Atomic-scale Mechanism Investigation Of Solid Electrolytes And Relevant Interfaces For Lithium Batteries

Replacing flammable organic liquid electrolytes in conventional lithium batteries with solid electrolytes can effectively overcome the safety issues and the bottleneck of energy density.However,preparing high-conductive solid electrolytes and constructing the robust interface between solid electrolytes and electrodes are the prerequisite of achieving this goal.This thesis employed aberration-corrected scanning transmission electron microscopy to observe the solid electrolytes and the interface between solid electrolytes and electrodes at atomic scale,revealing the relationship between their macroscopic properties and the atomic configurations,which led to a deep understanding of the ion transport mechanism of solid electrolytes and the relevant interface.The more targeted strategies were thus proposed for the optimization of solid electrolytes and the relevant interfaces.Chapter 1 introduced the characteristics of various solid electrolytes and the related development background,the development of transmission electron microscopy and summarized the application of current transmission electron microscopy approaches in the field of solid electrolytes.Chapter 2 introduced the preparation methods of various transmission electron microscopy specimens.Based on the nature of the samples and the requirements for imaging,they can be divided into four categories:Cu grid,Ar ion thinning,ultrathin sectioning and focused ion beam.In addition,the various transmission electron microscopes used in this research and the image processing methods were also introduced.In Chapter 3,through the atomic-scale observation of Li0.33La0.56TiO3 solid electrolyte,it was found that a type of two-dimensional defects different from the grain boundary was widespread.These two-dimensional defects tended to connect to each other to form closed loops,which isolated a considerable volume of solid electrolytes.By characterizing the composition and structure of the two-dimensional defects,it indicated that they were isostructural with the {001} crystal plane of γ-Li2TiO3,and the stoichioietry was[Li1.11TiO3]0.93-.First principles calculations demonstrated that Li+cannot diffuse across two-dimensional defects.Therefore,the enclosed solid electrolytes cannot participate in the overall ion transport,just like the voids in the solid electrolytes."Single-atom-layer traps" was coined to describe such phenomenon.It was estimated that the enclosed volume was equivalent to an increase in the porosity of the solid electrolyte by 15.1 vol%.For oxide solid electrolytes,the ion conductivity would thus decrease by 1 to 2 orders of magnitude.In addition,it was discovered that these two-dimensional defects tended to form under the loss of Li and/or O,which provided important guidance to avoid the formation of two-dimensional defects.In Chapter 4,it was demonstrated that the spinel Li4Ti5O12 and the rock salt Li7Ti5O12,like the layered cathode 0.54Li2TiO3-0.46LiTiO2,can also form the semicoherent interface with the perovskite solid electrolyte Li0.33La0.56TiO3.The lattice mismatch between electrodes and solid electrolytes can be reconciled by the periodic dislocations at the interfaces,and different lattice mismatches can lead to different strain behaviors.Different from ordinary physical connections,semi-coherent interfaces were based on much stronger covalent bonds.Therefore,the semi-continuous interfaces can remain stable,even though the electrode underwent a phase change between the spinel Li4Ti5O12 and the rock salt Li7Ti5O12 during the charging and discharging process.The batteries assembled with the semi-coherent composite electrode Li4Ti5O12Li0.33La0.56TiO3 still maintained 85%of the initial capacity after as many as 200 cycles,showing a good application prospect of the semi-coherent method.In Chapter 5,the Li0.38Sr0.44Ta0.7Hf0.3O2.95F0.05 perovskite solid electrolyte was atomically observed through advanced integrated differential phase contrast imaging technique,and a large amount of interstitial Li was discovered.Combining the projection of the interstitial Li on the<011>p and<001>p of Li0.38Sr0.44Ta0.7Hf0.3O2.95F0.05,five types of Li sites were determined in the 3D lattice of Li0.38Sr0.44Ta0.7Hf0.3O2.95F0.05.In addition,according to first principles computations,thermodynamically there existed only three types of interstitial Li sites,which was in sharp contrast to the two interstitial Li sites speculated by previous studies.This study demonstrated that the combination of integrated differential phase contrast and first principles computations indicated the good application prospects in determining Li sites in Li-containing materials and the relevant mechanism.Chapter 6 summarized the research in the full text and discussed the development orientation of transmission electron microscopy in the future.