In-situ Transmission Electron Microscopy Investigations On Transition Metal Oxides

Transition metal oxides exhibit a wealth of physical properties due to the strongly correlated interactions between the four dimensions of lattice,charge,orbital,and spin,and have been widely used in the energy and semiconductor fields.The evolution of the crystal structure and electronic structure of transition metal oxides plays a crucial role in their functionality under practical external field environments.Atomic-scale characterization combined with in-situ external field modulation to study the structural features of materials in equilibrium or quasi-equilibrium is an important way to explore the structure-activity relationships of transition metal oxide systems.In this dissertation,using in-situ scanning transmission electron microscopy,we characterize the atomic-scale crystal structure and electronic structure evolution of transition metal oxides under the external field in three parts as follows:First,the study of ion migration and stabilization mechanism on the surface of Ni-rich lithium-ion battery cathodes.Among the various layered transition metal oxides,Ni-rich Li Ni_0.8Mn_0.1Co_0.1O₂（NMC811）is widely used as the cathode material for lithium-ion batteries due to its high energy density.However,the loss of transition metal and oxygen ions at the surface during electrochemical cycling leads to capacity and discharge voltage degradation of NMC811 cathodes.Through in-situ electron beam irradiation by transmission electron microscopy,we observed the atomic-scale ion migration and surface reconstruction process of NMC811 at the（003）surface.Combined with first-principles calculations,we found that there is a Ni-O ion co-migration diffusion path on the surface of NMC811,and the ion migration path is accompanied by the formation of a large number of Li/Ni antisites.Due to the superexchange effect of Ni ion e_g orbitals in the interlayer linear Ni-O-Ni configuration,the Li/Ni antisites prevent the outward diffusion behavior of transition metal and oxygen ions,resulting in surface stabilization.This work provides a universal surface protection mechanism for Ni-rich layered oxide cathode materials and offers a method of designing cathodes for high-stability Li-ion batteries.Second,the study of cationic ordering transition mechanism in layered sodium-ion battery cathodes.The oxygen-redox activity in layered transition metal oxides is crucial for improving their energy density.Among them,the cationic ordering structure in the transition metal layer has a great influence on the orbital hybridization of oxygen2p orbitals with transition metal 3d orbitals,which can effectively regulate the redox activity and reversibility of oxygen ions.Based on the ribbon-ordered Na-ion layered oxide Na_0.6Li_0.2Mn_0.8O₂,we found that its discharge capacity and voltage decay are accompanied by the evolution from the ribbon ordering to disordered structure during the electrochemical cycles.Combining in-situ heating characterization and first-principles calculations,the Mn⁴⁺to Mn³⁺reduction-induced Mn O₆ octahedral Jahn-Teller distortion provides the ion migration path for the cationic ordering transition,which is the intrinsic reason for the oxygen-redox performance degradation.This work reveals the relationship between the ordering structure within transition metal layers and the oxygen-redox reversibility of Li/Na-ion layered oxide cathodes.Third,the study of ferroelectric order transition and phase transition mechanisms of fluorite oxides.Fluorite-structure ferroelectrics are the emerging ferroelectric oxides in recent years,including Hf O₂,Zr O₂,and their dopants.Compared with conventional perovskite ferroelectric films,fluorite-oxide films can maintain stable ferroelectric polarization at ultrathin thicknesses（less than 10 nm）and are highly compatible with the current silicon-based semiconductor technology.However,due to the coexistence of multiple phases in fluorite oxides,the polarization switching mechanism and the multiple phase transition processes have not been studied at the atomic scale.Using low-dose integral differential phase contrast scanning transmission electron microscopy（i DPC-STEM）imaging method,we observed the unit-cell-scale polarization switching and the coupled phase transition processes in Zr O₂ by in-situ electron irradiation.Through quantitative image analysis,we precisely measured the change of polarization values during the ferroelectric/antiferroelectric order transition and discovered the boundary conditions of the phase transition from the ferroelectric orthorhombic phase to the nonpolar monoclinic phase.In addition,we also found the phase transition path from the nonpolar tetragonal phase to the ferroelectric orthorhombic phase and the surface relaxation from the ferroelectric orthorhombic phase to the nonpolar tetragonal phase.This work reveals the atomic-scale polarization transition mechanism of fluorite-oxide ferroelectrics and their intrinsically coupled phase transition paths,providing new structural insights for the optimization of fluorite-oxide ferroelectrics.These atomic-scale results not only resolve the structure-activity relationships of multi-degree-of-freedom coupling in transition metal oxides,but also expand our knowledge of the correlation between material structure and macroscopic physical properties,and provide important guidance for optimal design studies of functional oxide devices.