Study On The Preparation And Performance Of Copper/Molybdenum-based Metal Oxide And Sulfide Electrode Materials

The structure of the electrode material is one of the factors that determine its electrochemical performance.Its electrochemical performance is closely related to the structural advantage of the electrode material in micro and nano scale.Therefore,it is of critical significance to understand the relationship between the structure change and the performance of battery during the electrochemical reaction.The extensive research and use of alkali metal anode provide an opportunity for the research and development of metal oxide and sulfide.There are many problems such as insufficient reaction and structural collapse in these bulk materials.The battery is a very complex and thermodynamically unstable closed system.In view of the diversity,randomness,and timeliness of the failure of electrode materials in the closed system,we have developed efficient and non-destructive characterization principles and methods,combined with visually three-dimensional reconstruction technology,realizing cross-scale and multi-modal diagnostic evaluation of material systems.We unraveled the structure evolution mechanism of electrode active materials during working,and clarified the structure-activity relationship between material structure and electrochemical performance,in order to guide the synthesis of electrode materials with high capacity,high rate and high stability.In this thesis,we focused on studying the preparation and performance of copper/molybdenum-based metal oxide and sulfide electrode materials:To study the“passivation”mechanism of metal oxide cathode particles based on the conversion reaction at the end stafe of the initial sodiation reaction.The bulk Cu O with a typical conversion reaction is selected as the model electrode,and its capacity in lithium and sodium ion batteries is compared.It is found that the sodium ion battery has a longer plateau at the end of discharge(0.1 V),and it has a lower capacity.Synchrotron-based high-energy X-ray diffraction and absorption spectroscopy techniques have analyzed the structure evolution of Cu O when lithium and sodium ions are inserted,and the content of Cu²⁺,Cu⁺,and Cu⁰in different charged states is quantitatively analyzed by absorption spectroscopy.Using synchrotron X-ray spectroscopy imaging technology,the"core-shell"phase transition process of a single Cu O particle during the first discharge(sodium insertion)was detected in Operando/in-situ on the micro-nano scale.We explored the relationship between the"passivation"at the end of the reaction and its lower capacity.The three-dimensional tomography technologyspatially demonstrated the"core-shell"reaction mechanism in the sodium insertion process of bulk Cu O,and finally solved the problem of low sodium battery capacity through the construction of micro-nano materials.To investigate the relationship between chemical doping and mechanical stability of metal sulfide materials that undergo typical conversion reactions in potassium half-cells.A small amount(2%)of divalent Co ions(Co²⁺)are doped into Cu S with a hexagonal structure using hydrothermal method.X-ray diffraction(XRD)and high-resolution transmission electron microscopy(HRTEM)were used to determine whether Co-containing impurities are generated,laying a foundation for the inveatigation on electrochemical performance.Synchrotron radiation imaging technology shows that the ordered hierarchical pores from"surface to bulk"in doped samples can guide the uniform diffusion of ions and avoid structural collapse caused by volume expansion.The scanning electron microscopy(SEM)and 3D reconstruction characterizations of the electrode before and after the cycle verified that doping can alleviate the volume expansion of Cu S,maintaining mechanical integrity and improving electrochemical performance.Through first-principles(DFT)calculations,the electron density distribution and bonding strength around Co and S atoms are analyzed.To investigate the relationship between the rate performance and pseudocapacitance charge storage behavior of metal sulfide(Mo S₂)electrode materials with different morphologies that have typical insertion-extraction reactions in potassium half-cells.The homogeneous multi-level rod-shaped,lamellar,and solid spherical Mo S₂ were synthesized by hydrothermal method.The morphology and structure characterization show that the Mo S₂ rods are evenly distributed on the current collector,forming a large number of porous channels.This structure can buffer volume expansion and inhibit the aggregation of Mo S₂ particles under fast charge/discharge conditions,ensuring excellent mechanical stability and cycle stability.The new ordered structure takes advantage of the two-dimensional Mo S₂ nanosheets,making them attractive materials for potassium battery applications.More importantly,the relationship between the excellent rate performance of the Mo S₂ potassium half-cell and the charge storage of the pseudocapacitor was explored.The rod-shaped Mo S₂ has the highest proportion of pseudocapacitance,and at the same time has the best rate performance.The pseudo-capacitance corresponding to the cyclic voltammetry(1.5 m V s^-1)sweep rate is as high as 74.8%.At current densities of 0.1 and 1.0 A g^-1,the specific capacities reach 320 and183 m Ah g^-1,respectively.