1117.3 mA h g-1 and 15000 cycling stability).And the bivalent cobalt ion was discovered mainly in the inner layers of V2C,while the trivalent cobalt ion was mainly presented in the interlayers of near-surface.The average valence of cobalt ions in V2C@Co MXene was finally calculated to be +2.12.This interlayer-control process and synchrotron spectroscopy analysis method can provide research ideas for two-dimensional layered MXene for high performance lithium storage and related energy applications.4.We designed the in situ electrochemical test device for synchronous light source,the in-situ synchrotron radiation X-ray spectroscopy technique was used to reveal the dynamic lithium storage mechanism of metal cation intercalated two-dimensional layered V2C MXene.The results of ex situ XRD indicated that the interlayer spacing of Sn4+ intercalated V2C(V2C@Sn)is reversibly expansion and shrinkage with the insertion and extration of lithium ions.The results of static XAS characterizations confirmed that the oxygen groups in the electrode contributed positively to the performance and the fluorine groups were electrochemical inactivity.Based on these research,we developed operando XAFS and Raman measurements to study the dynamic storage mechinism of lithium ion in V2C@Sn electrodes.Studies have shown that during the charge and discharge process,the valence of vanadium varied between+2.66 and +2.75,while the valence of tin varied from +3.35 to +3.42,which contributed to the main lithium storage properties.This in situ method of synchrotron radiational spectroscopy can provide an effective research tool for understanding the dynamic storage mechanism of layered energy materials.