In-situ Study Of Ionic Migration In The Electric Field By Transmission Electron Microscopy

Ions refer to charged particles formed by atoms or groups of atoms losing or gaining one or more electrons.It exists widely in nature,such as seawater,organisms,etc.At the same time,many functional materials are also composed of different types of anions and cations.As early as 1833,Faraday proposed the first law of electrolysis:when current flows through an electrolyte solution,the amount of chemically changing substances on the electrode?i.e.,the phase interface?is proportional to the amount of electricity passed in,clearly indicating the concept of ion transport and the nature of chemical reactions are related to the gain or loss of charge.The winner of the Nobel Prize in Chemistry in 1903,Arrhenius created the theory of ionization of solutions,which opened the way for people to understand,and apply ion transport materials.Based on the material's ion transport function,chemical primary batteries,secondary batteries,fuel cells,memristors and other devices have been developed.Also,it is found that biological nerve activity and muscle contraction are also related to the ion transport properties in the body.With the progress of society,people have higher and higher requirements for the function of ion transport materials.The development of new ion transport materials and the improvement and expansion of the functions of existing ion transport materials have become an important research direction in the field of materials.We all know that any material is composed of atoms.The type of atom,the spatial distribution of atoms in the material?material structure?,and the valence state of the atom determine the nature and function of the material.How to design and develop new functional materials?How to expand new functions of known materials?How to evaluate the functional changes of materials in service?This is the goal of material research.This requires people to establish an intrinsic relationship between the microstructure and properties of the material.In-depth research is needed on the microstructure and ion transport properties of ionic materials.This doctoral dissertation has carried out electron microscopy research on ion transport under the control of the in-situ electric field around solid ion transport materials.The following research results have been achieved:?1?Ionic migration in the electrochemical process is a basic scientific issue for the phase change behavior and has important technical research significance for various functional devices,especially lithium-ion batteries.Usually only one type of functional ions responses to the external electric field,cooperated by the valence change of other immobile ions in the host lattice matrix due to the charge neutralization.For example,the extraction and insertion of lithium are accompanied by the oxidation and reduction of the transition metal ions,Co³⁺/Co⁴⁺and Mn³⁺/Mn⁴⁺redox couples,in typical anode materials Li Co O₂ and Li Mn₂O₄,respectively.We constructed an all-solid-state battery with Li₄Ti₅O₁₂as the anode.During further delithiation,the Ti⁴⁺valence state remained stable,and the migration changes of O^2-and Li⁺under the action of the electric field were observed in situ at the atomic scale.A small number of oxygen anions were extracted firstly because of its lowest vacancy formation energy under 2.2 V,leading to the vertical displacement of oxygen.When the bias voltage is increased to 2.7 V,Li⁺and O^2-are simultaneously extracted from the[Li O₄]tetrahedron and form an ordered phase with both Li-and O-deficiency.We propose and verify the migration paths of O and Li through first-principles calculations.These results reveal a new cooperative migration method and may provide the latest insights into the migration process of lithium-ion conductors.?2?BiFeO₃?BFO?is a complex multiferroic material with good ferroelectric,antiferromagnetic,and piezoelectric effects.We applied a large electric field through AFM,and for the first time realized a large reversible deformation of BFO film?about 5%?,which is 10 times higher than the strain caused by the piezoelectric effect.The in-situ electron microscopy technique is used to characterize the lattice expansion and theoretical calculation simulation is also performed.It was found that at low voltage,an unreported oxygen-deficient phase firstly appeared in the BFO film.The lattice constant elongation of the thin film was observed in ex-situ high-voltage samples.And the change of Fe and O valence was characterized by EELS,indicating the atomic structure of BFO materials induced by electrochemically of oxygen ion adsorption/extraction.It is confirmed that the regulation of oxygen migration by electrochemical methods is a new method to realize the reversible large expansion of lattices.The writing arrangement of this doctoral dissertation is as follows:Chapter 1:Introduction,an overview of ion transport properties,applications,and important research directions of ion transport functional materials.Chapter 2:Introduction to the principles of scanning transmission electron microscopy,spatial resolution,principles of electron energy loss spectroscopy,spatial resolution and data processing methods,and the application of in-situ electron microscopy in the study of ionic migrations.Chapter 3:Sample growth and in-situ device preparation.Chapter 4:Atomic-scale study on the synergistic O^2-/Li⁺dual ion transportation.Chapter 5:BiFeO₃ thin film has a huge and reversible deformation due to oxygen vacancy migration in the electrochemical process.Chapter 6:Summary and Outlook.